Mass Wasting and Landslide: Relationship and Causes

Relationship between Mass Wasting and Landslide

Mass wasting (also called mass movement) 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. It is a broad term that includes several types of gravity-driven movements such as creep, slump, slide, flow, and fall.

A landslide is one specific type of mass wasting in which a mass of rock or soil moves down a slope along a well-defined surface or plane of failure. Thus, landslide is a subset of mass wasting — all landslides are mass wasting, but all mass wasting is not landslide. The relationship can be summarised as:

• Mass wasting is the general process; landslide is a particular form of it.
• Both are driven by the same fundamental force — gravity — acting on material resting on a slope.
• Both occur when the shear stress on a slope exceeds the shear strength of the material.
• Landslides involve a distinct sliding plane, whereas other mass-wasting types (creep, flow) do not.
• Factors that trigger mass wasting (slope steepening, water saturation, earthquakes, removal of vegetation) also trigger landslides.

Causes of Mass Wasting

Mass wasting occurs when the forces tending to move material downslope overcome the forces resisting movement. The causes are grouped as follows:

1. Gravity (the fundamental cause) Gravity provides the driving force. On a slope, gravity has two components: one perpendicular to the slope (normal stress) and one parallel to the slope (shear stress). When shear stress exceeds shear strength, movement begins. Steeper slopes have a larger shear component, so mass wasting is more likely.

2. Slope Angle and Steepening

• Natural steepening by river undercutting, wave erosion at cliff bases, or glacial carving.
• Artificial steepening by road cuts, quarrying, and excavation at the toe of a slope.
• Oversteepening reduces the resisting force and promotes failure.

3. Role of Water

• Water adds weight to the material, increasing shear stress.
• It reduces friction and cohesion between particles, lowering shear strength.
• Saturation causes pore-water pressure to rise, which pushes grains apart and triggers flow or slide.
• Freezing and thawing of water in cracks (frost wedging) loosens material.
• Heavy rainfall, snowmelt, and rapid infiltration are common immediate triggers.

4. Geological Structure and Rock Type

• Weak, soft, or clay-rich rocks (shale, clay, schist) are more prone to failure.
• Planes of weakness such as bedding planes, joints, fractures, and faults provide ready slip surfaces.
• A dip of strata parallel to the slope is especially dangerous.

5. Earthquakes and Volcanic Activity

• Seismic shaking increases shear stress and breaks inter-particle bonds.
• It can liquefy saturated sandy material, causing flows.
• Volcanic eruptions can trigger debris avalanches and lahars.

6. Removal of Vegetation

• Plant roots bind soil and add cohesion; removal by deforestation, forest fire, or overgrazing reduces strength.
• Loss of vegetation also increases surface runoff and infiltration, worsening saturation.

7. Weathering

• Chemical and mechanical weathering weakens rock and produces loose debris.
• Frost action, exfoliation, and hydrolysis prepare material for movement.

8. Human Activities

• Road and canal construction, mining, and building on steep slopes.
• Deforestation, slash-and-burn farming, and improper terracing.
• Irrigation and leakage from canals saturating slopes.
• Vibrations from blasting and heavy traffic.

9. Other Triggers

• Undercutting by rivers and glaciers.
• Rapid deposition of material on a slope (loading).
• Shrinkage and swelling of clay with moisture change.

Conclusion: Mass wasting and landslide share the same driving mechanism — gravity acting on slope material — but landslide is a specific, well-defined type of mass wasting. The causes are a combination of internal factors (slope, structure, rock type) and external triggers (water, earthquakes, human activity), and failure occurs when shear stress exceeds shear strength.

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OR

Floods and Geological Importance of Running Water

What are Floods?

A flood is the overflow of water from a river, stream, or other water body onto land that is normally dry. It occurs when the volume of water exceeds the carrying capacity of the channel, so water spreads over the floodplain. Floods may be caused by heavy and prolonged rainfall, rapid snowmelt, dam failure, cloudbursts, or deforestation that increases runoff. In geological terms, a flood is the extreme event of a river's discharge that exceeds bankfull stage and inundates the surrounding area.

Geological Importance of Running Water

Running water is the most important agent of erosion, transportation, and deposition on the Earth's surface. Its geological importance includes:

1. Sculpting of landforms: Rivers carve valleys, gorges, canyons, and waterfalls, shaping the landscape.
2. Erosion of the crust: It wears down continents and supplies sediment to lower areas and the sea.
3. Transportation of material: It carries dissolved, suspended, and bed load from highlands to basins.
4. Deposition and formation of plains: Deposited sediment forms floodplains, deltas, and alluvial fans, which are fertile and densely populated.
5. Rock cycle link: It converts igneous and metamorphic rocks into sedimentary rocks through erosion, transport, and deposition.
6. Groundwater recharge: Infiltration from rivers and floods recharges aquifers.
7. Soil formation: Flood-deposited silt renews soil fertility (as in the Nile, Ganges, and Terai plains).
8. Hazard and resource: Floods cause damage, but rivers also provide water, hydropower, irrigation, and navigation.

Role of Running Water in Erosion

Running water erodes by three main processes:

• Hydraulic action: The force of flowing water dislodges and lifts loose material and compresses air in cracks, breaking rock.
• Abrasion (corrasion): Sand, gravel, and boulders carried by the river grind and wear away the bed and banks.
• Solution (corrosion): Water dissolves soluble minerals such as limestone and gypsum, forming caves and karst features.

Erosion is greatest where the velocity is high — in steep upper courses, at waterfalls, rapids, and outer bends of meanders. Vertical erosion deepens valleys; lateral erosion widens them.

Role of Running Water in Deposition

Deposition occurs when the velocity of the river decreases and its carrying capacity falls. It takes place:

• On the inside of meanders, forming point bars.
• On floodplains, when floodwater spreads and slows, depositing silt and clay (alluvium).
• At the mouth of rivers, forming deltas where the river meets a sea or lake.
• At the base of steep slopes, forming alluvial fans.
• In reservoirs and lakes, forming beds of sediment.

Deposition produces sorted layers: coarse gravel and sand settle first, while fine silt and clay settle last, forming graded bedding. Over time, these deposits lithify into sandstone, shale, and conglomerate.

Conclusion

Floods are extreme discharges of rivers that inundate floodplains. Running water is the dominant geological agent that erodes, transports, and deposits material, shaping valleys, plains, deltas, and sedimentary rocks. Its dual role — destructive erosion in the upper course and constructive deposition in the lower course — makes it the most important sculptor of the Earth's surface.

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

How the internal structure is identified

The interior of the earth cannot be seen directly, so it is identified by indirect evidence:

1. Seismic waves (main evidence): Earthquake waves — P-waves (longitudinal) and S-waves (transverse) — travel through the earth. Their velocities change suddenly at certain depths, and S-waves cannot pass through liquids. These changes reveal boundaries and the physical state of each layer.
2. Density and pressure studies: Density and pressure increase with depth, which agrees with a layered earth.
3. Temperature and gravity data: The geothermal gradient and gravity anomalies support the layered model.
4. Meteorites: Meteorites (iron–nickel) give an idea of the earth's core composition.
5. Volcanic material: Lava and gases from volcanoes give information about the upper mantle.

Major internal boundaries

1. Crust–Mantle boundary (Mohorovičić discontinuity / Moho): At about 35 km below continents and 5–10 km below oceans. P- and S-wave velocities suddenly increase here. It separates the crust (sial/sima) from the mantle.
2. Mantle–Core boundary (Gutenberg discontinuity): At about 2,900 km depth. S-waves stop here, showing that the outer core is liquid. It separates the mantle from the core.
3. Outer core–Inner core boundary (Lehmann discontinuity): At about 5,150 km depth. P-wave velocity increases again, showing the inner core is solid.
4. Asthenosphere boundary (low-velocity zone): At about 100–200 km depth within the upper mantle; here waves slow down because the rock is partly molten. It is the source of magma and the layer over which plates move.

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b) Intensity of earthquake and why Nepal is highly earthquake-prone

Intensity of earthquake

Intensity is the measure of the effects (damage) produced by an earthquake at a particular place — how strongly the ground shook, how much buildings cracked or fell, and how people felt it. It is expressed in Roman numerals (I to XII) on the Modified Mercalli Intensity (MMI) scale. Intensity decreases away from the epicentre and depends on distance, local geology and building quality. It differs from magnitude, which measures the energy released at the focus and is the same for one earthquake everywhere.

