Coarse-Grained Gabbro (Greenland)
This photograph is of a coarse-grained gabbro from the Skaergaard intrusion from east Greenland. Gabbro is produced by the slow cooling and crystallization of basalt magma deep underground. As the magma crystallized, crystals settled to the floor of the magma body to accumulate in layers. This intrusion formed about 55 million years ago as plate tectonic widening of the North Atlantic moved the Iceland hot spot beneath eastern Greenland (further widening put the hot spot beneath modern Iceland). This photo shows crystals of olivine in pink, bluish purple, and bright green in the upper left and lower right. The other colorful crystals scattered about are pyroxene. The large black are on the right side is magnetite, but the striped black, gray, and white crystals are plagioclase.
This photo was taken in cross polarized light and the bright colors are an interference effect. The actual mineral colors are: plagioclase and olivine, colorless; pyroxene, pale brown; magnetite, black. The width of the area photographed is 8 mm. The specimen is a thin section, a thin slice of the rock 30 microns thick.
Photograph and text from Dr. Kurt Hollocher, Union College Department of Geology.
Duratrans print on LED lightbox.