As we've seen, plate margins are areas of weakness in the Earth's crust, and it is in these areas that earthquakes and volcanic activity occur.
Magma stored in the mantle makes use of the weaknesses to move up towards the surface. Where it breaks through to the surface it is know as extrusive rock. The most familiar features associated with this are volcanoes.
A volcano erupts when pressure from gasses and steam within the magma build up to sufficient pressure to fracture the surface, or layer of solid rock above them, allowing lava to flow onto the earth's surface.
If the lava is acidic and rich in silica it will harden and set quickly, moving only short distances from its source. It will form the typical volcano shape, a steep sided cone. Such volcanoes erupt violently because the rapidly hardened rock blocks the vents and allows pressure to build up under the blockage. When it finally fractures the blockage, the release of pressure can be likened to taking the lid off a bottle of fizzy drink that's been well shaken. Such volcanoes are called Composite Volcanoes. Examples are Mount Etna in Sicily, Mount St. Helens in the USA and Mount Vesuvius in Italy
If it is a basic lava, low in silica but rich in iron, it flows freely and sets slowly. This allows it to flow long distances from its source, forming layers of gently sloping lava. Such volcanoes are called Shield Volcanoes, and are typical of areas such as Hawaii.
Print out and read the information about Composite Volcanoes, then answer the following questions.
Click to go back