Why Nepal is one of the most potential countries for earthquake

1. Plate boundary location: Nepal lies on the collision boundary of the Indian and Eurasian plates (the Himalayan orogenic belt), one of the most active seismic zones of the world.
2. Continuing plate movement: The Indian plate is moving northward at about 2 cm/year and underthrusting the Eurasian plate; the accumulated strain is released suddenly as earthquakes.
3. Active faults: Many active thrust faults — Main Central Thrust (MCT), Main Boundary Thrust (MBT), Main Frontal Thrust (MFT) — run across Nepal, making it highly vulnerable.
4. Seismic zone classification: Most of Nepal lies in Seismic Zone V (very high damage risk) and the rest in Zone IV.
5. Weak structures and unplanned settlement: Poorly engineered buildings, dense population in valleys and lack of preparedness increase the damage from any earthquake.
6. Past records: The 1934 Bihar–Nepal earthquake (magnitude 8.0) and the 2015 Gorkha earthquake (magnitude 7.8) show that Nepal is repeatedly struck by great earthquakes.

a) Oceanic Trenches and Theories of Coral Reef Development

Oceanic Trenches

Oceanic trenches are long, narrow, steep-sided and deep depressions of the sea floor, generally 5,000–11,000 m deep, formed at convergent plate boundaries where a dense oceanic plate subducts beneath a lighter plate. They are the deepest parts of the ocean (e.g., Mariana Trench, ~11,000 m) and are marked by strong earthquakes, volcanic arcs and negative gravity anomalies.

Theories on Coral Reef Development

1. Darwin's Subsidence Theory (1842): A volcanic island sinks slowly while corals grow upward at a rate matching subsidence. First a fringing reef forms along the shore, then a barrier reef as the island sinks and a lagoon develops, and finally an atoll when the island disappears completely below sea level.

2. Daly's Glacial Control Theory (1915): During the Ice Age, sea level fell and low temperature killed corals. Later, when the ice melted and sea level rose, corals recolonized the drowned platforms, producing fringing reefs, barrier reefs and atolls.

3. Murray's Volcanic Platform Theory: Reefs grow on elevated submarine platforms or banks of volcanic origin; atolls form on these flat platforms without any subsidence.

4. Semper's and Agassiz's Theories: Semper related reef growth to subsidence plus solution of the limestone base; Agassiz explained barrier reefs and atolls by the outward growth of corals on a subsiding bank.

Conclusion: Darwin's subsidence theory is the most widely accepted, though later drilling (e.g., at Enewetak and Bikini atolls) showed that subsidence and glacial sea-level changes both played a role.

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b) Aquifer and Geological Work of Groundwater

Aquifer

An aquifer is a permeable rock formation (such as sandstone, limestone, gravel or fractured rock) that can store and transmit groundwater in usable quantities. It is bounded below by an impermeable layer (aquiclude) and is recharged by rainfall or seepage. Aquifers may be unconfined (water table open to the atmosphere) or confined/artesian (water trapped between impermeable layers under pressure).

Geological Work of Groundwater

1. Erosion (Solution): Groundwater charged with carbon dioxide forms weak carbonic acid, which dissolves limestone and dolomite, producing sinkholes, caverns, caves, underground channels and karst topography.

2. Transportation: Dissolved calcium carbonate and other salts are carried in solution through pores and joints; fine particles may also be washed down.

3. Deposition: When groundwater evaporates or loses carbon dioxide, it precipitates calcium carbonate as stalactites (hanging from cave roofs), stalagmites (growing from cave floors), pillars (when both join), and travertine deposits near springs. Silica and other minerals may also be deposited.

4. Weathering: Groundwater causes chemical weathering by solution and hydration, rotting rocks from within and preparing them for mass wasting.

5. Landform development: It creates karst landforms — sinkholes, dolines, uvalas, poljes, blind valleys, natural bridges and caverns — and controls springs, seepage and underground drainage.

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OR

a) Evolution of Eolian Landforms

Eolian landforms are produced by the erosional and depositional work of wind, mainly in hot deserts and coastal dunes where vegetation is sparse and loose sand is abundant.

Erosional Eolian Landforms

1. Deflation basins (blowouts): Wind removes loose sand and dust, leaving shallow depressions.
2. Mushroom rocks (gara): Wind abrasion is strongest near the ground, so the base of a rock is cut faster than the top, giving a mushroom shape.
3. Ventifacts: Pebbles and boulders polished and faceted by sand blasting.
4. Yardangs and zeugens: Ridges of hard rock left between furrows carved by abrasion in alternating hard and soft beds.
5. Inselbergs and pediments: Isolated residual hills rising from a planed rock floor.

Depositional Eolian Landforms

1. Sand dunes: Mounds of sand formed when wind velocity falls and sand is deposited. Types include barchan (crescent-shaped, horns downwind), transverse, longitudinal/seif, parabolic and star dunes.
2. Loess: Fine wind-blown silt and dust deposited far from the source, forming thick, fertile, vertical-cliffed deposits (e.g., Hwang Ho basin, China).
3. Sand sheets and sand drifts: Broad, gently undulating covers of sand.

Evolution: Wind erodes weak, dry, loose material, transports it by suspension, saltation and surface creep, and deposits it where velocity drops — behind obstacles, on lee slopes or far away as loess. Repeated erosion and deposition produce the full range of eolian landforms.

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b) Importance of Geology for Infrastructure Development and Disaster Mitigation

For Infrastructure Development

1. Site selection: Geological study of rock type, structure, slope and bearing capacity helps select safe sites for dams, bridges, roads, tunnels and buildings.
2. Foundation design: Knowledge of soil and rock strength decides the type and depth of foundation, preventing settlement and collapse.
3. Construction materials: Geology locates and evaluates aggregates, limestone, sand and building stone.
4. Tunnels and dams: Understanding folds, faults and joints prevents leakage, slipping and failure.
5. Groundwater and drainage: Geological study guides water supply and safe disposal of waste.

For Disaster Mitigation

1. Earthquake: Mapping of active faults and seismic zones helps design earthquake-resistant structures and enforce building codes.
2. Landslide: Study of slope, lithology and structure identifies unstable zones for early warning and safe land use.
3. Flood and erosion: Geological study of river behaviour and bank erosion supports embankments and watershed management.
4. Ground subsidence and sinkholes: Understanding karst and weak ground prevents damage in limestone terrain.
5. Land-use planning: Geological maps guide settlement away from hazardous zones, reducing loss of life and property.

Conclusion: Geology provides the basic scientific information needed for safe, economical and durable infrastructure and for reducing disaster risk.

a) Cross lamination and ripple marks

b) Rocks and minerals

c) Faults and joints

Q1. Differentiate between crystallography and mineralogy. Describe the major properties based on which the crystal systems are classified. [3+7]

A. Difference between Crystallography and Mineralogy [3]

Relation: Crystallography is a sub-discipline that supports mineralogy; a mineralogist uses crystallographic data to identify and classify minerals.

B. Major Properties Used to Classify Crystal Systems [7]

A crystal system is a group of crystals that possess the same degree of symmetry and the same number and arrangement of crystallographic axes. Crystals are classified into 7 systems on the basis of the following properties:

1. Number of crystallographic axes (a, b, c):

• Isometric: 3 equal axes (a₁ = a₂ = a₃)
• Tetragonal, Hexagonal, Trigonal, Orthorhombic: 3 axes
• Monoclinic, Triclinic: 3 axes of unequal length

2. Relative length of the axes:

• Isometric: all three axes equal
• Tetragonal: a = b ≠ c (vertical axis longer or shorter)
• Orthorhombic: a ≠ b ≠ c
• Monoclinic and Triclinic: all three unequal

3. Interaxial angles (α, β, γ):

• Isometric: α = β = γ = 90°
• Tetragonal: α = β = γ = 90°
• Orthorhombic: α = β = γ = 90°
• Monoclinic: α = γ = 90°, β ≠ 90°
• Triclinic: α ≠ β ≠ γ ≠ 90°
• Hexagonal/Trigonal: α = β = 90°, γ = 120°

4. Degree of symmetry: The total symmetry elements (axis, plane and centre of symmetry) decide the system. Isometric has the highest symmetry (4 three-fold axes); triclinic has the lowest (only centre of symmetry).

5. Position and orientation of the axes: Whether the axes are mutually perpendicular or inclined, and whether the vertical axis is vertical or inclined, decides the system.

Summary Table of the Seven Crystal Systems

Conclusion: The number, relative length and mutual inclination of the crystallographic axes, together with the degree of symmetry, are the fundamental properties on which the classification of crystals into seven systems is based.

