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Maharashtra HSC GFC practice questions

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Question 1Geology: Earth's Structure and Plate Tectonics

In the standard layered model of Earth's interior, which layer lies directly beneath the crust and is composed mainly of solid silicate rock rich in iron and magnesium?

  1. The mantle Correct answer.
  2. The inner core
  3. The asthenosphere only
  4. The outer core

Explanation

The mantle is the thick silicate layer between the crust and the core, dominated by iron- and magnesium-rich minerals such as olivine and pyroxene. The asthenosphere is only a weak, partly mobile zone within the upper mantle, not the whole layer beneath the crust.

Question 2Geology: Earth's Structure and Plate Tectonics

The Mohorovicic discontinuity, usually called the Moho, marks the boundary between which two regions?

  1. The outer core and the inner core
  2. The lithosphere and the asthenosphere
  3. The crust and the mantle Correct answer.
  4. The mantle and the outer core

Explanation

Seismic waves change speed sharply at the Moho because they pass from the lighter crustal rocks into denser mantle rocks. It is defined by this seismic velocity jump, not by a change in physical state.

Question 3Geology: Earth's Structure and Plate Tectonics

Which observation about seismic waves provides the strongest direct evidence that Earth's outer core is liquid?

  1. S waves do not pass through the outer core Correct answer.
  2. Surface waves travel only through continents
  3. P waves speed up in the outer core
  4. Love waves arrive before Rayleigh waves

Explanation

Shear waves cannot propagate through a fluid, so the S-wave shadow zone beyond about 103 degrees from the epicentre shows that the outer core lacks shear strength. This is the classic seismological proof of a liquid outer core.

Question 4Geology: Earth's Structure and Plate Tectonics

According to the theory of plate tectonics, the lithospheric plates move because they ride on which underlying layer?

  1. The outer core
  2. The continental crust
  3. The asthenosphere Correct answer.
  4. The mesosphere

Explanation

The asthenosphere is a hot, weak, partly molten zone of the upper mantle that deforms plastically and allows rigid lithospheric plates to glide over it. Plate motion is driven by convection and slab pull acting through this weak layer.

Question 5Geology: Earth's Structure and Plate Tectonics

Which type of plate boundary is characterised by two plates moving apart, with new oceanic crust forming from rising magma?

  1. Divergent boundary Correct answer.
  2. Oblique subduction zone
  3. Transform boundary
  4. Convergent boundary

Explanation

At divergent boundaries, such as the Mid-Atlantic Ridge, tensional forces pull plates apart and magma rises to fill the gap, creating new oceanic lithosphere. This process is called sea-floor spreading.

Question 6Geology: Earth's Structure and Plate Tectonics

A deep ocean trench, a volcanic island arc and a Wadati-Benioff zone of deep earthquakes together indicate which tectonic setting?

  1. Mid-ocean ridge
  2. Transform fault
  3. Continental rift valley
  4. Ocean-ocean convergent boundary Correct answer.

Explanation

Where two oceanic plates converge, the denser plate subducts, producing a trench, a Benioff zone of progressively deeper earthquakes, and an arc of volcanoes above the descending slab. This triad of features is diagnostic of ocean-ocean subduction.

Question 7Geology: Earth's Structure and Plate Tectonics

The Himalayas formed mainly as a result of which tectonic process?

  1. Sea-floor spreading at a ridge
  2. Collision of two continental plates Correct answer.
  3. Rifting of a continental plate
  4. Subduction of an oceanic plate

Explanation

Continental lithosphere is too buoyant to subduct deeply, so when the Indian and Eurasian plates converge, the crust thickens and is folded and thrust upward. This continent-continent collision produced the Himalayan mountain belt.

Question 8Geology: Earth's Structure and Plate Tectonics

Which of the following best describes the mechanism of sea-floor spreading?

  1. Oceanic crust is pushed upward to form continents
  2. Ocean water reacts with mantle rock to create new crust at trenches
  3. The sea floor sinks into trenches and is replaced by continental rock
  4. Magma rises at mid-ocean ridges and new crust moves laterally away from the ridge Correct answer.

Explanation

At mid-ocean ridges, decompression melting of the mantle produces magma that solidifies as new basaltic crust, and this crust is carried away symmetrically on both sides. Magnetic striping and age patterns confirm this lateral growth.

Question 9Geology: Earth's Structure and Plate Tectonics

Symmetric magnetic striping on either side of a mid-ocean ridge is best explained by which process?

