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Volcanoes

© Geoff Habiger

I can remember the fateful day in May, 1980 when Mt. St. Helens erupted and sent a cloud of ash and rock down its slope killing 57 people. It was the worst volcanic eruption in the United States in centuries. Yet, as devastating as the Mt. St. Helens explosion was, it was miniscule compared to the eruption of the island of Tambora in Indonesia in 1815. The ash from Tambora circled the globe three times and caused temperatures across the planet to fall so much that 1816 was known as the year without a summer. Sakurajima volcano in Japan erupts up to 400 times per year. In the Caribbean, Mt. Pelee erupted in 1902 sending a pyroclastic flow through the town of St. Pierre, killing 29,000 people. In 79 AD, one of the most famous (or infamous) volcanoes, Mt. Vesuvius erupted and killed nearly everybody in the town of Pompeii.

Volcanoes have been a part of our lives since man first walked the planet. They are a constant reminder of the active nature of our planet. Their eruptions provide soil rich in nutrients that have been a blessing to farmers for centuries. Volcanoes have long fascinated man, and been the source of much damage and destruction. In this article I will explain the four types of volcanoes and some of the hazards associated with them. The four types of volcanoes reflect the nature of the magma and the location of where the volcano forms.

The first type of volcano is the cinder cone. A cinder cone is a volcano, generally less than 500 meters in height that forms from the eruption of pyroclastic debris from a central vent. Cinder cone volcanoes tend to have steep slopes, generally at angles between 30 and 33 degrees. Cinder cones can form anywhere, even on the sides of larger volcanoes. Pyroclasts are any rock fragments that are blasted out of a volcano during an eruption. They should not be confused with lava that flows out of the ground.

Stratovolcanoes (or composite volcanoes) are composed of alternating layers of pyroclastic flows and lava flows representing both explosive and non-explosive volcanic activity. These alternating layers can change according to the history of the volcano, such that some volcanoes have mostly lava flows, while others have mostly pyroclastic layers. These differences in composition can make it difficult to determine the hazard a particular stratovolcano has. Many of the volcanoes in the Cascade Range are stratovolcanoes. Probably the most famous stratovolcano is Mt. Fuji in Japan. Stratovolcanoes are typically found on the leading edge of continents, where one tectonic plate is subducting (being driven under) another tectonic plate. When a stratovolcano erupts, it generally sends an ash cloud high into the atmosphere where it mushrooms out and is blown downwind. This type of eruption is known as a Plinian eruption, and named after Pliny the Younger who witnessed the eruption of Mt Vesuvius that killed his uncle Pliny the Older.