Ejecta

Ejecta image

For other uses, see Ejecta (disambiguation). Mt. St Helens eruption plume on 22 July 1980, showing ejecta in the form of pyroclastic material (ash)

Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma eruption volcanic vent, or crater, has traveled through the air or under water, and fell back on the ground surface or on the ocean floor.

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Volcanology

Typically in volcanology, ejecta is a result of explosive eruptions. In an explosive eruption, large amounts of gas are dissolved in extremely viscous lava; this lava froths to the surface until the material is expelled rapidly due to the trapped pressure. Sometimes in such an event a lava plug or volcanic neck forms from lava that solidifies inside a volcano's vent, causing heat and pressure to build up to an extreme with no way to escape. When the blockage breaks and cannot sustain itself any longer, a more violent eruption occurs, which allows materials to be ejected out of the volcano.

Ejecta can consist of:

  1. juvenile particles – (fragmented magma and free crystals)
  2. cognate or accessory particles – older volcanic rocks from the same volcano
  3. accidental particles – derived from the rocks under the volcano

These particles may vary in size; tephra can range from ash (<1/10in), lapilli (little stones from 1/10 in to 2 ½ in) or volcanic bombs (2.5 in+).

Planetary geology

See also: Panspermia

In planetary geology, the term "ejecta" includes debris ejected during the formation of an impact crater.

When an object massive enough hits another object with enough force, it creates a shockwave that spreads out from the impact. The object breaks and excavates into the ground and rock, at the same time spraying material known as impact ejecta. This ejecta is distributed outward from the crater's rim onto the surface as debris; it can be loose material or a blanket of debris, which thins at the outermost regions.

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