Physical Geography

Earth Science

Volcanoes

Volcanicity

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.

1. Hot Spots are believed to be the sources of magma that rises to the surface to form volcanoes. Have Hot Spots always been in the same places? What evidence is there to support your answer?
2. Why does a composite volcano have a characteristic cone shape?
3. What are Secondary Cones and Fumeroles? What materials would you expect to find ejected from each?
4. Why do you think such volcanoes are called 'composite' volcanoes?
5. Describe the course of magma from the magma chamber to the surface of the Earth through the main vent of a composite volcano, mentioning as many features of the volcano as you can.
6. Draw a cross-section sketch of a composite volcano, and include all the features mentioned in the web page you have printed. Label the features on your diagram.
7. Using text books, magazines or the Internet, discover the names and locations of four active European volcanoes.