As you have seen, earthquakes and volcanoes occur along the margins of tectonic plates. Below is a map of earthquakes which make a line down the center of the Atlantic Ocean, in a line traveling southward from Iceland. These earthquakes are nearly all shallow, and the associated volcanoes appear in clusters on the earthquake axes or as isolated events located off of the axes. The brownish red line shows the contact between the three major plates in the north Atlantic: The North American plate, the Eurasian plate, and the African plate.

Note the extensive development of volcanoes around the triple junction. Also be aware that the shallow earthquakes you see are associated with magmatic processes, yet the volcanoes are rare in many areas where there are earthquakes. Here, the mid atlantic ridge, an extensive underground mountain range has developed and this is the site where the North American plate is moving westward, and the Eurasian and African plates are moving eastward. The result is a big scar down the center of the Atlantic Ocean where magma is rising up from the mantle to fill in the gap that is being created.

Problem 5

Using your understanding of the composition of the lithosphere, what type of rock is being created by the magma being deposited along the mid ocean ridge? Do you think Iceland is composed of continental or oceanic rock? Why?

Volcanoes are also forming where they are not in association with either ocean ridge or other plate margins. Take a look off the coast of Africa and note that there are two areas where volcanoes cluster. These are the Canary Islands (north) and the Cape Verde Islands. Such volcanoes are described as "intraplate" volcanoes because they don't seem to be associated with plate margins. What type of material would you expect compose the Cape Verde and Canary Islands?

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