Volcanic bomb is pyroclastic rock that is a cooling of a mass of lava it flies thorough the air after eruption. If it is to be called a bomb, a specimens must be larger than 2, 5 inch diameter. Smaller specimens are known as Lapilli. Specimens up to 20 ft. (6 m) in diameter are known. Volcanic bombs are usually brown or red, weathering to a yellow-brown color. Specimens can become rounded as they fly through the air, although they may also be twisted or pointed. They may have a cracked, fine-grained, or glassy surface. There are several types of volcanic bomb, which are named according to their outward appearance and structure.
Color: Dark shades of red, brown, or green
Group: Extrusive
Minerals: Volcanic bombs commonly possess a basaltic or similar mafic composition.
Volcanic Bomb Classification
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 shaped; the spinning theory behind these bombs’ development has also given them the name ‘fusiform bombs’. Spindle bombs are characterized 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 (they 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 solidifies 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.
Volcanic Bomb Formation
A volcanic bomb is a mass of molten rock (2.5 inches) larger than 64 mm (2.5 inches), which occurs when a volcano removes viscous lava during eruption. They absorb the solid particles before they reach the soil. After the volcanic bombs have been removed from the volcano, extruded magmatic rocks occur due to their cooling. Volcanic bombs can be launched miles away from a blasting culvert and can often be aerodynamically shaped during their flight.
Where is It Found
Volcanic bombs are mostly produced by Strombolian and Vulcanian eruptions. These eruption types usually produce basaltic lava. Hence, volcanic bombs commonly possess a basaltic or similar mafic composition.
References
- Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
- Wikipedia contributors. (2018, October 18). Volcanic bomb. In Wikipedia, The Free Encyclopedia. Retrieved 15:22, May 14, 2019, from https://en.wikipedia.org/w/index.php?title=Volcanic_bomb&oldid=864612411
