Physical Geography: Earth's Interior, Rocks and Landforms
The layers inside the Earth, the three kinds of rock, moving plates and the mountains they build, earthquakes and volcanoes, and the landforms made by rivers, wind, ice, waves and underground water.
Mayon in the Philippines erupting. It is a composite (strato) volcano: layers of lava and ash have built a steep cone. Heat from inside the Earth drives volcanoes, earthquakes and the slow movement of plates. Image: Jaycee Esmeria, Wikimedia Commons · CC BY-SA 4.0
The ground under our feet looks still, but it is not. Deep inside, the Earth is hot. That heat moves huge plates of rock, lifts mountains and shakes the land. On the surface, rivers, wind, ice and waves slowly wear the rocks down and build new shapes. This page explains both: the forces from inside and the work done outside.
On this page you will learn:
The four spheres, and the crust, mantle and core
Igneous, sedimentary and metamorphic rocks
Continental drift, plates and the three kinds of mountain
Earthquake waves, the Richter scale and volcanoes
Weathering and erosion
Landforms of rivers, wind, glaciers, waves and underground water
Kinds of plateaus and plains
6,371 km
Average radius of the Earth
Centre to surface
2,900 km
Depth where the core begins
Gutenberg discontinuity
3
Kinds of rock
Igneous, sedimentary, metamorphic
The four spheres and the inside of the Earth
Rule in one line: the Earth is layered like an onion — a thin crust, a thick mantle and a metal core.
Land
Lithosphere
The solid rock and soil of the crust and the top of the mantle.
Water
Hydrosphere
Oceans, rivers, lakes, ice and water under the ground.
Air
Atmosphere
The blanket of air; its lowest layer is the troposphere.
Life
Biosphere
The narrow zone where land, water and air meet and life is found.
Life is found in the hydrosphere, the top of the lithosphere and the lower air (troposphere). The stratosphere above is too thin and cold for life.
The layers of the Earth: crust, mantle, outer core, inner core. The globe on the right is drawn to scale — see how thin the crust really is. The wedge on the left is not to scale. Image: Surachit, Wikimedia Commons · CC BY-SA 3.0
THREE MAIN LAYERS
Crust
Mantle
Core
Depth
About 5 km (oceans) to 30 km (continents)
Down to about 2,900 km
2,900 km to the centre
Made of
Light rocks
Hot, dense rock
Nickel and iron
Old name
SIAL and SIMA
—
NIFE
State
Solid
Solid; soft in the asthenosphere
Outer liquid, inner solid
Under big mountains like the Himalaya the crust is up to about 70 km thick.
KEY TERM
SIAL, SIMA and NIFE
Short names made from chemical symbols. SIAL = silica + aluminium: the light upper crust that makes the continents. SIMA = silica + magnesium: the heavier lower crust that forms the ocean floor. NIFE = nickel + iron: the core. The light SIAL 'floats' on the heavier SIMA.
SIAL: continents→SIMA: ocean floor→NIFE: core
Discontinuity
Between
Conrad
Upper crust (SIAL) and lower crust (SIMA)
Mohorovičić (Moho)
Crust and mantle
Repetti
Upper and lower mantle (named in some Indian books)
Gutenberg
Mantle and core, at about 2,900 km
Lehmann
Outer core and inner core, at about 5,150 km
A discontinuity is a boundary where earthquake waves suddenly change speed. The lithosphere is the crust plus the solid top of the mantle; below it the soft asthenosphere (up to about 400 km) is the main source of magma.
QUICK CHECK
PYQ · PPSC Principal 2020 — The biosphere is the zone of the Earth where life is found. It includes parts of I. Hydrosphere II. Lithosphere III. Stratosphere IV. Troposphere. Select the correct answer using the codes given below:
Tap an option to answer
Answer: D — I, II and IV
Life needs water, soil and breathable air: the hydrosphere, the top of the lithosphere and the troposphere. The stratosphere is left out.
Rocks and the rock cycle
Rule in one line: igneous rocks come from fire, sedimentary from layers, metamorphic from change.
THE THREE KINDS OF ROCK
Igneous
Sedimentary
Metamorphic
How formed
Magma or lava cools
Layers of bits pressed hard
Old rock changed by heat and pressure
Other name
Primary rocks
Layered rocks
Changed rocks
Examples
Granite, basalt
Sandstone, shale, limestone, coal
Marble, slate, quartzite, gneiss
Fossils
None
Often found
Rare
Magma that cools slowly inside the Earth forms big crystals (granite, an intrusive rock). Lava that cools fast on the surface forms fine grains (basalt, an extrusive rock). The Deccan Traps are made of basalt.