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OR

Q2. What are physical properties of minerals? Briefly describe the properties that are considered to identify them. [2+8]

A. Physical Properties of Minerals [2]

Physical properties of minerals are those observable and measurable characters of a mineral that depend on its chemical composition and internal atomic structure, and which can be determined by simple field or laboratory tests without destroying the mineral. They are the most convenient and cheapest means of identifying minerals in hand specimens. Examples: colour, streak, lustre, hardness, cleavage, fracture, form, specific gravity, diaphaneity, magnetism, taste and odour.

B. Properties Used for Identification of Minerals [8]

1. Colour: The colour of a mineral in hand specimen. It depends on the absorption of light by the mineral. Some minerals have a characteristic (idiochromatic) colour, e.g. pyrite — brass yellow, malachite — green, sulphur — yellow. Others show variable (allochromatic) colour due to impurities, e.g. quartz may be colourless, white, pink, purple or black. Hence colour alone is not a reliable property.

2. Streak: The colour of the powder of a mineral obtained by rubbing it on an unglazed porcelain plate (streak plate). It is more constant than colour. Examples: hematite — reddish brown streak; pyrite — greenish black; galena — lead grey. Minerals harder than the plate (H > 6.5) give no streak.

3. Lustre: The appearance of the mineral surface in reflected light. Types:

• Metallic — galena, pyrite
• Sub-metallic — hematite
• Vitreous (glassy) — quartz, calcite
• Resinous — sphalerite
• Pearly — talc, muscovite
• Silky — asbestos
• Adamantine — diamond
• Dull/Earthy — kaolinite

4. Hardness: The resistance offered by a mineral to scratching. It is measured by the Mohs scale of 10 minerals: 1 Talc, 2 Gypsum, 3 Calcite, 4 Fluorite, 5 Apatite, 6 Orthoclase, 7 Quartz, 8 Topaz, 9 Corundum, 10 Diamond. A mineral scratches all those below it and is scratched by those above it. Field aids: fingernail ≈ 2.5, copper coin ≈ 3.5, knife blade ≈ 5.5, glass ≈ 5.5.

5. Cleavage: The tendency of a mineral to break along definite smooth planes parallel to possible crystal faces, due to weak atomic bonds in those directions. Described by number, direction and quality:

• Mica — one perfect cleavage
• Calcite — three cleavages (rhombohedral)
• Galena — three cleavages at right angles (cubic)
• Fluorite — four cleavages (octahedral)

6. Fracture: The irregular break of a mineral when it does not possess cleavage. Types: conchoidal (quartz, obsidian), uneven (most minerals), hackly (native copper), earthy (kaolinite), fibrous (asbestos).

7. Form (Habit): The external shape in which a mineral occurs. Examples: cubic — pyrite, galena; octahedral — magnetite; rhombohedral — calcite; prismatic — quartz, tourmaline; tabular — barite; foliated — mica; fibrous — asbestos; granular — olivine; botryoidal — hematite; reniform — malachite; dendritic — native silver.

8. Specific Gravity: The ratio of the weight of a mineral to the weight of an equal volume of water. It depends on chemical composition and atomic packing. Examples: quartz — 2.65, calcite — 2.71, pyrite — 5.0, galena — 7.6, gold — 19.3. It is measured by a Jolly's spring balance or specific gravity bottle.

9. Diaphaneity (Transparency): The ability of a mineral to transmit light. Types: transparent (quartz, calcite), translucent (opal, gypsum), opaque (pyrite, galena).

10. Other Special Properties:

• Magnetism — magnetite, pyrrhotite
• Taste — halite (salty), sylvite (bitter)
• Feel — talc (soapy), graphite (greasy)
• Double refraction — calcite
• Luminescence — fluorite
• Radioactivity — uraninite, monazite

Conclusion: Among these, hardness, streak, lustre, cleavage and specific gravity are the most diagnostic properties, and a combination of several properties — not a single one — is always used for the correct identification of a mineral.

a) Major properties of any three rock-forming minerals

1. Quartz (SiO₂)

• Colour: Colourless when pure; white, grey, or smoky when impure.
• Lustre: Vitreous (glassy).
• 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 ending in pyramids.
• Occurrence: Chief constituent of granite, sandstone and quartzite; very resistant to weathering.

2. Feldspar (orthoclase KAlSi₃O₈ / plagioclase)

• Colour: White, pink, grey or flesh-red.
• Lustre: Vitreous to pearly.
• Hardness: 6 on Mohs scale.
• Cleavage: Two sets of cleavages meeting at right angles (orthoclase), giving blocky fragments.
• Streak: White.
• Crystal form: Monoclinic (orthoclase) or triclinic (plagioclase).
• Occurrence: Most abundant mineral group in the crust; essential in granite, syenite and gneiss. Plagioclase shows striations on cleavage faces.

3. Mica (muscovite and biotite)

• Colour: Muscovite — colourless, silvery-white; biotite — black to dark brown.
• Lustre: Pearly to vitreous.
• Hardness: 2–3 on Mohs scale (very soft).
• Cleavage: Perfect basal cleavage — splits into very thin, flexible, elastic sheets.
• Streak: White (muscovite); grey-brown (biotite).
• Crystal form: Monoclinic, tabular or platy.
• Occurrence: Common in granite, pegmatite, schist and gneiss; muscovite is used as an insulator and biotite is rich in iron and magnesium.

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b) Economic minerals of Nepal and their importance

Nepal possesses a variety of economic (industrial and metallic) minerals, though most occur in small, scattered deposits. The important ones are:

Importance in brief:

1. Industrial raw material — limestone, dolomite and magnesite support the cement, lime and refractory industries.
2. Energy and metal supply — iron, copper and graphite reduce dependence on imports for steel and electrical goods.
3. Foreign exchange — export of marble, gemstones and talc earns valuable foreign currency.
4. Employment and local economy — mining and mineral-based industries generate jobs, especially in rural hill districts.
5. Infrastructure development — cement and construction stone lower the cost of roads, dams and buildings.
6. Import substitution — domestic cement, lime and talc production saves the country large amounts of foreign currency.

Thus, though Nepal's mineral deposits are limited in size, they are of considerable economic value for industrial growth, employment and national development.

(a) Crystallinity and forms of minerals

Crystallinity is the degree of internal atomic order in a mineral.

• Crystalline: atoms in a regular repeating lattice; crystals are visible to the eye (quartz, garnet).
• Cryptocrystalline (microcrystalline): crystals too small to see without a microscope (chalcedony, flint).
• Amorphous (mineraloid): no long-range order (opal, volcanic glass).

Forms are the external shapes, reflecting internal symmetry and growth conditions.

• Euhedral (well-developed faces), subhedral (partly developed faces), anhedral (no crystal faces).
• Crystal habit: prismatic, acicular, tabular, bladed, platy, cubic, etc.
• Twinning: two or more crystals intergrown in a symmetrical way (e.g. in feldspar, staurolite).
• Aggregate forms: granular, fibrous, botryoidal, radiating, dendritic, massive.

(b) Space group and international notation

A space group is the complete set of symmetry operations that map a crystal structure onto itself. It combines the point-group operations (rotation, reflection, inversion) with the translations of the lattice and the translational symmetry elements, screw axes and glide planes. In three dimensions there are exactly 230 space groups. They arise from the 32 crystal classes (point groups) in the 14 Bravais lattices, with the possible placements of screw axes and glide planes. The number is not the simple product $32 \times 14$.

The international (Hermann-Mauguin) notation writes a space group as:

1. a capital letter for the lattice type: $P$ (primitive), $I$ (body-centred), $F$ (face-centred), $C$/$A$/$B$ (one face centred), $R$ (rhombohedral);
2. then symmetry symbols along the characteristic directions: numbers $1,2,3,4,6$ for rotation axes, $\bar{1},\bar{3},\bar{4},\bar{6}$ for roto-inversion axes, $m$ for mirror plane, subscripted numbers such as $2_1$, $4_1$ for screw axes, and $a, b, c, n, d$ for glide planes. A plane is written after the axis perpendicular to it, as in $2/m$.

Example: $P2_1/c$ (monoclinic) is a primitive lattice with a $2_1$ screw axis parallel to $b$ and a $c$-glide plane perpendicular to $b$. $Fm\bar{3}m$ (halite, NaCl) is a face-centred cubic lattice.

Field Geology and Plotting Geological Data on Maps

1. Field Geology (2 marks)

Field geology is the branch of geology that deals with the direct study of rocks, structures, and geological features in their natural setting in the field. It involves systematic observation, measurement, and recording of geological data such as rock types, attitudes of beds (strike and dip), faults, folds, joints, and fossils. Field geology forms the foundation of geological mapping, as all geological maps and cross-sections are ultimately based on field observations.

