Close Up--Plate Tectonics Theory

Shield Volcanoes

Shield volcanoes are the largest volcanoes on earth. The Hawaiian Islands and Iceland are examples. Typically the shield volcano is much broader than it is tall and has the look of a circular shield of the type used by knights-in-armor. The shield is built by successive flows of fluid basaltic lava.

Shield volcanoes occur in two different Plate tectonic settings one is the Intraplate Setting. The intraplate setting is not on the edge of a plate but is nearer the plates center. The classic example is the Hawaiian Islands that are due to a hot spot or mantle plume. That is hot magma is rising because there is an extremely hot part of the mantle below the ocean crust where Hawaii is located, and the hot spot melts the overriding Pacific Plate. The magma generated forms the volcanic islands. The Hawaiian Islands a little less than a million years old get successively older as one moves to northeast. The Pacific Plate is moving to the northeast and is dragging the Hawaiian Islands along with it. The other place where shield volcanoes are forming today is over the midocean ridge in the North Atlantic. Iceland is an example of this and is a place where basaltic magma from the ridge has risen all the way to the oceans surface.

Shield volcanoes are the largest volcanoes not only on earth but also in our whole solar system. The largest known volcanoe is Olypus Mons on Mars. Below is a photograph taken by Mariner 9 of Olympus Mons, with an inset of Hawaii shown in red at the same scale.

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