Original rock
After heat and pressure
Limestone
Marble
Shale or clay
Slate
Sandstone
Quartzite
Granite
Gneiss
Carbon-rich rocks such as coal
Graphite
Metamorphic pairs that exams ask. Graphite (the 'lead' of a pencil) is pure carbon formed by metamorphism, so it is classed with metamorphic rocks.
The rock cycle. Magma crystallises into igneous rock. Erosion carries bits away to settle as sediments, which harden into sedimentary rock. Burial with heat and pressure makes metamorphic rock, which can melt back into magma. Image: Carie Frantz, Wikimedia Commons · CC BY-SA 4.0
QUICK CHECK
PYQ · RSMSSB LDC 2024 — Which of the following is an example of Igneous rock?
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Answer: C — Granite
Granite forms when magma cools slowly underground. Sandstone and shale are sedimentary; marble is metamorphic (changed limestone).
QUICK CHECK
PYQ · PPSC DDSWO 2026 — Graphite is considered as which one of the following type of rocks?
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Answer: A — Metamorphic rocks
Graphite forms when carbon-rich rock is changed by great heat and pressure — the same process that turns limestone into marble.
QUICK CHECK
PYQ · PPSC JE 2021 — Lonar Lake, also known as Lonar crater, is a notified National Geo-heritage Monument, saline, soda lake. It is located in which of the following States?
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Answer: A — Maharashtra
Lonar is in Buldhana district of Maharashtra. A meteorite hit the Deccan basalt and made this round crater, which filled with salty water.
Moving plates and the mountains they build
Rule in one line: squeezing bends rocks into fold mountains; cracking lifts or drops blocks.
FROM DRIFTING CONTINENTS TO MOVING PLATES
1912
Alfred Wegener: continental drift
All continents were once one landmass, Pangaea, in one ocean, Panthalassa. It split into Laurasia (north) and Gondwanaland (south).
1960s
Sea-floor spreading
Harry Hess showed that new ocean floor forms at mid-ocean ridges and spreads outwards.
Late 1960s
Plate tectonics
The lithosphere is broken into large plates — seven major ones — that move a few centimetres a year.
WEGENER'S EVIDENCE
The coasts of South America and Africa fit like a jigsaw. The same fossils and rocks are found on both sides of the Atlantic. Old glacier deposits are found in India and Africa, now in warm lands.
Plates move apart at a divergent boundary (ocean ridge), slide past at a transform boundary, and collide at a convergent boundary, where one plate sinks (subduction — see the World Geography page). A hot spot can make volcanoes in the middle of a plate. Image: Jose F. Vigil, USGS, Wikimedia Commons · Public domain
THREE KINDS OF MOUNTAIN
Fold
Block
Volcanic
Made by
Plates squeeze; layers bend
Cracks (faults); blocks rise or sink
Lava and ash pile up
Young examples
Himalaya, Alps, Andes
Vosges, Black Forest
Fuji, Kilimanjaro
Old example
Aravalli, Appalachians
—
—
The Himalaya rose when the Indian plate moving north pushed into the Eurasian plate. It is still rising slowly.When the crust is pulled apart it cracks along faults. A block that stands up between two faults is a horst (a block mountain). A block that drops down is a graben (a rift valley). The Rhine valley lies in a graben between the Vosges and the Black Forest. Image: USGS; derivative by Gregors, Wikimedia Commons · Public domain
QUICK CHECK
PYQ · PPSC DDSWO 2026 — Horst and graben are related to which one of the following relief features?
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Answer: C — Block mountain
Both come from faulting: the raised block is a horst (block mountain) and the sunken block a graben (rift valley).
QUICK CHECK
PYQ · PPSC OHE 2016 — The oldest mountains in India are:
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Answer: A — Aravalis
The Aravalli are among the oldest fold mountains in the world. Long erosion has worn them into low, broken hills, so they are also called residual mountains. Guru Shikhar at Mount Abu is their highest point.
Earthquakes and volcanoes
Rule in one line: the Richter scale gives an earthquake's size; P-waves arrive first, surface waves do the most damage.
An earthquake starts at the focus deep inside; the point on the surface above it is the epicentre (more on the World Geography page). The waves spread out from the focus in three kinds.
EARTHQUAKE (SEISMIC) WAVES
P-waves
S-waves
Surface waves
Full name
Primary
Secondary
L-waves
Speed
Fastest; arrive first
Slower
Slowest
Pass through
Solids, liquids, gases
Solids only
Along the surface
Damage
Least
More
Most
S-waves cannot cross the outer core. That is how we know the outer core is liquid.
Richter magnitude
Class
Below 4.0
Minor
4.0 – 4.9
Light
5.0 – 5.9
Moderate
6.0 – 6.9
Strong
7.0 – 7.9
Major
8.0 and above
Great
Classes as used by the US Geological Survey. The scale is logarithmic: one step up means about 10 times bigger shaking and about 32 times more energy.
