Its composition leads geologists to surmise that it was probably formed from highly metamorphosed granite subjected to unimaginable heat and pressure. The exact origin of that granite is unknown, but its presence indicates continental crust and surface water were probably already present in those very ancient times.
Morton Gneiss: Minnesota's younger but prettier "Rainbow Granite" seen here in a quarry near Morton, MN has been used to decorate buildings around the country. It's also a popular material for countertops and cemetery headstones. The banding and severe swirling evident in the rock indicates that, at some point, it went through a period or periods of high heat and enormous pressure. Courtesy Mark Ryan It may interest you to know that Minnesota has its own ancient gneisses exposed in outcrops in the Minnesota River Valley.
The most well-known is the gneiss that's quarried around the town of Morton, Minnesota. At nearly 3. Known in the construction trade as Rainbow Granite , polished panels of the banded and severely swirled Archean -aged-aged migmatitic gneiss can be found decorating building facades throughout the country. An enterprising miner from Yellowknife has filed a claim on the Acasta gneiss site, and has been trying to market the ancient rock.
This doesn't set well with many in the geological community, who think the rare outcrop should be preserved for scientific study. They also say the prospector could be misrepresenting the public since not all the rock in the exposure dates back to 4 billion years, and it's very expensive to validate the age of any one piece. So how exactly has the Acasta gneiss been dated so precisely? Zircon crystals found in the rock's mineral structure trap uranium in their lattices when they form and can act as timekeepers through measuring the decay of the uranium into lead.
The half-life of uranium is a known number 4. But even zircon crystals aren't immune from 4 billion years of exposure to the elements. Things like naturally occurring radiation can damage or alter them and thus skew the measurements.
But by using an instrument called the Sensitive High-Resolution Ion Microprobe aka SHRIMP researchers are able to focus a beam of oxygen ions on a tiny unaffected segment of the zircon' s surface, remove atoms from it, and then analyze their isotopic composition. It should be noted that an older Canadian rock unit supposedly exists in the greenstone belt east of Hudson Bay, but there's still some contention regarding this, since the method of radiometric dating isn't the same that was used to date Acasta samples.
Whether the Acasta gneiss is the remaining crust of a protocontinent that existed when the Earth was still a relatively young, hot mass of accreted material remains a mystery at this point, but scientist named the time the Hadean for good reason: back then it must have been literally Hell on Earth. World View. How Is Gneiss Formed? More From Reference. What Are Athena's Personality Traits? Great Software Options for Small Offices. Minerals that melt at lower temperatures will approach their melting point but never actually melt.
The other minerals that can stand the higher temperatures will remain less changed by the process. Under the conditions of regional metamorphism, minerals will begin to migrate and segregate themselves which will cause the formation of a new material that has a different structure from the base material. The migration and segregation of minerals forms the characteristic bands that gneiss is known for.
In some cases there is a stark contrast between the bands but in others it is hard to differentiate. Gneiss forming from sedimentary rock is called paragneiss and can form from shale, sandstone or basalt while gneiss forming from igneous rock is called orthogneiss and typically forms from granite, gabbro, or diorite.
Most gneiss has very a similar composition to granite. The common mineral components are quartz, feldspar, micas, and silicates.
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