Caldera eruptions such as the one which produced Crater Lake, Oregon, were originally thought to be massive explosions which blew the top off the original volcano. Ferdinand Fouque, a French geologist, showed that the caldera could not have been formed by destruction of an earlier cone, since there was not enough material from the original cone in the deposits formed by the eruption. In his 1879 book on the eruption of Santorini, Greece in 1628 B.C. he concluded that the missing part of the volcano had sunk below sea level. Further evidence for this type of activity was found on Rannoch Moor in Glencoe, Scotland by Clough, Maufe and Bailey in 1909. A caldera eight kilometres in diameter formed during a major eruption 360 million years ago. Huge volumes of pyroclastic material had been expelled leaving the roof of the magma chamber without support, this caused it to collapse into the void. They named this process cauldron subsidence.
Cauldron subsidence forms calderas from a few kilometres across to as much as one hundred. This process may be compared to a piston moving down in its cylinder. Circular fractures open up and the roof of the magma chamber collapses and sinks. It is then covered by an accumulation of welded ignimbrites (pumice and ash deposits). Magma moves upwards through the ring fractures forming ring dikes which feed volcanic vents, pyroclastic flows continue from these vents.
Crater Lake volcano erupted 6800 years ago culminating in a cauldron subsidence event and the eruption of about 40 cubic kilometres of pumice and ash deposits. It has been estimated that a volume of 60 cubic kilometres would have been required to reconstruct the original 3600 metre volcanic cone which he named Mount Mazama. A huge eruption showered ash as far away as Alberta. The water filled caldera is nine kilometres in diameter with vertical walls 1200 metres high, half of this steep wall is hidden by the lake. Charles Bacon of the United States Geological Survey confirmed in a recent study that cauldron subsidence had taken place from vents supplied with magma from the ring fractures. He suggested that there were two phases of eruption. An early single vent eruption during which 30 cubic kilometres of magma erupted causing the roof of the magma chamber to collapse and form the floor of the present caldera. 13 cubic kilometres of material then erupted during the second phase of eruption from vents along the ring fractures.
Some of the biggest volcanoes are resurgent calderas. There may be earlier eruptions but they are obliterated by the scale of these calderas. Apart from the size, the main feature of these volcanoes is the slow post eruption uplift (or resurgence) of the floor. This uplift may be as much as one kilometre. A resurgent caldera is marked by a large topographical depression with an elevated area at the centre. The Valles caldera in New Mexico and the Yellowstone caldera are examples.
The Oka Hills described in the Fall 1997 issue of WAT ON EARTH show the characteristic ring dykes produced during the formation of a caldera. Figure 2 shows ring dykes associated with the Oka Hills complex. Inclusions found in the rocks provides a sampling of the cover rocks present at the time of formation.
- Gold, D.P. and Marchand, M. GAC/MAC Guide Book, Geology of The Monteregian Hills, 1969.
- Francis, P. 1993 Volcanoes, A Planetary Perspective, Clarendon Press.