ABSTRACT: Formation of Volcanic Eruptions and Possibilities of Their Prevention
E. A. Skobelin
Magmatic sources of volcanoes are sills of basic and ultrabasic magma inside water-rich sedimentary rocks folded as closed anticlinal uplifts. Sills convert water into steam which accumulates in the hinge of the anticline. Here the strength of the oversill complex becomes weaker because of differing thickness of the sill in the hinge and limbs of the anticline. There is a very fissured and permeable zone created at the contacts of the sill and country rocks.
Pyroclastic eruptions take place if a water-rich bed overlies the sill, because of accumulation of excess steam pressure that can not be withstood by the oversill complex. After a break-through, the steam moves through the fissured, very permeable zone near the sill contacts. Steam velocity increases toward the place of this break-through, Owing to effect of pulverizer the steam transports fragmented portions of magma, solidified rock, and country rock upward. After exhaustion of the steam, magma streams into the conduit, filling it partly or completely. In the last case, lava eruption also takes place.
Lava eruptions happen if the water-rich bed is below a sill. Generated steam moves directly into the sill chamber and squeezes magma out from its upper part. After a break-through (often explosively) and frothing a steam-rich magma fills up the conduit and moves upward by the mechanism of gas-lift, known for oilers. Movement of steam from the undersill complex through the sill contributes to heat transfer into the conduit of a volcano and sometimes makes possible the existence of boiling lava lakes in a crater.
Basic and ultrabasic sills of large thickness are able to interact intensively with country rocks, assimilating or completely melting them. The extent of interaction determines the composition of magma. There may be different combinations of conditions and mechanisms for eruptions in nature.
During periods between eruptions a new accumulation of steam forms. Such accumulation and high pressure may be prevented by output of steam through geothermal wells. This suggests a possibility of prevention of volcanic eruptions.
AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990