The surface of volcanic domes are generally very rough, with numerous spines that have been pushed up by the magma from below. Most dome eruptions are preceded by explosive eruptions of more gas rich magma, producing a tephra cone into which the dome is extruded. Volcanic domes can be extremely dangerous. because they form unstable slopes that may collapse to expose gas-rich viscous magma to atmospheric pressure. Craters and Calderas Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and tephra. Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km.
Calderas form as a result of collapse of a volcanic structure.
The collapse results from evacuation of the underlying magma chamber. In shield volcanoes, like in Hawaii, the evacuation of the magma chamber is a slow drawn out processes, wherein magma is withdrawn to erupt on from the rift zones on the flanks. In stratovolcanoes the collapse and formation of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive eruptions that form extensive fall deposits and pyroclastic flows. Calderas are often enclosed depressions that collect rain water and snow melt, and thus lakes often form within a caldera. The caldera formed about 6800 years ago as a result of the eruption of about 75 km 3 of rhyolite magma in the form of tephra, found as far away as Canada, accompanied by pyroclastic flows that left thick deposits of tuff on the flanks of the volcano.