Structure of the Earth
The Earth is almost a sphere. These are its main layers, starting with the outermost:
- crust - relatively thin and rocky
- mantle - has the properties of a solid, but can flow very slowly
- outer core - made from liquid nickel and iron
- inner core - made from solid nickel and iron.
It is difficult to study the structure of the earth, because the crust is too thick to drill through, so most information about the earth is collected through siesmic waves produced by earthquakes or a man-made explosion.
Tectonic plates and Continental Drift
The lithosphere is made up of the crust and the top part of the mantle. It is relatively cold and rigid.
The lithosphere is then broken into large pieces called tectonic plates.
The lithosphere is less dense than the mantle, so floats on top of it.
The tectonic plates move about 2.5 cm each year. This called continental drift.
The theory of continental drift was proposed by Wegner. However he was not a geologist, so he could not prove this. He was disbelived by other scientists until the 1960s, when new sea floor spreading evidence was found. Subsequent research led to Wegner's theory slowly becoming accepted.
The theory of plate tectonics
The mantle is the zone between the core and the crust.
It is relatively cold and rigid near to the crust.
Deeper down the mantle becomes hotter and less rigid and it flows slowly. Energy transfer from the Earth's core, increases the mantle temperature with depth.
Huge convection currents slowly move the tectonic plates around the Earth.
Most scientist now accept the theory of plate tectonics because:
- it explains a wide range of evidence
- it has been discussed and tested by many scientists
The oceanic plate is more dense than the continental plate.
At an ocean margin, the oceanic plate cools.
When an oceanic plate and a continental plate collide, the oceanic plate sinks, pulling more of the oceanic plate under the continental plate.
The oceanic plate partially melts as it reaches the hotter part of the mantle.
This process is called subduction.
Earthquakes and Volcanoes
Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happens most at the boundaries between plates.
Magma is less dense than the crust so it can rise up through weak spots in the crust. This can cause volcanoes.
Magma is molten rock under the Earth's surface. When it erupts to the surface it is called Lava.
Some volcanic eruption produce runny lava, whilst others produce thick lava.
People choose to live near volcaonoes because the soil is very fertile.
Geologist study volcanoes to try to predict when the next volcanic eruption is going to be and to reveal more about the structure of the Earth.
Although geologists are able to predict future eruptions, it is still not with a100% certainty.
Igneous rocks are made when molten rock cools down and solidifies.
The slower the rock cools the larger the crystals made. For example, gabbro has larger crystals than basalt because the molten rock that formed gabbro cooled more slowly.
- basalt is rich in iron - it formed from runny lava produced in a fairly safe volcanic eruption
- rhyolite is rich in silica - it formed from thick lava produced in an explosive eruption.