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The lava dome formed by a complex series of lava extrusions, supplemented occasionally by internal inflation of the dome by shallow intrusions of dacite magma into its molten core.
Extrusions of lava produced short (200-400 m) and thick (20-40 m) flows piled on top of one another.
Of the lithic inclusions 85 per cent are medium grained gabbros with an average diameter of 6 cm." The high mafic mineral content of gabbroic inclusions makes a small but significant decrease in the overall Si OPhotomicrograph of Mount St. The most abundant phenocrysts are plagioclase which are embedded in a much finer-grained groundmass containing grass end microphenocrysts. Geologists are in general agreement concerning the crustal source of the dacitic magma beneath Mount St. Experimental data from the assemblage of minerals in the dacite indicate that just prior to the May 18, 1980 eruption the upper part of the magma chamber was at a temperature of 930°C and at a depth of about 7.2 km.
Photographed in polarised light with 2 mm width of view (dacite sample ' DOME-1 photo by A. Major-element and trace-element abundances in the 1986 dacite lava flow at Mount St. The analysis was performed on dacite groundmass and phenocrysts without lithic inclusions.
The stability of this third dome, along with decrease in the frequency of earthquakes and phreatic steam eruptions in the ten years after October 1986, indicate that the volcano, again, may be approaching a period of dormancy.