Mass Wasting and Landslide

Relationship Between Mass Wasting and Landslide

Mass wasting is the downslope movement of rock, soil, and debris under the direct influence of gravity, without the direct aid of moving water, wind, or ice as a transporting medium. It is a general, broad term that covers all gravity-driven slope movements.

Landslide is one specific type of mass wasting in which a mass of rock or soil moves downslope along a well-defined surface of rupture or a narrow zone of shear.

Relation: Landslide is a subset of mass wasting. Every landslide is a form of mass wasting, but every mass wasting process is not a landslide. Mass wasting includes falls, topples, slides, flows, and creep; landslide strictly refers to the slide-type movement (rotational or translational sliding along a failure plane). Thus landslide is one category within the wider classification of mass wasting.

Causes of Mass Wasting

Mass wasting occurs when the shear stress acting on a slope exceeds the shear strength of the slope material. The causes can be grouped as follows:

1. Internal (Inherent) Causes

(a) Gravity: The fundamental driving force. The component of gravity acting parallel to the slope (shear stress) pulls material downslope. Steeper slopes have a larger driving component.

(b) Slope Angle (Gradient): Steep slopes are unstable because the downslope component of gravity is large. Slopes steeper than the angle of repose of the material are especially prone to failure.

(c) Nature of Slope Material: Weak, unconsolidated, clay-rich, or highly fractured materials have low shear strength and fail easily. Clay minerals absorb water and lose cohesion.

(d) Geological Structures: Bedding planes, joints, faults, and foliation planes dipping parallel to the slope provide ready-made surfaces of weakness along which sliding occurs.

(e) Weathering: Chemical and mechanical weathering reduces rock strength, loosens grains, and produces weak regolith that is easily moved.

2. External (Triggering) Causes

(a) Water Content: Water is the most important trigger. It adds weight (increasing shear stress), reduces friction and cohesion between particles, and builds pore-water pressure that pushes grains apart. Saturation after heavy rain or snowmelt commonly triggers landslides.

(b) Undercutting of Slopes: River erosion at the base, wave action along coasts, and road cutting or quarrying remove support from the toe of a slope, causing failure.

(c) Earthquakes: Seismic shaking increases shear stress and liquefies saturated sandy material, triggering massive slides and flows.

(d) Volcanic Activity: Eruption-related shaking, melting of snow/ice, and loading by lava or ash can cause debris avalanches and lahars.

(e) Human Activities: Deforestation removes root binding; construction, irrigation, reservoir filling, and mining overload or destabilize slopes; blasting causes vibration.

(f) Overloading: Addition of weight by buildings, water, snow, or volcanic deposits increases driving stress.

(g) Removal of Vegetation: Roots hold soil together; their loss reduces cohesion and increases infiltration.

(h) Freeze–Thaw Action: Repeated freezing and thawing loosens rock and soil, promoting creep and falls.

---

OR

Floods and Geological Work of Running Water

What are Floods?

A flood is the overflow of water from a stream or river beyond its normal channel onto land that is normally dry. It occurs when the volume of water exceeds the channel's capacity, usually due to heavy or prolonged rainfall, rapid snowmelt, dam failure, or deforestation in the catchment area. Floods may be flash floods (sudden, short duration, high velocity) or river floods (slow, prolonged, large areal extent).

Geological Importance of Running Water

Running water is the most important agent of erosion on the Earth's surface. Its geological importance lies in three functions:

1. Erosion — removal and wearing away of rock and soil.
2. Transportation — carrying the eroded material downstream.
3. Deposition — laying down the transported material to form new landforms.

Role in Erosion

Running water erodes by:

• Hydraulic action: The force of moving water dislodges and breaks rock fragments.
• Abrasion: Sand, gravel, and boulders carried by the stream grind against the bed and banks.
• Attrition: Rock fragments collide with one another and become smaller and rounded.
• Solution: Soluble rocks such as limestone are dissolved by weakly acidic water.

Erosional landforms produced include V-shaped valleys, gorges, canyons, waterfalls, rapids, plunge pools, and potholes.

Role in Transportation

Material is moved as dissolved load (in solution), suspended load (fine particles carried in the water), and bed load (sand, gravel, and boulders rolled or dragged along the bed). The transporting capacity depends on the velocity and discharge of the stream.

Role in Deposition

Deposition occurs when the stream's velocity falls — on entering a lake, sea, or plain, or on the inside of a meander. Deposited material forms:

• Alluvial fans and cones at the base of mountain slopes.
• Flood plains — flat, fertile land along river banks.
• Natural levees — raised banks formed by repeated flooding.
• Deltas — at river mouths, e.g., the Ganges–Brahmaputra delta.
• Meanders, ox-bow lakes, and point bars in the lower course.

Summary

Running water continuously sculpts the land by eroding uplands, transporting debris, and depositing it in lowlands. This process produces fertile plains and deltas that support agriculture and settlement, while also causing floods and riverbank erosion — making running water the most powerful and economically significant geological agent.

a) Identification of the internal structure of the earth and its major internal boundaries

The internal structure of the earth cannot be observed directly, so it is identified mainly by indirect evidence:

1. Seismic waves (earthquake waves) — The behaviour of P-waves and S-waves gives the most reliable picture. P-waves travel through solid, liquid and gas, while S-waves travel only through solids. The sudden change in the velocity of these waves at certain depths shows that the earth is made of layers of different composition and physical state. S-waves disappear beyond a depth of about 2900 km, proving that the outer core is liquid.
2. Density and pressure studies — The average density of the earth (about 5.5 g/cm³) is much greater than that of surface rocks (about 2.7 g/cm³), so the interior must contain denser material.
3. Temperature and gravity data — The increase of temperature with depth (geothermal gradient) and the variation of gravity indicate a layered interior.
4. Meteorites — Meteorites contain iron and nickel in the same proportion believed to exist in the core.
5. Volcanic material — Lava and gases coming from the interior give information about the materials below the crust.

Major internal boundaries (discontinuities)

b) Intensity of earthquake and why Nepal is highly prone to earthquakes

Intensity of earthquake is the measure of the degree of damage and the strength of shaking produced by an earthquake at a particular place on the earth's surface. It depends on the magnitude of the earthquake, the distance from the epicentre, the depth of the focus and the local geological conditions. It is measured by the Modified Mercalli scale (I to XII), which is a descriptive scale based on observed effects on people, buildings and the ground. It differs from magnitude, which measures the energy released at the focus and is measured by the Richter scale.

Why Nepal is one of the most potential countries for earthquakes

1. Location on the Himalayan collision zone — Nepal lies on the boundary of the Indian plate and the Eurasian plate. The Indian plate is moving northwards at about 5 cm per year and is being subducted under the Eurasian plate. The accumulated strain is released suddenly as earthquakes.
2. Presence of active faults — The Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT) run across Nepal. Movement along these faults causes frequent earthquakes.
3. Young and unstable geology — The Himalaya is a young, rising mountain system; the rocks are weak, folded and fractured, so even moderate earthquakes cause heavy damage.
4. Unplanned settlement and weak construction — Rapid, unplanned urbanisation and non-engineered buildings in Kathmandu and other cities increase the risk of damage.
5. History of great earthquakes — The 1934 Bihar–Nepal earthquake (magnitude about 8.0) and the 2015 Gorkha earthquake (magnitude 7.8) show that Nepal lies in a high seismic zone (Zone V of the seismic zoning map).
6. Secondary hazards — Landslides, avalanches, glacial lake outburst floods (GLOFs) and soil liquefaction in the Terai add to the damage.

For these reasons Nepal is regarded as one of the most earthquake-prone countries in the world.

Question a) Oceanic Trenches and Theories on Coral Reef Development

Oceanic Trenches

Oceanic trenches are long, narrow, steep-sided, and deep depressions on the ocean floor. They are the deepest parts of the Earth's surface and are typically found along the margins of continents or island arcs, where tectonic plates converge. The deepest known trench is the Mariana Trench in the western Pacific Ocean, with a maximum depth of about 11,034 m (Challenger Deep).

Formation: Oceanic trenches form at subduction zones, where a dense oceanic plate descends beneath a less dense continental or oceanic plate. As the oceanic plate bends downward, it creates a deep, V-shaped depression on the seafloor. Trenches are often associated with volcanic island arcs, earthquakes, and mountain building.

Theories on Coral Reef Development

The development of coral reefs has been explained mainly by three theories:

1. Darwin's Subsidence Theory (1842):

• Proposed by Charles Darwin after his voyage on HMS Beagle.
• According to this theory, coral reefs develop around a subsiding volcanic island.
• Stages:
  • Fringing reef: Coral grows in shallow waters around the island's shore.
  • Barrier reef: As the island subsides, the coral continues to grow upward, leaving a lagoon between the reef and the island.
  • Atoll: When the island completely submerges, only a ring-shaped coral reef (atoll) remains around the lagoon.
• Evidence: The presence of lagoons, drowned volcanic peaks beneath atolls, and thick coral limestone deposits.

2. Daly's Glacial Control Theory (1915):

• Proposed by Reginald Daly.
• According to this theory, coral reefs formed after the Pleistocene glaciation.
• During glacial periods, sea level dropped, exposing and eroding pre-existing coral reefs. The flat surfaces were later submerged when glaciers melted and sea level rose.
• Coral then grew upward on these eroded platforms, forming reefs and atolls.
• Evidence: Flat-topped drowned reefs and the uniformity of lagoon depths (about 30–40 fathoms).

