VOLCANOES AND EARTHQUAKES
Earth’s structure and surface
Core: Temp. can reach above 5000c. Inner core is a solid ball of mostly iron and nickel, outer core is semi-liquid and consists mostly of iron.
Mantle: 2900km thick, made up of silicate rocks in a dense liquid state with increasing density with depth. The layer above is the asthenosphere, and is semi-molten. The very top of the mantle is rigid.
Crust: the outer layer of the Earth. The crust and the rigid top part of the mantle are together called the lithosphere. Continental crust is 30-70km thick, oceanic crust is 6-10km thick.
The lithosphere is divided into slabs called tectonic plates, which are moving due to convection currents in the asthenosphere. The places where they meet are called boundaries or plate margins.
PLATE TECTONICS THEORY
This is the idea that the Earth’s lithosphere is made up of many plates that are moved around by convection currents.
1. Radioactive decay of some elements in the core (e.g. uranium) generates heat.
2. When lower parts of the asthenosphere heat up they become less dense and rise.
3. As they move towards the top of the asthenosphere they cool down, become more dense and slowly sink.
4. These circular movements of semi-molten rock are called convection currents.
5. They create drag on the base of the tectonic plates, which causes them to move.
1. As far back as 1620 people noticed the close fit between the east coast of S America and the west coast of Africa. There were suggestions the continents were once joined together, but most people believed they were fixed in place.
2. In 1912 Alfred Wegener proposed the theory of continental drift, and that all the continents were once joined, as a supercontinent called Pangaea.
3. He based his theory on geological evidence and fossil records, but he couldn’t back the theory up with a mechanism that explained the movement.
4. 1950s, paleomagnetism added further evidence.
5. 1960s, sea floor spreading provided the mechanism for continental drift.
Evidence for the theory
· Areas of S America and W Africa have rocks of the same age and composition, such as the Permian fossil reptile called mesosaurus. Unlikely they migrated or evolved.
· Scratches on rocks in Brazil match those in W Africa.
· The age, rock type and distribution of some mountain ranges can be matched up, for example those in Scotland are similar to those on the east cost of America.
· The same living organisms can be found on different continents- unlikely they migrated or evolved.
· For example, the some earthworms are found in New Zealand and USA, so before they could travel between them.
· Similar glacial deposits are found in Antarctica, Africa, S America, Australia and India. By fitting…