Volcanic lavas, chemical compositions

The dark spot is of special interest to researchers because it represents a kind of volcanic outflow unusual on Io. Most such outflows are white, yellow, or red, due to the presence of sulfur. The new outflow is much darker than any that had been observed before, suggesting that its chemical composition differs significantly from others on the moon s surface. Scientists believe the dark spot may be a new lava flow produced during a violent eruption on the moon s surface observed by the Hubble Space Telescope in June 1997. That eruption released a plume of gas nearly 100 km (75 miles) high into the moon s atmosphere. 

The volcanoes in the Transantarctic Mountains and in Marie Byrd Land of West Antarctica erupted lava flows and pyroclastic ash that was deposited on the surface of the ice sheets. The ash was subsequently buried by snow and was thereby incorporated into the ice. The resulting ash layers now serve a useful purpose in the study of the ice sheets because they are unique event horizons whose age can be determined by isotopic methods (e.g., Folco et al. 2007). In addition, these horizons have preserved a record of the deformation of the ice sheets that is revealed by mapping their outcrop patterns on the bare-ice surfaces in the ablation zones. The chemical composition of the ash has been used to identify the volcanoes from which certain ash layers were erupted, while the sulfate concentration and the acidity (pH) of the ice above an ash layer provide clues to the amount of sulfuric acid that was injected into the stratosphere (Palais 1985). The volcanic dust and sulfuric acid in the stratosphere can cause temporary cooling of the global climate as demonstrated by the eruptions of Krakatau (Indonesia) in 1883, Mount St. Helens (Washington) in 1980, El Chichon (Mexico) in 1982, and Mount Pinatubo (Philippines) in 1992 (Holland and Petersen 1995 Thompson and Mosley-Thompson 1981 Kyle et al. 1981 Self etal. 1981). 

Fig. 16.26 The chemical compositions of the McMurdo Volcanics in the Melbourne volcanic province of northern Victoria Land range widely from basanites to phonolites, trachytes, and even rhyolites. Most of the volcanoes in this province include the full range of chemical compositions of lava flows. Bn = basanite, Tp = tephrite, Ftp = phonotephrite, Tpp =...

Macdonald GA, Katsura T (1964) Chemical composition of Hawaiian lavas. J Petrol 1 172-177 Mankinen EA, Cox A (1988) Paleomagnetic investigation of some volcanic rocks from the McMurdo volcanic province, Antarctica. J Geophys Res 93 11599-11612 Mason RA, Smith JV, Dawson JB, Treves SB (1982) A reconnaissance of trace elements in anorthoclase megacrysts. Mineral Mag 46 7-11... 

Obsidian is a volcanic rock that forms when lava cools very quickly. The chemical elements in the melt freeze in random order. The result is obsidian or volcanic glass, which has no orderly internal structure and can be of any elemental composition. 

Rhyolite is an example of even more siliceous (as much as 70% Si02), or acidic, volcanic material. Rhyolite is a product of rather extreme chemical fractionation relative to the average composition of Earth. It is a common material, but hardly abundant in comparison with basalt, or even andesite. Rhyolitic lavas mainly appear in regions where less extreme types of lavas predominate. Such silica- and alkali-rich materials as rhyolites melt at lower temperatures than basalts or andesites and produce viscous liquids. On cooling, many fail to crystallize, but produce obsidian glass. Some are so forcibly ejected that they erupt as shards of glass, producing widespread falls of volcanic ash and pumice. 

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