At depth beneath a volcano, magma accumulates. Magma is a mix of solid minerals, liquid rock, and gasses. The gasses include CO2 and sulfur, but most is water, and much of the water penetrates from the surface far above. At depth, these gasses are compressed, but as the magma rises – they expand. They expand, it has been calculated, so much so that 1 cubic meter of rhyolitic magma would increase to ~63 cubic meters by the time it reaches the surface. If it reaches the surface quickly, the expansion is explosive – this means BOOOM!!
When a particular eruption is accompanied by a great explosion of magmatic gasses, a mix of rock, ash particles and superheated gas is produced: this mix is denser than the surrounding atmosphere and forms a moving monster of destruction called a pyroclastic flow. These flows can have temperatures between 200°C – 700°C and attain speeds of ~80 km/h. When this flow resembles an avalanche on fire, it is called a “nuee ardente.” If it mixes with snow, which vaporizes immediately, or surface waters, it forms a flow called a “lahar.”
When pyroclastic flows comes to rest, the rock that forms with the cooling of the pyroclastic material is called “ignimbrite.” Since we can’t directly observe pyroclastic flows and survive, we use the study of ignimbrites to deduce the processes, rheology and depositional mechanisms of these flows.
Ignimbrites retain the solid parts of the pyroclastic flows: they are a mix of volcanic ash, pumice fragments, and scattered lithic fragments that have been introduced into the magma during transport from depth, incorporated from the volcanic crater or even include non-volcanic country rocks. If the flows were hot enough, the material can be welded together when deposited; if the deposits are hot and thick and heavy enough, the ash and pumice can be compressed to thin lenticular shapes and layers.
And I can think of no better place to study ignimbrites than those shown in this photo from Santorini: the explosive eruption about 1625 BC blew up ~60 km3 of the island as well as the resident civilization that may have entered legend as Atlantis. Ignimbrites entered the geologic record and form part of the complex eruptive history of Santorini.
Photo credit: A. Georgakopoulos