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Roger Weller, geology instructor
by Travis Clay
It is very widely known that volcanoes can be extremely violent and destructive. The most violent eruptions are known to destroy towns and wildlife, and even take lives. When people think of volcanoes many of them think the lava flow and the large amounts of ash coming from the volcanic vent. A lot of attention is put on the dangers of volcanoes if you get too close. However, one of the more violent and dangerous aspects of volcanoes can get to you even if you are not close, Volcanic Bombs.
Volcanic Bombs are a mass of molten rock formed when large fragments of lava are ejected from the vent of a volcano. During flight they cool into solid or semi solid rock before hitting the ground. These violent bombs are shaped while flying through the air and usually take aerodynamic forms. Volcanic bombs are usually 64 millimeters in diameter and mostly produce by Strombolian and Vulcanian eruptions. Since these eruptions usually produce basaltic lava volcanic bombs are commonly a basaltic or similar mafic composition. They can travel a long distance from the vent of the volcano erupting and have been known to travel even further when they hit the ground and begin to roll at high speeds. These are known as cannonball bombs. Volcanic bombs can be the most dangerous aspect of a volcano.
It is obvious to see how Volcanic Bombs and be an extreme hazard to anyone or anything even remotely close to an eruption. Volcanic bombs are large and extremely fast moving. They can be up to five to six meters in diameter and move at a rate of 200-400 meters per second. They have also been know to be thrown up to five kilometers from the vent of a volcano so even by standers who believe they are at a safe distance are in danger from these flying hazards. In 1993 during the eruption of Galeras in Colombia six people were killed and several were severely injured from Volcanic Bombs. Volcanic bombs are also known to explode from internal gas pressure, however this is rare and most of the damage from these molten rocks is mainly cause by impact. Volcanic bombs take many different shapes and sizes, so they are categorized into different groups.
There are six different types of Volcanic Bombs.
Ribbon Bombs form from moderately fluid magma ejected as irregular blobs. The
blobs break up into small segments, which fall to the ground intact and look
like ribbons. Cow Pie Bombs are formed when highly fluid magma falls from
moderate height. So they are not able to solidify and are still liquid when they
hit the ground. They flatten or splash and form irregular roundish disks, which
looks like cow-dung. Bread-crust bombs are formed if the outside of the lava
bombs solidifies during their flights but not the inside. When the inside
solidifies and expand its cracks the outer surface. Cored bombs are bombs that
have rinds of lava circling a core of previously merged lava. The core contains
fragments from a previous eruption. Spherical Bombs form from high to moderately
fluid magma. The surface tension of Spherical Bombs pulls the ejected material
into spheres. Lastly, Spindle Bombs are formed when lava fragments are ejected
and begin spinning at a high speed. As they cool and continue to spin during
flight these bombs begin looking elongated or almond shaped.
Volcanic Bombs are undoubtedly dangerous and destructive. However, they are also one of the more interesting and spectacular aspects of a volcanic eruption. During an eruption it is amazing to see the transformation of these bombs as they go from lava fragments to solid molten rock. Volcanic Bombs are very exciting because of the fact that in the matter of seconds they can take so many shapes and sizes while flying through the air. Volcanic Bombs are truly amazing and one of the little wonders of geology.