5.1 Volcano Eruption Basics

In 1980, Mount St. Helens blew up in the costliest and deadliest volcanic eruption in United States history. The eruption killed 57 people, destroyed 250 homes, and swept away 47 bridges. Mount St. Helens today still has minor earthquakes and eruptions, and now has a horseshoe-shaped crater with a lava dome inside. The dome is made of pb_glossary id=”1812″]viscous[/pb_glossary] lava that oozes into place [1].

Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in size, style, and composition of the erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano.

Volcanoes with a mafic composition will typically not have very explosive eruptions, but the lava erupted will be fast moving. This mafic lava often moves down mountainsides and cools rapidly into unique textures that are either ropey called “Pahoehoe” or rough and rocky called “A’a”.

Felsic magma is very viscous, and it does not flow smoothly. Most felsic magma will stay deeper in the crust and will cool to form intrusive igneous rocks such as granite and diorite. If felsic magma rises into a magma chamber, it may be too viscous to move, so it tends to get stuck.

However, intermediate magma is also highly viscous, and it contains dissolved volatile gases. These gases become trapped by the magma, and the magma chamber begins to build in pressure. When the magma finally can erupt as lava, it does so very violently and explosively, as we have seen at Mount St. Helens.

***See 5.8  for Text and Media Attributions