Glossary
<https://pubs.usgs.gov/gip/volcus/page42.html>
Maintained by John Watson
Updated 06.24.97
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Glossary | |
| Andesite | A volcanic rock containing 53-63% silica with a moderate viscosity when in a molten state. |
| Ash | Fragments less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions. |
| Basalt | A volcanic rock consisting of less than 53% silica with a low viscosity when in a molten state. |
| Caldera | A large volcanic depression, commonly circular or elliptical when seen from above. |
| Composite volcano | A steep-sided volcano composed of many layers of volcanic rocks, usually of high-viscosity lava and fragmented debris such as lahar and pyroclastic deposits. |
| Dacite | A volcanic rock containing 63-68% silica with a high viscosity when in a molten state. |
| Dome | A steep-sided mound that forms when viscous lava piles up near a volcanic vent. Domes are formed by andesite, dacite, and rhyolite lavas. |
| Fumarole | A vent that releases volcanic gases, including water vapor (steam). |
| Lahar | A flowing mixture of water and rock debris that forms on the slopes of a volcano, sometimes referred to as debris flow or mudflow. The term comes from Indonesia. |
| Lava | Molten rock that erupts from a vent or fissure; see magma. |
| Magma | Molten rock that contains dissolved gas and crystals, formed deep within the Earth. When magma reaches the surface, it is called lava. |
| Phreatic eruption | A type of volcanic explosion that occurs when water comes in contact with hot rocks or ash near a volcanic vent, causing steam explosions. |
| Pumice | A light-colored volcanic rock containing abundant trapped gas bubbles formed by the explosive eruption of magma. Because of its numerous gas bubbles, pumice commonly floats on water. |
| Pyroclastic flow | A hot, fast-moving and high-density mixture of ash, pumice, rock fragments, and gas formed during explosive eruptions. |
| Pyroclastic surge | Same process as pyroclastic flow but of much lower density. |
| Rhyolite | A volcanic rock containing more than 68% silica with a very high viscosity when in a molten state. |
| Shield volcano | A volcano shaped like an inverted warrior's shield with long gentle slopes produced by eruptions of low-viscosity basaltic lava. |
| Silica | The molecule formed of silicon and oxygen (SiO2) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma's silica content, the greater its viscosity or "stickiness." |
| Vent | The opening at the Earth's surface through which volcanic materials (magma and gas) escape. |
| Volcano | A vent in the surface of the Earth through which magma erupts and also the landform that is constructed by the erupted material. |
| Volcanic landslide | The downslope movement of soil, rock debris, and sometimes glacial ice, with or without water, from the flank of a volcano. |
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