3. Murray's Volcanic Theory (1880):

• Proposed by John Murray.
• According to this theory, coral reefs develop on submarine volcanic peaks or elevated seafloor platforms.
• Coral grows upward on these elevated bases until it reaches sea level, forming reefs and atolls.
• Evidence: The presence of volcanic rocks beneath some coral reefs.

Conclusion: Among these, Darwin's subsidence theory is the most widely accepted, as it explains the sequence from fringing reef to barrier reef to atoll. However, modern studies suggest that reef development is influenced by a combination of subsidence, sea-level changes, and antecedent topography.

---

Question b) Aquifer and Geological Work of Groundwater

Aquifer

An aquifer is a geological formation (rock or sediment) that is permeable and porous enough to store and transmit groundwater in usable quantities. It acts as an underground reservoir from which water can be extracted through wells and springs.

Characteristics:

• High porosity (to store water) and high permeability (to transmit water).
• Examples: Sand, gravel, sandstone, fractured limestone, and fractured volcanic rocks.

Types:

1. Unconfined aquifer (water table aquifer): The upper boundary is the water table, which is open to the atmosphere.
2. Confined aquifer (artesian aquifer): Bounded by impermeable layers (aquicludes) above and below; water is under pressure, and wells may flow without pumping.

Geological Work of Groundwater

Groundwater performs significant geological work through erosion, transportation, and deposition, mainly by chemical processes.

1. Erosion:

• Solution: Groundwater containing dissolved carbon dioxide forms weak carbonic acid, which dissolves limestone and other soluble rocks (e.g., gypsum, rock salt). This creates caverns, caves, and sinkholes.
• Karst topography: Characterized by sinkholes, caves, disappearing streams, and underground drainage.

2. Transportation:

• Groundwater transports dissolved minerals (calcium carbonate, silica, iron) in solution.
• It also moves fine particles through pores and fractures.

3. Deposition:

• When groundwater rich in dissolved calcium carbonate reaches a cave, it precipitates as speleothems:
  • Stalactites: Hang from the cave ceiling.
  • Stalagmites: Grow upward from the cave floor.
  • Pillars/columns: Form when stalactites and stalagmites join.
• Travertine and tufa: Deposits formed at springs and hot springs.
• Petrified wood and concretions: Minerals precipitated around buried organic matter or sediment grains.
• Geodes: Hollow rock cavities lined with crystals.

Conclusion: Groundwater is a powerful geological agent, especially in limestone regions, where it creates distinctive karst landforms and underground caves. It also plays a vital role in weathering, soil formation, and the rock cycle.

---

OR

Question a) Evolution of Eolian Landforms

Eolian landforms are features formed by the erosional and depositional work of wind. They are most prominent in arid and semi-arid regions where vegetation is sparse, and loose sand and dust are abundant. The evolution of eolian landforms involves three main processes: erosion, transportation, and deposition.

1. Erosional Eolian Landforms

a) Deflation:

• Removal of loose sand and dust by wind, creating deflation hollows or blowouts (shallow depressions).
• In extreme cases, deflation can lower the land surface to the water table, forming oases or deflation basins (e.g., Qattara Depression, Egypt).

b) Abrasion:

• Sandblasting action of wind-borne particles wears down rock surfaces.
• Produces ventifacts (wind-faceted stones), mushroom rocks (pedestal rocks), and yardangs (streamlined ridges).

c) Attrition:

• Sand grains collide with each other, becoming rounded and polished.

2. Depositional Eolian Landforms

a) Sand Dunes:

• Mounds or ridges of sand formed by wind deposition.
• Types:
  • Barchan: Crescent-shaped dune with horns pointing downwind; formed where sand supply is limited.
  • Transverse dune: Long ridges perpendicular to wind direction.
  • Longitudinal dune: Ridges parallel to wind direction (e.g., Seif dunes).
  • Star dune: Complex dune with multiple arms; formed where wind direction varies.
  • Parabolic dune: U-shaped dune with horns pointing upwind; formed where vegetation anchors the arms.

b) Loess:

• Fine, wind-blown silt and clay deposited far from the source.
• Forms thick, fertile, and homogeneous deposits (e.g., Loess Plateau of China).

3. Factors Controlling Eolian Landform Evolution

• Wind velocity and direction: Stronger winds transport more material; prevailing wind direction determines dune orientation.
• Sand supply: Abundant sand leads to larger dunes.
• Vegetation: Sparse vegetation promotes eolian activity; vegetation stabilizes dunes.
• Climate: Arid climates with low rainfall and high evaporation favor eolian processes.
• Topography: Obstacles like rocks or hills trap sand, initiating dune formation.

4. Evolution Sequence

1. Weathering breaks rocks into loose particles.
2. Wind erosion removes and transports fine particles (deflation, abrasion).
3. Transportation carries particles by suspension, saltation, and surface creep.
4. Deposition occurs when wind velocity decreases, forming dunes and loess.
5. Stabilization may occur when vegetation colonizes dunes, converting them into fixed dunes.

Conclusion: Eolian landforms are dynamic features that evolve through the interaction of wind, sediment supply, climate, and vegetation. They are characteristic of desert and coastal environments and provide important clues about past and present wind patterns.

---

Question b) Importance of Geology for Infrastructure Development and Disaster Mitigation

Geology plays a vital role in infrastructure development and disaster mitigation by providing essential information about the Earth's materials, structures, and processes. Its importance can be discussed under the following headings:

A. Importance for Infrastructure Development

1. Site Selection:

• Geological studies help select suitable sites for dams, bridges, roads, tunnels, buildings, and airports.
• Factors such as rock type, soil strength, slope stability, and groundwater conditions are evaluated.

2. Foundation Design:

• Geology determines the bearing capacity of soil and rock, which is essential for designing safe foundations.
• Weak or unstable ground may require special foundation treatments (e.g., piling, grouting).

3. Construction Materials:

• Geology identifies sources of construction materials such as sand, gravel, limestone, and building stone.
• Quality and quantity of materials are assessed before construction.

4. Tunnels and Underground Works:

• Geological mapping helps predict rock conditions, faults, and fractures along tunnel alignments.
• This reduces construction risks and costs.

5. Water Resources:

• Geology helps locate groundwater for drinking, irrigation, and industrial use.
• It also assesses water quality and sustainability.

6. Environmental Impact Assessment:

• Geological studies help predict the impact of construction on land, water, and ecosystems.
• They guide mitigation measures to reduce environmental damage.

B. Importance for Disaster Mitigation

1. Earthquake Hazard Assessment:

• Geology identifies active faults, seismic zones, and soil conditions that amplify ground shaking.
• This information is used for earthquake-resistant design and land-use planning.

2. Landslide Hazard Assessment:

• Geological studies identify unstable slopes, weak rocks, and landslide-prone areas.
• Mitigation measures include slope stabilization, drainage, and retaining walls.

3. Flood Hazard Assessment:

• Geology helps understand floodplain characteristics, river behavior, and groundwater conditions.
• This guides flood control measures and safe settlement planning.

4. Landslide and Debris Flow:

• Geology assesses the risk of debris flows, mudflows, and rockfalls in mountainous regions.
• Early warning systems and land-use zoning can reduce losses.

5. Volcanic Hazard Assessment:

• Geology studies volcanic history and eruption patterns to assess risk.
• Helps in evacuation planning and hazard zoning.

6. Tsunami Hazard Assessment:

• Geology identifies subduction zones and past tsunami deposits to estimate tsunami risk.
• Guides coastal planning and early warning systems.

C. Conclusion

Geology provides the scientific basis for safe, sustainable, and cost-effective infrastructure development. It also helps in identifying, assessing, and mitigating natural hazards such as earthquakes, landslides, floods, and volcanic eruptions. Therefore, geological studies should be an integral part of planning and development projects to ensure safety and reduce disaster risk.

TU BBS — BSc First Year Difference between (any two) — 5×2 = 10 marks

---

a) Cross lamination and Ripple marks

---

b) Rocks and Minerals

---

c) Faults and Joints

---

Note: Any two of the above may be written as required.

Either: Crystallography vs mineralogy, and basis of classifying crystal systems (3 + 7)

Difference between crystallography and mineralogy (3 marks)

Basis of classification of crystal systems (7 marks)

Crystals are grouped into six (seven, if trigonal is separated from hexagonal) crystal systems on the basis of three things.

1. Number and length of crystallographic axes. Three axes ($a, b, c$) are used, or four ($a_1, a_2, a_3, c$) for the hexagonal system. The axes may be equal or unequal in length.
2. Angles between the axes ($\alpha = b\wedge c$, $\beta = a\wedge c$, $\gamma = a\wedge b$), whether all $90^\circ$, or some oblique.
3. Symmetry: the planes, axes and centre of symmetry present, which fix the axial relations.

The cubic system has the highest symmetry (up to 9 planes and 13 axes of symmetry) and triclinic the lowest (at most a centre of symmetry).

---

Or: Physical properties of minerals and those used for identification (3 + 7)

Physical properties (3 marks)

Physical properties are the observable characteristics that result from a mineral's chemical composition and internal atomic structure. They can usually be tested in the field without destroying the specimen, so they are the first tool for identifying a mineral.

