Most volcanoes have steep sides but sometimes they can slope down or even be flat. The volcanoes above sea level are the best known, but the most volcanoes lie beneath the sea, formed along the global oceanic ridge. 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. 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.
Lava is magma that breaks the surface and erupts from a volcano. If the magma is very fluid, it flows rapidly down the volcano’s slopes. Lava that is more sticky and less fluid moves slower. Lava flows that have a continuous, smooth, ropy, or billowy surface are called pahoehoe flows. But aa flows have a jagged surface composed of loose, irregularly shaped lava chunks.
Once cooled, pahoehoe forms smooth rocks, while aa forms jagged rocks. The words pahoehoe and aa are Hawaiian terms that describe the texture of the lava All volcanoes are formed by the accumulation of. 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 earths crust. High temperatures and pressures are needed to form magma. Volcanic activity ranges from emission of gases, non-explosive lava emissions to extremely violent explosive bursts that may last many hours. The types of eruptions determine the relative volumes and types of volcaniclastic material and lava flows, consequently the shapes and sizes of volcanoes. A volcanic event occurs when there is a sudden or continuing release of energy caused by near-surface or surface magma movement. The energy can be in the form of earthquakes, gas-emission at the surface, release of heat, explosive release of gases and the non-explosive extrusion or intrusion of magma.
An event could be non-destructive without release of solids or magmatic liquid, or if there is anything to destroy, could be destructive with voluminous lava flows or explosive activity. A volcanic event can include an eruptive pulse usually an explosion with an eruption plume, but also non-explosive surges of lava. A pulse may last a few seconds to minutes. Then an eruptive phase that may last a few hours to days and consist of numerous eruptive pulses that may alternate between explosions and lava surges. Or a single eruption or eruptive episode, composed of several phases that may last a few days or even months. Some volcanoes may form completely within a few weeks or months.
Others, such as shield volcanoes and composite volcanoes may show high order discontinuities such as major chemical changes, volcano-tectonic events like caldera collapse, or long erosional intervals. During a single eruption, styles of activity and types of products may change within minutes or hours, depending upon changes in magma composition, volatiles, or other magma chamber and vent conditions. Volcanic eruptions and eruptive phases are traditionally classified according to a wide range of criteria. Many have been given names from volcanoes where a certain type of behavior.
Common eruptions types are Plinian, Hawaiian, Strombolian, and Vulcanian. Volcanoes occur when magma makes its way to the surface of the earth. One type of volcanic eruption is a rift eruption. A rift eruption occurs at a diverging boundary. They are non-violent eruptions, and mafic magma is usually involved. Subduction eruptions occur at a subduction boundary.
They are usually very violent, and involve felsic magma. Hot Spot eruptions occur in the middle of a plate, not at a boundary. They are non-violent. Magma is molten rock found in the mantle. Mafic magma is low in silica, dark colored magma and does not trap gases, forms basalt.
Felsic magma is rich in silica, light colored magma, .