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Anatomy of a Volcano

What's the difference between lava and magma? What are volcanic vents, dikes, and fissures? In this anatomy of a volcano, explore the basic geological features of a volcano such as Mt. St. Helens as well as the deadly materials released during volcanic eruptions.

ByLexi KrockNova

diagram of volcano

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To learn more about the various numbered parts of this volcano diagram, read on.
© WGBH Educational Foundation

There are many different kinds of volcanoes, ranging from the Hawaiian type, which produces gentle, effusive eruptions that tourists can observe from mere steps away, to the andesitic variety, which can produce violent, life-threatening eruptions with little warning. Though volcanologists study all types of volcanoes, the latter kind is of greatest concern since it is capable of killing thousands of people, destroying entire cities and forests, and severely disrupting local economies.

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Many volcanoes famous for their destructive power, including Mt. St. Helens, Galeras, Nevado del Ruiz, and Mt. Vesuvius, are andesitic volcanoes. Below, get to know the major features and products of an andesitic volcano, which forms when two tectonic plates rub against each other and generate enough friction and heat to create magma from melted rock. This magma surges through the surface of the earth, then solidifies, resulting over time in a classic volcano cone.

Mt. St. Helens
Mt. St. Helens, a grande dame among andesitic volcanoes, released a huge plume of ash on July 22, 1980.

1. Ash

Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than a tenth of an inch in diameter—or slightly larger than a pinhead. Volcanic ash is quite different from the soft, fluffy ash that results from burning wood, leaves, or paper. It is hard, does not dissolve in water, and can be extremely small—ash particles less than 1/1,000th of an inch in diameter are common. It is also extremely abrasive (similar to finely crushed window glass), mildly corrosive, and electrically conductive, especially when wet.

Volcanic ash is created during explosive eruptions by the shattering of solid rocks and the violent separation of magma into tiny pieces. Explosive eruptions result when groundwater heated by magma abruptly converts to steam and also when magma reaches the surface so that volcanic gases dissolved in the molten rock expand and escape into the air extremely rapidly. Hot ash and gas rise quickly to form a towering eruption column directly above the volcano.

Ash from the 1993 eruption of Unzen volcano in Kyushu, Japan rises halfway up the walls of a house.
© Corbis

2. lava flow

Lava flows are masses of magma that pour onto the Earth's surface during an effusive eruption; they include both moving lava and the resulting solidified deposits. Lava flows come in a great variety of shapes and sizes. This is due to the wide range in lava discharge during eruptions, characteristics of the erupting vent and topography over which lava travels, and viscosity of the different lava types.

lava flow
A molten river of lava runs through the Valle del Bove during an eruption by Mount Etna in Sicily.

3. lava dome

Lava domes are rounded, steep-sided mounds built by magma that is highly resistant to flow, usually either dacite or rhyolite. Such magmas are typically too viscous to move far from the vent before cooling and crystallizing. Domes may consist of one or more individual lava flows.

lava dome
An aerial view of the 600-foot lava dome in the crater of Washington's Mt. St. Helens, as seen between the May and July eruptions in 1980.

4. lava

Lava is the word for magma when it erupts onto the Earth's surface. Geologists also use the term to describe the solidified deposits of lava flows and fragments hurled into the air by explosive eruptions (for example, lava bombs or blocks). Lava is from the Italian word for stream, which is derived from the verb lavare—to wash.

A fast-moving lava flow from Mt. Etna in Sicily. Lava flows can also move slowly, especially as they begin to cool and develop a hard crust.

5. vent

Vents are openings in the Earth's crust from which magma and volcanic gases escape onto the ground or into the atmosphere. Vents may consist of a single, circular-shaped structure, a large elongated fissure and fracture, or a tiny ground crack. The release of volcanic gases and the eruption of molten rock result in a wide assortment of edifices, ranging from enormous shield volcanoes and calderas to fumaroles and small hornitos.

Steam rises from a vent on Mount Asahidake, a volcano on Hokkaido, Japan.

6. tephra

Tephra is a general term for fragments of volcanic rock and lava that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Tephra includes large, dense blocks and bombs, and small, light rock debris such as scoria, pumice, reticulite, and ash.

