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Geology of Mount Shasta

Figure 6

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Figure 6: Drawings illustrating the processes that produce the diverse lavas erupted on and around Mount Shasta. The cross section of the upper crust on the left shows a schematic magma reservoir with its "roof" at a depth of 10 to 12 kilometers beneath the mountain. The drawing of three such reservoirs on the right illustrates three end-member processes that may modify the compositions of rising basaltic magmas. During fractional crystallization (a), crystals enriched in magnesium, iron, and calcium grow from the melt along the floor and walls of the reservoir. The removal of the crystals leaves the remaining liquid depleted in these components and enriched in others -- such as silicon, sodium, and potassium -- which give the magma an andesitic or dacitic composition. During assimilation (b) the basaltic magma fractures its wallrocks and engulfs the resulting blocks. As these blocks melt and dissolve, their components are added to the surrounding magma and modify its composition. Finally, during magma mixing (c) two batches of magma with different compositions encounter one another beneath the mountain and blend together to create a new magma with an intermediate composition. These drawings depict the three processes as separate, but in nature they are likely to occur together. For example, heat released by crystallization may enable a magma to warm and dissolve blocks of cold wall rock it could not otherwise assimilate.

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