This is a drawing of magma changing chemical composition within the chamber.
Click on image for full size
Magma consists of remelted material from Earth's crust and fresh material from the aesthenosphere, relatively near the Earth's surface.
When magma is erupted onto the surface in the form of lava, it becomes rock; basalt, andesite, dacite, rhyolite, and other forms of silicate rock. On subsequent eruptions of the volcano, magma leaves the chamber composed of differing chemical constituents, which result in the different forms of silicate rock listed above.
As shown in this drawing, magma in the "chamber" evolves over time into chemical compositions which result in different forms of rock, as if the magma were "cooking". It is too hot within the chamber itself for rock to crystallize, as shown in the picture, but on subsequent eruptions, lava which pours out of the chamber is composed of different material. Magma can even change composition in the middle of an eruption, as reported in the ad79 eruption of Mt. Vesuvius.
Scientists examine the lava flows of a volcano to study the chemical history of the magma inside the chamber.
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