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Medicine Lake Volcano

Medicine Lake volcano

Medicine Lake volcano

  • Exhibition Text

    • 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. 

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  • For Educators

    • Topic: Earth Science

      Subtopic: Volcanoes

      Keywords: Medicine Lake (Cal.), Geology, Volcanoes, Magmas, Lava, Volcanic eruptions, Cascade Range, Volcanology

      Audience: General

In This Section

Basalt Tablet

Basalt tablet

The composition of the magma that formed this tablet was basically unchanged by its journey from the mantle to the surface, where it erupted to form a smooth, sinuous lava flow.



This pumice has the same composition as the obsidian next to it.



Obsidian is a volcanic glass, a comparatively rare and crystal-poor form of rhyolite, the most silica-rich lava.

Remnants of a Buried Forest

Remnants of a buried forest

Glass Mountain began with an explosive eruption of ash that blanketed the surrounding forest, killing all the trees.

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