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Medicine Lake volcano

Part of Hall of Planet Earth.

Medicine Lake Volcano AMNH/R.Mickens

Medicine Lake is a broad shield volcano, built up by successive basalt flows. In contrast, the other Cascade volcanoes are tall peaks made mostly of andesite lavas. The magma for all the volcanoes in the Cascade Range originates near the subducting plate, about 120 kilometers below the surface. At Medicine Lake, the magma rises rapidly enough that it remains basalt. In the other volcanoes, the magma rises much more slowly, allowing it to cool and partially crystallize, thereby changing its composition to andesite.

Diverse lavas
The variety of lava types at Medicine Lake illustrates the complexity of arc volcanism. The magma that feeds the volcano originates in the mantle as basalt, and sometimes it reaches the surface to erupt unchanged. At other times, the magma collects in chambers and cools, causing crystals to form and settle out. The remaining magma, changed in composition, erupts as the different types of lava you see here.