Unit 4: Rocks and mass movements - Subjective Questions

Definition: A rock is a naturally occurring solid aggregate of one or more minerals, mineraloids, or organic materials that forms part of the Earth's crust.

Main criteria for classification:

• Origin: Rocks are classified as igneous, sedimentary, or metamorphic according to how they form.
• Mineral composition: The types and proportions of minerals present help identify a rock.
• Texture: Grain size, shape, arrangement, and degree of crystallinity are important characteristics.
• Structure: Features such as layers, foliation, joints, and vesicles are considered.
• Mode of formation: Cooling of magma, deposition and lithification of sediments, or alteration by heat and pressure determines the rock group.

Formation: Igneous rocks form when molten magma or lava cools and solidifies.

Classification:

• Intrusive or plutonic rocks: Form when magma cools slowly below the Earth's surface. They usually have coarse crystals. Example: granite.
• Extrusive or volcanic rocks: Form when lava cools rapidly at or near the surface. They generally have fine crystals or a glassy texture. Examples: basalt and obsidian.

Characteristics:

• They are generally hard and resistant to erosion.
• Fossils are normally absent because of the high temperatures involved.
• Their texture depends mainly on the rate of cooling.
• Large mineral crystals indicate slow cooling, while small crystals indicate rapid cooling.

Origin: Sedimentary rocks form from material deposited by water, wind, ice, or gravity and later compacted and cemented.

Major stages:

• Weathering and erosion of pre-existing rocks.
• Transportation and deposition of sediments.
• Compaction due to overlying pressure.
• Cementation by minerals precipitated from groundwater.

Classification:

• Clastic rocks: Form from fragments of older rocks, such as sandstone and shale.
• Chemical rocks: Form when dissolved minerals precipitate, such as limestone and gypsum.
• Organic rocks: Form from accumulated plant or animal remains, such as coal.

Characteristics: They commonly occur in layers, may contain fossils, and often have bedding planes.

Formation: Metamorphic rocks develop when existing rocks are altered by heat, pressure, chemically active fluids, or a combination of these factors without completely melting.

Types:

• Contact metamorphism: Caused mainly by heat from a nearby magma body. Limestone may change into marble.
• Regional metamorphism: Occurs over large areas during mountain building under intense pressure and temperature. Shale may become slate and later schist.
• Dynamic metamorphism: Results mainly from pressure and friction along faults.

Textural classification:

• Foliated rocks: Minerals are arranged in layers or bands, as in slate, schist, and gneiss.
• Non-foliated rocks: Minerals lack a layered arrangement, as in marble and quartzite.

Conclusion: The three groups are linked through the rock cycle, in which one type may be transformed into another through weathering, burial, melting, cooling, or metamorphism.

The rock cycle is the continuous set of geological processes through which rocks are formed, altered, destroyed, and re-formed.

• Magma cools and crystallizes to form igneous rocks.
• Weathering, erosion, transportation, deposition, compaction, and cementation convert exposed rocks into sedimentary rocks.
• Burial and increased heat and pressure transform igneous or sedimentary rocks into metamorphic rocks.
• Melting of any rock produces magma, which may later cool to form a new igneous rock.
• Uplift exposes buried rocks to weathering and erosion.
• Metamorphic and sedimentary rocks can also be uplifted, weathered, and redeposited.

Thus, no rock type is permanent; each may change into another over geological time.

Weathering is the in-situ breakdown or decomposition of rocks at or near the Earth's surface.

Differences:

• Weathering: Disintegration or decomposition occurs without movement of the material.
• Erosion: Weathered material is removed and transported by agents such as running water, wind, glaciers, or waves.
• Mass wasting: Soil, rock, and debris move downslope mainly under the force of gravity, often with little or no transporting medium.

Weathering weakens rocks and produces regolith. Erosion and mass wasting then remove and redistribute this material. Therefore, weathering commonly prepares material for transport, while erosion and mass wasting involve movement.

