The Structure of the Earth
Earth is made of layers:
- Crust: thin outer layer (5–70 km thick) — the part we live on
- Mantle: thick layer of semi-molten rock (silicate) that flows very slowly
- Outer core: liquid iron and nickel
- Inner core: solid iron and nickel
Tectonic Plates
The crust is cracked into around 15–20 large pieces called tectonic plates. These fit together like a jigsaw puzzle. They carry the crust (and the top part of the mantle, together called the lithosphere) on top of the convecting mantle below.
Plates move very slowly — about 2–10 cm per year — driven by convection currents in the mantle. Hot mantle rock rises, spreads sideways, takes the plates with it, then sinks again as it cools.
The idea that continents were once joined and have drifted apart is called continental drift. Alfred Wegener proposed it in 1912. Evidence includes matching shapes of continents, identical fossils on opposite sides of the Atlantic, and matching rock types.
Types of Plate Boundaries
| Boundary type | Movement | Results |
|---|---|---|
| Divergent (constructive) | Plates move apart | Volcanoes, rift valleys, new ocean floor (e.g. Mid-Atlantic Ridge) |
| Convergent (destructive) | Plates move together | Subduction, trenches, fold mountains, volcanoes (e.g. Pacific Ring of Fire) |
| Transform (conservative) | Plates slide past each other | Earthquakes, no volcanoes (e.g. San Andreas Fault, California) |
| Collision | Two continental plates converge | Mountain ranges without volcanoes (e.g. Himalayas) |
Mountain Building
The world's highest mountain ranges formed at convergent collision boundaries:
- Himalayas: Indian plate colliding with Eurasian plate over 50 million years
- Alps: African plate pushing into Eurasian plate
- Andes: oceanic Nazca plate subducting under the South American plate
When plates collide, rocks are crumpled and folded upwards into fold mountains.
The Himalayas are still growing by about 5 mm per year as India continues to push into Asia. Fossils of marine creatures have been found in Himalayan rocks — proof that this land was once beneath an ancient ocean called the Tethys Sea.
Real world: The San Andreas Fault and California earthquakes
The San Andreas Fault in California is a transform (conservative) plate boundary where the Pacific Plate slides northward past the North American Plate. No volcanoes form here, but the friction causes constant earthquakes. San Francisco was devastated by a major earthquake in 1906 and continues to be earthquake-prone — engineers now design all new buildings in the city to withstand seismic shaking.
Real world: The East African Rift Valley
In East Africa, the African continent is slowly being pulled apart along a divergent plate boundary called the East African Rift Valley. The land is sinking, forming a string of deep lakes (like Lake Tanganyika and Lake Victoria), and volcanoes like Kilimanjaro have formed along the rift. In millions of years, East Africa may separate from the rest of Africa and form a new ocean.
Real world: GPS confirms plate movement
Scientists use GPS satellites to measure tectonic plate movement directly. Australia is moving northward at about 7 cm per year — fast enough that GPS coordinates used in navigation systems have to be regularly updated, or maps would become inaccurate. India moves about 5 cm per year into Asia, directly measurable from space.
Where do you see this in real life? Earthquakes in Japan, volcanic eruptions in Iceland, and mountain building in the Himalayas are all driven by the same slow movement of tectonic plates happening right now beneath our feet.