Volcanic bomb

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Fusiform lava bomb. Capelinhos Volcano, Faial
Island, Azores.

Volcanic bomb found in the Cinder Cones region of
the Mojave National
Preserve.

Volcanic bomb at Vulcania (Puy-de-Dôme)

A volcanic bomb is a mass of molten rock (tephra) larger than 65 mm
(2.5 inches) in diameter, formed when a volcano ejects viscous
fragments of lava
during an eruption. They cool into solid fragments before they reach the ground.
Because volcanic bombs cool after they leave the volcano, they do not have grains
making them extrusive igneous rocks.
Volcanic bombs can be thrown many kilometres from an erupting vent, and often
acquire aerodynamic shapes during their flight. Bombs can be extremely large;
the 1935 eruption of Mount Asama in Japan
expelled bombs measuring 5–6 m in diameter up to 600 m from the vent. Volcanic
bombs are a significant volcanic hazard, and can cause severe injuries and death
to people in an eruption zone. One such incident occurred at Galeras volcano in Colombia in
1993; six people near the summit were killed and several seriously injured by
lava bombs when the volcano erupted unexpectedly.

Volcanic bombs are known to occasionally explode from internal gas pressure
as they cool, but contrary to some claims in popular culture (specifically, the
1997 film Volcano),
explosions are rare; in most cases most of the damage they cause is from impact.
Bomb explosions are most often observed in 'bread-crust' type bombs.[1]

[edit]
Types of bombs

Bombs are named according to their shape, which is determined by the fluidity
of the magma from which they are formed.

Ribbon or cylindrical bombs form from highly to moderately fluid
magma, ejected as irregular strings and blobs. The strings break up into small
segments which fall to the ground intact and look like ribbons. Hence, the
name- ribbon bombs. These bombs are circular or flattened in cross section,
are fluted along their length, and have tabular vesicles.

Spherical bombs also form from high to moderately fluid magma. In
the case of spherical bombs, surface tension plays
a major role in pulling the ejecta into spheres.

Spindle, fusiform, or almond/rotational bombs are formed by the
same processes as spherical bombs, though the major difference being the
partial nature of the spherical shape. Spinning during flight leaves these
bombs looking elongated or almond shape, the spinning theory behind these
bombs' development has also given them the name 'fusiform bombs'. Spindle
bombs are characterised by longitudinal fluting, one side slightly smoother
and broader than the other. This smooth side represents the underside of the
bomb as it fell through the air.

Cow pie bombs are formed when highly fluid magma falls from
moderate height (so the bombs do not solidify before impact) which are still
liquid when they strike the ground. They consequently flatten or splash and
form irregular roundish disks which resemble cow-dung.

Bread-crust bombs are formed if the outside of the lava bombs
solidify during their flights. They may develop cracked outer surfaces as the
interiors continue to expand.

Cored bombs are bombs that have rinds of lava enclosing a core of
previously consolidated lava. The core consists of accessory fragments of an
earlier eruption, accidental fragments of country rock or in rare cases bits
of lava formed earlier during the same eruption.

[edit]
References

^ [1] Volcanic bombs, Glendale Community College

[edit]
External links

Wikimedia Commons has media related to: Volcanic bomb

http://www.gc.maricopa.edu/earthsci/imagearchive/bombs.htm

http://volcanoes.usgs.gov/images/pglossary/bomb.php

Retrieved from "http://en.wikipedia.org/w/index.php?title=Volcanic_bomb&oldid=454640481"

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This page was last modified on 9 October 2011 at
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