How do you know what is inside a volcano like a magma chamber?

Kyla Godbehere

Hi Kyle,

There are a number of ways in which volcanologists collect evidence that magma chambers exist. One is with looking at the patterns of small earthquakes in the summit regions of volcanoes. It is known that molten rock is unable to generate earthquakes (it is hard to imagine something that is fluid "breaking'). At some volcanoes where there are many seismometers it is possible to "see" regions beneath the summit where there are no earthquakes, usually surrounded by regions where there are many earthquakes. The volcanologists will probably decide that the non-earthquake regions are the magma chamber.

Another way in which magma chambers are detected is by measuring the tilting of the ground surface at the summit of the volcano. A magma chamber can be thought of as a balloon, or perhaps a bunch of interconnected balloons. When magma from deeper in the Earth comes up into the magma chamber, it inflates. If you try an experiment with a regular balloon, you might be able to picture what happens more easily. Take the balloon and inflate it just a little, and stop. Draw two dots on the balloon with a pen, then inflate it again. What happens to the two points? You can think of the two points as points on the ground above the magma chamber. With very sensitive instruments, geologists can detect very very small movements of the earth's surface, and see if the magma chamber below is inflating or deflating. Geologists can also measure the tilting of the ground above the magma chamber.

A final way that scientists have evidence of magma chambers comes from the chemistry of the lavas that are erupted. Geochemists think the have a pretty good idea of the composition of magma that is generated in the Earth's mantle. However, the compositon of the lava that gets erupted on to the surface often is different. It may have too much olivine, or not enough olivine, or other more subtle differences. It is often very easy to explain these differences by processes that go on in magma chambers. For example, if some basalt magma sits in a magma chamber for a while, olivine crystals start to crystallize out of the magma. These crystals are dense and they sink. An eruption that produces lava with a defficiency of olivine may have come from the upper part of the magma chamber whereas lava with too much olivine may have come from the bottom.

Finally (again) there are some examples in the world where very old and extinct volcanoes have been eroded down far enough so that their magma chambers (now solidified) are exposed on the surface.

Sincerely,

Scott Rowland

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