INTRODUCTION
Volcano, mountain or hill formed by the accumulation of materials erupted through one or more openings (called volcanic vents) in the earth's surface. The term volcano can also refer to the vents themselves. Most volcanoes have steep sides, but some can be gently sloping mountains or even flat tablelands, plateaus, or plains. The volcanoes above sea level are the best known, but the vast majority of the world's volcanoes lie beneath the sea, formed along the global oceanic ridge systems that crisscross the deep ocean floor. According to the Smithsonian Institution, 1511 above-sea volcanoes have been active during the past 10,000 years, 539 of them erupting one or more times during written history. On average, 50 to 60 above-sea volcanoes worldwide are active in any given year; about half of these are continuations of eruptions from previous years, and the rest are new.
Volcanic eruptions in populated regions are a significant threat to people, property, and agriculture. The danger is mostly from fast-moving, hot flows of explosively erupted materials, falling ash, and highly destructive lava flows and volcanic debris flows (see Volcano Hazards below). In addition, explosive eruptions, even from volcanoes in unpopulated regions, can eject ash high into the atmosphere, creating drifting volcanic ash clouds that pose a serious hazard to airplanes.
VOLCANO FORMATION
All volcanoes are formed by the accumulation of magma (molten rock that forms below the earth's surface). Magma can erupt through one or more volcanic vents, which can be a single opening, a cluster of openings, or a long crack, called a fissure vent. It forms deep within the earth, generally within the upper part of the mantle (one of the layers of the earth’s crust), or less commonly, within the base of the earth's crust. High temperatures and pressures are needed to form magma. The solid mantle or crustal rock must be melted under conditions typically reached at depths of 80 to 100 km (50 to 60 mi) below the earth’s surface.
VOLCANIC MATERIALS
Three different types of materials may erupt from an active volcano. These materials are lava, tephra (rock fragments), and gases. The type and amount of the material that erupts from an active volcano depends on the composition of the magma inside the volcano.
TYPES OF VOLCANOES
Volcanoes come in different shapes and sizes, depending on the makeup of the magma, the style of the eruption, and how often they erupt. The major types of volcanoes, roughly in order of increasing size, are cinder cones, composite volcanoes (also called stratovolcanoes), shield volcanoes, calderas, and plateaus. Calderas and plateaus are shaped differently than traditional volcanoes—neither has a mountain-like shape.
Volcano, mountain or hill formed by the accumulation of materials erupted through one or more openings (called volcanic vents) in the earth's surface. The term volcano can also refer to the vents themselves. Most volcanoes have steep sides, but some can be gently sloping mountains or even flat tablelands, plateaus, or plains. The volcanoes above sea level are the best known, but the vast majority of the world's volcanoes lie beneath the sea, formed along the global oceanic ridge systems that crisscross the deep ocean floor. According to the Smithsonian Institution, 1511 above-sea volcanoes have been active during the past 10,000 years, 539 of them erupting one or more times during written history. On average, 50 to 60 above-sea volcanoes worldwide are active in any given year; about half of these are continuations of eruptions from previous years, and the rest are new.
Volcanic eruptions in populated regions are a significant threat to people, property, and agriculture. The danger is mostly from fast-moving, hot flows of explosively erupted materials, falling ash, and highly destructive lava flows and volcanic debris flows (see Volcano Hazards below). In addition, explosive eruptions, even from volcanoes in unpopulated regions, can eject ash high into the atmosphere, creating drifting volcanic ash clouds that pose a serious hazard to airplanes.
VOLCANO FORMATION
All volcanoes are formed by the accumulation of magma (molten rock that forms below the earth's surface). Magma can erupt through one or more volcanic vents, which can be a single opening, a cluster of openings, or a long crack, called a fissure vent. It forms deep within the earth, generally within the upper part of the mantle (one of the layers of the earth’s crust), or less commonly, within the base of the earth's crust. High temperatures and pressures are needed to form magma. The solid mantle or crustal rock must be melted under conditions typically reached at depths of 80 to 100 km (50 to 60 mi) below the earth’s surface.
VOLCANIC MATERIALS
Three different types of materials may erupt from an active volcano. These materials are lava, tephra (rock fragments), and gases. The type and amount of the material that erupts from an active volcano depends on the composition of the magma inside the volcano.
TYPES OF VOLCANOES
Volcanoes come in different shapes and sizes, depending on the makeup of the magma, the style of the eruption, and how often they erupt. The major types of volcanoes, roughly in order of increasing size, are cinder cones, composite volcanoes (also called stratovolcanoes), shield volcanoes, calderas, and plateaus. Calderas and plateaus are shaped differently than traditional volcanoes—neither has a mountain-like shape.