Properties used for identification (7 marks)

1. Colour: the colour of the mineral in hand specimen. It is diagnostic for minerals with constant colour (malachite, green) but unreliable where impurities vary it (quartz can be clear, pink, purple or smoky).
2. Streak: the colour of the powder on an unglazed porcelain plate. It is more constant than colour (hematite always gives a red-brown streak, whatever its colour).
3. Lustre: the way light is reflected: metallic (galena), sub-metallic, or non-metallic (vitreous in quartz, pearly in talc, silky in asbestos, resinous, greasy, earthy).
4. Hardness: resistance to scratching, measured by the Mohs scale from 1 (talc) to 10 (diamond): talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. Handy field tests are the fingernail (2.5), copper coin (3.5), knife blade (5.5) and window glass (5.5).
5. Cleavage and fracture: cleavage is the tendency to split along smooth planes of weak bonding (perfect in mica, three directions in calcite, cubic in halite). Where there is no cleavage, the surface of breakage is a fracture (conchoidal in quartz, uneven, splintery, hackly).
6. Crystal form and habit: the characteristic shape (prismatic, tabular, acicular, fibrous, botryoidal, massive).
7. Specific gravity: the ratio of the mineral's weight to that of an equal volume of water, e.g. quartz 2.65, galena about 7.5. A mineral that feels "heavy for its size" has a high value.
8. Other tests: tenacity (brittle, malleable, flexible, elastic), transparency (transparent, translucent, opaque), magnetism (magnetite), taste (halite), effervescence with dilute HCl (calcite), and feel (talc soapy, graphite greasy).

No single property is enough, so several are combined, for example: colourless, vitreous, hardness 7, conchoidal fracture and no cleavage identify quartz.

a) Major properties of any three rock-forming minerals

1. Quartz (SiO₂)

• Colour: Usually colourless, white, or grey; may be pink, smoky, or purple (amethyst).
• Lustre: Vitreous (glassy), sometimes greasy.
• Hardness: 7 on Mohs scale — scratches glass.
• Cleavage: Absent; shows conchoidal (curved shell-like) fracture.
• Streak: White.
• Crystal form: Hexagonal, often as six-sided prisms with pyramidal ends.
• Occurrence: Chief constituent of granite, sandstone, and quartzite; very resistant to weathering.

2. Feldspar (orthoclase KAlSi₃O₈ / plagioclase)

• Colour: White, pink, cream, or grey.
• Lustre: Vitreous to pearly.
• Hardness: 6 on Mohs scale.
• Cleavage: Two sets of cleavages meeting at right angles (orthoclase) — gives blocky fragments.
• Streak: White.
• Crystal form: Monoclinic (orthoclase) or triclinic (plagioclase).
• Occurrence: Most abundant mineral group in the crust; major component of granite and other igneous rocks.

3. Mica (muscovite — white mica; biotite — black mica)

• Colour: Muscovite is colourless to silvery-white; biotite is black to dark brown.
• Lustre: Pearly to vitreous.
• Hardness: 2–3 on Mohs scale — easily scratched by a fingernail or copper coin.
• Cleavage: Perfect basal cleavage — splits into very thin, flexible, elastic sheets.
• Streak: White.
• Crystal form: Monoclinic, tabular or flaky.
• Occurrence: Common in granite, gneiss, and schist.

---

b) Economic minerals of Nepal and their importance

Nepal is rich in a variety of economic (industrial and metallic) minerals, though most are still exploited on a small scale.

Overall importance:

1. Industrial raw material — supports cement, glass, ceramic, and steel industries.
2. Employment generation — mining and mineral-based industries create jobs.
3. Foreign exchange — export of cement, talc, and gemstones earns foreign currency.
4. Infrastructure development — supplies construction materials for roads, bridges, and buildings.
5. Reduction of imports — local production reduces dependence on imported cement, iron, and salt.
6. Source of energy — coal, natural gas, and peat supplement conventional energy.
7. Balanced regional development — mineral-based industries can be set up in remote areas, reducing migration.

a) Crystallinity and forms of minerals

Crystallinity: A mineral is crystalline when its atoms, ions or molecules are arranged in a regular, repeating three-dimensional pattern (the crystal lattice). The flat faces and sharp edges of a well-formed crystal are the outward expression of this internal order. The degree of crystallinity depends on the conditions of formation:

• Crystalline (holocrystalline): ordered lattice throughout, e.g. quartz, halite, garnet. Slow cooling or crystallisation from a melt or solution favours this.
• Cryptocrystalline: crystallites too small to be resolved by the naked eye but still ordered, e.g. chalcedony (SiO₂).
• Amorphous (non-crystalline): no long-range order, e.g. opal, obsidian (volcanic glass). Rapid cooling or loss of volatiles prevents ordering. Such substances are strictly mineraloids.

Forms of minerals: the external shape or habit, controlled by the internal symmetry and the growth environment.

1. Euhedral: crystal bounded by its own well-developed faces (e.g. a perfect cube of pyrite).
2. Subhedral: only some faces developed.
3. Anhedral: no faces developed, irregular grains (e.g. quartz in granite).
4. Habit types: prismatic (quartz), tabular (barite), acicular (needle-like, natrolite), fibrous (asbestos), bladed (kyanite), dendritic (native copper, pyrolusite), botryoidal (hematite), reniform, mammillary, stalactitic (calcite), foliated (mica), granular (olivine).
5. Aggregate forms: columnar, radiating, concretionary, oolitic, earthy, massive (no definite shape, e.g. massive hematite).
6. Twins: two or more individuals of the same species intergrown in a definite crystallographic relation, e.g. contact twin (gypsum swallow-tail), penetration twin (staurolite cross), polysynthetic twin (albite striations in plagioclase).

---

b) Space group and international space notation

Concept of space group: A space group is the complete set of symmetry operations, both point-group operations (rotation, reflection, inversion, roto-inversion) and translational operations (lattice translations, screw axes, glide planes), that leave a crystal structure unchanged. It describes the full three-dimensional periodicity of the structure. There are exactly 230 space groups in three dimensions (Fedorov and Schönflies, 1891). They arise by combining the 32 crystal classes (point groups) with the 14 Bravais lattices and adding screw axes and glide planes: 73 are symmorphic and the remaining 157 are non-symmorphic.

International (Hermann–Mauguin) notation: A space group symbol has two parts.

1. Lattice letter: P (primitive), A, B, C (one-face centred), I (body-centred), F (all-face centred), R (rhombohedral).
2. Symmetry symbols: up to three symbols giving the symmetry elements along the principal directions of the crystal system. A screw axis is written $n_m$ (e.g. 2₁, 3₁, 4₂, 6₃): rotation of $360^\circ/n$ combined with translation of $m/n$ of the repeat distance. A glide plane is written a, b, c (axial glide), n (diagonal glide) or d (diamond glide); a mirror plane is m; a roto-inversion axis is $\bar3$, $\bar4$, $\bar6$.

Examples:

The notation is compact, unambiguous and universal, so it is used in crystallography, mineralogy and X-ray structure determination.

Field Geology, Plotting Geological Data and Cross-Sections

1. Field Geology (2 marks)

Field geology is the branch of geology concerned with the direct study of rocks, structures and geological features in the field (at natural outcrops) rather than in the laboratory. It involves observation, measurement of the attitude of beds (strike and dip), collection of samples and data, and interpretation to reconstruct the geological history of an area. It is the base of all geological work, since laboratory conclusions must finally be checked against the field.

2. Methods of Plotting Geological Data on Maps (8 marks, with cross-sections)

Field data are plotted on a base (topographic) map to prepare a geological map.

(a) Outcrops and lithology. Contacts between rock units are traced by walking along them and drawn on the map. Each unit gets its own colour or symbol. The map needs a scale, north arrow and grid references.

(b) Strike and dip. The attitude of a bed is recorded with a clinometer compass: strike is the direction of the horizontal line in the bed, and dip is the maximum inclination measured perpendicular to the strike. The standard map symbol is a long line for the strike, a short tick perpendicular to it pointing in the dip direction, and the dip angle written beside it. For example, a symbol with "30" and a tick pointing south on an E–W line means the bed strikes E–W and dips $30^\circ$ south.

(c) Contour (topographic) intersection and the rule of V's. The trace of an inclined bed on the map is found by intersecting the bed surface with the topographic contours. The outcrop trace forms a V where it crosses a valley, and the V points in the direction of dip (downstream if the bed dips downstream more gently than the stream falls, upstream if the bed dips upstream). Horizontal beds follow the contours, and vertical beds cross valleys in straight lines.

(d) Three-point method. If the elevation of a boundary is known at three points (boreholes or outcrops), the strike is the line joining the two points of equal elevation (found by interpolation), and the dip is calculated from the height difference and the horizontal distance perpendicular to that strike line.

(e) Structural data. Faults are drawn as thick lines with symbols for the downthrown side or movement, folds by axial traces with anticline/syncline symbols, and joints and many attitude readings are analysed on rose diagrams or stereonets.

(f) Finishing the map. The data are transferred to a fair copy, a legend (rock types, symbols), title, scale and north arrow are added, and the cross-section line (A–B) is marked.

3. Preparation of a Cross-Section

A geological cross-section is a vertical profile along a chosen line on the map, showing the rock units and structures beneath the surface.

