This is a drawing of magma changing chemical composition within the chamber.
Click on image for full size
When magma is erupted onto the surface in the form of lava, it becomes silicate rock. With each different eruption of the volcano, lava which comes to the surface is made of slightly different chemicals, and forms slightly different rocks.
Thus, magma inside the chamber is a little like something left in the refrigerator too long. Each time the door is opened, it looks a little different.
Scientists examine the layers of lava flows of a volcano to study the chemical history of the magma inside the chamber.
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