A volcano is a vent at the Earth's surface through which 'magma' and associated gases 'erupt', and also the cone built by eruptions. A volcano that is currently erupting or showing signs of unrest ('earthquakes', gas emissions) is considered active. A volcano that is not currently active but which could become active again is considered dormant. Extinct volcanoes are those considered unlikely to erupt again. Volcanic eruptions are one of Earth's most dramatic and violent agents of change. They pose significant geologic hazards because their eruptions and associated activities can affect large areas and go on for extended periods of time. Many kinds of volcanic activity can endanger the lives of people and property, and the affects of these activities are felt both close to and far away from the volcano. Explosive eruptions can spread lava, gas and other materials over a wide area, and may drastically alter the landscape. Slow eruptions or flows can also alter landscapes, while associated earthquakes, atmospheric effects, landslides, and floods all may damage or destroy property and threaten human lives. Some volcanic eruptions are mild and slow, while others are powerful and dramatic. An eruption happens when magma, gases, or steam break through vents in the Earth's surface. A mild eruption may simply discharge steam and other gases, or quietly extrude lava. A strong eruption can consist of violent explosions that send great clouds of gas-laden debris into the atmosphere, or may consist of explosions that blast sideways from a collapsed portion of the volcano, as happened in the '1980 eruption of Mount St. Helens'. Eruptions can alter the land and water locally through lava flows, lahars, pyroclastic flows, and landslides. An eruption cloud of ash and gas may spread the impact of a volcano over many miles or even around the Earth. A 'lava flow' is molten rock that has reached the surface of the Earth. It may flow quickly or slowly, but destroys everything in its path, including vegetation and manmade structures, and may bury homes and agricultural land under tens of feet of hardened black rock. People are rarely able to use land buried by lava flows or to sell it for more than a small fraction of its previous worth. Lava flows usually do not travel far from their volcanic source. Lava entering the sea poses special risks. With temperatures higher than 2,000 degrees Fahrenheit (1,100 degrees Celsius), lava can instantly transform seawater to steam, causing explosions that blast hot rocks, water, and molten lava fragments into the air. A lava delta, created as lava enters the sea, looks like a stable platform that extends tens to hundreds of feet into the ocean. However, the lava delta may not be well supported and can collapse into the ocean with little or no warning. A  'lahar' is a mixture of volcanic ash, rock, debris, and water that can travel quickly down the slopes of a volcano. They are generated when a high volume of hot or cold water mixes with ash and rock and starts down slope. The water may come from melting snow or ice, heavy rainfall during an eruption, or the breakout of a lake. When moving, a lahar looks like a mass of wet concrete. As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice or from the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size. But as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size. A 'pyroclastic flow' is a rapidly-moving mixture of hot, dry rock fragments, ash, and hot gases which knocks down, buries, or burns everything in its path. Pyroclastic flows are caused by explosive eruptions or by the collapse of a lava flow, can reach temperatures as high as 1,300 degrees Fahrenheit (700 degrees Celsius), and may melt snow and ice to cause lahars. These flows vary considerably in size and speed, but even relatively small flows can destroy buildings, forests, and farmland. Even on the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hot ash and gases. Volcanic 'landslides' are common and can be caused by an eruption or associated heavy rainfall, by an earthquake under the volcano, or by the collapse of a slope weakened by underlying volcanic activity. A landslide caused by collapse of part of the volcano's cone may also trigger an eruption as pressure on the underlying volcanic systems is decreased. Historically, landslides have caused explosive eruptions, buried river valleys with tens to hundreds of feet of rock debris, generated lahars, triggered waves and tsunami, and created deep horseshoe-shaped craters. Moving rapidly and with great momentum, a large volcanic landslide may flow up and over ridges, and may cause damage far from the volcano. 'Tephra' is 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. These fragments may be as small as ash or as large as several feet in diameter. Tephra includes combinations of 'pumice', glass shards, crystals from different types of minerals, and shattered rocks. Large tephra typically falls back to the ground near the volcano while smaller fragments are carried away by wind. Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of miles downwind from a volcano. Ash usually covers a much larger area and disrupts the lives of far more people than the other more lethal types of volcano hazards. Ash fall may injure livestock and crops, collapse buildings, damage communications and power-supply facilities, cause driving and visibility problems, damage or disable aircraft, and cause respiratory and eye irritation problems in people. Magma contains dissolved gases that are released into the atmosphere during eruptions, primarily as acid aerosols (tiny acid droplets), compounds attached to tephra particles, and microscopic salt particles. Volcanic gases may also escape continuously into the atmosphere from the soil, volcanic vents, 'fumaroles', and hydrothermal systems. The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide, and hydrogen fluoride. Sulfur dioxide gas can lead to acid rain and air pollution downwind from a volcano, and large amounts may lead to lower surface temperatures and promote depletion of the Earth's ozone layer. Concentrations of carbon dioxide gas can be lethal to people, animals, and vegetation, while hydrogen fluoride can contribute to acid rain and is a powerful irritant that can deform or kill animals. Scientists monitor active volcanoes and try to anticipate when an eruption will occur. Volcano monitoring methods detect and measure changes in the state of a volcano caused by magma movement beneath the volcano. Rising magma typically will trigger numerous earthquakes, cause swelling or subsidence of a volcano's summit or flanks, and lead to the release of volcanic gases from the ground and vents. In the United States, the USGS Volcano Hazards Program has established a series of 'volcano warning schemes' that are used to notify the public and civil authorities of impending volcanic activity or eruptions. Volcanic activity since 1700 has killed more than 260,000 people, destroyed entire cities and forests, and severely disrupted local economies for months to years. Even with our improved ability to identify hazardous areas and warn of impending eruptions, increasing numbers of people face certain danger. Scientists face a formidable challenge in providing reliable and timely warnings of eruptions to so many people at risk. The USGS 'Volcano Hazards Program' provides 'current updates of worldwide volcanic activity' and additional information on various types of volcanic hazards