Geology Bread crust bombs (Bombes de croûte de pain)

It is no accident that an entire class of pyroclasts has the name of breadcrust bombs. Breadcrust bombs are characteristic of a specific type of volcanic eruption that is called Vulcanian after the volcano Vulcano in Italy (the original volcano!). These bombs form degassed magma that forms a plug on the conduit. When bubbles collect below this cap they exert pressure until the plug fails and an eruption happens.

Magma that formed the plug has enough residual gas dissolved in the magma to form bubbles, but only after the outside has cooled and solidified. Therefore, bubble formation and expansion causes the outer crust to crack. Although there are many different types of fracture patterns, some bombs look very similar to the crusty bread shown above. If we break open these bombs, we see larger bubbles in the middle, just as we saw in the bread.








source: http://all-geo.org/erratics/2011/01/bubbling-up/


and http://blogs.agu.org/mountainbeltway/2011/01/31/aw30-the-bake-sale/





and http://blogs.agu.org/mountainbeltway/2011/01/24/east-wall-of-bakers-quarry/