LYCOS RETRIEVER 
Volcanoes: Lavas
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Over the last 2 million years, volcanoes have been depositing lava, tephra, and ash in particular areas of the globe (Figure 2). These areas occur at hot spots, rift zones, and along plate boundaries where tectonic subduction is taking place.
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This feature is an example of a class of volcanoes that are considerably smaller than the broad shield volcanoes. The summit consists of a single, very circular caldera with a smooth floor that predates the ejecta from two large impact craters. The lower flanks of the volcano, including portions of the impact craters, have been buried by the material that makes up the surrounding plains. This superpositional relationship indicates that the plains were emplaced subsequent to both the volcano and the large impact craters on the volcano. The plains are probably made up of lava supplied from Tharsis Montes that flowed down the sides of the broad uplift associated with the Tharsis shields. Both the plains and the volcano are cut by a graben, indicating tectonic activity subsequent to the emplacement of the plains.
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"Geologists generally group volcanoes into four main kinds – cinder cones, composite volcanoes, shield volcanoes, and lava domes. Cinder cones are the simplest type of volcano. They are built from particles and blobs of congealed lava ejected from a single vent." This online booklet, published by the U.S. Geological Survey, is an outstanding introduction to the geology of volcanos. A print version is ... available via snail mail request.
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Some basaltic magmas can produce very large slightly sloping volcanoes, 6 to 12°, that have gently flowing magmas called shield volcanoes. Shield volcanoes can be up to 9,000 meters tall. The volcanoes of the Hawaiian Islands are typical of this type. Extruded materials from this type of volcano mainly consist of low-viscosity basaltic lava flows (Figure 3).
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Low viscosity lavas produce shield volcanoes, named after their appearance, which is much like a Roman shield. They have relatively little explosive activity, and are characterized by lava flows, which create their slowly sloping sides of 15° or less. Lava flows are distinguished by the Hawaiian terms pahoehoe, referring to the smooth and curving pattern appearance, created when the top crust of the lava is pulled and twisted by the flow beneath it, and aa, an exceedingly rough rubbly surface with angular and jagged fragments (figure 2). figure 2
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[One] way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions (Cas & Wright, 1987):
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