By far the greatest number of bombs produced during a volcanic eruption are simply irregular in shape. They are generally very vesicular and lumpy and are called scoria or cinders. The photographs that follow are of volcanic bombs that have a somewhat streamlined shape and were generally ejected during the final stages of the volcanic eruption.
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Bread Crust Bombs
Bread crust bombs are created when the bomb cools, producing a hard crust that insulates the inside keeping it hot and full of dissolved gases. The gases continue to expand and break the crust that has already formed. Occasionally the molten interior squeezes out (see "pie filling" bomb) or if the gas expansion is violent, the bomb explodes (see explosion bomb).
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CORE BOMBS
A
core of basaltic scoria.
(Bottom picture pair) A core of basalt and a coarse grained gabbro. (Close-up photo of core is on the right.)
As the name implies, core bombs have formed around a distinct core of solid material. The core can be any kind of a rock that is stable enough to withstand the conditions associated with the volcanic eruption. Commonly the core of core bombs is an older fragment of the volcano, e.g., a piece of basalt. In other cases, it is a piece of country rock ripped up by the ascending molten mass. Occasionally the core can be carried up from great depths and represent the lower crust or even the upper mantle.
The top photo shows a core of basaltic scoria, perhaps a remnant of a former eruption that was caught up in a more recent emission of molten ejecta. The bomb also shows a delicate flange of basalt on the left that formed as a result of its flight through the air.
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COW PIE BOMBS OR COW PAT BOMBS
A cow pie or cow pat bomb is so named
because of its resemblance to the cow droppings one would encounter while walking across a
cow pasture. Actually the bomb was very plastic when it formed and was capable of flowing
after it hit the ground. The implication is that the bomb was not cooled by a long flight time or was very hot when ejected. In addition to its plastic flow, cow
pie bombs were also sticky enough to hold (weld) pieces of cinder to its cooling
crust. This particular cow pie bomb is also a slag bomb (see explanation
below).
Another fine example of a cow pie bomb is shown here. The photo on the left is the "top" side of the bomb and the photo on the right is the "bottom". In either case, it is obvious that the bomb was very fluid after it fell on the side of the volcano and flowed downslope becoming encrusted with cinders, both from below and those falling from above. Cow pie bombs are relatively rare.
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Explosion Bomb
The above picture shows a bomb that has had a minor explosion of rapidly expanding gas. After the bomb was ejected from the volcanic vent, it created a hard crust or shell that insulated the still plastic interior. Within this material, volcanic gas was still expanding. This gas produced a bubble that burst leaving this crater in the side of the bomb.
These photos are of different perspectives of the same explosion bomb. Here the expanding gases were more violent, blasting the bomb apart.
Another
explosion bomb about 4 inches across.
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FUSIFORM BOMBS
These are the "classic" types of bombs referred to in textbooks. They are typically very streamlined, often with elongate "ears" stretching beyond the central bulge. Often, the bomb and the ears are part of a much larger ribbon bomb in which the delicate ribbons separate as the fluid mass flies through the air or break off on impact.
This fusiform bomb has become welded to volcanic cinder and scoria, an
indication that the volcanic ejecta remains hot and plastic after it comes to
rest along the flanks of the volcano.
The heat associated with volcanic ejecta is also demonstrated in this image of a
fusiform bomb that has welded to cinder and has also become deformed by plastic
flow.
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SQUEEZE OUT BOMB After the bomb formed a hard crust the inside continued to expand. This expansion was not explosive but was able to crack the newly formed crust and allow some of its plastic interior to extrude outward. (See also the Tear Drop Bomb below.)
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RIBBON BOMB
Ribbon Bombs, as the name implies are long thin bands of lava ejected from the
volcanic vent. These very plastic bombs can be ejected directly or can
form as long streamers pulled off of larger bombs as they fly through the
air. Regardless of their mode of formation, they are usually very fragile
and break upon impact.
Another
ribbon bomb about 6 inches in length showing three distinct inclusions
reminiscent of peas in a pod.
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ROTATION BOMB
This fusiform bomb
probably had elongate ears that may have stretched into streamers as it was
ejected from the volcano. It also achieved some rotation either from the
force of the ejection or by the aerodynamics of its flight. What ever the
reason, the bomb displays at definite twist.
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SLAG BOMBS
Slag bombs have the appearance of smelter slag in that they are very vesicular (porous) and have a very fine grained, almost glassy, exterior. They are, however, bombs in that they were ejected from the volcanic vent and have taken on a streamlined shape.
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TEARDROP BOMB
Teardrop bombs have the classic shape of a teardrop or raindrop. They are asymmetrical
fusiform bombs that are elongate in one direction away from the central
bulge. This teardrop bomb also displays the Squeeze Out effect in that the
solid crust was breached and the still plastic interior expanded outwards.
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