1. Select the section line A–B, preferably perpendicular to the strike so that true dips and thicknesses are shown.
2. Draw the topographic profile by transferring the elevation of each contour crossing to a vertical scale (same as the horizontal scale, or exaggerated).
3. Mark the points where A–B cuts contacts, faults and fold axes, and transfer them to the profile.
4. Plot the dips at each contact with a protractor, in the correct direction (use the apparent dip if the line is not perpendicular to the strike).
5. Extend the beds downward with constant thickness until they meet a fault or other boundary.
6. Show the structures: faults as inclined lines with arrows, folds as arches and troughs, and unconformities as wavy lines.
7. Add legend, title, scale and orientation (A at one end, B at the other).

Cross-sections help to interpret subsurface structure, find true thickness and depth, locate ore bodies, groundwater and petroleum traps, and plan engineering sites.

---

OR

Formation and Classification of Faults

1. How Faults are Formed (2 marks)

A fault is a fracture in rock along which the two blocks have moved relative to each other. Faults form when stress exceeds the strength of the rock and it fails by brittle fracture instead of folding:

• Tension gives normal faults (crustal extension, e.g. East African Rift).
• Compression gives reverse and thrust faults (e.g. the Himalayan thrusts: MCT, MBT, MFT in Nepal).
• Shear gives strike-slip faults (e.g. San Andreas Fault).
• Uplift, subsidence and gravity sliding can also cause faults.

2. Classification of Faults (8 marks)

Basis 1: relative movement of the blocks (dip-slip faults)

The hanging wall is the block above an inclined fault plane and the footwall the block below.

Normal fault: the hanging wall moves down relative to the footwall. Caused by tension, with a steep plane.

   Footwall   \
  ____________ \
               \ Hanging wall moves down

Reverse fault: the hanging wall moves up relative to the footwall. Caused by compression, with a steep plane (above $45^\circ$).

                 / Hanging wall moves up
   Footwall     /

Thrust fault: a reverse fault with a gentle plane (below $45^\circ$); the hanging wall is pushed far over the footwall. Example: the Main Boundary Thrust of Nepal.

        ===> Hanging wall (overriding)
     ______________________
   Footwall (overridden)

Basis 2: direction of slip

• Dip-slip fault: slip is along the dip of the fault plane (normal, reverse, thrust).
• Strike-slip fault: slip is horizontal, parallel to the strike of the plane, which is usually vertical. It is dextral (right-lateral) if the opposite block appears to move to the right, and sinistral (left-lateral) if to the left.
• Oblique-slip fault: slip has both dip and strike components.

 Map view of a dextral fault
      ---->
   ============  fault line
      <----

Basis 3: attitude of the fault relative to the strike of the beds

• Strike fault: fault strike is parallel to the strike of the beds.
• Dip fault: fault strike is parallel to the dip direction of the beds.
• Oblique fault: fault strike is oblique to the strike of the beds.

Group patterns: parallel normal faults may form step faults, a horst (raised block between two normal faults) and graben (dropped block between two normal faults, e.g. rift valleys).

Faults control earthquakes, mineralisation, groundwater, springs and valleys, and affect dams, tunnels and bridges.

a) Usefulness and stepwise plotting of a plane in stereographic projection

Usefulness: Stereographic projection represents three-dimensional orientation data (planes and lines) on a two-dimensional sheet. Many measurements can be plotted, compared and statistically analysed together. It is used to find true dip and strike from apparent dips, to determine plunge and pitch of lineations, to find the intersection of two planes (e.g. a fold axis), to construct fabric diagrams, to solve rotation problems (such as restoring tilted beds), and to prepare maps and cross-sections from field data.

Plotting a plane (example: strike N40°E, dip 30° SE):

1. Prepare the net. Place tracing paper over a stereonet (Wulff, equal-angle, or Schmidt, equal-area). Mark the centre with a pin, draw the primitive circle and mark north.
2. Mark the strike. Measure 40° clockwise from north along the primitive circle and mark the point (and its opposite end, at 220°).
3. Align the strike with the N–S diameter. Rotate the tracing paper about the centre until the strike mark lies at the N (or S) end of the net's N–S line. The strike line now lies along the N–S diameter, so the dip is measured along the E–W diameter.
4. Mark the dip. Count 30° from the primitive circle inward along the E–W diameter, on the side towards which the plane dips (here the south-east side, which is now the east or west half of the net depending on the rotation). Trace the great circle that passes through this point and through the two ends of the strike line. This great circle is the plane.
5. Plot the pole (optional). The pole is the normal to the plane. It lies on the E–W diameter on the side opposite the dip direction, at an angular distance from the net centre equal to the dip ($30^\circ$), i.e. $90^\circ$ from the dip point on the great circle. Mark it.
6. Rotate back. Turn the tracing paper back until the north marks coincide again. The great circle and its pole now give the plane's true orientation, and the intersection of two such great circles gives the trend and plunge of their line of intersection.

---

b) Schistosity, foliation and lineation

Schistosity: a penetrative planar fabric of medium to high-grade metamorphic rocks (schists), in which platy and prismatic minerals (mica, chlorite, talc, graphite) are aligned in parallel, wavy or lenticular layers coarse enough to see with the naked eye. It forms by recrystallisation under directed pressure during regional metamorphism, with the platy minerals growing perpendicular to the maximum compressive stress. It is a coarser, more strongly developed form of slaty cleavage.

Foliation: any penetrative planar fabric in a rock produced by parallel arrangement of minerals or compositional layers: slaty cleavage (slate), schistosity (schist) and gneissic banding (gneiss). It develops when platy or elongate minerals rotate or grow perpendicular to the maximum compressive stress during deformation and metamorphism.

Lineation: a penetrative linear fabric element: aligned elongate minerals (hornblende, kyanite), stretched pebbles or fossils, mineral streaks, fold axes and intersection lineations where two planar surfaces meet. It may be primary (igneous flow, deposition) or secondary (deformation and metamorphism).

Development:

• Directed pressure causes rotation and recrystallisation of platy minerals into parallelism perpendicular to the maximum stress, producing foliation.
• Alignment of prismatic minerals and stretching of pebbles, fossils and aggregates in the direction of extension produces lineation.
• Intersection of two foliations (e.g. bedding and cleavage) gives an intersection lineation, commonly parallel to the fold axis.
• With increasing metamorphic grade, slaty cleavage passes into schistosity and then gneissic banding.

a) Antiform and synform

Note: Antiform and synform describe the shape of the fold only, while anticline and syncline describe the age relation of rocks in the core (anticline: oldest rocks in the core; syncline: youngest in the core). Where the succession is normal, antiform = anticline and synform = syncline; where the sequence is overturned or unknown, an antiform may be a syncline and vice versa.

b) Sills and dykes

c) Apparent dip and true dip

Relation: $\tan(\text{apparent dip})=\tan(\text{true dip})\times\sin\theta$, where $\theta$ is the angle between the apparent dip direction and the strike.

Define the process of erosion and denudation. Describe the geological process and importance of running water in detail. [3+7]

1. Erosion and Denudation

Denudation is the collective, overall process by which the Earth's surface is stripped, worn down, and lowered. It is a broad term that includes all the processes — weathering, mass wasting, erosion, and transportation — that together reduce the elevation of the land. Denudation is essentially the "uncovering" or "laying bare" of the underlying rocks as the overlying material is removed.

Erosion is one specific component of denudation. It is the process by which rock material is loosened, dissolved, or worn away from the surface and simultaneously removed (transported) from its place of origin by a moving agent such as running water, wind, ice, or waves. Erosion involves two inseparable actions: detachment of particles and removal/transport of those particles. If material is loosened but not carried away, the process is only weathering or disintegration, not erosion.

Key distinction: Denudation is the umbrella term (the total lowering of the land); erosion is one of the tools that accomplishes denudation. Erosion cannot occur without a transporting medium; denudation can occur through weathering and mass wasting even without a transporting medium.

---

2. Geological Processes and Importance of Running Water

Running water is the single most important agent of erosion and denudation on the Earth's surface, because water covers about 71% of the globe and rainfall is widespread. It operates through three stages: erosion, transportation, and deposition, producing characteristic landforms at each stage.

A. Geological Processes of Running Water

1. Erosional Processes

Running water erodes the land in four ways:

• Hydraulic action: The sheer force of flowing water compresses air into cracks and joints in rocks; when the water recedes, the compressed air expands explosively, shattering the rock.
• Abrasion (corrasion): Sand, gravel, and rock fragments carried by the river act like cutting tools, grinding and scouring the river bed and banks.
• Attrition: Rock fragments carried by the river collide with one another and become progressively smaller and smoother.
• Solution (corrosion): Water dissolves soluble rocks such as limestone, chalk, and gypsum, carrying them away in solution.

2. Transportation Processes

The eroded material (called load) is transported in four ways:

• Traction: Large boulders are rolled or dragged along the bed.
• Saltation: Pebbles and coarse sand bounce along the bed in a hopping motion.
• Suspension: Fine silt and clay are carried floating within the water.
• Solution: Dissolved minerals are carried invisibly in solution.

3. Depositional Processes

Deposition occurs when the river's velocity and carrying capacity fall — on entering a lake or sea, on a gentle slope, or on the inside of a meander bend. The coarsest material is dropped first, the finest last. This produces alluvium (fertile river deposits).