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2. Methods of Plotting Geological Data on Maps (8 marks)

Geological data collected in the field are plotted on a base map (topographic map) to prepare a geological map. The main methods are:

(a) Plotting of Outcrops and Lithology

The location of each rock outcrop is marked on the base map using a GPS or by resection with a compass. The rock type at each point is recorded using standard symbols, colours, or patterns. Boundaries between different rock units (contacts) are drawn by joining outcrops of the same rock type, taking into account topography and the attitude of the beds.

(b) Plotting of Attitude (Strike and Dip)

The attitude of a bed is measured with a clinometer compass and plotted using the standard T-shaped symbol:

• The long line of the T represents the strike direction.
• The short line (tick) points in the direction of dip.
• The number written near the symbol gives the dip amount in degrees.

Example: A symbol with a long line trending N30°E and a tick pointing SE with "40" means the bed strikes N30°E and dips 40° toward the southeast.

(c) Plotting of Folds

Fold axes and axial traces are plotted using symbols:

• Anticline: a line with two outward-pointing arrows.
• Syncline: a line with two inward-pointing arrows.
• The plunge direction and amount are written beside the symbol.

(d) Plotting of Faults

Faults are plotted as thick lines on the map. The hanging wall side is marked with a short tick or "U" (upthrown) and "D" (downthrown) symbols. The dip and slip direction are indicated by arrows.

(e) Plotting of Joints and Lineaments

Joint trends are plotted as rose diagrams or as short line segments at each station. Lineaments are drawn as straight lines on the map.

(f) Contouring and Structural Contours

Structure contours (lines of equal elevation of a marker bed) are drawn by connecting points of equal elevation on the same bed. These help in determining the true dip, locating outcrop patterns, and constructing cross-sections.

(g) Use of Symbols and Legend

All plotted features are explained in a legend placed on the map, showing rock types, structures, and symbols used.

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3. Preparation of Cross-Sections (8 marks)

A geological cross-section is a vertical profile drawn along a chosen line on the map, showing the subsurface distribution of rocks and structures.

Steps:

Step 1: Selection of section line A straight line (A–B) is drawn on the map perpendicular to the general strike of the beds, so that the true dip is visible.

Step 2: Preparation of the section plane A horizontal line is drawn on a graph paper representing the ground surface. The horizontal scale is kept the same as the map scale; the vertical scale may be equal or exaggerated.

Step 3: Plotting topography The elevation of the ground along the section line is taken from the contour lines and plotted as a profile.

Step 4: Plotting geological boundaries The points where the section line intersects geological contacts, faults, and fold axes are marked on the profile.

Step 5: Plotting dip of beds At each boundary point, the bed is drawn inclined at its true dip angle. The direction of dip is taken from the map. Beds are extended downward to the required depth.

Step 6: Showing structures Folds are drawn as curved beds (anticlines arching upward, synclines troughing downward). Faults are drawn as inclined or vertical lines with arrows showing relative movement.

Step 7: Labelling and legend Each rock unit is labelled (e.g., limestone, sandstone) and a legend is added. The vertical scale, horizontal scale, and orientation of the section are written.

Importance:

Cross-sections help in interpreting subsurface geology, locating ore bodies, planning tunnels, and understanding the three-dimensional structure of the area.

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OR

Formation of Faults and Classification of Faults

1. Formation of Faults (1 mark)

A fault is a fracture in the Earth's crust along which displacement of rocks has taken place. Faults are formed when the stress (compressional, tensional, or shear) acting on rocks exceeds their strength, causing them to break and slip along the fracture plane. The movement may be sudden (earthquake) or slow (creep).

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2. Classification of Faults (9 marks)

Faults are classified on the basis of:

1. Dip of the fault plane (relative to the horizontal)
2. Direction of slip (relative movement of blocks)
3. Apparent movement (as seen in cross-section)

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(a) Classification Based on Dip of Fault Plane

1. High-angle fault: The fault plane dips more than 45° from the horizontal. Most normal faults are high-angle.

2. Low-angle fault: The fault plane dips less than 45°. Thrust faults are usually low-angle.

3. Vertical fault: The fault plane is vertical (dip = 90°).

4. Horizontal fault: The fault plane is horizontal (dip = 0°); rare.

        High-angle        Low-angle         Vertical
           /                 /                 |
          /                 /                  |
         /                 /                   |
        /                 /                    |

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(b) Classification Based on Direction of Slip

1. Normal Fault (Gravity Fault)

• The hanging wall moves down relative to the footwall.
• Caused by tension (stretching) of the crust.
• Dip is usually steep (60°–90°).
• Results in extension and formation of rift valleys and horsts.

        Footwall   Hanging wall
           \         |
            \        |  ↓ (down)
             \       |
              \      |
               \_____|

2. Reverse Fault

• The hanging wall moves up relative to the footwall.
• Caused by compression.
• Dip is usually steep (>45°).
• Results in shortening of the crust.

              /  ↑ (up)
             /
            /  Hanging wall
           /
    ______/
   Footwall

3. Thrust Fault

• A special type of reverse fault with a low dip angle (<45°, often <30°).
• Hanging wall moves up and over the footwall.
• Caused by strong compression.
• Produces large-scale crustal shortening (e.g., Himalayan thrusts — MCT, MBT).

   Hanging wall  →→→ (over)
   ______________
    \  Footwall
     \
      \

4. Strike-slip Fault

• Movement is parallel to the strike of the fault plane.
• Blocks move horizontally past each other.
• Caused by shear stress.
• Two types:
  • Dextral (right-lateral): opposite block moves to the right.
  • Sinistral (left-lateral): opposite block moves to the left.
• Example: San Andreas Fault.

   ← ← ←
   ———————
   ———————
   → → →

5. Oblique-slip Fault

• Combination of dip-slip and strike-slip movement.
• Blocks move both vertically and horizontally.

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(c) Classification Based on Apparent Movement

1. Dip-slip fault: Movement is mainly along the dip direction (normal, reverse, thrust).

2. Strike-slip fault: Movement is mainly along the strike direction.

3. Oblique-slip fault: Movement is inclined to both dip and strike.

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Summary Table

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Conclusion: Faults form due to stress exceeding rock strength. They are classified by dip, slip direction, and apparent movement, each type having distinct geometry and geological significance.

a) Usefulness of stereographic projection [2]

Stereographic projection is a method of representing the three-dimensional orientation of planes and lines on a two-dimensional surface (the equatorial plane of a reference sphere). It is useful because:

• It allows the true angular relationships (dip and strike, plunge and trend) of geological planes and lines to be measured and compared directly on paper.
• It permits rapid graphical solution of problems such as finding the intersection of two planes, the angle between two planes, the true dip from apparent dips, and the mean orientation of a set of field measurements.
• It is essential in structural geology for analysing folds, faults, joints, foliations and lineations, and for constructing cross-sections and block diagrams.

Stepwise process of plotting a plane [3]

1. Draw the primitive circle: Draw a circle of convenient radius on tracing paper and mark N, E, S, W; this represents the horizontal equatorial plane.
2. Plot the strike (or the pole): Measure the strike of the plane from north and mark it on the primitive circle (e.g. N30°E). Alternatively, plot the pole directly from its trend.
3. Plot the dip direction: Rotate the tracing paper so that the strike line coincides with the N–S diameter of the net (the vertical great circle).
4. Plot the dip amount: Along the E–W diameter, count the dip angle inward from the primitive circle using the graduated great circles of the net (e.g. 40°), and mark the point.
5. Draw the great circle: Trace the great circle of the net passing through this point and the two ends of the strike line; this great circle is the stereographic projection of the plane.
6. Plot the pole (optional): The pole of the plane lies 90° from the great circle, i.e. count 90° − dip along the same E–W diameter; mark and label it.
7. Restore orientation: Rotate the tracing paper back to its original position and read off the strike and dip to check the plot.

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b) Schistosity [1]

Schistosity is a type of penetrative planar foliation developed in medium- to high-grade metamorphic rocks (schists) by the parallel preferred orientation of platy and prismatic minerals such as mica, chlorite, talc and hornblende. It gives the rock a layered or "schistose" appearance and causes it to split along these planes.

Development of foliation and lineation [4]

Foliation is a general term for any penetrative planar structure in a metamorphic rock along which the rock tends to split. It develops as follows:

• Definition: Foliation is the parallel arrangement of platy, tabular or prismatic minerals, or of compositional layers, in a metamorphic rock.
• Development: Under directed (differential) stress during metamorphism, minerals grow with their longest or flattest faces perpendicular to the maximum compressive stress. Recrystallisation and rotation of platy minerals (mica, chlorite) align them into parallel planes. With increasing grade, this produces slaty cleavage (low grade), schistosity (medium grade) and gneissose banding (high grade). Segregation of light and dark minerals into alternate layers also produces compositional foliation.