  1. Repeated reversals of Earth's magnetic field recorded as crust forms and spreads Correct answer.
  2. Tidal forces aligning magnetic minerals
  3. Meteorite impacts magnetising the sea floor
  4. Local magnetic anomalies caused by iron ore deposits

Explanation

As basaltic magma cools at the ridge, magnetic minerals lock in the direction of the field at that time. When the field reverses, stripes of normal and reversed polarity form, and spreading makes them mirror images on both flanks.

Question 10Geology: Earth's Structure and Plate Tectonics

Which driving force of plate motion is considered the most important by most modern geologists?

  1. Tidal drag from the Moon
  2. Slab pull from the dense, descending edge of a subducting plate Correct answer.
  3. Centrifugal force of Earth's rotation
  4. Expansion of the core

Explanation

The negative buoyancy of cold, dense lithosphere sinking at subduction zones provides the dominant force, called slab pull. Mantle convection and ridge push also contribute, but slab pull is generally the largest single component.

Question 11Geology: Earth's Structure and Plate Tectonics

Alfred Wegener's continental drift hypothesis was initially rejected mainly because he could not provide a satisfactory explanation for what?

  1. The existence of fossil evidence across continents
  2. The mechanism that could move continents across the ocean floor Correct answer.
  3. The matching shapes of continental coastlines
  4. The presence of glacial deposits in the Southern Hemisphere

Explanation

Wegener assembled strong evidence from fossils, rock sequences and glacial deposits, but he lacked a credible driving mechanism. Plate tectonics later supplied that mechanism through sea-floor spreading and mantle convection.

Question 12Geology: Earth's Structure and Plate Tectonics

Which seismic discontinuity separates the mantle from the core?

  1. The Mohorovicic discontinuity
  2. The Gutenberg discontinuity Correct answer.
  3. The Conrad discontinuity
  4. The Lehmann discontinuity

Explanation

The Gutenberg discontinuity, at roughly 2,900 km depth, marks the mantle-core boundary and is detected by a strong drop in P-wave velocity and the termination of S waves. The Conrad discontinuity is within the crust, and the Lehmann is within the core.

Question 13Geology: Earth's Structure and Plate Tectonics

The rigid outer shell of Earth that is broken into tectonic plates is called the lithosphere. Which statement about it is correct?

  1. It lies below the asthenosphere
  2. It is entirely molten
  3. It consists only of continental crust
  4. It includes the crust and the uppermost rigid part of the mantle Correct answer.

Explanation

The lithosphere is defined by its mechanical rigidity, comprising the crust together with the uppermost solid mantle above the asthenosphere. Its thickness varies, being greatest beneath old continental interiors.

Question 14Geology: Earth's Structure and Plate Tectonics

At a transform boundary such as the San Andreas Fault, the dominant relative motion of the plates is best described as which of the following?

  1. Horizontal sliding past one another Correct answer.
  2. Spreading apart perpendicular to the fault
  3. Direct head-on collision
  4. Vertical uplift without lateral motion

Explanation

Transform boundaries accommodate lateral shear, with plates sliding horizontally past each other and neither crust created nor destroyed. Earthquakes there are typically shallow and strike-slip in character.

Question 15Geology: Earth's Structure and Plate Tectonics

A geologist maps a region with a linear rift valley, shallow normal-fault earthquakes and basaltic volcanism, with no trench nearby. Which boundary type is most consistent with these features?

  1. Continental collision zone
  2. Transform boundary
  3. Divergent boundary Correct answer.
  4. Ocean-continent convergent boundary

Explanation

Extension stretches the crust, producing normal faults, a rift valley and decompression melting that feeds basaltic volcanism. The absence of a trench rules out subduction, and collision would produce compression and folding.

Question 16Geology: Earth's Structure and Plate Tectonics

The Wadati-Benioff zone is best defined as which of the following?

  1. A zone of no seismic activity beneath continents
  2. A horizontal layer of shallow earthquakes at a mid-ocean ridge
  3. A dipping plane of earthquake foci that traces a descending subducting slab Correct answer.
  4. A region of the outer core where S waves vanish

Explanation

Earthquake hypocentres in subduction zones deepen progressively inland along a dipping plane, following the cold, brittle subducting slab to depths of up to about 700 km. This geometry provided early confirmation of subduction.

Question 17Geology: Earth's Structure and Plate Tectonics

Which statement correctly describes the difference between the epicentre and the focus (hypocentre) of an earthquake?

