Interior of Mount St. Helens

Simplified cross-section of Mount St. Helens. Mount St. Helens is a stratovolcano made of layers of lava, pyroclastic deposits (tephra), and mudflow deposits. This diagram is for the volcano prior to the 1980 eruption. The summit dome formed in about 1600 A.D., filling the throat of the volcano. A conduit fed the magma towards the surface. The Goat Rocks dome was extruded in the 1940's. Numerous thin dikes cut across the older rock and interior of the volcano. The magma chamber in this diagram is much shallower than it is actually thought to be.

Figure from:

Foxworthy, B.L., and Hill, M., 1982, Volcanic eruptions of 1980 at Mount St. Helens: The First 100 Days: U.S. Geological Survey Professional Paper 1249, 124 pages.

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