Volcano Types
Geologists classify volcanoes according to their type or style of volcanism: how they erupt and how that eruption type creates the shape of the volcano we see at the Earth’s surface. You have already learned how to describe a volcano by its origins, location, and hazards. You have also learned why the different rock types (based on silica content) creates different levels of eruptions. Generally, the more silica you have in the lava, the more explosive the eruption. You will now discover that these descriptions can be tied together into types of volcanism: shield volcanoes, composite volcanoes, dome volcanoes, and cinder cone volcanoes. We’ll also discuss calderas as a special type of volcano; some would call these a super volcano.
To help you to organize your notes, create a table similar to Table 8.2 (p 256) in your book. Use the following headings: Volcano Type, Shape, Silica Content of Magma, Viscosity, Rock Type Formed, Tectonic Boundary, Tectonic Plate (Oceanic, Continental or Both), and Examples. Try to fill out this table as you to go through the textbook. Here, I have provided some “Volcano Identification Cards.”
Shield Volcano (pp 254 – 257)
A little bit more about the shape and eruption: Shield volcanoes are formed from very low viscosity lava. Because this lava is so “thin” it doesn’t form steeply sloped volcanoes. Imagine trying to build a pyramid with gravy. The gravy would keep sliding around the table, forming a pool. Slowly, as the gravy cooled and thickened, it would solidify. This is how basaltic lava behaves. Slowly it would build up to form a volcano shaped with a broad shield profile.
Pyroclastic debris is a relatively minor product of shield volcano eruptions. Some pyroclastics thrown from the Hawaiian volcanoes have special names as the early Hawaiians believed that the volcanoes were manifestations of the Hawaiian goddess, Pele, goddess of volcanoes (and lightning, dance, and fire). One small mineral crystal that is often formed from the Hawaiian volcanoes is olivine (which is the August birthstone, Peridot). When thrown from the volcano, it soars through the air and cools forming long strings of green crystals, called Pele’s Hair. Sometimes larger amounts of olivine soar through the air and take on a raindrop shape. These are called Pele’s tears.
Composite Volcano (p 258)
A little bit more about the shape and eruption: Composite volcanoes are also called stratovolcanoes – and for good reason! The prefix “strato-” comes from the Latin stratum, which means layer. These volcanoes erupt in alternating styles: sometimes with andesite lava and other times with pyroclastic debris or tephra. This creates a cone with alternating layers of andesite rock and ash. Now if we relate this back to what we know about the volcano cycle and rock strength, if the last eruption left a solidified plug of andesite in the vent, the next eruption would be massive. Andesite is of intermediate strength and would hold back an immense amount of building pressure!