Some of Earth's oldest surviving rock is banded gneiss over four billion years old
On an island about 300 kilometres north of Yellowknife in Canada sits the Acasta Gneiss, metamorphosed between 3.58 and 4.031 billion years ago and among the most ancient intact pieces of crust anywhere. Gneiss gets its name from an old German word for spark, because the striped rock glitters.
Gneiss forms when existing igneous or sedimentary rock is cooked and squeezed deep underground, at temperatures above 300 degrees Celsius and pressures of roughly 2 to 15 kilobars, sometimes more. The result is almost always striped: alternating dark and light bands of grains big enough to see with the naked eye. Unlike schist, gneiss has only a weak tendency to split along those layers. British and international geological bodies define it by texture alone, with layers over 5 millimetres thick, regardless of what the rock started as.
The stripes record pressure from one direction. Under uneven stress, often a shearing motion like sliding the top of a deck of cards one way and the bottom the other, the rock flows like plastic and its platy minerals line up in sheets perpendicular to the squeeze. Dark bands are rich in magnesium and iron minerals; pale bands hold more feldspar and quartz. Sometimes the layers simply inherit an original stack of sandstone and shale, now quartzite and mica, and sometimes chemical reactions sort the minerals, a process still not fully understood.
Varieties tell their own stories. Augen gneiss, from the German for eyes, has oval feldspar grains that finer material has flowed around. Migmatite looks half gneiss, half granite, and is usually read as rock caught in the act of partial melting. Geologists name gneisses carefully by colour and minerals, as in garnet-biotite paragneiss, since so many rock types can end up this way; orthogneiss comes from igneous rock and paragneiss from sedimentary.
Gneiss dominates the ancient stable cores of continents. Scotland's Lewisian gneiss runs through the Outer Hebrides, the Morton Gneiss of Minnesota may be the oldest intact block of crust in the United States, and India's Peninsular Gneiss spans 3,400 to 2,500 million years. It is also practical: a variety called facoidal gneiss is used widely in Rio de Janeiro's buildings, and crushed gneiss goes into asphalt.
Source: Gneiss