KEY TERM
Magnitude and intensity
A seismograph records the waves. The Richter scale (Charles Richter, 1935) gives the magnitude — the energy released. The Mercalli scale gives the intensity — how much damage people see at one place.
PYQ · PPSC TM 2015 — Earthquakes are classified as small, moderate, major or great based on Richter scale (a measure of energy released during the earthquake). On Richter scale a moderate earthquake will measure:
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Answer: C — 5.0 – 5.9
Moderate is 5.0–5.9. Then 6.0–6.9 is strong, 7.0–7.9 major and 8.0 or more great; 4.0–4.9 is light.
QUICK CHECK
PYQ · PSSSB Jail Warder 2024 — How is the magnitude of an earthquake determined?
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Answer: A — Richter Scale
Magnitude is given on the Richter scale. The seismograph is only the recording instrument; the Mercalli scale measures intensity. There is no 'Igneous Scale'.
Shape
Shield volcano
Runny basalt lava spreads far, making gentle slopes. Example: Hawaii.
Shape
Composite volcano
Layers of thick lava and ash build a steep cone. Examples: Fuji, Mayon.
Activity
Active, dormant, extinct
Active erupts now and then; dormant is sleeping; extinct is not expected to erupt again.
Melted rock is magma while it is underground and lava once it comes out. Most volcanoes lie on the Pacific Ring of Fire.
QUICK FACT
Barren Island in the Andaman Sea, north-east of Sri Vijaya Puram (Port Blair), has India's only active volcano. Nearby Narcondam island is usually described as a dormant volcano.
QUICK CHECK
PYQ · PPSC DDSWO 2026 — Active volcano in Andaman & Nicobar Islands is found in _______.
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Answer: A — Barren Island
Barren Island. It woke up in 1991 after a long quiet spell and has erupted again since. Baratang is known for mud volcanoes, not a lava volcano.
Weathering and erosion
Rule in one line: weathering breaks rock where it stands; erosion carries the pieces away.
TWO DIFFERENT JOBS
Weathering
Erosion
Rock moves?
No, breaks in place
Yes, carried away
Caused by
Heat, frost, water, plants
Rivers, wind, ice, waves
Result
Loose bits and soil
New landforms
Together, weathering and erosion slowly lower the land. This wearing down is called denudation.
Physical weathering: rock cracks when water freezes in it, or when hot days and cold nights make the outer layers peel off like an onion (common in deserts).
Chemical weathering: rain with carbon dioxide dissolves limestone; oxygen rusts iron in rocks.
Biological weathering: roots grow into cracks and burrowing animals loosen the rock.
QUICK CHECK
PYQ · PPSC AEE 2024 — In which sphere does weathering primarily occur, and how does it affect the other spheres?
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Answer: D — The lithosphere; it breaks down rocks into soil that supports plant life in the biosphere.
Weathering acts on rocks, so it happens in the lithosphere. The soil it makes feeds plants, linking the lithosphere to the biosphere.
Landforms made by rivers, wind, ice, waves and underground water
Rule in one line: every agent cuts in one place and drops its load in another.
Agent
Cuts (erosion)
Drops (deposition)
River (fluvial)
V-shaped valley, gorge, waterfall, meander, oxbow lake
Floodplain, levee, delta
Wind (eolian)
Mushroom rock
Sand dunes (barchan), loess
Glacier
U-shaped valley, cirque
Moraine
Sea waves
Cliff, sea cave, arch, stack
Beach, spit, bar
Underground water (karst)
Sinkhole, cave
Stalactite, stalagmite, pillar
Landforms by agent. 'Fluvial' means made by rivers, 'eolian' by wind (after Aeolus, the Greek god of winds), 'karst' by underground water dissolving limestone.
A RIVER'S WORK FROM HILLS TO SEA
1
Young river
Fast in the hills: cuts V-shaped valleys, gorges and waterfalls
↓
2
Mature river
Slow on the plain: winds in loops called meanders
↓
3
Oxbow lake
A loop gets cut off and is left as a curved lake
↓
4
Delta
At the mouth, the river drops its silt in a fan shape
A mushroom rock in the White Desert of Egypt. Wind carrying sand blows close to the ground, so it wears away the lower part faster and leaves a thin stem under a wide cap. This is an eolian (wind-made) landform. Image: Asgalal, Wikimedia Commons · CC BY-SA 4.0
KEY TERM
Barchan and loess
A barchan is a crescent-shaped (half-moon) sand dune; its horns point the way the wind blows. Loess is a thick deposit of fine wind-blown dust; the Loess Plateau of China is the best-known example.
