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Pipe, Volcanic

The central conduit by which magma rises through a volcano is termed a volcanic pipe. A volcanic pipe may be anywhere from a few yards to about 0.5 mi (0.8 km) in width. When a volcano ceases to erupt, its pipe generally becomes plugged by a column of solidified magma mixed with angular fragments of rock ripped from the walls of the pipe. This solid column is also termed a pipe (or neck, or plug). Erosion may strip the cone from around such a plug to create a free-standing pillar.

A pipe forms when magma from a deep reservoir drills or blasts upward. One mechanism by which this is occurs involves convection, that is, vertical circulation driven by the density difference between hotter and cooler magmas: hotter magma rises, cooler magma sinks. Magma in a narrow vertical pipe quickly loses heat to surrounding rocks, and the magma thus cooled sinks along the sides of the pipe while hot, fresh magma ascends in the pipe’s center. This central fountain erodes chunks of rock from the pipe’s roof, extending the pipe upward. These chunks are transported by down-convecting magma to the reservoir below, where they are melted down and assimilated. This process enables a pipe to rise through many miles of rock without having to push rocks aside.

Magma containing large amounts of dissolved gas can widen a pipe explosively by a mechanism resembling that of an erupting geyser . If magma reaches the surface via a relatively narrow pipe and encounters substantial groundwater , a large steam explosion may occur: the pipe explodes at the top. This suddenly removes weight from the magma column in the pipe, reducing the pressure on magma deeper down. Gas dissolved in this deeper magma boils out explosively, blowing still more material out of the top of the pipe and further reducing the pressure on magma still deeper down. A series of explosive eruptions can thus propagate downward to great depth. The rubble-choked pipe left after such an eruption is termed a diatreme.

If an ascending pipe full of hot magma encounters a layer of groundwater but conditions are not right for a downward-propagating explosion, a simple steam explosion at the surface may result that excavates a large, shallow crater or maar. Maars closely resemble meteor impact craters because they do not rise above the terrain surrounding them.

See also Crater, volcanic; Fissure; Magma chamber; Volcanic eruptions; Volcanic vent

Pipe, volcanic The central conduit by which magma rises through a volcano is termed a volcanic pipe. A volcanic pipe may be anywhere from a few yards to about 0.5 mi (0.8 km) in width. When a volcano ceases to erupt, its pipe generally becomes plugged by a column of solidified magma mixed with angular fragments of rock ripped from the walls of the pipe. This solid column is also termed a pipe (or neck, or plug). Erosion may strip the cone from around such a plug to create a free-standing pillar. Source for information on Pipe, Volcanic: World of Earth Science dictionary.

Kimberlite eruption

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Kimberlite eruption, small but powerful volcanic eruption caused by the rapid ascent of kimberlites—a type of intrusive igneous rock originating in the asthenosphere—through the lithosphere and onto the surface of the Earth. Kimberlites are thought to rise through a series of fissures in the rock. They form vertical pipelike structures that penetrate the surrounding rock. Unlike other kinds of eruptions, magma does not collect in a subsurface reservoir prior to the eruption. In addition, many surface depressions resulting from kimberlite eruptions contain deposits of diamonds.

As the kimberlite pipes approach the surface, decreasing pressure above allows some of the volatile materials in the magma (such as water and carbon dioxide) to become gaseous, and these gases expand rapidly. Should the pipes encounter rock layers containing groundwater, the water is vaporized and additional expansion occurs. Such expansion widens the pipes and produces an explosive event at the surface as upward-rushing gases dislodge rocks and create a craterlike depression.

The last kimberlite eruption is thought to have taken place more than 25 million years ago, and some scientists note that most occurred during the Cretaceous Period (146 million to about 65.5 million years ago). Since that time, depressions caused by kimberlite eruptions have undergone substantial erosion. During and after the eruption, the depression is often filled with breccia, a type of lithified sedimentary rock consisting of angular and subangular fragments rather than rounded clasts. Breccias that form during kimberlite eruptions are made up of rising kimberlite and the walls of the surrounding rock. When eroded, such a depression exposes a vertical funnel-shaped pipe that resembles a volcanic neck with the exception of the brecciated filling. If the eruption was explosive, these pipes, called diatremes, typically assume carrot-shaped profiles. In cases where the eruption is slower and corrodes the surrounding rock, diatremes may be bowl-shaped.

During their ascent, kimberlite pipes may pass through a region of the lower lithosphere called the diamond stability field, an area of high pressure where carbon can be transformed into diamonds. Diamonds that intersect the rising pipe may be pushed along by or carried within the magma to the surface. Although there is evidence that diamonds and other ejected materials can fall several kilometres away from the crater during an explosive event, most present-day discoveries of kimberlite diamonds occur within the remains of eroded craters.

Kimberlite eruption, small but powerful volcanic eruption caused by the rapid ascent of kimberlites—a type of intrusive igneous rock originating in the asthenosphere—through the lithosphere and onto the surface of the Earth. Kimberlites are thought to rise through a series of fissures in the rock.