1. Geography No title

2. The structure of the Earth. Crust Mantle Outer core Inner core

3. Mantle convection. Mantle convection is the slow creeping motion of Earth's rocky mantle. It is caused by convection currents, carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, is divided into a number of plates that are continuously being created and consumed at their opposite plate boundaries. Accretion occurs as mantle is added to the growing edges of a plate, usually associated with seafloor spreading. This hot added material cools down by conduction and convection of heat. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction at an ocean trench.

4. Theory of Earth’s history. According to the theory of Earth’s continental drift, the world was originally made up of a single continent through most of geologic time. That continent eventually separated and drifted apart, forming into the seven continents we have today. The first comprehensive theory of continental drift was suggested by the German meteorologist Alfred Wegener in 1912. The hypothesis asserts that the continents consist of lighter rocks that rest on heavier crustal material similar to the manner in which icebergs float on water. Wegener contended that the relative positions of the continents are not rigidly fixed but are slowly moving at a rate of about one yard per century.

5. P waves P-waves are a type of elastic wave that can travel through gases, solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs. The name P-wave is often said to stand either for primary wave, as it has the highest velocity and is therefore the first to be recorded; or pressure wave, as it is formed from alternating compressions and rarefactions.

6. S waves A type of seismic wave, the S-wave, secondary wave, or shear wave, sometimes called an elastic S wave, is one of the two main types of elastic body waves, so named because they move through the body of an object, unlike surface waves.

7. Subduction Subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle. These regions of mantle downwellings are known as "Subduction Zones". A subduction zone is an area on Earth where two tectonic plates move towards each other and one slides under the other. Rates of subduction are typically measured in centimetres per year. The average rate of convergence is approximately 8 centimetres per year.

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