Mechanical weathering breaks rocks into smaller fragments without changing their chemical composition.

• Frost action: Water enters cracks, freezes, expands, and widens the cracks. Repeated freezing and thawing produces angular fragments.
• Thermal expansion: Repeated heating and cooling cause minerals to expand and contract at different rates, resulting in cracking and exfoliation.
• Salt weathering: Salt crystals grow in pores and cracks, exerting pressure on rock surfaces.
• Exfoliation: Outer layers peel away from massive rocks because of unloading or repeated thermal stress.
• Pressure release: Removal of overlying material allows rock to expand and form curved fractures.

These processes increase surface area, produce regolith, create debris slopes, and accelerate erosion and mass movement.

Chemical weathering changes the mineral composition of rocks through reactions with water, oxygen, carbon dioxide, or acids.

• Solution: Soluble minerals dissolve directly in water.
• Carbonation: Carbon dioxide combines with water to form weak carbonic acid, which dissolves carbonate rocks such as limestone.
• Oxidation: Oxygen combines with minerals containing iron, producing iron oxides and weakening the rock.
• Hydrolysis: Water reacts with silicate minerals, especially feldspar, to form clay minerals.
• Hydration: Minerals absorb water and expand, causing weakening.

Chemical weathering is favoured by warm temperatures, abundant moisture, thick vegetation, high carbon dioxide levels, and good drainage. It is especially effective in humid tropical and warm temperate regions.

These processes often operate together. For example, plant roots widen cracks mechanically, while organic acids released by vegetation chemically attack minerals.

The rate and type of weathering depend on several environmental and geological factors:

• Climate: Warm and wet climates favour chemical weathering, whereas cold or dry climates commonly favour physical weathering.
• Temperature: Higher temperatures generally accelerate chemical reactions.
• Water availability: Moisture promotes solution, hydrolysis, carbonation, and biological activity.
• Rock type: Minerals differ in resistance; quartz is relatively resistant, while feldspar weathers more readily.
• Rock structure: Joints, faults, bedding planes, and fractures allow water and air to penetrate deeply.
• Topography: Steep slopes encourage removal of weathered material, exposing fresh rock, while gentle slopes may permit thick weathering mantles to develop.
• Vegetation: Roots and organic acids enhance both mechanical and chemical weathering.

In limestone regions, weakly acidic water dissolves calcium carbonate through carbonation. This produces distinctive karst landforms.

• Lapiaz or karren: Grooves and channels formed on exposed limestone surfaces.
• Sinkholes or dolines: Closed depressions formed by solution or collapse.
• Caves: Underground cavities developed along joints and bedding planes.
• Stalactites: Deposits hanging from cave roofs.
• Stalagmites: Deposits rising from cave floors.
• Poljes: Large, flat-floored depressions surrounded by limestone hills.
• Disappearing streams: Surface streams that sink through swallow holes.

The intensity of karst development depends on soluble limestone, abundant water, joints, and sufficient carbon dioxide in soil and groundwater.

Arid environments:

• Desert pavement: A surface covered by closely packed resistant stones after fine material is removed.
• Talus or scree slopes: Accumulations of angular fragments at the bases of cliffs.
• Exfoliation domes: Rounded rock masses formed by peeling of outer layers.
• Rock pedestals: Irregular forms produced when wind abrades softer lower sections.

Cold environments:

• Block fields: Areas covered by frost-shattered rock fragments.
• Frost-riven cliffs: Cliffs broken by repeated freezing and thawing.
• Talus cones: Cone-shaped accumulations of frost-shattered debris below slopes.

These landforms result mainly from temperature changes, frost action, pressure release, and limited chemical alteration.

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

Slope stability depends on the balance between driving forces and resisting forces.

