Saturday, August 2, 2014

Geo 730: July 31, Day 577: Colorful Clays

Very shortly- within a mile or so- after the vesicular/amygdaloidal basalt near Green Peter Dam, one starts to encounter large banks of brilliantly colored clay. These are not "deposits," in the normal sense of the word, but in-place alteration of the bedrock by hot water, or hydrothermal fluids. During the initial stages of hydrothermal alteration- generally, low temperature, low pressure- the effects are similar and comparable to pervasive weathering of the host rock. In particular, feldspars, which comprise 60-70% or more of volcanic rock, change to clays. Mafic minerals such as olivine, pyroxine and amphiboles are much less in volume, but contribute various oxides and hydroxides that pigment the rock. The pigment here is likely mostly iron oxides and hydroxides. Depending on the availability of oxygen, iron pigmentation can range from red, orange, and yellow, to green in reducing environments. So originally, the rock here may have looked very much like what we saw back at Green Peter Dam!

Photo unmodified. June 19, 2012. FlashEarth location (uncertain, but looks right, and close).

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