Volcanoes are natural Earth structures that occur when Earth’s tectonic plates (see the “Plate Tectonics” post) converge or diverge, pushing rock up into a mountainous shape and creating a rupture for magma from the mantle to escape to the surface. The word “volcano” comes from “Vulcano”, a volcanic island, which in turn is named after Vulcan, the Roman mythological god of fire and forges.
Structure of Volcanoes: Volcanoes are divided into multiple parts. Beneath the primary volcano structure is the magma chamber. This is the reservoir of magma that is pushed up during an eruption. The conduit is the tunnel structure that magma is pushed up through when the volcano erupts. Dikes are offshoots of the main conduit that lead into sills, small chambers of contained magma, or into a parasitic cone, a secondary escape point for magma. The throat of a volcano is where the conduit widens slightly to allow the magma to exit through the vent, the primary escape point for magma. The vent is usually enclosed by a crater. The sides of a volcano are called flanks, and are covered by layers of lava and ash after an eruption.
Types of Volcanoes: There are multiple types of volcanoes. The most iconic type, composite volcanoes (also known as stratovolcanoes), are tall, conical mountains that erupt violently. Shield volcanoes are extremely wide volcanoes that are wider than they are tall. They resemble a warrior’s shield lying on the ground. Fissure vents are simply cracks in the Earth from which magma emerges. Lava domes are dome-shaped structures often formed after an eruption and serve as the main eruption point from that point on. Mount Saint Helens, which erupted violently in 1980, completely altering the landscape around it, is a prime example of a lava dome. Cinder cones are, as the name suggests, small conical structures that usually erupt only once. Supervolcanoes are colossal and extremely dangerous volcanoes, usually with large calderas, that have cataclysmic eruptions but luckily have very long time spans between eruptions. The range of ash clouds from a supervolcano eruption can threaten the entire continent they are part of. Yellowstone Caldera is an example of a supervolcano. Submarine volcanoes and subglacial volcanoes form on the ocean floor and beneath glaciers, respectively.
Eruption: When a volcano erupts, it releases a number of things. When magma reaches the surface, it is termed lava, to avoid confusion with the magma in the Earth’s mantle. Lava may flow down the sides of the volcano. There are three main types of lava. Pahoehoe (pronounced “pa-hoy-hoy”) lava is smooth and fluid lava. A’a (pronounced “Ah-ah”) lava is slow moving, glowing and extremely viscous lava. Pillow lava is formed by underwater eruptions, where lava quickly cools upon contact with water and forms pillow-like shapes. Contrary to popular belief, lava is not the most hazardous output of volcano eruptions. In fact, ash clouds are the most dangerous thing to be expelled from a volcano. Ash can pollute air, making it unbreathable, as well as warm up the upper atmosphere and block out sunlight. Hot particles suspended in ash clouds can also partially melt airplane turbines, hampering their ability to function correctly. Landslides caused by lava eruptions can enter rivers to for extremely hazardous lahar flows, which are large, mudslide-like flows where mounds of dirt, rock and ash are carried by water. Mount Rainier, one of the 16 dangerous Decade Volcanoes, could produce very dangerous lahar flows that could flood the entire Puyallup River Valley in Washington State if it erupted.