B. Landforms Produced by Running Water

Erosional landforms:

• V-shaped valley / gorge / canyon: Vertical erosion in the upper course cuts deep, narrow valleys. Example: the Kali Gandaki gorge.
• Waterfalls and rapids: Formed where a river crosses a hard-rock band over softer rock (e.g., Davis Falls, Pokhara).
• Pot holes: Circular depressions drilled into the river bed by abrasion.
• Meanders and ox-bow lakes: Lateral erosion in the middle course produces winding curves; a cut-off meander becomes an ox-bow lake.
• River terraces: Remnants of former floodplains left standing after renewed down-cutting.

Depositional landforms:

• Alluvial fans and cones: Formed where a stream leaves a mountain front and spreads its load.
• Flood plains: Flat, fertile land built up by repeated flooding.
• Natural levees: Raised banks formed by coarse sediment dropped at flood time.
• Delta: Formed where a river enters a sea or lake and deposits its load, e.g., the Sundarbans (Ganges–Brahmaputra) delta.
• Point bars: Deposits on the inner (convex) side of a meander.

C. Importance of Running Water

1. Geological importance:

• It is the chief agent of denudation and landform evolution, carving valleys, gorges, and plains.
• It transports and deposits sediments, building deltas, flood plains, and alluvial fans.
• It recycles rock material and contributes to the formation of sedimentary rocks.
• It shapes drainage patterns and controls the regional topography.

2. Economic and human importance:

• Fertile soil: River alluvium forms the most productive agricultural land (e.g., the Terai plains of Nepal).
• Water supply: Rivers supply drinking water, irrigation water, and water for industry.
• Hydropower: Running water is a major source of renewable energy (e.g., Nepal's hydropower potential).
• Transportation: Navigable rivers serve as cheap means of transport.
• Fisheries and recreation: Rivers support fishing, tourism, and rafting.
• Sand and gravel: River beds are a source of construction materials.

3. Hazards:

• Floods, bank erosion, and landslides cause loss of life, land, and property — a serious problem in Nepal's monsoon season.

Conclusion

Denudation is the total process of wearing down the land, of which erosion is the most active part. Running water, through hydraulic action, abrasion, attrition, and solution, erodes, transports, and deposits material, creating both destructive and constructive landforms. It is thus the most powerful and economically most valuable geological agent shaping the Earth's surface.

---

OR

What are geological structures? Differentiate between primary and secondary geological structures with examples and figures. [2+8]

1. Geological Structures

Geological structures are the features produced in rocks by deformation — that is, by the forces (stress) acting on rocks that cause them to bend, break, fold, or fracture. They record the direction and intensity of the tectonic forces that acted on the rock mass after (or during) its formation. They are studied under structural geology.

Geological structures are broadly classified into two types:

1. Primary structures — formed at the time of formation of the rock.
2. Secondary structures — formed after the rock has already formed, due to later deformation.

---

2. Difference Between Primary and Secondary Geological Structures

Figures

Primary structures (undisturbed, horizontal beds):

  ─────────────────────────────  ← bedding plane
  ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
  ─────────────────────────────
  ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒
  ─────────────────────────────
  ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓
        (Horizontal strata)

Secondary structures — Fold (Anticline and Syncline):

        Anticline            Syncline
          /\                  \  /
         /  \                  \/
        /    \                /  \
       /      \              /    \
  ────/        \────────────/      \────
      (Upward arch)      (Downward trough)

Secondary structures — Fault (with displacement):

  ────────────╲
               ╲  ← fault plane
                ╲────────────
   (hanging wall block has slipped down
    relative to the footwall block)

Secondary structures — Joints (fractures without displacement):

  │    │   │      │
  │    │   │      │
  │    │   │      │
  │    │   │      │
   (Parallel fractures)

---

3. Explanation of Primary Structures

These are formed during the formation of the rock itself:

• Bedding planes: The boundary surfaces between successive layers (beds) of sedimentary rock.
• Cross-bedding: Inclined layers within a bed, formed by migrating sand dunes or ripples.
• Graded bedding: A bed in which grain size decreases from bottom to top, formed by a single depositional event (e.g., turbidity current).
• Ripple marks: Small wavy ridges formed on sand by water or wind currents.
• Mud cracks: Polygonal cracks formed when wet mud dries and shrinks.
• Vesicular structure: Small cavities in volcanic rocks formed by escaping gas bubbles.

4. Explanation of Secondary Structures

These are formed by tectonic deformation after rock formation:

• Folds: Bending of rock layers due to compressional stress. An anticline is an upward arch (older rocks in the core); a syncline is a downward trough (younger rocks in the core).
• Faults: Fractures along which rocks have been displaced. Types include normal, reverse, and strike-slip faults.
• Joints: Fractures along which there has been no displacement. They are very common in rocks and control weathering and drainage.
• Unconformity: A surface representing a gap in the geological record, where deposition was interrupted by erosion or non-deposition.

Conclusion

Geological structures record the history of rock formation and later deformation. Primary structures tell us about the environment of deposition, while secondary structures tell us about the tectonic forces that acted on the rocks. Both are essential for understanding the geological history and for engineering and mineral exploration work.

a) Typical properties of crust, mantle and core [5]

The Earth is divided into three main concentric layers — crust, mantle and core — on the basis of chemical composition and physical state.

1. Crust

• It is the outermost, thinnest and lightest solid layer of the Earth.
• Average thickness: about 5–10 km under the oceans (oceanic crust) and 30–70 km under the continents (continental crust).
• Average density: about 2.7 g/cm³ (continental) and 3.0 g/cm³ (oceanic).
• Chiefly composed of silicate rocks — granite (continental, rich in Si and Al, called SIAL) and basalt (oceanic, rich in Si and Mg, called SIMA).
• It is brittle, rigid and broken into plates; it floats over the mantle.
• Temperature increases with depth, reaching about 400–600 °C at its base.
• It is separated from the mantle by the Mohorovičić discontinuity (Moho).

2. Mantle

• It lies below the crust and above the core, extending from the Moho to a depth of about 2,900 km.
• Thickest layer, about 2,900 km thick, and makes up nearly 84% of the Earth's volume.
• Density ranges from about 3.3 g/cm³ (upper) to 5.5 g/cm³ (lower).
• Composed mainly of iron- and magnesium-rich silicates (olivine, pyroxene, peridotite).
• The upper mantle (asthenosphere) is partially molten and plastic, and is the source of magma and the seat of convection currents that drive plate movement.
• It is separated from the core by the Gutenberg discontinuity.

3. Core

• It is the innermost and central layer, extending from 2,900 km to the centre at 6,371 km.
• It is divided into a liquid outer core (2,900–5,150 km) and a solid inner core (5,150–6,371 km).
• Density is very high — about 9.9 g/cm³ in the outer core and 13.6 g/cm³ in the inner core.
• Composed mainly of iron and nickel (Ni–Fe alloy), hence called NIFE.
• Temperature is extremely high, about 5,000–6,000 °C.
• The outer core is in a molten state; its movement generates the Earth's magnetic field. The inner core is solid due to immense pressure.
• It is separated from the mantle by the Gutenberg discontinuity.

---

b) Possibility of earthquake occurrence in Nepal Himalaya and its impact [2+3]

Possibility of occurrence [2] Nepal lies in one of the most seismically active zones of the world. The Indian Plate is moving northward at about 2 cm per year and colliding with the Eurasian (Tibetan) Plate. This collision has produced the Himalaya and has stored a huge amount of strain energy along active faults such as the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT). When this accumulated strain exceeds the rock strength, it is suddenly released as an earthquake. Historical records show major earthquakes in 1255, 1505, 1803, 1833, 1934 (8.4 M), 1988 (6.6 M) and 2015 (7.8 M). Since the last great earthquake in this region occurred long ago and strain is still accumulating, seismologists consider a major earthquake (above 8 M) highly probable in the Nepal Himalaya in the near future. Thus, the possibility of earthquake occurrence in Nepal is very high.

Impacts [3]

1. Loss of life and property: Earthquakes cause heavy casualties and destroy houses, schools, hospitals and other structures. The 2015 Gorkha earthquake killed nearly 9,000 people and damaged about 800,000 houses.
2. Landslides and avalanches: Shaking triggers landslides in the fragile Himalayan terrain, blocking rivers and roads and causing further damage. The 2015 earthquake triggered thousands of landslides and an avalanche on Everest.
3. Damage to infrastructure: Roads, bridges, hydropower projects, water supply and communication systems are badly damaged, disrupting transport, electricity and communication for a long time.
4. Economic loss and displacement: Huge amounts of money are needed for rescue and reconstruction, which slows national development. Many people become homeless and are forced to live in temporary shelters.
5. Impact on heritage and tourism: Historical monuments, temples and heritage sites (e.g., Dharahara, Kasthamandap) are destroyed, and trekking and tourism decline, affecting the livelihood of many people.

TU BBS — BSc First Year

Option A

a) Plate Tectonics and Evolution of the Theory [2+3]

Plate tectonics is the scientific theory that the Earth's outer rigid shell, the lithosphere (crust + uppermost rigid mantle), is broken into several large and small slabs called plates that float over the plastic, partially molten asthenosphere and move slowly relative to one another. The relative motion of these plates — diverging, converging or sliding past each other — is responsible for most earthquakes, volcanoes, mountain building and the present arrangement of continents and oceans.