A fountain of hot lava and tephra erupts from a crater in Hawaii's Kilauea East Rift in 1983.

7. caldera

A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. Calderas are different from craters, which are smaller, circular depressions created primarily by explosive excavation of rock during eruptions.

Mount Aso vents a large cloud of steam at Japan's Aso National Park. The mountain's caldera is one of the world's largest.

8. lahar

Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form in a variety of ways, chiefly from the rapid melting of snow and ice by pyroclastic flows, intense rainfall on loose volcanic rock deposits, the breakout of a lake dammed by volcanic deposits, and as a consequence of debris avalanches.

A lahar from the Unzen Volcano eruption in Kyushu, Japan tore through this street in Shimabara, destroying cars and homes in the process.
© Corbis

9. Fissure

On volcanoes, a fissure is an elongated fracture or crack at the surface from which lava erupts. Fissure eruptions typically dwindle to a central vent after a period of hours or days. Occasionally, lava will flow back into the ground by pouring into a crack or an open eruptive fissure, a process called drainback; sometimes lava will flow back into the same fissure from which it erupted.

A close-up of a crack in the volcanic surface of a cooled lava flow from the crater of Mt. St. Helens.
© Corbis

10. dike

Dikes are tabular or sheet-like bodies of magma that cut through and across the layering of adjacent rocks and then solidify. They form when magma rises into a fracture or creates a new crack by forcing its way through rock. Hundreds of dikes can invade the cone and inner core of a volcano, often along zones of structural weakness.

An exposed dike, approximately five feet wide, at the caldera of Mauna Loa Volcano in Hawaii.

11. magma

Magma is molten or partially molten rock beneath the Earth's surface. When magma erupts onto the surface, it is called lava. Magma typically consists of a liquid portion (often referred to as the melt); a solid portion made of minerals that crystallized directly from the melt; solid rocks incorporated into the magma from along the conduit or reservoir, called xenoliths or inclusions; and dissolved gases. Magma collects inside a volcano's magma chamber before it erupts (see diagram).

Magma from the belly of Kilauea in Hawaii collects as it flows out to the Pacific Ocean.
© Corbis

Volcano Glossary

block: a solid rock fragment greater than three inches in diameter that is ejected from a volcano during eruption

bomb: a partially molten lava fragment greater than three inches in diameter that is ejected from a volcano during eruption; many bombs acquire rounded aerodynamic shapes as they fly through the air

dacite: one of the most common rock types associated with enormous Plinian-style eruptions; dacite lava consists of 63 to 68 percent silica, erupts at temperatures of between 800 and 1000°C, and is often light gray in color after cooling

effusive eruption: an eruption characterized by an outpouring of lava onto the ground (as opposed to the fragmentation of lava into the air during violently explosive eruptions)

fumarole: vent from which volcanic gases escape into the atmosphere; these usually occur along fissures or on the surface of lava flows that have cooled enough to have a crust

hornito: a small cone that forms on the surface of a lava flow when hot lava is forced up through an opening in the cooled surface of a flow and accumulates

inclusion: a rock or liquid that is enclosed in another foreign kind of rock or mineral

pumice: a light, porous volcanic rock that forms during explosive eruptions; all types of magma form pumice

pyroclastic flow: a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 63 miles/hour or more

reticulite: a light, porous volcanic rock that forms during explosive eruptions

rhyolite: rhyolite lava erupts at 700 to 850°C and contains a silica content of over 68 percent; it can also include the minerals quartz, feldspar, and biotite, which harden with a glasslike texture

scoria: a bubbly and glassy lava rock high in iron content

xenolith: a rock fragment that is foreign to the mass in which it is embedded

Editor's Notes

Some of the text in this feature is adapted from materials produced by the U.S. Geological Survey. An extensive volcano glossary can be found at

National corporate funding for NOVA is provided by Draper. Major funding for NOVA is provided by the David H. Koch Fund for Science, the NOVA Science Trust, the Corporation for Public Broadcasting, and PBS viewers.