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Aa: (pronounced ah'-ah) a type of lava that forms a thick, rough crust.
Active volcano: a volcano that has erupted in the past 10,000 years, may be currently erupting, and is expected to erupt again.
Caldera: a large, cauldron-shaped depression caused by the collapse of the ground when the magma chamber is empty.
Cinder cone volcano: the most common and smallest type of volcano, usually not more than 300 m (1000 ft) tall, produced by pyroclastic flows. An example of a cinder cone volcano is Paricutin in Mexico.
Composite volcano: also known as stratovolcano, formed by a combination of lava flows and pyroclastic explosions that form alternating layers. Composite volcanoes are steep and often very tall. Examples of composite volcanoes are Mt. Fuji in Japan, Mt. Kilimanjaro in Tanzania, and Mt. Rainier in Washington.
Crater: an indentation at the top of a volcano.
Dormant volcano: a volcano that is not currently erupting, but is expected to erupt again.
Extinct volcano: a volcano that has not erupted in the past 10,000 years, and is not expected to erupt again.
Hot spots: volcanically active places that lie above mantle plumes.
Island arc: a chain of volcanic islands created when a tectonic plate passes over a hot spot. The Hawaiian Islands are an example of an island arc.
Lahar: a fast-moving volcanic mudflow composed of ash and water.
Lapilli: intermediate-sized fragments of material (2-64 mm, or 0.08-2.5 in) that are ejected in a pyroclastic explosion.
Lava: magma that reaches Earth's surface.
Magma: molten rock below Earth's surface.
Magma chamber: a large area deep underground filled with magma.
Mantle plume: a column of hot rock that rises up to the surface from Earth's interior.
Pahoehoe: (pronounced pah-hoy-hoy) a type of lava that forms a thin, ropy crust when it hardens.
Pillow lava: a smooth, rounded type of lava flow that erupts underwater.
Pyroclastic flow: a high-temperature mixture of hot ash and fragments of lava that erupts with great force and speed. Pyroclastic flows are the most dangerous type of volcanic eruptions, traveling at speeds of up to 200 km/hr (125 mph) with temperatures of up to 700 degrees C (1300 degrees F).
Shield volcanoes: volcanoes that form from fluid lava that builds up slowly over time, creating a wide, gently-sloping mountain shaped like a warrior's shield. Examples of shield volcanoes are Kilauea and Mauna Kea in Hawaii.
Tephra: any material ejected during a pyroclastic explosion, regardless of size.
Vent: an opening at the top or side of a volcano where lava erupts.
Volcanic ash: very small (less than 2 mm, or 0.08 in) fragments of material that are ejected in a pyroclastic explosion.
Volcanic bomb: a mass of molten lava that is ejected during a pyroclastic explosion and hardens in the air. Volcanic bombs range in size from 2.5 in (64 mm) to several yards (meters) in diameter, and can weigh several tons.