How Volcanoes Erupt
All volcanoes share the same basic features. The magma collects in magma chambers that
can be 160 kilometers (100 miles) beneath the surface. As the rock heats, it expands, which
creates even more pressure. As a result, the magma seeks a way out pushing toward the
surface, the magma seeps through cracks in the Earth’s crust called vents. Eventually, the
magma reaches the surface; when it comes out, we call it an eruption. The worderuption
is used in other contexts, as well. An eruption can be an outburst or explosion, a violent
and sudden occurrence, like when a crowd erupts in anger. But an eruption can also be
a spreading of something like a rash on your skin, gradual and relatively calm. These two
definitions are similar to the two kinds of eruptions that we see in volcanoes.
Types of Eruptions
Every geological formation is unique. Their composition and construction depend on so
many factors, that it would be impossible for two formations to be exactly alike. In the
same way, each volcano and its eruptions are unique. However, we tend to see two major
kinds of eruptions. We talked about eruption to mean both a violent explosion or a sort of
silent spreading. These are the two types of volcanic eruptions that we see–explosive and
non-explosive eruptions. When we think of volcanic eruptions, we often think of huge clouds
of volcanic ash ejected high into the atmosphere and then thick rivers of red lava snaking
down the mountainside. In reality, these two phenomena rarely occur in the same volcano.
Volcanic eruptions tend to be one or the other.
Explosive Eruptions
Figure 8.9: An explosive eruption from the Mayon Volcano in the Philippines in 1984. ( 25 )