A volcanic bomb is a mass of molten rock (tephra

Tephra

200px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at center of the photo is [[rhyolitic]] tephra from [[Hekla]]....

) larger than 65 mm (2.5 inches) in diameter, formed when a volcano

Volcano

2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 ejects viscous

Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 fragments of lava

Lava

Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...

 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

Crystallite

Crystallites are small, often microscopic crystals that, held together through highly defective boundaries, constitute a polycrystalline solid. Metallurgists often refer to crystallites as grains.- Details :...

 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

Mount Asama

is an active complex volcano in central Honshū, the main island of Japan. The volcano is the most active on Honshū. The Japan Meteorological Agency classifies Mount Asama as rank A. It stands above sea level on the border of Gunma and Nagano prefectures...

 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

Galeras

Galeras is an Andean stratovolcano in the Colombian department of Nariño, near the departmental capital Pasto. Its summit rises above sea level. It has erupted frequently since the Spanish conquest, with its first historical eruption being recorded on December 7, 1580...

 volcano in Colombia

Colombia

Colombia, officially the Republic of Colombia , is a unitary constitutional republic comprising thirty-two departments. The country is located in northwestern South America, bordered to the east by Venezuela and Brazil; to the south by Ecuador and Peru; to the north by the Caribbean Sea; to the...

 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.

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
  Surface tension
  Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
  
  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.

External links

• http://www.gc.maricopa.edu/earthsci/imagearchive/bombs.htm
• http://volcanoes.usgs.gov/images/pglossary/bomb.php