Special Paper No. 8
The Role of Explosive Fractionation in the Genesis of the Alkaline and Calc-Alkaline Rocks. Special Paper No. 8 - C.E. Williams (2003). Upper mantle partial melts provide the universal progenitor for the evolved rock families. The evolution from basalt to rhyolite or basalt to trachyte requires the virtual elimination of Mg, Fe and Ca in order to achieve a relative increase in Si and alkalis. This may be effected partially by crystal differentiation and segregation but mainly by selective expulsion of early Mg-Fe-Ca-rich rock-forming mineral phenocrysts in explosive volcanism and a continuing elimination of later phenocrysts, each with less silica than the residual melt thus leaving a rhyolitic or trachytic end product in the upper reaches of the subvolcanic magmatic system. This felsic residue may crystallize as diorites, granodiorites, granites or nepheline syenites but grades downward into gabbroic types.
The prime mover in initiating the high P.T.X. conditions and subsequent surface penetration, is the mantle plume. Instant reduction of hydraulic pressures ensures the ballistic expansion of high tension mantle carbonates and sulphates etc. to produce an effervescent gaseous reaction phase with its own chemical regime of simple compounds. Adiabatic chilling blocks the vent and restoration of pressure and temperature revives silicate polymerization in an OH-rich atmosphere producing mantling amphiboles, mica and oxidation products.
The much studied pumices of Mt. Pinatubo and other complexes worldwide confirm this in the field.
Click here to download this publication (PDF 2.3Mb)
At 08:32 Sunday morning May 18th, 1980, Mount St. Helens erupted.
Shaken by an earthquake measuring 5.1 on the Richter scale, the north face of this tall symmetrical mountain collapsed in a massive rock debris avalanche. Nearly 230 square miles of forest was blown over or left dead and standing. At the same time a mushroom-shaped column of ash rose thousands of feet skyward and drifted downwind, turning day into night as dark, grey ash fell over eastern Washington and beyond. The eruption lasted 9 hours, but Mount St. Helens and the surrounding landscape were dramatically changed within moments.
In 1982 the President and Congress created the 110,000-acre National Volcanic Monument for research, recreation, and education. Inside the Monument, the environment is left to respond naturally to the disturbance.