One step further back in time from today’s active volcanoes are people who picnic in the White Mountains of New Hampshire, enjoy the autumn colors in the Blue Ridge of Shenandoah National Park, and hike in the rugged Big Bend National Park of Texas. Many of the rocks in these areas were formed by eruptions or by intrusion of magma into the Earth’s crust many millions of years ago. Because volcanic activity has been so important in shaping the Earth, watching active volcanoes today provides a window through which we can glimpse and reconstruct the early volcanic history of our planet.
As we increase our knowledge about volcanic processes, by studying volcanoes erupting today as well as those that have lain dormant for hundreds to thousands of years, we increase our ability to predict when and how volcanoes will erupt. Accurate predictions, presented in terms that are meaningful to public officials, will minimize the number of lives lost and the social and economic upheaval that an eruption can cause.
Thick layers of volcanic rocks form the Superstition Mountains, located about 60 kilometers east of Phoenix, Arizona. The consolidated deposits of pyroclastic flows, lava flows and domes, and lahars in the Superstition Mountains and adjacent areas testify to a period of intense volcanism about 17 to 25 million years ago in central Arizona. (Photograph by D.W. Peterson.)
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. |
Scientist surveying lava dome at Redoubt Volcano. (Photograph by G. McGimsey.)
The metric units used in this publication can be converted to English units by using the approximate conversions given below:
| Length | |
|---|---|
| 1 kilometer | 0.6 of a mile |
| 1 meter | 39.37 inches |
| 1 centimeter | 0.4 inch |
| 1 millimeter | 0.04 inch |
| Area | |
| 1 sq. kilometer | 0.4 sq. mile |
| 1 sq. meter | 1.2 sq. yards |
| 1 sq. centimeter | 0.155 sq. inch |
| Temperature | |
| To convert °Celsius to °Fahrenheit, multiply °C by 1.8 and add 32. | |
| To convert °Fahrenheit to °Celsius, subtract 32 from °F and divide the result by 1.8. | |