A volcano is a vent in Earth’s surface from which lava, rock, ash, and hot gases erupt. Most volcanoes are located along the boundaries of tectonic plates, although some, such as those that built the Hawai’ian Islands, are found over hot spots. Scientists suspect that hot spots occur over mantle plumes.

Scientists use a variety of different classification systems to categorize volcanoes for study. This map layer classifies volcanoes as either composite, cinder cone, shield, fissure, caldera, or unknown types. Composite volcanoes, also called stratovolcanoes, are built by two types of eruption; lava flow and pyroclastic flow. Often the eruption begins with an explosion of ash and debris and ends with thick lava flows. The material from these eruptions builds up forming steep slopes with a crater at the top. One of Earth’s most famous volcanoes—Krakatau, located in Indonesia—is classified as a composite volcano.

Cinder cone volcanoes, sometimes called pyroclastic cones, are formed from the build-up of small, loose, pyroclastic debris. Since these hot fragments are small, they cool quickly as they erupt, which prevents them from sticking together. As a result in locations with high winds, the volcanic vent may be located upwind of the cone. An example of this type of volcano is Cerro Negro in Nicaragua.

Shield volcanoes, primarily composed of the igneous rock, basalt, are building over longer periods of time as layer after layer of lava flows and hardens. These appear to be dome-shaped mountains. One example of this type is Mauna Loa, in Hawai’i.

Fissure vents follow the paths of dikes (magma filled fractures in the rock). These dikes may run out the side of another type of volcano or along the ground near a rift zone where two plates are spreading. Iceland has a number of fissure vents as the island sits on two diverging plates, the North American Plate and the Eurasian Plate.

Caldera volcanoes, like Crater Lake in Oregon, United States, are large oval depressions formed by the collapse of the magma chamber after a previous eruption.

The United States Geologic Survey estimates that Earth is home to about 1,500 active volcanoes. Use this layer to explore the awesome force of nature shaping our landscape.

Use this Map Layer in the Classroom

Tectonic Plates and Physical Features: In this activity, students will analyze maps of tectonic plates to predict the location of physical features.

Earth's Major Volcanoes: In this activity, students build an understanding of where volcanoes come from, how they form, and their impact on human civilization and the environment.