The Perito Moreno Glacier in Argentina. A glacier is a slow river of ice. It scrapes valleys into a U shape and drops the rocks it carries as moraines. Image: Luca Galuzzi (Lucag), Wikimedia Commons · CC BY-SA 2.5A limestone cave in New Zealand — a karst landform. Water dripping from the roof leaves lime behind: stalactites hang down from the ceiling, stalagmites grow up from the ground. When the two meet they make a pillar. Image: Pseudopanax at English Wikipedia, Wikimedia Commons · Public domain
REMEMBER
StalacTites hang from the Top (ceiling); stalagMites Mount up from the ground.
QUICK CHECK
PYQ · PPSC ICS 2016 — Landscapes produced by the work of winds are called
Tap an option to answer
Answer: A — Eolian
Eolian (aeolian) landforms are made by wind — mushroom rocks, barchans, loess. Karst is underground water, fluvial is rivers, glacial is ice.
Plateaus and plains
Rule in one line: a plateau is a raised flat table; a plain is low flat land, often built by rivers.
Between ranges
Intermontane plateau
Ringed by mountains. Tibet, the highest, is called the 'roof of the world'.
At the foot
Piedmont plateau
Lies at the foot of mountains, with a plain or sea on the other side.
Lava
Volcanic plateau
Built by sheets of lava. The north-west Deccan (Deccan Traps) is one.
Kinds of plateau. Plateaus are often rich in minerals.
Plain
How it forms
Example
Depositional (aggradational)
Rivers lay down layer on layer of silt
Indo-Gangetic plain
Erosional (peneplain)
Old highland worn almost flat
Parts of old shields
Glacial (till plain)
Ice sheets drop clay and stones
North Europe, North America
Lake (lacustrine)
An old lake fills up and dries
Kashmir valley
'Aggradation' means building up by deposition; 'degradation' means wearing down by erosion.
QUICK CHECK
PYQ · PPSC OHE 2016 — The Great Plain of North India is a ______________ plain
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Answer: A — Aggradational
The Indus, Ganga and Brahmaputra have filled a huge low area with alluvium, so the plain is built up — aggradational. A till plain is glacial; an abyssal plain is on the deep ocean floor.
Traps and practice
Last revision — where the options try to fool you.
COMMON TRAPS
SIAL is the continents, SIMA the ocean floor — not the other way round.
Marble and slate are metamorphic; granite and basalt are igneous.
Horst and graben belong to block mountains, not fold mountains.
Richter = magnitude; Mercalli = intensity; seismograph = the instrument.
S-waves stop at the liquid outer core; P-waves go through.
Eolian = wind, fluvial = river, karst = underground water.
The Aravalli are the oldest mountains in India; the Himalaya are the youngest.
CHECK YOURSELF
A river flowing slowly across a plain makes a big loop. Years later the loop is cut off. What is left behind?
Show answer
An oxbow lake — a curved lake beside the new, straighter course of the river.
Wegener 1912: Pangaea split into Laurasia and Gondwanaland
Moderate earthquake 5.0–5.9 on the Richter scale
Mushroom rock and barchan: wind; oxbow lake and delta: river; moraine: glacier
Indo-Gangetic plain is aggradational; Tibet is an intermontane plateau
❧
Frequently Asked Questions
What are the layers of the Earth?
From outside in: the crust, the mantle (down to about 2,900 km), the liquid outer core and the solid inner core.
What do SIAL, SIMA and NIFE mean?
SIAL is silica and aluminium (the continental crust), SIMA is silica and magnesium (the ocean-floor crust), and NIFE is nickel and iron (the core).
What is the difference between igneous, sedimentary and metamorphic rocks?
Igneous rocks form when magma or lava cools (granite, basalt). Sedimentary rocks form from layers pressed together (sandstone, limestone). Metamorphic rocks are old rocks changed by heat and pressure (marble, slate).
Which type of rock is graphite?
Metamorphic. It is carbon that has been changed by great heat and pressure.
Who gave the theory of continental drift?
Alfred Wegener in 1912. He said all continents were once joined as Pangaea, which later broke apart.
What are horst and graben?
Features of block mountains made by faults. A horst is a raised block; a graben is a sunken block, also called a rift valley.
What is the difference between the Richter and Mercalli scales?
The Richter scale measures magnitude, the energy released. The Mercalli scale measures intensity, the damage seen at a place. A moderate earthquake is 5.0–5.9 on the Richter scale.
Where is India's active volcano?
On Barren Island in the Andaman Sea, part of the Andaman and Nicobar Islands.
What are eolian landforms?
Landforms made by wind, mostly in deserts — mushroom rocks, sand dunes such as barchans, and loess deposits.
Why is the Great Plain of North India called aggradational?
Because it was built up by deposits of silt brought by the Indus, Ganga and Brahmaputra and their tributaries.
