Even with our improved ability to identify hazardous areas and warn of impending eruptions, increasing numbers of people face volcanic danger. Clearly, scientists face a formidable challenge in providing reliable and timely warnings of eruptions to so many people at risk. The best preparation is to simply heed the warnings given by scientists who specialize in this arena. Evacuation is the best way to protect yourself and your family.

A general term for fragments of volcanic rock and lava that, regardless of size, are blasted into the air by explosions or carried upward by hot gases in an eruption column. Such fragments range in size from less than 1/10 of a millimeter to more than one meter in diameter. Large tephra falls back to the ground on or close to the volcano, and progressively smaller tephra are carried farther away by wind. Volcanic ash, the smallest tephra fragments, can be carried hundreds to thousands of kilometers from a volcano.

An Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and/or river valleys. When moving, lahars look like masses of wet concrete that carry rock debris ranging in size from clay to gravel to boulders more than 10 meters in diameter. Eruptions may trigger lahars directly by quickly melting snow and ice or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption; rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. No matter how a lahar forms, as it flows downstream, it will always change its speed, size and relative amount of water and rock debris.

Streams of molten rock that either extrude quietly from a vent or are fed by lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. Fluid basalt flows may extend tens of kilometers from their source, and the leading edges of basalt flows can move at velocities as much as 10 kilometers per hour on steep slopes. Basalt ('bA-"'solt) is a dark gray to black, dense to fine-grained igneous rock. The flow front of a basalt flow on a shallow slope typically advances less than one kilometer per hour. Where basalt lava flows are confined within channels or lava tubes, however, velocities can reach over 30 miles per hour. Viscous andesite ('an-di-"zIt) flows move only a few kilometers per hour and rarely extend more than 8 kilometers from their vent. Viscous dacite and rhyolite flows often form steep-sided mounds called lava domes right over an erupting vent. Andesite, dacite and rhyolite are all very acidic volcanic rock.

High-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. They may also result from the non-explosive eruption of lava when parts of dome or thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.

Large masses of rock and soil that fall slide or flow very rapidly under the force of gravity. These mixtures can move in a wet or dry state, or both. Landslides commonly originate as massive rockslides or avalanches, which during movement disintegrate into fragments ranging in size from small particles to enormous blocks hundreds of meters across. If the landslide consists of enough water and clay particles, the landslide may transform into a lahar as it travels down a river valley.

All magmas contain dissolved gases that are released into the atmosphere during and between eruptions. Water vapor is the most abundant gas released, followed by carbon dioxide and sulfur gases (predominantly sulfur dioxide and hydrogen sulfide). Magma also releases minor amounts of others gases such as hydrogen, carbon monoxide, hydrochloric acid and hydrofluoric acid. The two most hazardous volcanic gases to people, animals, agriculture and property are sulfur dioxide and carbon dioxide. Sulfur dioxide gas can lead to acid rain downwind from a volcano. Large explosive eruptions that inject tremendous volumes of sulfur dioxide gas into the stratosphere can contribute to global cooling. Since carbon dioxide gas is heavier than air and collects in soil and low spots, people and animals walking into these low areas may suffocate and vegetation may die. Also, a few historic eruptions have released sufficient fluorine-compounds gas to deform or kill animals grazing in areas of ash fall; the fluorine compounds tend to be concentrated on fine-grained ash particles.