Evolution of the theory:

1. Continental Drift (Alfred Wegener, 1912): Wegener proposed that all continents were once joined into a single supercontinent, Pangaea, surrounded by one ocean, Panthalassa, which broke up and drifted apart. Evidence: jig-saw fit of coastlines (Africa–South America), matching fossils (Glossopteris flora, Mesosaurus), similar rock formations and glacial deposits on now-separated continents. His theory was rejected because he could not explain the driving force.

2. Sea-floor Spreading (Harry Hess, 1960s): Mapping of the ocean floor revealed the mid-oceanic ridge system, and palaeomagnetic studies showed symmetrical magnetic stripes on either side of the ridge. Hess explained that new oceanic crust forms at the ridge by upwelling magma and spreads outward, while old crust is destroyed at deep-sea trenches. This supplied the missing mechanism.

3. Plate Tectonics (1960s–70s): Combining continental drift, sea-floor spreading, palaeomagnetism and earthquake/volcano distribution, the modern theory was formulated. It explains the Earth's dynamics in terms of the movement, creation and destruction of lithospheric plates driven by mantle convection, ridge push and slab pull. It is now the unifying theory of geology.

---

b) Minerals and Process of Mineral Formation [1+4]

Minerals are naturally occurring, inorganic, homogeneous solids with a definite chemical composition and an ordered internal (crystalline) atomic structure. Examples: quartz (SiO₂), feldspar, calcite, mica, olivine.

Processes of mineral formation:

1. Crystallisation from magma and lava (igneous process): As molten magma/lava cools, atoms arrange themselves in an orderly pattern and crystallise. Slow cooling at depth produces large crystals (e.g., granite — quartz, feldspar, mica); rapid cooling at the surface produces fine crystals or glass (e.g., basalt, obsidian). Fractional crystallisation gives different minerals at different temperatures — olivine first, quartz last.

2. Precipitation from solution (sedimentary process): When mineral-rich water evaporates or becomes saturated, minerals precipitate. Example: rock salt (halite) and gypsum from evaporating seas; calcite (CaCO₃) from lime-rich water forming limestone and travertine.

3. Metamorphic recrystallisation: Under high temperature and pressure, existing minerals recrystallise into new minerals without melting. Example: limestone → marble (calcite), shale → slate, sandstone → quartzite; garnet and kyanite form in schists and gneisses.

4. Biological and hydrothermal processes: Organisms secrete hard parts (calcium carbonate shells → limestone; silica skeletons → chert). Hot mineral-rich solutions rising through fractures deposit ore minerals in veins — e.g., quartz, galena (PbS), sphalerite (ZnS), gold and copper sulphides.

---

OR

a) Causes of Mass Movement and Its Impact [5]

Mass movement (mass wasting) is the downslope movement of rock, soil and debris under the direct influence of gravity, without the aid of a transporting medium such as water, wind or ice.

Causes:

1. Gravity: The fundamental driving force; steeper slopes have a greater component of gravity acting downslope.
2. Slope angle and topography: Steep, undercut or over-steepened slopes (by rivers, roads, quarrying) are unstable.
3. Water content: Heavy rainfall, snowmelt or irrigation saturates the material, increases its weight and pore-water pressure, and reduces friction — a major trigger of landslides and mudflows.
4. Geological structure: Weak, jointed, fractured or clay-rich rocks, and bedding/foliation planes dipping downslope, favour failure.
5. Earthquakes and volcanic activity: Shaking loosens material and triggers rockfalls, avalanches and slumps.
6. Human activities: Deforestation, slope cutting for roads and buildings, unplanned excavation, mining and overgrazing remove vegetation cover and root binding, destabilising slopes.
7. Freeze–thaw and weathering: Alternate freezing and thawing loosens rock and soil on high mountain slopes.

Impacts:

• Loss of life and property: Landslides bury settlements, kill people and livestock, and destroy houses, roads and bridges.
• Blockage of rivers: Debris dams may form, causing flash floods when they burst (e.g., in the Nepal Himalaya).
• Soil erosion and loss of fertility: Fertile topsoil is removed, reducing agricultural productivity.
• Damage to infrastructure: Highways, hydropower projects, irrigation canals and communication lines are disrupted.
• Sedimentation: Reservoirs and river beds silt up, reducing water storage and increasing flood risk.
• Economic loss: Costly rescue, relief, rehabilitation and reconstruction; disruption of tourism and trade.

---

b) Glaciers and Associated Landforms [1+4]

Glaciers are large, slow-moving, perennial masses of ice formed by the compaction and recrystallisation of snow on land, which move downslope or outward under their own weight. They are of two main types: valley (alpine) glaciers and continental ice sheets.

Landforms associated with glaciers:

A. Erosional landforms:

1. Cirque (corrie): A bowl-shaped, steep-walled hollow at the head of a glacier, formed by glacial scouring; when filled with water it becomes a tarn lake.
2. Arete and Horn: A sharp, knife-edged ridge between two cirques is an arete; where three or more cirques meet, a sharp pyramidal peak called a horn (e.g., Matterhorn) is formed.
3. U-shaped valley (glacial trough): A river valley widened and deepened by glacial erosion into a U-shaped cross-section.
4. Hanging valley: A tributary valley joining the main glacial trough high above its floor, often ending in a waterfall.
5. Roche moutonnée and crag-and-tail: Asymmetrical rock hummocks smoothed on the up-glacier side and steep on the down-glacier side.
6. Fjord: A drowned glacial trough invaded by the sea, with steep sides and deep water.

B. Depositional landforms:

1. Moraine: Accumulation of unsorted rock debris deposited by a glacier — lateral (along sides), medial (between two glaciers), terminal/end (at the snout) and ground moraine (beneath the ice).
2. Drumlin: A smooth, elongated, egg-shaped mound of till, blunt end facing up-glacier.
3. Esker: A long, winding ridge of sand and gravel deposited by meltwater streams flowing under or within the glacier.
4. Kame and Kettle: Kames are small mounds of stratified drift; kettles are depressions formed by the melting of buried ice blocks, often filled with lakes.
5. Outwash plain: A gently sloping plain of sorted sand and gravel spread by meltwater beyond the terminal moraine.
6. Erratics: Large boulders carried far from their source and left on different bedrock.

Difference between (any two) [5×2=10]

---

a) Lithosphere and Asthenosphere

In short: The lithosphere is the rigid outer shell; the asthenosphere is the plastic, weaker layer beneath it that allows the plates to move.

---

b) Divergent and Convergent Plate Boundaries

In short: Divergent boundaries pull plates apart and create new crust; convergent boundaries push plates together and destroy crust.

---

c) Igneous Rocks and Metamorphic Rocks

In short: Igneous rocks form by cooling of molten material, while metamorphic rocks form by the transformation of existing rocks under heat and pressure.

Major Crystal Systems and Comparison of Isometric, Triclinic and Monoclinic Systems

1. Major Crystal Systems (2 marks)

On the basis of the number of crystallographic axes, their relative lengths, and the interfacial angles between them, all crystals are grouped into seven crystal systems:

1. Cubic (Isometric) — 3 equal axes, all at 90°
2. Tetragonal — 3 axes, two equal + one unequal, all at 90°
3. Hexagonal — 4 axes (3 equal horizontal + 1 vertical), horizontal axes at 60°
4. Trigonal (Rhombohedral) — 4 axes (3 equal + 1), angles not 90°
5. Orthorhombic — 3 unequal axes, all at 90°
6. Monoclinic — 3 unequal axes, one inclined (two at 90°, one oblique)
7. Triclinic — 3 unequal axes, all inclined (no angle 90°)

---

2. Differences between Isometric, Triclinic and Monoclinic Systems (8 marks)

The three systems are compared on the basis of axial lengths (a, b, c) and axial angles (α, β, γ), where α is the angle between b and c, β between a and c, and γ between a and b.

Explanation of the differences

Isometric system: The three crystallographic axes are equal in length and mutually perpendicular. Because of this high symmetry, light travels with the same velocity in all directions, so isometric minerals are isotropic (they show a single refractive index and remain dark under crossed nicols). Example: halite forms cubes; garnet forms dodecahedra.

Monoclinic system: The three axes are unequal. The b-axis is perpendicular to both a and c (α = γ = 90°), but the a and c axes are inclined to each other (β > 90°). This gives only one plane of symmetry and one two-fold axis. The inclined axes make the mineral anisotropic. Example: gypsum (selenite) and orthoclase show oblique faces.

Triclinic system: The three axes are unequal and none of the axial angles is 90°. The crystal has the least symmetry — only a centre of symmetry. It is strongly anisotropic. Example: kyanite (bladed crystals) and plagioclase feldspar.

Figures (described)

• Isometric: a cube — three equal axes X, Y, Z meeting at 90° at the centre.
• Monoclinic: a box-like form with the front face tilted — b-axis perpendicular to the a–c plane, while a and c are inclined to each other at β > 90°.
• Triclinic: a completely tilted, oblique box — all three axes unequal and all angles oblique.