Lineation is a general term for any penetrative linear structure in a metamorphic rock.

• Definition: Lineation is the parallel alignment of elongate minerals or of linear structures (fold axes, intersection lines) in a metamorphic rock.
• Development: It develops by the preferred orientation of prismatic or needle-like minerals (hornblende, kyanite, tourmaline) parallel to the direction of maximum extension or to the movement direction. It also forms as intersection lineation where two foliation planes cut each other, as crenulation lineation on folded foliation, and as mineral lineation by the alignment of elongate grains during deformation.

Thus foliation is planar and lineation is linear; both are produced by the same tectonic deformation and are commonly developed together in metamorphic rocks.

(a) Antiform and synform

(b) Sills and dykes

(c) Apparent dip and true dip

If $\theta$ is the true dip, $\phi$ the apparent dip and $\beta$ the angle between the vertical section and the strike direction, then

$\tan\phi = \tan\theta \,\sin\beta .$

For $\beta = 90^\circ$ (section perpendicular to strike), $\phi = \theta$; for $\beta = 0^\circ$ (section along strike), $\phi = 0$.

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 general term for the sum total of all processes that wear down, level, and lower the land surface of the Earth. It is a collective term that includes three closely related processes:

• Weathering — the in-situ (in place) breakdown and decay of rocks by physical, chemical, and biological agents; no transport is involved.
• Erosion — the removal and transportation of weathered or loose rock material from one place to another by a moving agent (running water, wind, ice, waves).
• Mass wasting — the downslope movement of material under gravity (creep, slide, flow, slump).

Erosion may therefore be defined as the geological process by which surface material is loosened, dissolved, or worn away from one part of the Earth's crust and simultaneously transported and deposited elsewhere by a natural transporting agent. Its three stages are detachment → transportation → deposition.

Denudation is thus the broader process; erosion is one of its components. Denudation lowers the land, while erosion supplies the material that is carried away and deposited as sediment.

2. Geological Process and Importance of Running Water

Running water is the most important single agent of erosion and denudation on the continents, because it is universal in occurrence, acts over the whole land surface, and operates both chemically and mechanically. Its work is conveniently described in three stages.

A. Work of Running Water

(i) Erosional work

Running water erodes by four means:

1. Hydraulic action — the sheer force of flowing water plucks loose material and compresses air into cracks, loosening jointed rock.
2. Abrasion (corrasion) — the load carried by the stream acts as a cutting tool and wears away the bed and banks.
3. Attrition — rock fragments collide with one another and become smaller and more rounded downstream.
4. Solution (corrosion) — water dissolves soluble rocks such as limestone, gypsum, and rock salt.

The characteristic landforms produced are:

• In the upper (youthful) course: V-shaped valleys, gorges, canyons, waterfalls, rapids, plunge pools, pot holes, and river terraces.
• In the middle (mature) course: meanders, ox-bow lakes, river cliffs, and broad open valleys.
• In the lower (old) course: flood plains, natural levees, deltas, distributaries, and estuaries.

(ii) Transportation work

The load is carried in four ways: traction (rolling and dragging along the bed), saltation (bouncing), suspension (fine silt and clay held in the water), and solution (dissolved load). The total load carried depends on the velocity and discharge of the river.

(iii) Depositional work

Deposition takes place when the velocity of the river falls — on entering a lake or sea, on the inside of a meander bend, or after a flood. The resulting features are alluvial fans and cones (at the mountain front), flood plains and natural levees (along the course), and deltas (at the mouth, e.g. the Ganges–Brahmaputra delta, the Nile delta).

B. Importance of Running Water

1. Sculpting the land surface: Running water is the chief agent that carves valleys, gorges, and canyons and reduces highlands to plains, thus playing a leading role in the gradation (denudation) of the continents.
2. Formation of fertile plains: Deposition by rivers produces extensive alluvial plains (Indo-Gangetic Plain, Terai of Nepal), which are the granaries of densely populated regions.
3. Soil formation and renewal: Flood-deposited silt renews soil fertility every year, as in the Nile and Ganges flood plains.
4. Water supply and irrigation: Rivers supply water for drinking, irrigation, industry, and hydroelectric power (e.g. Kulekhani, Kali Gandaki, Trishuli projects in Nepal).
5. Navigation and transport: Large rivers and their deltas provide cheap inland waterways and connect the interior with the sea.
6. Economic deposits: River action concentrates placer deposits of gold, tin, diamonds, and heavy minerals, and provides sand and gravel as construction material.
7. Groundwater recharge: River beds and flood plains recharge aquifers, sustaining wells and springs.
8. Hazards and land degradation: The same process causes soil erosion, gully formation, landslides, and floods, damaging farmland and settlements — a serious problem in the Chure and Middle Hills of Nepal.
9. Recreational and ecological value: Rivers, waterfalls, and lakes formed by river action support tourism (rafting, boating) and aquatic ecosystems.

Conclusion

Running water is the most widespread and effective agent of denudation. Through its combined erosional, transportation, and depositional work, it continuously lowers the land, fills basins with sediment, creates fertile plains and deltas, and supplies water and power to human society — while also creating flood and erosion hazards that must be managed by soil conservation, afforestation, and river training.

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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 geometric features or arrangements of rock masses — such as their attitude, shape, and internal arrangement — produced either at the time of formation of the rocks or by later deformation. They record the geological history of a region and are studied in structural geology. They are broadly classified into primary structures (formed at the time of rock formation) and secondary structures (formed after the rock has consolidated, by tectonic forces).

2. Difference between Primary and Secondary Geological Structures

Figures

Primary structures

   Cross-bedding (cross-section)          Ripple marks (surface view)

        \  \  \  \  \                          /\  /\  /\  /\
         \  \  \  \  \                        /  \/  \/  \/  \
   ---------------------------            -------------------------
   (inclined foreset beds within          (wavy symmetrical ridges on
    horizontal beds)                       the bedding plane)

Secondary structures

        Anticline            Syncline              Fault
          /\                  \  /              ______
         /  \                  \/              /      \
    ____/    \____          ____/\____        /        \
   (upward arch)          (downward trough)  (displacement along
                                              a fracture plane)

Conclusion

Primary structures are syngenetic and record the conditions of deposition or cooling, whereas secondary structures are epigenetic and record the tectonic deformation a rock body has suffered. Both are essential for interpreting the geological history, structure, and stability of a region.

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

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

1. Crust

• Outermost, thin, brittle and solid shell of the Earth.
• Thickness: about 5–70 km (5–10 km under oceans, 30–70 km under continents).
• Two types: sial (continental crust — rich in silica and alumina, lighter, granitic) and sima (oceanic crust — rich in silica and magnesia, heavier, basaltic).
• Average density about 2.7–3.0 g/cm³; temperature increases with depth.
• Separated from the mantle by the Mohorovičić discontinuity (Moho).
• It is broken into several rigid plates (lithospheric plates) that float over the asthenosphere.

2. Mantle

• Lies between the crust and the core, about 2,900 km thick.
• Divided into upper mantle (asthenosphere) and lower mantle.
• Composed mainly of silicates of iron and magnesium (olivine, pyroxene, peridotite).
• Density about 3.3–5.5 g/cm³; temperature rises from about 500 °C to 3,700 °C.
• The asthenosphere (80–200 km depth) is partially molten, plastic and mobile — it is the source of magma and the layer over which plates move.
• Separated from the core by the Gutenberg discontinuity.

3. Core

• Innermost layer, extending from 2,900 km to 6,371 km depth; radius about 3,471 km.
• Composed mainly of nickel and iron (NiFe).
• Two parts: outer core (liquid, 2,900–5,150 km) and inner core (solid, 5,150–6,371 km) due to immense pressure.
• Density about 9.5–13 g/cm³; temperature about 4,000–6,000 °C.
• The liquid outer core generates the Earth's magnetic field by dynamo action; the core is the main source of the Earth's internal heat.

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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 (Seismic Zone IV and V). The Himalaya was formed by the collision of the Indian Plate with the Eurasian Plate, and India is still moving northward at about 2 cm/year. This continuous convergence accumulates strain energy along major thrust faults — the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT). When the accumulated stress exceeds the rock strength, the rocks rupture suddenly, releasing energy as an earthquake. Historical records show major events in 1255, 1505, 1803, 1833, 1934 (8.4 M) and 2015 (7.8 M), and the central Nepal segment (the "seismic gap") has not ruptured for centuries, so a large earthquake is expected in the future. Hence the occurrence of earthquakes in the Nepal Himalaya is certain and recurring.

