What causes earthquakes?
The earth's outer layer or crust is broken into pieces called tectonic plates which are constantly moving towards, away, from or past each other. Because continents are part of these plates, they also move. An earthquake occurs when the rocks break and move as a result of stresses caused by plate movements. Most earthquakes occur on the edge of plates, especially where one plate is forced under another. The epicentre is the point on the Earth's surface directly above the source of the earthquake. The source, also known as the focus, can be as deep as 700 kilometres. Smaller earthquakes occur much more frequently than large ones and most cause little or no damage. A very large earthquake can be followed by a series of smaller events called aftershocks during a period of adjustment which may last for several months.
What causes volcanoes to erupt?
The Earth's crust is made up of huge slabs called plates, which fit together These plates sometimes move. Between the Earth's crust and the mantle is a substance called magma which is made of rock and gases. When two plates collide, one section slides on top of the other, the one beneath is pushed down. Magma is squeezed up between two plates. A volcano erupts when the pressure within the molten magma below the surface becomes stronger than the strength of the rocks on the surface that make up the volcano. The magma then finds the path of least resistance to the surface and erupts. That pressure comes both from the buoyancy of the magma and from gas bubbles inside the magma that want to expand. The magma pressure comes from the expansion of gas bubbles that are within the magma. The heat source for all erupted materials comes from natural radioactive decay within the Earth. The concentration of radioactive elements in the Earth is not really very high, but the volume of the Earth is great enough so that there is enough of a contribution of heat from the decay to produce melting. The material that melts (magma) has a lower density and it migrates to the surface to erupt