  1. The focus is the point of rupture within the Earth, and the epicentre is the point on the surface directly above it Correct answer.
  2. Both terms refer to the same point on the surface
  3. The epicentre is deep underground, and the focus is at the surface
  4. The focus is the area of greatest surface damage, and the epicentre is the fault plane

Explanation

Energy is released at the focus, which lies at depth along the fault, and the epicentre is its vertical projection onto the ground surface. Seismic stations use the epicentre to locate the earthquake.

Question 18Geology: Earth's Structure and Plate Tectonics

The shadow zone for P waves between about 103 and 143 degrees from an epicentre is caused mainly by which phenomenon?

  1. Reflection of P waves from the Moho
  2. Complete absorption of P waves by the crust
  3. Total internal reflection within the mantle
  4. Refraction of P waves at the core-mantle boundary Correct answer.

Explanation

P waves bend downward as they enter the denser core, leaving a ring on the far side of the Earth where they are not recorded directly. The effect is due to refraction at the mantle-core boundary, not absorption.

Question 19Geology: Earth's Structure and Plate Tectonics

Oceanic lithosphere becomes denser and thicker with age, which helps explain why older oceanic plates tend to do which of the following?

  1. Subduct more readily at convergent margins Correct answer.
  2. Float higher than young plates
  3. Melt completely at mid-ocean ridges
  4. Stop moving along transform faults

Explanation

Cooling oceanic lithosphere thickens and becomes denser, increasing its negative buoyancy and making subduction easier. This is why old, cold oceanic plates are associated with steep subduction and strong slab pull.

Question 20Geology: Earth's Structure and Plate Tectonics

Which evidence was most decisive in establishing that the continents once formed a single landmass called Pangaea?

  1. The presence of mountains on every continent
  2. Matching glacial deposits, fossils and rock sequences across now-separated continents Correct answer.
  3. The existence of deserts at all latitudes
  4. The uniform depth of the oceans

Explanation

Glacial striations, Permo-Carboniferous coal beds and fossils such as Glossopteris and Mesosaurus occur in matching patterns across continents now far apart. Such correlations are very unlikely unless the landmasses were once joined.

Question 21Geology: Earth's Structure and Plate Tectonics

Why do deep-focus earthquakes rarely occur below about 700 km depth?

  1. The subducting slab warms and becomes too ductile to fracture Correct answer.
  2. The core absorbs all seismic energy at this depth
  3. The mantle is liquid below this depth
  4. Seismic waves cannot travel deeper than 700 km

Explanation

At greater depths the subducting slab heats up and loses brittleness, so it deforms by creep rather than by sudden fracture. The lower limit of deep earthquakes therefore reflects a change in slab rheology, not a barrier to seismic waves.

Question 22Geology: Earth's Structure and Plate Tectonics

A chain of volcanic islands becomes progressively older away from a mantle hot spot. This pattern is best explained by which process?

  1. Sea-floor spreading at a mid-ocean ridge
  2. Repeated reversals of the geomagnetic field
  3. A subducting slab moving steadily inland
  4. A stationary hot spot beneath a moving lithospheric plate Correct answer.

Explanation

A hot spot is a fixed mantle plume that melts the plate above it; as the plate moves over the plume, older volcanoes are carried away and new ones form. The Hawaiian-Emperor chain is the classic example.

Question 23Geology: Earth's Structure and Plate Tectonics

Which of the following best explains why the asthenosphere behaves plastically while the lithosphere above it remains rigid?

  1. The asthenosphere has no pressure acting on it
  2. The asthenosphere is made of liquid iron
  3. The lithosphere is hotter than the asthenosphere
  4. The asthenosphere is closer to its melting temperature and deforms by creep over long times Correct answer.

Explanation

Higher temperature and small amounts of partial melt make the asthenosphere weak, so it flows slowly by solid-state creep on geological timescales. The cooler lithosphere remains rigid and can transmit stress as a plate.

Question 24Geology: Earth's Structure and Plate Tectonics

Which of these is the best example of a modern divergent plate boundary?

  1. The Japan Trench
  2. The San Andreas Fault
  3. The Mid-Atlantic Ridge Correct answer.
  4. The Andes mountain belt

Explanation

The Mid-Atlantic Ridge is a spreading centre where the North American and Eurasian plates move apart and new oceanic crust forms. The Andes, San Andreas and Japan Trench are respectively convergent, transform and subduction settings.

