Plate movement


VOLCANOES


l Magma derived from the earth’s mantle is usually quite runny, so gases can escape from it easily.


l The fissures through which the magma emerges are usually large. This reduces pressure on the moving magma.


The lava that emerges at constructive plate margins may build up gently sloping mountains as well as mid-ocean ridges, such as the Mid-Atlantic Ridge. High lava plateaux of basalt rock were also built in this way. An example is the Antrim Plateau, about which you will learn more on page 19.


You learned on page 13


that some volcanoes are formed over places called hot spots.


Hot Spots Some parts of the earth’s mantle are particularly hot.


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These places are called hot spots. There are about 120 known hot spots in the world. They are stationary and most of them are situated far from plate boundaries. A narrow column of extra hot molten magma


(called a plume) rises up from each hot spot and may force its way up through the earth’s crust. When magma reaches the surface, it forms a volcanic mountain or sometimes a volcanic island. Hawaii, Iceland and the Canary Islands are all volcanic islands situated over hot spots. The Hawaiian Islands are a line of volcanic


islands. Each active volcano was formed over the same stationary hot spot. As the Pacific Plate moved, each ‘older’ volcano was carried away from the hotspot and so became inactive. (See Figure 3.)


Pacific Ocean


Mount Kilimanjaro, Africa’s highest mountain, is located over a hot spot. Why is there snow on the top of Kilimanjaro, which is situated close to the Equator?


Active volcano formed over the hot spot Older inactive volcanoes


The Hawaiian Islands


3 Mantle Hot spot 15


Crust


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