The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on an occasional schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Wednesday, January 22, 2014

January 22. Gneiss

By Richard I. Gibson

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Gneiss, pronounced "nice," is from a German word that means spark and it’s a metamorphic rock that has been heated and pressurized so much that the chemicals have been rearranged to form minerals in distinct bands, often alternating light and dark minerals.

Four pieces of gneiss
Metamorphism means “changed form” and that can happen to any rock, from granite or basalt to sandstone and limestone. Granite is pretty common in the continental crust of the earth, so granites from the Precambrian are often metamorphosed into granite gneiss, but the original rock could be almost anything, as long as the chemistry is right.

Because Precambrian rocks have been around so long, it’s no surprise that they may have been metamorphosed repeatedly. The layers that you see on the side of a piece of gneiss are not depositional beds like you would find in sandstone or shale, but they were formed with that preferred orientation under high heat and pressure. These surfaces are called foliation to distinguish them from sedimentary bedding.

There are as many kinds of gneiss as you want – geologists use modifiers to provide more information, so it might be a garnet gneiss, or a quartzo-feldspathic gneiss, or an amphibolite gneiss, granite gneiss, or a dozen other names. They just tell you some of the dominant minerals in the rock, or maybe something about its likely origin.

The other common type of metamorphic rock is schist, which generally has more mica in it than gneiss. We’ll talk about that another day.

2 comments:

  1. [Terrific blog! I am enjoying each one, and going back here from March to catch up on items I glossed and missed the first time, when only listening] Is there a general term in geology that serves as a catch-all for all the specifically named types of rocks... like gneiss, basalt, schist, chert, etc.?

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    1. There are three main groups of rock types, igneous (from molten rock), sedimentary (from pieces of sand mud etc that settle into sediments), and metamorphic (heat and pressure change pre-existing rocks). This may help - https://www.learner.org/interactives/rockcycle/types.html Thanks for the nice words!!

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