Question 25Geology: Rocks, Minerals and the Rock Cycle

Which mineral is the most abundant primary constituent of granite and is commonly identified by its glassy, often pink or flesh-coloured appearance on a fresh surface?

  1. Orthoclase feldspar Correct answer.
  2. Muscovite mica
  3. Gypsum
  4. Calcite

Explanation

Granite is a coarse-grained felsic intrusive rock dominated by alkali feldspar, quartz and mica or amphibole. Orthoclase is the pinkish potash feldspar that gives granite much of its colour. Calcite and gypsum are not granite-forming minerals.

Question 26Geology: Rocks, Minerals and the Rock Cycle

On Mohs scale of hardness, a mineral that can scratch calcite but is itself scratched by fluorite has a hardness of about

  1. 7
  2. 5
  3. 2
  4. 3.5 Correct answer.

Explanation

Mohs scale orders minerals by scratch resistance: calcite is 3 and fluorite is 4. A mineral that scratches calcite but is scratched by fluorite must fall between them, so about 3.5 is correct. Values of 2, 5 and 7 lie outside this bracket.

Question 27Geology: Rocks, Minerals and the Rock Cycle

Basalt is best described as

  1. a clastic sedimentary rock
  2. a fine-grained mafic extrusive rock Correct answer.
  3. a coarse-grained felsic intrusive rock
  4. a foliated metamorphic rock

Explanation

Basalt forms from rapid cooling of mafic lava at the surface, giving a fine-grained texture, and it is rich in ferromagnesian minerals. Its dark colour and fine grain distinguish it from coarse intrusive rocks and from foliated or clastic rocks.

Question 28Geology: Rocks, Minerals and the Rock Cycle

A magma that cools very slowly deep beneath the surface produces an igneous rock with which texture?

  1. Vesicular
  2. Fine-grained
  3. Coarse-grained Correct answer.
  4. Glassy

Explanation

Slow cooling gives crystals a long time to grow, so the resulting intrusive rock is coarse-grained and individual minerals are visible to the eye. Rapid cooling instead produces fine-grained or glassy textures.

Question 29Geology: Rocks, Minerals and the Rock Cycle

Which sedimentary rock is formed mainly by chemical precipitation of calcium carbonate from solution?

  1. Conglomerate
  2. Limestone Correct answer.
  3. Shale
  4. Sandstone

Explanation

Limestone can form by chemical and biochemical precipitation of calcium carbonate, for example in warm shallow seas. Sandstone, conglomerate and shale are clastic rocks built from transported fragments of varying grain size.

Question 30Geology: Rocks, Minerals and the Rock Cycle

Which metamorphic rock typically shows a well-developed foliation defined by parallel alignment of platy mica minerals?

  1. Marble
  2. Hornfels
  3. Schist Correct answer.
  4. Quartzite

Explanation

Schist is a medium-grade foliated metamorphic rock in which micas grow large enough to be seen and align in parallel bands. Quartzite, marble and hornfels are generally non-foliated.

Question 31Geology: Rocks, Minerals and the Rock Cycle

The rock cycle is best understood as

  1. a closed loop in which any rock type can be transformed into any other Correct answer.
  2. a process that requires volcanic eruptions at every step
  3. a one-way sequence from igneous to sedimentary to metamorphic rock
  4. a process that operates only on the ocean floor

Explanation

The rock cycle is a set of interconnected processes by which igneous, sedimentary and metamorphic rocks are continually recycled through melting, weathering, erosion, deposition, burial and metamorphism. Any rock type can be converted into another, and no single direction is compulsory.

Question 32Geology: Rocks, Minerals and the Rock Cycle

Aphanitic texture in an igneous rock indicates that

  1. the rock has been metamorphosed
  2. crystals are too small to be seen with the unaided eye Correct answer.
  3. the rock contains abundant gas cavities
  4. the rock consists almost entirely of glass

Explanation

Aphanitic means fine-grained: the crystals formed quickly and are microscopic, so the rock looks uniform to the naked eye. It differs from glassy texture, where no crystals form at all.

Question 33Geology: Rocks, Minerals and the Rock Cycle

Which property of a mineral is most directly determined by its internal arrangement of atoms and is observed as regular flat surfaces on a broken specimen?

  1. Specific gravity
  2. Lustre
  3. Streak
  4. Cleavage Correct answer.

Explanation

Cleavage is the tendency of a mineral to break along planes of weak bonding in its crystal structure, producing flat shiny surfaces. Lustre describes reflected light, streak is powder colour and specific gravity is density relative to water.

