Interior of the Earth
We cannot dig to the centre. Scientists learn about the inside from earthquake waves, which change speed in different layers.
| Layer | What it is like | Thickness (about) |
|---|---|---|
| Crust | Solid rock we live on. Thin under oceans, thicker under continents. | 5–70 km |
| Mantle | Very hot, partly soft rock that flows very slowly. | ~2,900 km |
| Outer core | Liquid iron and nickel. | ~2,200 km |
| Inner core | Solid iron and nickel, hottest part. | ~1,200 km radius |
The crust plus the top hard part of the mantle is called the lithosphere.
Plate tectonics
The lithosphere is broken into about 7 big and many small plates. Heat inside the mantle makes them move a few centimetres a year (about as fast as your nails grow).
- Push together (convergent): land folds into mountains, like the Himalayas, or one plate dives under the other.
- Pull apart (divergent): magma comes up and makes new crust, like the mid-ocean ridges.
- Slide past (transform): plates rub and get stuck, then slip suddenly. This causes earthquakes.
Most earthquakes and volcanoes happen near plate edges.
Weathering, erosion and agents of gradation
Weathering = breaking rock in place. Heat and cold make rock swell and shrink; water freezes in cracks; roots grow into rocks; rain water slowly dissolves some rocks.
Erosion = carrying the broken pieces away. Deposition = dropping them somewhere else.
Gradation is the whole levelling work: high land is worn down and low land is filled up. The agents of gradation are:
| Agent | Wears away (erosion) | Builds (deposition) |
|---|---|---|
| Running water (rivers) | V-shaped valleys, waterfalls | Flood plains, deltas |
| Wind | Mushroom rocks in deserts | Sand dunes, loess |
| Glaciers (moving ice) | U-shaped valleys | Moraines (heaps of rock) |
| Sea waves | Cliffs, sea caves, arches | Beaches |
Earthquakes and landslides
An earthquake is sudden shaking of the ground when rocks along a plate edge slip. The point inside the Earth where it starts is the focus; the point just above it on the surface is the epicentre. Strength is measured on the Richter or moment magnitude scale by a seismograph.
A landslide is rock and soil sliding down a slope. Heavy rain, cutting slopes for roads and removing trees make it more likely.
Stay safe: in a quake, Drop, Cover and Hold under a strong table; stay away from glass. In hills, avoid steep, cracked slopes in heavy rain.
Avalanches, GLOFs and duststorms
- Avalanche: a large mass of snow slides fast down a mountain, often after fresh heavy snow or warming.
- GLOF (Glacial Lake Outburst Flood): lakes form in front of melting glaciers, held back by loose rock. If this wall breaks (from heavy rain, an ice fall or a quake), the lake empties suddenly and floods valleys below. Warming makes such lakes grow.
- Duststorm: strong, dry winds lift loose dust and sand into a thick moving wall. Common in dry north-west India before the monsoon.
We cannot stop these hazards, but early warning, safe building and planting trees reduce the damage.
Key formulas and definitions
- Layers (outside → inside): Crust → Mantle → Outer core (liquid) → Inner core (solid)
- Lithosphere = crust + top solid mantle, broken into plates
- Weathering (break in place) → Erosion (carry away) → Deposition (drop) = Gradation
- Agents of gradation: running water, wind, glaciers, sea waves
- Focus = start point inside; Epicentre = point on the surface above it
- GLOF = Glacial Lake Outburst Flood
Worked examples
1. A rock on a hill cracks after many hot days and cold nights. Is this weathering or erosion?
Weathering, because the rock breaks in place and does not move.
2. A river carries sand to the sea and a delta forms. Name the two processes.
Erosion (carrying the sand) and deposition (dropping it to form the delta).
3. Why are the Himalayas called young fold mountains?
They formed quite recently in Earth's history when the Indian plate pushed into the Eurasian plate, folding the land. They are still rising.
4. A plate moves 5 cm a year. How far does it move in 1 lakh (1,00,000) years?
5 × 1,00,000 = 5,00,000 cm = 5,000 m = 5 km.
5. A glacier lake in the hills grows bigger every summer. Why is this a danger?
The lake is held back by loose rock. More water means more pressure; if the wall breaks, a GLOF can flood villages downstream.
Common mistakes
- Mixing up weathering and erosion. Weathering breaks rock in place; erosion carries it away.
- Thinking the mantle is liquid like water. It is mostly solid rock that flows very slowly; the outer core is the liquid layer.
- Saying the epicentre is deep inside the Earth. The focus is inside; the epicentre is on the surface.
- Thinking plates move fast. They move only a few centimetres a year, but over millions of years this builds mountains.