Overview and images of gneiss as a rock type are here: GNEISS
I recently posted a picture of migmatite from Norway. Today I am itching to post another one. These rocks are really beautiful and fortunately very common in certain areas and versatile as well.
Where can we expect to find such rocks? They form deep in the crust where rocks are ductile because of enormous pressure and high temperature. Such conditions prevail under the forming mountain ranges where crust is thicker than usually. Mountains eventually erode away and rocks that were formed deep in the crust become exposed. So, if we see migmatite or gneiss, we can pretty safely say that big mountains once stood here. Migmatites are usually very old because it takes time to completely wear away mountains. Norway is a mountainous country but this migmatite didn’t form during the Caledonian orogeny (approximately 400…500 Ma) which is mostly responsible for the mountainous terrain in Norway. These mountains are still there and we have to wait few hundred million years more to see the migmatites that formed underneath them. The migmatite or migmatized gneiss I am presenting today formed one billion years before. Its age could be anywhere between 1.8…1.5 billion years. It is part of a Norwegian basement in western part of the country which is not covered by allochthonous rocks.
This migmatized gneiss is collected in Norway (near Hornindal). It is only 8 cm across.
So how does one distinguish between gneiss and migmatite? waviness? I almost asked in response to the previous post, but resisted; maybe it’s too difficult. But we have migmatites not far from where I live, and I would like to be able to recognize them. They certainly are beautiful, I agree.
Thanks.
Gneiss is banded. Dark layers of flat biotite sheets (mostly) alternating with quartz/feldspar layers. Migmatite seems to a mixture of two rock types — dark-colored metamorphic and light-colored igneous. Sometimes it is hard to distinguish and sometimes gneiss smoothly transforms to migmatite. I usually say migmatized gneiss when I am not sure