• Slope angle: Steeper slopes have a greater downslope component of gravity.
• Material strength: Strong, coherent rocks resist movement better than loose or clay-rich materials.
• Water content: Water adds weight, raises pore-water pressure, and may reduce friction.
• Rock structure: Joints, faults, and bedding planes can provide planes of weakness.
• Vegetation: Roots bind soil and intercept rainfall, increasing stability.
• Undercutting: Rivers, waves, glaciers, and construction may remove support at the slope base.
• Vibrations: Earthquakes and blasting can trigger failure.
• Climate: Intense rainfall, freeze-thaw action, and rapid snowmelt often promote movement.

Mass-wasting processes can be classified using the manner of movement and the material transported.

According to movement:

• Falls: Material detaches and moves through the air from a steep slope.
• Topples: Blocks rotate forward and fall from a cliff.
• Slides: A coherent mass moves along a distinct shear surface.
• Flows: Water-rich soil or debris moves like a viscous fluid.
• Creep: Very slow, gradual downslope movement of soil or rock.

According to material:

• Rockfall or rockslide: Dominated by bedrock.
• Debris flow: Contains a mixture of coarse fragments, sand, and mud.
• Mudflow: Contains a high proportion of fine sediment and water.
• Earthflow: Involves fine-grained, water-saturated earth moving downslope.

The categories may overlap, and one process can transform into another during movement.

Soil creep is the extremely slow, persistent downslope movement of soil and regolith under gravity.

Causes:

• Repeated expansion and contraction caused by heating and cooling.
• Wetting and drying of soil.
• Freezing and thawing.
• Growth and movement of plant roots.
• Burrowing by animals.
• Disturbance by cultivation and human activity.

Identifying features:

• Tilted fence posts, poles, and trees.
• Bent tree trunks with curved lower portions.
• Terracettes or small step-like ridges on slopes.
• Small cracks and uneven ground surfaces.

Effects: Soil creep gradually alters slope profiles, damages roads and buildings, and supplies loose material for faster mass movements.

All are gravity-driven, but they differ in speed, water content, material, and movement style.

Landslides and slumps create distinctive erosional and depositional landforms.

• Head scarp: A steep break in slope at the upper edge of the failed mass.
• Crown: The relatively undisturbed ground above the head scarp.
• Main body: The displaced material that moves downslope.
• Back-tilted blocks: Blocks in a slump that rotate backward toward the source area.
• Transverse cracks: Cracks formed across the moving mass.
• Toe or toe bulge: A compressed and often hummocky accumulation at the lower end.
• Hummocky terrain: Irregular mounds and depressions produced by disrupted blocks.
• Sag ponds: Water-filled depressions formed in closed hollows on the displaced mass.

Together, these features help identify the type and direction of slope movement.

Causes:

• Intense or prolonged rainfall.
• Rapid snowmelt or glacier melt.
• Volcanic eruptions and the remobilization of ash.
• Wildfire, which removes vegetation and reduces infiltration.
• Earthquakes that loosen slopes.
• Sudden failure of natural or artificial dams.

Geomorphic effects:

• Erosion of steep channels and valley sides.
• Deposition of poorly sorted debris at fan-shaped debris-flow fans.
• Formation of mudflow levees and lobes.
• Burial of valleys, roads, settlements, and agricultural land.
• Modification of drainage networks and valley floors.

These flows are highly mobile because water reduces internal friction and allows material to travel long distances.

Water is a major trigger of mass movement because it increases the weight of a slope, reduces friction between particles, and may create buoyancy and seepage forces.

The relationship between total stress, pore-water pressure, and effective stress is:

where:

• is effective stress,
• is total normal stress,
• is pore-water pressure.

As rainfall infiltrates a slope, increases. Therefore, decreases. Since the shear strength of soil is related to effective stress, a decrease in lowers resistance to failure. In simplified form:

where is shear strength, is cohesion, and is the angle of internal friction. Thus, intense or prolonged rainfall can transform a stable slope into an unstable one.