---

OR

Optical Mineralogy and Optical Properties of Minerals

1. Optical Mineralogy (2 marks)

Optical mineralogy is the branch of mineralogy that deals with the study of minerals by means of polarized light, using a petrographic (polarizing) microscope. It involves the preparation of thin sections of rocks (about 0.03 mm thick) and the determination of optical properties such as refractive index, birefringence, pleochroism, extinction angle and optic sign. It is the most important method for identifying rock-forming minerals.

---

2. Optical Properties of Minerals (8 marks)

(a) Colour and Pleochroism

Minerals show colour in plane-polarized light. Some minerals show different colours when the stage is rotated — this is pleochroism (e.g., biotite shows pale yellow to dark brown). If three colours appear, it is trichroism (e.g., hornblende).

(b) Refractive Index (RI)

The RI of a mineral is the ratio of the velocity of light in air to that in the mineral. It is determined by the Becke line method or by immersion in liquids of known RI. Isotropic minerals have one RI; anisotropic minerals have two or three.

(c) Relief

Relief is the apparent roughness or prominence of a mineral grain against the mounting medium. It depends on the difference between the RI of the mineral and that of the Canada balsam (RI = 1.54).

• High relief — RI much greater than balsam (e.g., garnet, olivine)
• Low relief — RI close to balsam (e.g., quartz, feldspar)

(d) Birefringence

The difference between the highest and lowest refractive indices of an anisotropic mineral is called birefringence (Δ = n₁ − n₂). It is shown as interference colours under crossed nicols. High birefringence gives high-order colours (e.g., calcite); low birefringence gives grey or white (e.g., quartz).

(e) Extinction

When an anisotropic mineral is rotated under crossed nicols, it becomes dark at certain positions — this is extinction. The angle between the extinction position and a crystallographic direction is the extinction angle:

• Straight extinction — parallel to cleavage (e.g., pyroxene)
• Inclined extinction — oblique to cleavage (e.g., hornblende)
• Undulose extinction — wavy, due to strain (e.g., quartz)

(f) Twinning

Many minerals show twinning under crossed nicols, appearing as two or more parts with different extinction positions. Common types are simple twinning (e.g., orthoclase) and polysynthetic twinning (e.g., plagioclase).

(g) Cleavage

Cleavage is the tendency of a mineral to break along definite planes. In thin section it appears as parallel straight lines. The number of cleavage directions and the angle between them help identify minerals (e.g., mica — one perfect cleavage; pyroxene — two cleavages at ~90°; hornblende — two cleavages at ~120°).

(h) Interference Figures

When a mineral is viewed under crossed nicols with the Bertrand lens, characteristic interference figures are obtained. These give the optic sign (uniaxial or biaxial, positive or negative) and help determine the optic axial angle (2V). Examples: quartz shows a uniaxial positive figure; mica shows a biaxial negative figure.

(i) Isotropism and Anisotropism

• Isotropic minerals (cubic system) remain dark under crossed nicols in all positions — e.g., garnet, halite.
• Anisotropic minerals (all other systems) become dark only at extinction positions — e.g., quartz, calcite.

---

Conclusion: Optical mineralogy, through the petrographic microscope, provides a rapid and reliable means of identifying minerals by their optical properties — colour, pleochroism, relief, birefringence, extinction, twinning and interference figures — which together give a complete optical signature of the mineral.

a) Distribution of economic minerals in Nepal [5]

Nepal lies along the collision zone of the Indian and Tibetan plates, so its mineral wealth is largely controlled by the geology of the four east–west trending tectonic zones. The main economic minerals and their distribution are:

1. Metallic minerals

   • Iron: Deposits occur at Fulchoki and Labsi (Kathmandu), Jirikhimti (Terhathum), Baitadi, and Ramechhap. Most are low-grade magnetite–hematite ores.
   • Copper: Found at Khandbari and Bhotebas (Sankhuwasabha), Bandipur (Tanahun), Baitadi, and Ilam. Small-scale mining was done at Khandbari in the past.
   • Zinc and lead: Associated with the Ganesh Himal and Rasuwa–Sindhupalchok areas; galena and sphalerite occur in dolomite.
   • Cobalt, nickel, tungsten, molybdenum: Reported from the Higher Himalaya and Midland belts, but not yet economically exploited.
   • Gold: Placer gold occurs in the Karnali, Bheri, Mahakali, and Sunkoshi river gravels; primary gold in the Higher Himalaya.

2. Industrial minerals

   • Limestone: Abundant in the Lesser Himalaya — Chovar, Surkhet, Udayapur, Sindhuli, Arghakhanchi; used for cement.
   • Dolomite: Widely distributed in the Lesser Himalaya, e.g. Chovar, Dhading.
   • Magnesite: Large deposits at Kharidhunga (Dolakha) — one of the biggest in Asia.
   • Talc: Dolakha, Sindhuli, Bhaktapur.
   • Clay, silica sand, feldspar: Chovar, Kathmandu Valley, and the Terai; used in bricks, glass, and ceramics.
   • Graphite: Ilam, Sindhupalchok, Dhading.
   • Mica: Nuwakot, Sindhuli, Dhading.
   • Salt: Mustang (Tibetan border) and Terai salt springs.

3. Energy and other minerals

   • Coal: Dang (Tosh, Salyan), Chautara, Kathmandu Valley; mostly lignite to sub-bituminous.
   • Petroleum and natural gas: Traces in the Terai (Nepalgunj, Dailekh); exploration is ongoing.
   • Gemstones: Ruby and sapphire in Chumar (Dhankuta), Ilam; tourmaline, aquamarine in Sankhuwasabha.
   • Building stones: Marble at Godavari, granite and quartzite in many parts of the Lesser Himalaya.

Conclusion: Nepal is rich in industrial and building minerals (limestone, dolomite, magnesite, talc, marble) but poor in high-grade metallic and energy minerals. Most deposits are small, scattered, and of low grade, so only limestone, dolomite, magnesite, talc, and gemstones are mined commercially.

---

b) Causes of twinning in crystals and their types [5]

Twinning is the intergrowth of two or more crystals of the same mineral in a symmetrical manner, such that one part is the mirror image of the other or is rotated by a definite angle about a crystallographic axis. The two parts share a common twin plane, twin axis, or twin centre.

Causes of twinning

1. Growth twinning (primary): During crystallisation, two nuclei or two parts of the same crystal grow in a symmetrical orientation because of slight irregularities in the growth environment. Example: penetration twins of pyrite and fluorite.
2. Deformation twinning (secondary): Mechanical stress or pressure acting on a crystal after its formation causes gliding along lattice planes, producing twins. Example: calcite and dolomite.
3. Transformation twinning: A change in crystal structure on cooling or heating (polymorphic transformation) leaves the two orientations intergrown. Example: quartz (high → low quartz).
4. Accidental or growth accidents: Rapid crystallisation, impurities, or sudden changes in temperature and concentration cause two crystals to unite symmetrically.
5. Twinning by reflection, rotation, or inversion: Depending on whether the symmetry operation relating the two parts is a reflection, a rotation, or an inversion.

Types of twins

1. Contact twin: The two individuals meet along a common plane (twin plane) and appear as mirror images. Example: gypsum (swallow-tail twin), spinel.
2. Penetration twin: The two individuals interpenetrate each other irregularly. Example: pyrite (iron cross), fluorite, staurolite (cruciform twin).
3. Simple twin: Only two individuals are involved. Example: orthoclase (Carlsbad twin).
4. Multiple or repeated twin: More than two individuals are intergrown in a repeated manner. Example: plagioclase (albite twin), calcite.
5. Polysynthetic twin: Many thin lamellae alternate in a repeated fashion. Example: plagioclase feldspar.
6. Cyclic twin: Three or more individuals are twinned about the same axis in a ring. Example: aragonite (pseudo-hexagonal), rutile.
7. Geniculate twin: The two parts bend at an angle, giving a knee-shaped appearance. Example: rutile, cassiterite.

Conclusion: Twinning may arise during growth, by deformation, or by structural transformation, and is classified on the basis of the symmetry operation and the number of individuals involved. It is an important diagnostic property in mineral identification.

a) Rock-forming minerals vs. accessory minerals

Note: The distinction is relative, not absolute — e.g. quartz is rock-forming in granite but may be accessory in a basalt; olivine is essential in peridotite but accessory in basalt.

---

b) Crystals

A crystal is a solid body in which the constituent atoms, ions or molecules are arranged in a regular, repeating, three-dimensional geometric pattern (internal lattice), bounded by flat, naturally formed plane surfaces called crystal faces. This orderly internal arrangement gives a crystal a definite chemical composition, a characteristic geometric shape, and fixed physical properties such as cleavage, symmetry and optical behaviour. Examples: quartz (hexagonal prism), halite (cube), garnet (dodecahedron), diamond (octahedron).

Crystallography vs. Mineralogy

In short, crystallography is a branch of mineralogy concerned only with crystal geometry and symmetry, whereas mineralogy is the parent science that studies all aspects of minerals.

