Mt st helens dating

12 Apr

The third dacite lava dome began to appear on October 18, 1980 above the lip of a 25-metre-diameter feeding conduit. Helens' new lava dome is composed of 74 million cubic meters of dacite flows and intrusions built up within the crater between October 18, 1980, and October 26, 1986 The view is toward the north looking over the lava dome into the 1980 blast zone (photo by Lyn Topinka of the US Geological Survey, after Pringle, Ref. After October 18, 1980, this third and newest composite dome of dacite began to appear.By October 1986 this newest lava dome had grown within the horseshoe-shaped crater to be an immense structure up to 350 m high and up to 1,060 m in diameter (see Figures 1 and 2).

Some Indians of the Pacific Northwest variously called Mount St.Helens "Louwala-Clough," or "smoking mountain." The modern name, Mount St.Helens, was given to the volcanic peak in 1792 by Captain George Vancouver of the British Royal Navy, a seafarer and explorer.The conventional K-Ar dating method was applied to the 1986 dacite flow from the new lava dome at Mount St. Porphyritic dacite which solidified on the surface of the lava dome in 1986 gives a whole rock K-Ar 'age' of 0.35 ± 0.05 million years (Ma).Mineral concentrates from the dacite which formed in 1986 give K-Ar 'ages' from 0.34 ± 0.06 Ma (feldspar-glass concentrate) to 2.8 ± 0.6 Ma (pyroxene concentrate). The fundamental dating assumption ('no radiogenic argon was present when the rock formed') is questioned by these data. Helens dacite argue that significant 'excess argon' was present when the lava solidified in 1986.