Impact [3]

• Loss of life and property: The 2015 Gorkha earthquake killed about 9,000 people and destroyed over 800,000 houses, with damage of billions of rupees.
• Damage to infrastructure: Roads, bridges, hydropower projects, schools, hospitals and heritage sites (Dharahara, temples) are damaged or collapsed.
• Landslides and avalanches: Shaking triggers landslides in the weak, steep Himalayan slopes, blocking rivers and causing floods; the 2015 event caused an avalanche on Everest.
• Economic setback: Heavy expenditure on relief and reconstruction slows national development and increases poverty.
• Social and psychological effects: Displacement, loss of livelihood, orphaned families and long-lasting trauma.
• Environmental impact: Debris, glacial lake outburst risk and disturbance of drainage systems.

Thus Nepal must invest in earthquake-resistant construction, early warning systems and public awareness to reduce future losses.

a) Plate tectonics and evolution of the theory

Plate tectonics is the theory that the rigid lithosphere is broken into about seven major and several minor plates that move slowly over the plastic asthenosphere. Their interaction at divergent (constructive), convergent (destructive) and transform (conservative) boundaries causes earthquakes, volcanism, mountain building (for example the Himalaya, formed by the India–Eurasia collision) and ocean-basin formation.

Evolution of the theory

1. Continental drift (Alfred Wegener, 1912): all continents were once joined as the supercontinent Pangaea, surrounded by the ocean Panthalassa, and broke up about 200 million years ago. Evidence: jigsaw fit of coastlines (Africa and South America), matching fossils (Glossopteris, Mesosaurus), matching rocks and mountain belts, and Permo-Carboniferous glacial deposits on separated continents. It was rejected because Wegener could not explain a driving force.
2. Mantle convection (Arthur Holmes, 1929): convection currents in the mantle proposed as the driving mechanism.
3. Sea-floor spreading (Harry Hess, 1960s): new oceanic crust forms at mid-oceanic ridges and moves away. Supporting evidence includes symmetrical palaeomagnetic stripes on both sides of ridges, ocean-floor age increasing away from the ridge, and deep-sea trenches with Benioff zones marking subduction.
4. Plate tectonics (late 1960s): Wilson, Morgan, McKenzie and Le Pichon combined drift and spreading into the unified theory. Plates are driven by mantle convection, ridge push and slab pull.

b) Minerals and their formation

Minerals are naturally occurring, inorganic, homogeneous solids with a definite chemical composition and an ordered crystalline internal structure, for example quartz (SiO$_2$), calcite (CaCO$_3$) and feldspar.

Processes of mineral formation

1. Crystallisation from magma or lava: minerals crystallise in order as the melt cools (Bowen's reaction series: olivine, pyroxene, amphibole, biotite, then feldspar and quartz). Slow cooling gives coarse crystals; rapid cooling gives fine crystals.
2. Precipitation from solution: evaporation or cooling of saturated water deposits halite, gypsum and calcite; hot springs deposit silica and calcite.
3. Metamorphic recrystallisation: heat and pressure change existing minerals into new ones in the solid state (garnet, kyanite, and calcite recrystallising to marble).
4. Hydrothermal and metasomatic processes: hot mineral-rich fluids deposit ore minerals in veins and fractures (quartz veins with gold, galena PbS, sphalerite ZnS).
5. Weathering and residual concentration: leaching of soluble material leaves insoluble minerals such as bauxite and iron oxides.
6. Biological processes: organisms form calcite, aragonite or silica in shells, corals and bones.

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OR

a) Causes and impact of mass movement

Mass movement is the downslope movement of rock, soil and debris under the direct influence of gravity.

Causes

1. Gravity and slope angle: steep or over-steepened slopes increase shear stress.
2. Geological structure: joints, bedding planes, faults and foliation form weak planes.
3. Water: heavy rain, snowmelt and groundwater raise pore-water pressure and reduce friction.
4. Rock type and weathering: weak, clay-rich or deeply weathered material fails easily.
5. Vegetation removal: deforestation and overgrazing remove root binding.
6. Human activities: road cutting, quarrying, mining, construction and irrigation on slopes.
7. Earthquakes and volcanic activity: shaking triggers landslides and rockfalls.

Impacts

• Loss of life and property, and burial of settlements and roads.
• River blockage, landslide dams and outburst floods.
• Damage to highways, bridges, canals and hydropower projects.
• Loss of fertile soil and farmland.
• Sedimentation of reservoirs, reducing their life.
• Disruption of transport and communication, a serious problem in the hills of Nepal during the monsoon.

b) Glaciers and associated landforms

Glaciers are large, slowly moving masses of ice formed by the compaction and recrystallisation of snow on land, flowing downslope under their own weight (for example the Khumbu Glacier in Nepal).

Erosional landforms

1. Cirque (corrie): armchair-shaped hollow at the head of a glacier.
2. Arete: sharp ridge between two cirques.
3. Horn: pyramidal peak left by erosion of three or more cirques (Matterhorn).
4. U-shaped valley (glacial trough): river valley widened and deepened by ice.
5. Hanging valley: tributary valley left high above the main trough, often with a waterfall.
6. Fjord: glacial trough drowned by the sea.
7. Roche moutonnée: asymmetrical bedrock knob, smoothed on the stoss (up-ice) side and plucked and rough on the lee side.

Depositional landforms

1. Moraines: unsorted glacial debris (till) as lateral, medial, terminal and ground moraines.
2. Drumlin: streamlined, egg-shaped hill of till; it is steeper and blunter on the stoss (up-ice) side and tapers gently on the lee side in the direction of ice flow.
3. Esker: long, winding ridge of stratified sand and gravel deposited by a meltwater stream in or under the ice.
4. Kame: irregular mound of stratified drift at the ice margin.
5. Outwash plain: gently sloping plain of sorted sand and gravel spread by meltwater.
6. Erratics: large boulders carried far from their source and left on different bedrock.

Differences Between the Given Pairs

a) Lithosphere and Asthenosphere

b) Divergent and Convergent Plate Boundaries

c) Igneous Rocks and Metamorphic Rocks

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

1. Major Crystal Systems (2 marks)

On the basis of the relative lengths of the three crystallographic axes (a, b, c) and the angles between them (α, β, γ), all crystals are grouped into seven crystal systems:

1. Isometric (Cubic) — a = b = c; α = β = γ = 90°
2. Tetragonal — a = b ≠ c; α = β = γ = 90°
3. Hexagonal — a₁ = a₂ = a₃ ≠ c; α = β = 90°, γ = 120°
4. Trigonal (Rhombohedral) — a = b = c; α = β = γ ≠ 90°
5. Orthorhombic — a ≠ b ≠ c; α = β = γ = 90°
6. Monoclinic — a ≠ b ≠ c; α = γ = 90°, β ≠ 90°
7. Triclinic — a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90°

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

(a) Isometric (Cubic) System

Axial characters: Three mutually perpendicular axes of equal length: $a = b = c, \quad \alpha = \beta = \gamma = 90°$

Symmetry: Highest symmetry of all systems — four 3-fold axes of symmetry, three 4-fold axes, six 2-fold axes, nine planes of symmetry and a centre of symmetry.

Characters of forms: Crystals are equidimensional (roughly spherical or cubic in outline). Common forms are cube, octahedron, dodecahedron, tetrahedron.

Examples: Halite (NaCl), galena (PbS), pyrite (FeS₂), garnet, diamond, spinel, fluorite.

        ___________            /\        /\
       /          /|          /  \      /  \
      /          / |         /____\____/____\
     |__________|  |        |      |    |    |
     |          |  |        |  cube|    |    |
     |          |  /        |______|____|____|
     |__________|/            \    /      \  /
                                \  /        \/
        Cube                    Octahedron

(b) Triclinic System

Axial characters: Three axes of unequal length, all inclined to one another at oblique angles; none of the angles is 90°: $a \neq b \neq c, \quad \alpha \neq \beta \neq \gamma \neq 90°$

Symmetry: Lowest symmetry of all systems — only a centre of symmetry (and sometimes one 1-fold axis). No plane or axis of symmetry is present.

Characters of forms: Crystals are completely unsymmetrical in appearance; no two faces are alike in shape or size. Common forms are pinacoid, dome, sphenoid.

Examples: Plagioclase feldspar (albite–anorthite series), microcline, kyanite, axinite, amblygonite.