Option A: Stereographic Projection in Structural Geology

1. Application of Stereographic Projection (2 marks)

Stereographic projection is a graphical method of representing three-dimensional orientation data (planes and lines) on a two-dimensional surface (the projection plane or primitive). In structural geology it is applied to:

1. Representation of planar features — bedding, foliation, cleavage, joints, faults and axial planes are plotted as great circles or poles.
2. Representation of linear features — lineation, fold axes, intersection lineations and slickenside striations are plotted as points.
3. Determination of angles — true dip, apparent dip, plunge, pitch (rake) and the angle between two planes or two lines are read directly from the net.
4. Fold analysis — plotting of poles to bedding (π-diagram) and of lineations (β-diagram) to locate fold axes and axial planes.
5. Structural contour and block-diagram construction — used to solve three-point problems and to construct cross-sections.
6. Analysis of stress and strain — plotting of fault-plane solutions, slip lineations and strain ellipsoid axes.
7. Statistical analysis — contouring of pole density (equal-area projection) to bring out preferred orientations of fabrics.

---

2. Procedure of Plotting a Plane (8 marks)

Apparatus required

• Stereographic net (Wulff net for equal-angle, Schmidt net for equal-area)
• Tracing paper, drawing pin, pencil, protractor, ruler

Basic principle

Every plane passing through the centre of the reference sphere cuts the sphere in a great circle. When this great circle is projected on to the equatorial plane, it appears as an arc. The pole of a plane is the point 90° away from the great circle, on the line normal to the plane.

Procedure

Step 1 — Draw the primitive. Place the tracing paper on the net and fix it with a pin at the centre. Trace the outer circle (the primitive) and mark N, E, S, W.

Step 2 — Mark the strike direction. Measure the strike angle from North in a clockwise direction and mark this point on the primitive. This point represents the line of strike.

Step 3 — Rotate to the E–W or N–S diameter. Rotate the tracing paper until the strike point coincides with either the N–S or the E–W diameter of the net (whichever is convenient).

Step 4 — Plot the great circle. Along the diameter chosen, count the dip angle from the primitive towards the centre and mark the point. Then trace the great circle that passes through this point and through the two ends of the diameter. This arc is the projection of the plane.

Step 5 — Plot the pole. Count 90° from the point marked in Step 4, along the same diameter, in the opposite direction. The point so obtained is the pole of the plane. Alternatively, the pole lies on the diameter at a distance of (90° − dip) from the centre.

Step 6 — Restore orientation. Rotate the tracing paper back to its original position so that the reference marks again coincide with N, S, E, W of the net. The great circle and the pole now give the true orientation of the plane.

Example

Plot a plane with strike N 30° E and dip 40° towards SE.

1. Trace the primitive and mark N, E, S, W.
2. Measure 30° clockwise from N and mark point A on the primitive — this is the strike direction N 30° E.
3. Rotate the tracing paper until A lies on the N–S diameter.
4. Since the dip is towards SE, the plane dips to the right of the strike direction. Count 40° from the primitive along the E–W diameter towards the centre and mark point B. Trace the great circle passing through B and the two ends of the N–S diameter. This arc represents the plane.
5. Count 90° from B in the opposite direction (i.e. 50° from the centre towards the primitive) and mark point P — this is the pole of the plane.
6. Rotate the tracing paper back until the reference marks coincide with N, S, E, W. The great circle and pole P now represent the plane striking N 30° E and dipping 40° SE.

Precautions

• Always measure strike clockwise from North.
• Measure dip from the primitive towards the centre.
• The pole always lies 90° from the great circle, on the side opposite to the dip direction.
• Use the same net (Wulff or Schmidt) throughout a single problem.

---

Option B: Unconformity

1. Definition (1 mark)

An unconformity is a surface of erosion or non-deposition that separates younger strata above from older strata below, representing a gap in the geological record (a period of time during which no deposition took place, or during which erosion removed previously formed rocks).

---

2. Methods of Formation (9 marks)

Unconformities are formed by the following processes:

(a) Uplift and emergence. The sea floor or land surface is uplifted above the base level of erosion. Deposition stops and the exposed rocks begin to be eroded.

(b) Subaerial erosion. The uplifted rocks are subjected to weathering and erosion by agents such as running water, wind, ice and waves. A part or all of the earlier formed strata are removed, producing an irregular erosion surface.

(c) Non-deposition. Even without uplift, a long pause in sedimentation (a diastem or hiatus) may occur. No new material is added, so a time gap develops between the older and younger beds.

(d) Subsidence and renewed deposition. The eroded surface is later submerged, and new sediments are laid down unconformably over the old erosion surface. The basal bed of the younger sequence often contains pebbles or fragments of the older rocks (a basal conglomerate).

(e) Deformation and metamorphism (for angular unconformity). The older strata are folded, faulted or tilted, then eroded to a plane surface, and younger horizontal beds are deposited over them. The discordance in dip between the two sets produces an angular unconformity.

(f) Igneous activity. Intrusion of igneous bodies followed by erosion and later sedimentation may produce a non-conformity, where sedimentary rocks rest on igneous or metamorphic basement.

---

3. Types of Unconformity

(i) Angular Unconformity

The older beds are tilted or folded and are overlain by younger beds that are nearly horizontal. There is a clear angular discordance between the dips of the two sequences. It indicates a period of deformation followed by erosion and renewed deposition. Example: the Cambrian sandstone resting on folded Precambrian rocks.

(ii) Disconformity

The beds above and below the unconformity are parallel to each other, but the surface of separation is an irregular erosion surface (with channels, valleys or a basal conglomerate). It indicates a period of uplift and erosion without tilting. Example: the Mississippian–Pennsylvanian boundary in the Grand Canyon.

(iii) Non-conformity

Sedimentary rocks rest directly on igneous or metamorphic rocks (the basement complex). The older rocks were intruded, metamorphosed, uplifted and eroded before the younger sediments were deposited. Example: the Cambrian sandstone resting on Precambrian granite.

(iv) Paraconformity

The beds above and below are parallel and the contact appears flat and undisturbed, but a time gap is proved by palaeontological evidence (fossils of different ages). It is the most difficult type to recognise in the field. Example: the Ordovician–Silurian boundary in some regions.

(v) Blended unconformity

The erosion surface is covered by younger sediments so that the actual contact is not exposed; the unconformity is inferred from the difference in the age of the fossils or from the presence of a weathered zone.

(vi) Local unconformity

The unconformity is of limited areal extent, formed by local uplift or by a local pause in deposition. It passes laterally into a conformable sequence.

(vii) Regional unconformity

The unconformity extends over a large area (a whole continent or a major part of it) and represents a major diastrophic event. It is of great stratigraphic importance.

---

4. Criteria for Recognition (supporting points)

1. Basal conglomerate or pebble bed at the base of the younger sequence.
2. Irregular erosion surface with channels, valleys or buried topography.
3. Difference in dip between the two sequences (angular unconformity).
4. Contrast in lithology and in the degree of induration or metamorphism.
5. Palaeontological break — fossils of the two sequences belong to different ages.
6. Presence of a weathered zone or residual soil at the contact.
7. Truncation of folds, faults or dykes at the erosion surface.

a) Mohr circle

The Mohr circle is a graphical representation of the state of stress at a point, plotted on a set of axes where the horizontal axis is normal stress (σ) and the vertical axis is shear stress (τ). Any point on the circle gives the normal and shear stress acting on a plane of a particular orientation. The circle is constructed using the principal stresses σ₁ and σ₃: its centre lies on the σ-axis at (σ₁ + σ₃)/2 and its radius is (σ₁ − σ₃)/2.

Uses of the Mohr circle:

1. It gives the normal stress (σ) and shear stress (τ) on any plane of known orientation in a stressed body, without solving long equations.
2. It determines the principal stresses (σ₁ and σ₃) and the orientation of the principal planes (planes of zero shear stress).
3. It gives the maximum shear stress (equal to the radius of the circle) and the orientation of the planes on which it acts.
4. It is used to analyse the stress conditions at which rocks fail — by comparing the circle with a failure envelope (Coulomb–Mohr criterion), one can predict whether a rock will undergo brittle fracture or shear failure.
5. It helps in interpreting the relationship between stress and the orientation of faults, joints and other structures in structural geology.

---

b) Faults

A fault is a fracture or zone of fractures in rocks along which there has been appreciable displacement of the two blocks relative to each other, parallel to the fracture surface. The movement may be vertical, horizontal or oblique.

Recognition of faults in the field:

1. Displacement of beds and structures — Marker horizons, dykes, veins, coal seams or any planar feature are found offset or truncated across a plane; matching the two displaced parts proves faulting.
2. Fault plane features — Slickensides (polished, striated surfaces), slickenside lineations, drag folds, and crushed or powdered rock (fault gouge, fault breccia, mylonite) along the plane indicate movement.
3. Topographic and geomorphic evidence — Fault scarps, fault-line scarps, triangular facets, fault valleys, sag ponds, offset streams, truncated spurs and aligned springs or vegetation lines.
4. Stratigraphic evidence — Repetition or omission of strata in a vertical section, sudden change in thickness or facies, and abrupt juxtaposition of rocks of very different ages or types.
5. Structural evidence — Abrupt change in dip or strike of beds, sudden termination of folds or dykes against a plane, and presence of a zone of intense fracturing or shearing.
6. Geophysical and other evidence — Seismic activity along the plane, gravity or magnetic anomalies, and alignment of earthquake epicentres.

TU BBS — BSc First Year

Difference between (any two) [5×2=10]

a) Anticline and Syncline folds

---

b) Lineation and Foliation

---

c) Sills and Dykes

---

Note: Any two of the above pairs may be written for full marks.