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How can a volcano erupt mafic lava one time, and then felsic lava another time?
- Kelly; Cullman, AL
There are several ways, but one of the common ones happens when the magma in a magma chamber has time to separate (called "differentiation" ). Just like lighter grease will bubble to the top of a pan of spaghetti sauce, so will felsic magma bubble upwards in a magma chamber. So what you can get is a violent felsic explosion, followed by an intermediate or mafic eruption after the pressure is released and magma from deeper in the magma chamber can be released. For example: Crater Lake in Oregon. The initial eruption blew off the top of Mt. Mazama and made the hole for Crater Lake, but then a mafic cinder cone erupted inside the caldera and formed Wizard Island.
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