          /\
         /  \___
        /___/   \
        \   \    \
         \___\____\
        Triclinic (no right angle, all axes oblique)

(c) Monoclinic System

Axial characters: Three axes of unequal length, of which two (a and c) are inclined obliquely to each other while the third (b) is perpendicular to both: $a \neq b \neq c, \quad \alpha = \gamma = 90°, \quad \beta \neq 90°$

Symmetry: Intermediate symmetry — one 2-fold axis of symmetry (the b-axis), one plane of symmetry (010) and a centre of symmetry. The symmetry axis is inclined (oblique) to the other two axes, hence the name "monoclinic" (one inclined angle).

Characters of forms: Crystals are prismatic, tabular or bladed; the prism faces are inclined to the vertical axis, giving a "leaning" or wedge-like appearance. Common forms are prism, pinacoid, dome, sphenoid.

Examples: Gypsum (CaSO₄·2H₂O), orthoclase (K-feldspar), muscovite mica, hornblende, augite, epidote.

           /|
          / |
         /  |____
        |  /    /
        | /    /
        |/____/
      Monoclinic (b-axis vertical, a and c oblique)

Summary Table of Differences

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OR

Optical Mineralogy and Optical Properties of Minerals

1. What is Optical Mineralogy? (2 marks)

Optical mineralogy is the branch of mineralogy that deals with the study of minerals by means of polarized light, using an instrument called the petrographic (polarizing) microscope. It involves the determination of the optical properties of minerals — such as refractive index, birefringence, pleochroism, extinction angle and optic sign — by examining thin sections of rocks or crushed mineral grains mounted in Canada balsam. It is the most important method for identifying rock-forming minerals in thin section.

2. Optical Properties of Minerals in Detail (8 marks)

(a) Colour and Pleochroism

Colour is the property observed in plane-polarized light. When a coloured mineral shows different colours as the stage is rotated under plane-polarized light, the phenomenon is called pleochroism (e.g., biotite, hornblende). If three colours are seen in three directions it is trichroism (e.g., hypersthene).

(b) Refractive Index (R.I.)

It is the ratio of the velocity of light in air to that in the mineral: $n = \frac{v_{\text{air}}}{v_{\text{mineral}}}$ It is determined by the Becke line method or by immersion methods. Isotropic minerals have a single R.I.; anisotropic minerals have two or three (e.g., quartz ≈ 1.54, calcite ≈ 1.66).

(c) Relief

The apparent roughness or prominence of a mineral grain in thin section depends on the difference between its R.I. and that of the mounting medium (Canada balsam, n = 1.54). High relief (e.g., garnet, olivine) shows thick dark borders; low relief (e.g., quartz, feldspar) shows faint borders.

(d) Birefringence

The difference between the highest and lowest refractive indices of an anisotropic mineral: $\delta = n_{\max} - n_{\min}$ It is determined from the interference colours seen under crossed nicols. High birefringence gives high-order white colours (e.g., calcite); low birefringence gives grey/black (e.g., quartz).

(e) Interference Colour

When anisotropic minerals are viewed between crossed nicols, they show brilliant colours called interference colours, which depend on birefringence and thickness of the section. These are compared with the Michel-Lévy colour chart.

(f) Extinction

When a mineral becomes completely dark between crossed nicols on rotation of the stage, it is called extinction. It occurs every 90°. Types: straight extinction (extinction parallel to cleavage/twinning), inclined extinction (angle ≠ 0°), and undulose extinction (wavy, due to strain, e.g., quartz).

(g) Extinction Angle

The angle between the crystallographic direction (cleavage/twinning) and the vibration direction of the mineral at extinction. It is measured with a graduated stage. It may be 0° (straight), or an inclined angle (e.g., 30° in hornblende).

(h) Twinning

Under crossed nicols, twinned crystals show alternate light and dark bands on rotation. Common in plagioclase (albite twinning — parallel stripes) and microcline (cross-hatched/grid twinning).

(i) Optic Axis and Optic Sign

Anisotropic minerals have one (uniaxial) or two (biaxial) optic axes. The optic sign (positive or negative) is determined from the interference figure seen in convergent light. Uniaxial minerals give a dark cross figure; biaxial minerals give a dark cross with curved arms.

(j) Interference Figure

A pattern of coloured rings and a dark cross obtained in convergent polarized light when the optic axis is vertical. It helps determine uniaxial/biaxial character, optic sign and optic axial angle (2V).

(k) Cleavage

Under the microscope, cleavage appears as straight, parallel, closely spaced lines in plane-polarized light. The number of cleavage sets and their angle help identify minerals (e.g., one set in mica, two sets at ~90° in pyroxene, two sets at ~60°/120° in amphibole).

(l) Zoning

Some minerals show concentric or parallel zones of different colours or extinction positions, indicating compositional variation during growth (e.g., plagioclase, garnet).

(m) Inclusions

Foreign materials trapped inside a mineral grain appear as small dark or coloured spots; they help infer the paragenesis of the mineral.

(n) Alteration

Minerals often show alteration products along cracks or borders (e.g., olivine → serpentine, feldspar → clay). This is useful in deciphering the history of the rock.

a) Distribution of economic minerals in Nepal

Nepal lies along the collision zone of the Indian and Tibetan plates, and this Himalayan orogeny has produced a variety of economic minerals. Their distribution is as follows:

1. Metallic minerals

• Iron: Deposits occur at Phulchowki (Lalitpur), Those (Ramechhap), Labdikhola and Bhainse (Makwanpur), and Jirikhimti (Terhathum). Most are small and of low grade.
• Copper: Associated with the Lesser Himalaya; known occurrences at Bhotang (Sindhupalchok), Kalitar (Makwanpur), and in Gulmi, Palpa and Darchula.
• Lead and zinc: Found at Ganesh Himal, Phakuwa (Sankhuwasabha), and in the Bagmati and Gandaki zones, usually as galena and sphalerite.
• Cobalt, nickel, tungsten and molybdenum: Reported from the Higher Himalaya and Lesser Himalaya, but not in workable quantities.
• Gold: Placer gold occurs in the sands of the Bheri, Karnali, Seti, Budhi Gandaki and Marsyangdi rivers; primary gold at Lungri (Rolpa).

2. Non-metallic/industrial minerals

• Limestone: Abundant throughout the Lesser Himalaya and Siwalik; large deposits at Chovar, Surkhet, Udayapur, Sindhuli and Hetauda — the basis of the cement industry.
• Dolomite and magnesite: Dolomite is widespread; magnesite at Kharidhunga (Dolakha).
• Talc: Kharidhunga, Sindhuli and Baitadi.
• Mica: Widespread in the Higher Himalaya; notable at Chainpur, Bhojpur and Taplejung.
• Graphite: Ilam, Dhankuta, Sindhuli and Sankhuwasabha.
• Quartz, feldspar, clay, and talc: Used in ceramics and glass; clay deposits in the Churia and Terai.
• Gemstones: Ruby and sapphire at Chumar–Ruyil (Dolakha), tourmaline at Hyakule (Sankhuwasabha), and aquamarine in Taplejung.

3. Fuel minerals

• Coal: Tertiary coal in the Siwalik and Lesser Himalaya — Dang (Tosh, Salyan), Chautara, Kathmandu valley, and Ilam.
• Petroleum and natural gas: Traces in the Siwalik foreland basin; exploration is ongoing.
• Peat: In the Kathmandu valley and Terai.

4. Construction materials

• Sand, gravel, boulders and riverbed aggregates are abundant in the Terai rivers; building stone from the Siwalik and Lesser Himalaya.

Conclusion: Nepal is rich in non-metallic and construction minerals but poor in high-grade metallic and fuel minerals. Most deposits are small, scattered and of low grade, so mining is largely limited to limestone, dolomite, talc, coal and construction aggregates.

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b) Causes of twinning in crystals and their types

Twinning is the intergrowth of two or more crystals of the same mineral in a definite, symmetrical orientation, such that one part is the mirror image of the other across a common plane or axis.

Causes of twinning

1. Growth twinning (primary): During crystallization, two or more nuclei or growing crystals attach in a symmetrical way because the atomic arrangement at the contact is favourable. This lowers the surface energy of the aggregate. Common in minerals like aragonite, staurolite and pyrite.
2. Transformation twinning (secondary): A crystal changes from a high-temperature (high-symmetry) form to a low-temperature (low-symmetry) form on cooling. The new lattice cannot fit the old one everywhere, so different parts of the crystal twin to relieve the strain. Example: sanidine (orthoclase) and leucite.
3. Deformation twinning (secondary): Mechanical stress (pressure, shearing) applied to a crystal causes gliding along lattice planes, producing twin lamellae. Example: calcite and dolomite.
4. Polysynthetic twinning: Repeated twinning on the same law produces many thin parallel lamellae, as in plagioclase feldspar.

