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
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.
The solid rock and soil of the crust and the top of the mantle.
Oceans, rivers, lakes, ice and water under the ground.
The blanket of air; its lowest layer is the troposphere.
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.
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.
| 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 |
Rocks and the rock cycle
Rule in one line: igneous rocks come from fire, sedimentary from layers, metamorphic from change.
| Original rock | After heat and pressure |
|---|---|
| Limestone | Marble |
| Shale or clay | Slate |
| Sandstone | Quartzite |
| Granite | Gneiss |
| Carbon-rich rocks such as coal | Graphite |
Moving plates and the mountains they build
Rule in one line: squeezing bends rocks into fold mountains; cracking lifts or drops blocks.
- 1912Alfred Wegener: continental drift
All continents were once one landmass, Pangaea, in one ocean, Panthalassa. It split into Laurasia (north) and Gondwanaland (south).
- 1960sSea-floor spreading
Harry Hess showed that new ocean floor forms at mid-ocean ridges and spreads outwards.
- Late 1960sPlate tectonics
The lithosphere is broken into large plates — seven major ones — that move a few centimetres a year.
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.
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.
| 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 |
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.
Runny basalt lava spreads far, making gentle slopes. Example: Hawaii.
Layers of thick lava and ash build a steep cone. Examples: Fuji, Mayon.
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.
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.
Weathering and erosion
Rule in one line: weathering breaks rock where it stands; erosion carries the pieces away.
- 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.
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 |
- 1Young riverFast in the hills: cuts V-shaped valleys, gorges and waterfalls
- 2Mature riverSlow on the plain: winds in loops called meanders
- 3Oxbow lakeA loop gets cut off and is left as a curved lake
- 4DeltaAt the mouth, the river drops its silt in a fan shape
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.
StalacTites hang from the Top (ceiling); stalagMites Mount up from the ground.
Plateaus and plains
Rule in one line: a plateau is a raised flat table; a plain is low flat land, often built by rivers.
Ringed by mountains. Tibet, the highest, is called the 'roof of the world'.
Lies at the foot of mountains, with a plain or sea on the other side.
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 |
Traps and practice
Last revision — where the options try to fool you.
- 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.
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.
- Boundary between crust and mantle
- Moho (Mohorovičić)
- Boundary between mantle and core
- Gutenberg, about 2,900 km
- Core is made of
- Nickel and iron (NIFE)
- Limestone after heat and pressure
- Marble
- Gave continental drift
- Alfred Wegener, 1912
- Raised block between two faults
- Horst
- India's active volcano
- Barren Island
- Crescent-shaped sand dune
- Barchan
- Lime deposit hanging from a cave roof
- Stalactite
- Plain built up by river silt
- Aggradational plain
- Crust → mantle (to 2,900 km) → outer core (liquid) → inner core (solid)
- SIAL continents, SIMA ocean floor, NIFE core
- Igneous: granite, basalt; sedimentary: sandstone, shale; metamorphic: marble, slate, graphite
- 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