Question 34Geology: Rocks, Minerals and the Rock Cycle

A mineral sample is dark, heavy, strongly magnetic and has a black streak. It is most probably

  1. magnetite Correct answer.
  2. galena
  3. hematite
  4. pyrite

Explanation

Strong magnetism combined with a dark colour and black streak identifies magnetite, an iron oxide. Hematite is weakly magnetic at most and gives a red-brown streak, galena is non-magnetic with a grey streak, and pyrite is non-magnetic with a greenish-black streak.

Question 35Geology: Rocks, Minerals and the Rock Cycle

Which sedimentary structure indicates deposition from currents that reverse direction, such as tidal action?

  1. Graded bedding
  2. Cross-bedding
  3. Herringbone cross-bedding Correct answer.
  4. Ripple marks of symmetrical form

Explanation

Herringbone cross-bedding consists of sets of inclined laminae dipping in opposite directions, recording alternating current directions typical of tidal flats. Graded bedding records a single waning flow, while symmetrical ripples reflect wave oscillation.

Question 36Geology: Rocks, Minerals and the Rock Cycle

Regional metamorphism of a shale during increasing temperature and pressure typically produces which sequence of rocks?

  1. Shale, sandstone, conglomerate, limestone
  2. Shale, marble, quartzite, hornfels
  3. Shale, slate, phyllite, schist, gneiss Correct answer.
  4. Shale, basalt, gabbro, peridotite

Explanation

Progressive regional metamorphism of a clay-rich protolith increases grain size and foliation, giving slate, then phyllite, then schist and finally gneiss at the highest grades. The other sequences mix sedimentary, igneous or non-foliated rocks that do not follow this path.

Question 37Geology: Rocks, Minerals and the Rock Cycle

The main difference between conglomerate and breccia is that

  1. conglomerate is fine-grained while breccia is coarse-grained
  2. conglomerate contains rounded clasts while breccia contains angular clasts Correct answer.
  3. conglomerate is metamorphic while breccia is sedimentary
  4. conglomerate is chemically precipitated while breccia is clastic

Explanation

Both are coarse clastic sedimentary rocks, but conglomerate has rounded pebbles that were transported and abraded, whereas breccia has angular fragments that were not carried far. Rounding therefore records transport history.

Question 38Geology: Rocks, Minerals and the Rock Cycle

Which mineral group includes talc, kaolinite and muscovite and is characterised by sheet-like silicate structures?

  1. Chain silicates
  2. Island silicates
  3. Sheet silicates Correct answer.
  4. Framework silicates

Explanation

Sheet silicates have silicon-oxygen tetrahedra linked in continuous layers, which gives them platy habits and perfect basal cleavage. Talc, kaolinite and muscovite all belong to this group.

Question 39Geology: Rocks, Minerals and the Rock Cycle

A dark-coloured, dense, coarse-grained intrusive igneous rock composed mainly of olivine and pyroxene is

  1. rhyolite
  2. gabbro
  3. andesite
  4. peridotite Correct answer.

Explanation

Peridotite is an ultramafic rock dominated by olivine with pyroxene, and it forms much of the upper mantle. Gabbro is mafic but contains plagioclase feldspar, while rhyolite and andesite are extrusive rocks.

Question 40Geology: Rocks, Minerals and the Rock Cycle

Which metamorphic rock forms by contact metamorphism of limestone around an igneous intrusion?

  1. Quartzite
  2. Marble Correct answer.
  3. Slate
  4. Gneiss

Explanation

Marble is the metamorphic product of limestone, and contact metamorphism around an intrusion is one way it forms. Slate and gneiss come from shale under regional conditions, and quartzite comes from sandstone.

Question 41Geology: Rocks, Minerals and the Rock Cycle

Which statement best explains why quartz is chemically very resistant to weathering at the Earth's surface?

  1. It dissolves readily in rainwater
  2. It has a perfect basal cleavage that sheds water
  3. Its framework silicate structure is strongly bonded and stable under surface conditions Correct answer.
  4. It is the softest common mineral

Explanation

Quartz has a three-dimensional framework of strongly bonded silicon-oxygen tetrahedra with no easy cleavage, so it resists chemical attack and physical breakdown. This is why quartz survives to become the dominant mineral of many sandstones.

Question 42Geology: Rocks, Minerals and the Rock Cycle

Which of these is a foliated metamorphic rock formed at the highest metamorphic grade?

