Mount RainierDominating the skyline of Seattle stands the highest peak (4392 m/14411 ft) in the Casc

Mount RainierDominating the skyline of Seattle stands the highest peak (4392 m/14411 ft) in the Cascade Mountain Range, Mount Rainier. Along with other notable volcanoes in the Cascades, Mount Rainier formed as a result of subduction along the Cascadia subduction zone, where melting and crustal contamination generates a high silica, high water content magma. This magma composition presents at the surface in a very particular way; large stratovolcanoes with the potential for large, explosive eruptions.Located on the leading edge of the Cascades, Mount Rainier is located just 87 km/54 miles to the south-east of Seattle. Parts of the city of 634,535 people, as well as the city of Tacoma (202,010 people) and surrounding towns are built on ash and lahar deposits from Rainier.Along with the 20 or so major stratovolcanoes in the Cascade Arc, Rainier is capable of large, explosive eruptions similar to that of Mount St Helens in 1980. The geologic record shows that continuous eruptions have been occurring at Rainier for 500,000 years, fluctuating between periods of higher and lower eruption rates (We are in the latter).The eruptions themselves however, are not the only, nor the biggest threat the volcano poses. The mountain is home to 26 major glaciers which feed several rivers (Cowlitz, Nisqually, Puyallup, Carbon and White) that during an eruption would be vulnerable to lahars created by the sudden release of glacial meltwater. The USGS estimates that 150,000 people live on top of past lahar deposits from Rainier, including areas of Seattle and nearby Tacoma.Hydrothermal alteration on the mountain is also of concern. A mixture of magmatic fluids, hydrothermal fluids, and steam have acted to chemically alter the volcanic rocks, making them weaker in the process. Widespread alteration on the steep sided flanks of Rainier makes them vulnerable to collapse, a process evident in the geological record. Approximately 5,600 years ago, 2-3 km^3 of mostly altered material from the summit and N-E flank detached from the mountain, creating a lahar that reached as far as the Puget Sound. This event, known as the Osceola Mudflow, created a horseshoe shaped crater not unlike that created after Mt St Helens erupted, which has since refilled with later lavas. Most of the heavily altered rock from the volcano was removed in this event, but some does remain on the upper west flank and presents a collapse hazard. These flank failure events can prove particularly hazardous as they do not necessarily require volcanic or seismic activity to occur; instead failing due to their own structural weakness.There are no current signs of renewed activity at Mount Rainier, which is heavily monitored for any activity that suggests the contrary. Rainier has not had a major eruption in recent times, but its eventual awakening will no doubt make headline news worldwide.USGS: Mount Rainier - http://on.doi.gov/PctRUIHydrothermal Alteration on Rainier - John, D. A., Sisson, T. W., Breit, G. N., Rye, R. O., & Vallance, J. W. (2008). Characteristics, extent and origin of hydrothermal alteration at Mount Rainier Volcano, Cascades Arc, USA: Implications for debris-flow hazards and mineral deposits. Journal of Volcanology and Geothermal Research, 175(3), 289-314. - http://bit.ly/1hdKn4QThe Osceola Mudflow - Vallance, J. W., & Scott, K. M. (1997). The Osceola Mudflow from Mount Rainier: Sedimentology and hazard implications of a huge clay-rich debris flow. Geological Society of America Bulletin, 109(2), 143-163. - http://bit.ly/1KKcmWyDeep magma reservoir below Mt Rainier - http://on.fb.me/1IECxvmImage Credits - Andrew E. Larsen - http://bit.ly/1OY1nrh, Poyson - http://bit.ly/1Hj5zvd, Hazard Map - USGS http://on.doi.gov/1JG1p6WRJW -- source link

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