Volcanism, or volcanic activity, is the venting of liquid magma at the surface of the earth. Occasionally explosive, the process is important in producing continental and oceanic crust. Volcanoes are hills or mountains that form around the vent and consist of cooled magma, rock fragments, and dust from the eruptions.
Pieces of rock that are blown out of a volcano are called pyroclasts or pyroclastic debris. Pyroclasts may also be beads of liquid magma that supercool in the air during descent to form glassy shards of rock. Pyroclastic flows are dense, cloudlike mixtures of hot gas and pyroclastic debris that flow down a volcano's sides like an avalanche. These flows can be especially deadly—for example, 30,000 people were killed by a scalding pyroclastic flow on the Caribbean island of Martinique in 1902.
Craters and calderas. The crater is the circular depression at the top of the volcano. A caldera is a larger depression at least 1 kilometer in diameter that forms at the top of the volcano when the summit is destroyed during an eruption or when the crater floor collapses into the magma chamber below.
Types of volcanoes. There are three kinds of volcanoes: composite, shield, and cinder cone.
Composite volcanoes (stratovolcanoes) have been the sources of some of the more famous and destructive eruptions, such as those of Mount St. Helens, Vesuvius, and Krakatoa. Built up over millions of years, they consist of alternating layers of lava and pyroclastic debris that can approach slopes as steep as 45 degrees. They are characterized by long periods of dormancy, or inactivity, that can last for up to hundreds of thousands of years. How violent an eruption is depends on the temperature of the lava and the amounts of silica and dissolved gas in the lava.
Composite volcanoes are located along the circum‐Pacific belt and the Mediterranean belt, which mark the boundaries of colliding crustal plates. The circum‐Pacific belt, also known as the “Ring of Fire,” runs along the west coasts of South and North America, through the Aleutian Islands south of Alaska, and along the east coasts of Asia and Indonesia.
Shield volcanoes are broad, cone‐shaped hills or mountains made from cooled lava flows. The sides are very gently dipping and rarely exceed 10 degrees from the horizontal because the lavas have a low viscosity and spread quickly after eruption. (Viscosity is defined as resistance to flow; a lava with high viscosity flows sluggishly.) A spatter cone is a smaller feature that usually develops on a cooling lava flow from a shield volcano. Gas and lava are ejected through a small vent, building up a steep‐sided cone that resembles an appendage.
A cinder cone (pyroclastic cone) is composed of pyroclastic material (not lavas) ejected from a vent and commonly has slopes of about 30 degrees.
Volcanic domes. If a magma is thick and viscous and does not easily flow, it may form a volcanic dome. Volcanic domes are steep sided or rounded and form near the volcanic vent, creating a plug that can trap gases, build up internal pressures, and lead to violent explosions.
Lava floods. Nonvolcanic lavas called lava floods or plateau basalts are often associated with deep cracks in the continental crust. Although volcanoes don't form, huge amounts of very nonviscous, “runny” lavas pour from the rift and spread for hundreds of square kilometers. Repeated outpourings of lava have reached thicknesses of 2 kilometers or more in the geologic past.