  1. Phyllite
  2. Schist
  3. Gneiss Correct answer.
  4. Slate

Explanation

Foliation becomes coarser and minerals segregate into light and dark bands as grade rises. Gneiss represents high-grade metamorphism with distinct banding, following slate, phyllite and schist in order of increasing grade.

Question 43Geology: Rocks, Minerals and the Rock Cycle

Porphyritic texture, in which large crystals are set in a finer groundmass, most directly indicates

  1. two distinct stages of cooling at different rates Correct answer.
  2. complete melting of the rock after crystallisation
  3. metamorphic recrystallisation under stress
  4. deposition of crystals from water

Explanation

Large phenocrysts grow slowly at depth, and the finer groundmass records later, faster cooling, for example after eruption. The texture therefore records a two-stage cooling history.

Question 44Geology: Rocks, Minerals and the Rock Cycle

Which mineral is the chief ore of aluminium and is extracted from bauxite deposits formed by intense tropical weathering?

  1. Bauxite Correct answer.
  2. Sphalerite
  3. Chalcopyrite
  4. Haematite

Explanation

Bauxite is a residual ore rich in aluminium hydroxides, concentrated when intense tropical weathering leaches silica and alkalis from aluminous rocks. Haematite is an iron ore, chalcopyrite a copper ore and sphalerite a zinc ore.

Question 45Geology: Rocks, Minerals and the Rock Cycle

A clastic sedimentary rock consists of grains of mixed size, from clay to pebbles, with poor sorting and angular fragments. This most likely indicates

  1. deposition far from the source after long transport
  2. chemical precipitation in a shallow lagoon
  3. slow accumulation of wind-blown dust
  4. deposition close to the source with rapid burial Correct answer.

Explanation

Poor sorting and angular clasts mean the sediment was transported only a short distance and buried quickly before abrasion could act. Long transport would round the grains and sort them by size.

Question 46Geology: Rocks, Minerals and the Rock Cycle

Which process converts loose sediment into solid sedimentary rock?

  1. Melting and recrystallisation
  2. Rapid chilling of lava
  3. Compaction and cementation Correct answer.
  4. Foliation under directed pressure

Explanation

Lithification of sediment involves compaction as overburden squeezes out water, followed by cementation when minerals precipitate in the pore spaces. Melting, foliation and chilling belong to igneous and metamorphic processes.

Question 47Geology: Rocks, Minerals and the Rock Cycle

Which feature of a mineral is tested by rubbing it on an unglazed porcelain plate?

  1. Hardness
  2. Cleavage
  3. Lustre
  4. Streak Correct answer.

Explanation

Streak is the colour of the fine powder left when a mineral is rubbed on unglazed porcelain, and it is often more constant than the colour of the bulk specimen. Hardness, cleavage and lustre are assessed by other tests.

Question 48Geology: Rocks, Minerals and the Rock Cycle

A rock composed of alternating light and dark mineral bands, with abundant quartz and feldspar and a granular texture, is most likely

  1. marble
  2. shale
  3. obsidian
  4. gneiss Correct answer.

Explanation

Banded segregation of light and dark minerals at high metamorphic grade is the defining feature of gneiss. Marble is non-foliated, shale is a fine clastic sedimentary rock and obsidian is a glassy volcanic rock.

Question 49Geology: Geomorphic Processes and Landforms

Which geomorphic agent is chiefly responsible for the formation of a V-shaped valley in the youthful stage of a river?

  1. Lateral erosion by the river
  2. Deposition of alluvium along the banks
  3. Vertical downcutting by the river Correct answer.
  4. Glacial plucking of the valley floor

Explanation

In the youthful stage, a river has a steep gradient and high energy, so erosion is dominantly vertical, deepening the channel faster than it widens. This concentrated downcutting produces the narrow, steep-sided V-shaped profile typical of mountain torrents. Lateral erosion and deposition become dominant only in later stages.

Question 50Geology: Geomorphic Processes and Landforms

A river emerging from a mountain front onto a plain suddenly loses gradient and spreads its load. The landform built at this break of slope is:

  1. An alluvial fan Correct answer.
  2. A plunge pool
  3. A meander scar
  4. A natural levee

Explanation

When a stream leaves a steep upland and enters a flatter plain, its velocity and carrying capacity drop abruptly, so coarse sediment is deposited in a fan-shaped spread radiating from the apex. This is the classic alluvial fan of piedmont zones. Meander scars, plunge pools and levees form under different conditions along the channel course.