Types of twins

By mode of formation:

• Growth twins — formed during crystallization.
• Transformation twins — formed by polymorphic change.
• Deformation twins — formed by stress.

By geometry:

• Contact twins: The two individuals meet along a single plane (twin plane); e.g. gypsum swallow-tail twin, spinel twin of spinel.
• Penetration twins: The two individuals interpenetrate each other; e.g. cross twin of staurolite, iron cross twin of pyrite, Carlsbad twin of orthoclase.
• Polysynthetic (multiple) twins: Many individuals twinned repeatedly on the same law, giving a striped or lamellar appearance; e.g. albite twin of plagioclase.
• Cyclic twins: Three or more individuals twinned in a ring; e.g. aragonite trilling.

By twin law:

• Normal (twin plane) law — reflection across a plane.
• Parallel (twin axis) law — rotation of 180° about an axis.
• Complex law — combination of both.

Conclusion: Twinning arises from growth, transformation or deformation processes and is classified by its mode of origin, geometry and twin law. It is an important diagnostic feature in mineral identification.

a) Rock forming minerals vs. Accessory minerals

Note: The distinction is not absolute — a mineral that is accessory in one rock (e.g. quartz in a basalt) may be rock forming in another (e.g. quartz in granite); hence the classification is based on the abundance of the mineral in that particular rock.

b) Crystals

A crystal is a solid, homogeneous, naturally occurring (or artificially produced) body in which the constituent atoms, ions or molecules are arranged in a definite, orderly, three-dimensional, repeating geometric pattern (internal lattice). This ordered internal arrangement gives a crystal a definite chemical composition, a definite external geometric form bounded by flat faces, and characteristic physical properties such as cleavage, symmetry and anisotropy. Examples: quartz, halite (common salt), diamond, calcite.

Crystallography vs. Mineralogy

In short, crystallography studies the form and internal order of crystalline solids, whereas mineralogy studies minerals as a whole, using crystallography as one of its essential tools.

Part 1: Stereographic projection

Applications in structural geology (2)

Stereographic projection represents three-dimensional orientation data on a two-dimensional net. It is used to:

• plot planes (bedding, foliation, joints, faults) as great circles and their poles as points;
• find the angle between two planes or lines, and the true dip from apparent dips;
• find the line of intersection of two planes (fold axis, intersection lineation);
• determine fold axes and axial planes using $\pi$ and $\beta$ diagrams;
• rotate data to restore beds to horizontal (unfolding) and analyse fault slip and palaeostress;
• assess rock-slope stability (wedge and planar failure) in road and tunnel work.

Plotting a plane (8)

Example: plane with strike N30°E and dip 40° towards SE (dip direction $120^\circ$).

1. Place tracing paper over the equal-angle (Wulff) or equal-area (Schmidt) net, fixed at the centre with a pin. Mark north and trace the primitive circle.
2. Mark the strike on the primitive: measure $30^\circ$ east of north and mark the point and its opposite end (N30°E and S30°W).
3. Rotate the tracing paper until the strike mark lies on the N–S line of the net.
4. The dip direction (SE) now lies on the east side. Count $40^\circ$ inward from the eastern edge along the E–W diameter and mark point B.
5. Trace the great circle (meridian) that passes through the two strike ends and point B. This is the plane.
6. To plot the pole, count $90^\circ$ from B along the E–W diameter across the centre, to the west; this is $50^\circ$ west of the centre. Mark it.
7. Rotate the paper back so north coincides with north. The pole plunges $90^\circ - 40^\circ = 50^\circ$ towards $300^\circ$ (N60°W), opposite to the dip direction.

Check: the pole is $90^\circ$ from every line in the plane; the dip is read as the angular distance from the primitive to the great circle along the E–W line.

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OR

Unconformity

An unconformity is a buried surface of erosion or non-deposition separating younger strata from older rocks, representing a gap (hiatus) in the geological record.

Formation

1. Older rocks are deposited (and may be folded, tilted or intruded).
2. They are uplifted above base level (by tectonic movement or fall in sea level) so that deposition stops.
3. The exposed surface is eroded or remains a surface of non-deposition for a long time.
4. Subsidence or a rise in sea level submerges the surface.
5. New sediments are deposited on it, burying the surface as an unconformity.

Unconformities thus form by a combination of uplift and erosion, non-deposition, and later subsidence with renewed sedimentation.

Types

1. Angular unconformity: younger beds lie at an angle to the tilted or folded older beds below. It records deposition, deformation, erosion and renewed deposition.
2. Disconformity: younger and older beds are parallel, but the contact is an irregular erosion surface (channels, relief, weathered zone).
3. Nonconformity: sediments lie on eroded igneous or metamorphic rocks such as granite or gneiss.
4. Paraconformity (pseudo-conformity): beds are parallel and the contact looks like an ordinary bedding plane; the gap is shown only by missing fossil zones.
5. Local and regional unconformities: by extent, a local one is a small break (for example a channel cut into river beds), and a regional one extends over large areas of a continent.

a) Mohr circle

The Mohr circle is a graphical representation of the state of stress at a point. On a plot of normal stress ($\sigma$) against shear stress ($\tau$), all possible combinations of $\sigma$ and $\tau$ acting on planes of all orientations through a point lie on a circle. For a two-dimensional stress state with principal stresses $\sigma_1$ and $\sigma_3$ ($\sigma_1 > \sigma_3$), the circle has:

• Centre at $\left(\dfrac{\sigma_1 + \sigma_3}{2},\ 0\right)$ on the $\sigma$-axis
• Radius $= \dfrac{\sigma_1 - \sigma_3}{2}$

The point on the circle farthest to the right gives the maximum normal stress ($\sigma_1$), the point farthest to the left gives the minimum normal stress ($\sigma_3$), and the highest and lowest points give the maximum shear stress, $\tau_{max} = \dfrac{\sigma_1 - \sigma_3}{2}$, acting on planes at $45°$ to the principal stress directions.

Uses of the Mohr circle:

1. Determination of stresses on any plane: It gives the normal and shear stresses acting on a plane of any given orientation, without solving lengthy equations.
2. Finding principal stresses and principal planes: The maximum and minimum normal stresses ($\sigma_1$ and $\sigma_3$) and the orientation of the planes on which they act (where $\tau = 0$) can be read directly from the circle.
3. Finding maximum shear stress: The maximum shear stress and the planes on which it acts (at $45°$ to the principal planes) are obtained at once.
4. Tectonic and structural interpretation: In geology, the Mohr circle (with the Mohr–Coulomb failure envelope) is used to explain the orientation of faults and fractures. Rocks fail by shear when the circle touches the failure envelope; the point of tangency gives the angle of the fault plane, which is why faults form at about $30°$ to the direction of maximum compressive stress ($\sigma_1$).
5. Comparison of stress states: Circles for different stress conditions can be drawn on the same axes to compare which state is nearer to failure, useful in rock mechanics and engineering design.

b) Faults and their recognition in the field

A fault is a fracture or zone of fractures in the Earth's crust along which the rocks on either side have moved relative to each other, i.e. there has been displacement parallel to the fracture surface. The fracture surface along which the movement takes place is called the fault plane. Faults are produced when the stresses acting on rocks exceed their strength, and the movement may be sudden (causing earthquakes) or slow and gradual.

Criteria for recognizing faults in the field:

1. Displacement of beds or markers: The same bed, vein, dyke or any marker (e.g. a fossil horizon or a road) is found offset or displaced across a line, showing that movement has taken place.
2. Repetition or omission of strata: In a vertical section, a set of beds may be repeated (normal fault) or some beds may be missing/omitted (reverse or thrust fault) compared with the normal succession.
3. Fault plane features: A distinct plane or zone of crushed, broken rock is seen. The rocks along it are brecciated, mylonitized or clayey (fault gouge, fault breccia), and the zone is often weathered or mineralized.
4. Slickensides and drag folds: Polished, striated surfaces (slickensides) formed by friction during movement, and bending of beds adjacent to the fault (drag folds) indicate the sense of movement.
5. Topographic and physiographic evidence: Fault scarps (steep linear cliffs), fault-line scarps, triangular facets, aligned springs, sudden changes in stream courses, waterfalls, sag ponds and linear valleys or ridges commonly mark fault traces.
6. Juxtaposition of unlike rocks: Rocks of very different type or age, or of different metamorphic grade, are found in direct contact along a sharp line, which is unusual in a normal succession.
7. Offset of geomorphic or man-made features: Streams, terraces, fences, roads, canals or walls are displaced across a line, clearly indicating recent movement along a fault.

Answer any two of the following.

a) Anticline and syncline

b) Lineation and foliation

c) Sills and dykes