Acasta gneiss radiometric dating
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. 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. 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. To give you a better idea of the vast amount of time we're talking about here, let's first reduce it to a more comprehendible time-frame. Acasta gneiss outcrop location:
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. Composed mostly of the minerals quartz and feldspar , the Acasta gneiss was formed during the Hadean , the earliest eon in Earth's history. Minnesota's younger but prettier "Rainbow Granite" seen here in a quarry near Morton, MN has been used to decorate buildings around the country. The most well-known is the gneiss that's quarried around the town of Morton, Minnesota. But even zircon crystals aren't immune from 4 billion years of exposure to the elements. Found on an island in the extreme and very isolated northern regions of Canada's Northwest Territories, the Acasta gneiss has been radiometrically dated to be upwards to 4. 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. It's also a popular material for countertops and cemetery headstones. 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. We modern humans wouldn't appear for the first time until sometime in the show's last couple hours. 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: The half-life of uranium is a known number 4. The event took place at Macalester College in St. 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. This doesn't set well with many in the geological community, who think the rare outcrop should be preserved for scientific study. The Earth itself is estimated to be just a half-billion years older, so the Acasta gneiss pronounced nice is some of the very earliest crustal rock still existing on Earth's ever-changing surface. Things like naturally occurring radiation can damage or alter them and thus skew the measurements. You'd still be watching it in , when non-avian dinosaurs suddenly go extinct about three-and-a-half weeks before the end of the video. The exposure is about kilometers miles from Yellowknife , so the only practical way to get there is by float plane. After finishing his talk about the rock cycle, and as everyone began examining the variety of rock types spread out on lab tables in several rooms, Jeff brought out the chunk of ancient gneiss for everyone to see. 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. The ancient gneiss is named after the Acasta River, located east of Great Bear Lake , where the outcrop was first found in the s. 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. To give you a better idea of the vast amount of time we're talking about here, let's first reduce it to a more comprehendible time-frame. Paul, and was led by Jeff Thole , laboratory supervisor and instructor in the college's Geology Department. For a rock unit to withstand 4 billion years of the rock cycle - where the forces of erosion and plate tectonics are constantly at work wearing down, reworking and remelting rocks - that's quite a feat if you think about it.
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