Structure of the Earth
Geophysics uses physics to study the Earth. The Earth has layers. The crust is a thin solid skin (about 5 km thick under the oceans and 30–70 km under continents). Below it is the mantle, about 2 900 km of hot rock that is solid but flows very slowly. The outer core is liquid iron and nickel and the inner core is solid because of the great pressure. The temperature rises with depth to over 5 000 °C at the centre. Most of what we know comes from seismic waves.
Plate tectonics, earthquakes and volcanoes
The crust and the top of the mantle form the lithosphere, which is broken into large tectonic plates that move a few centimetres a year, carried by convection currents in the mantle. Where plates meet there are three kinds of boundary:
| Boundary | Movement | Result | Example |
|---|---|---|---|
| Divergent | Plates move apart | New crust forms; rift valleys, mid-ocean ridges | The East African Rift Valley, Mid-Atlantic Ridge |
| Convergent | Plates move together | One plate sinks beneath the other; fold mountains, volcanoes, deep ocean trenches | The Andes, Himalayas |
| Transform | Plates slide past each other | Earthquakes | San Andreas Fault |
Seismic waves and earthquakes
An earthquake is a shaking of the ground when rocks along a fault suddenly slip and release stored energy. The point underground where it starts is the focus; the point on the surface above it is the epicentre. The energy spreads as seismic waves, recorded by a seismometer on a seismogram.
| Wave | Type | Speed | Passes through liquid? |
|---|---|---|---|
| P (primary) | Longitudinal | Fastest, 5–8 km/s | Yes |
| S (secondary) | Transverse | Slower, about 60% of P | No |
| Surface (L) | Rolling and sideways | Slowest | Along the surface only |
S waves do not pass through the outer core, so there is an S-wave shadow zone on the opposite side of the Earth. This is how scientists found that the outer core is liquid. The size of an earthquake is its magnitude on the Richter scale (logarithmic): each step is about 10 times larger in amplitude and 32 times more energy.
Choose two earthquake magnitudes. See how many times more energy the larger one releases.
Using geophysics
| Method | What is measured | Use |
|---|---|---|
| Seismic surveys | Echoes of waves from rock layers | Finding oil, gas and water |
| Gravity surveys | Small changes in g caused by rocks of different density | Locating ore bodies and dense minerals |
| Magnetic surveys | Changes in the Earth’s magnetic field | Finding iron ores and buried structures |
| Electrical resistivity | How well the ground conducts | Locating groundwater for wells and boreholes |
Earthquake safety
Build houses and schools with strong frames and flexible joints, away from faults and steep slopes. During an earthquake, drop, cover and hold on under a strong table; stay away from windows and walls. After the shaking stops, move to open ground and expect aftershocks. In Tanzania the greatest risk is in the Rift Valley zones.
Key terms
- Lithosphere
- The rigid outer layer of the Earth: the crust and top of the mantle.
- Epicentre
- The point on the Earth’s surface directly above the focus of an earthquake.
- Seismometer
- An instrument that records earthquake waves.
- Magnitude
- A measure of the size of an earthquake on the Richter scale.
Summary
- The Earth has a crust, mantle, liquid outer core and solid inner core.
- Plates move at divergent, convergent and transform boundaries, causing earthquakes and volcanoes.
- P waves are longitudinal and travel through everything; S waves are transverse and not through liquids.
- Geophysical surveys find water, minerals and oil.