Magma consists of remelted material from Earth’s crust and fresh material from other regions near the Earth’s surface.
When magma is erupted onto the surface in the form of lava, it becomes silicate rock. On subsequent eruptions of the volcano, magma leaves the chamber composed of slightly different chemicals.
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.