Thursday, February 7, 2013

Minnesota Geology Pictures - Pillowed Greenstone

In Gilbert, Minnesota, is a site of 2,700 million year old pillowed greenstone.  Basaltic lava would have originally have erupted into an ocean or sea forming rounded structures called pillows.  Through weak metamorphosism, involving higher temperatures and pressures, original minerals found in the basalt like plagioclase and pyroxene changed to chlorite and epidote.  The edges of the pillows have crystals much smaller than the interior.  As the lava erupted into the colder water, the edges cooled faster resulting in small crystal growth, while the interior cooled relatively slower resulting in larger crystals.

Much later in time, during the late Wisconsin glaciation, glaciers from the Rainy Provenance scoured the area, largely cutting the pillows in half and leaving numerous striations across the pillows.

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