Volcanic bomb most of the damage they cause is from impact, or subsequent fire damage. Bomb explosions are most often observed in "bread-crust" type bombs. Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed. Volcanic bomb A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks.
Oceanic crust frozen in the basalt. A symmetrical pattern of positive and negative magnetic lines emanates from the mid-ocean ridge. New rock is formed by magma at the mid-ocean ridges, and the ocean floor spreads out from this point. When the magma cools to form rock, its magnetic polarity is aligned with the then-current positions of the magnetic poles of the Earth. New magma then forces the older cooled magma away from the ridge. This process results in parallel sections of oceanic crust of alternating magnetic polarity. Oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of a tectonic