New geologic mapping has discovered a 'frozen blob of magma' in the desert ranges of northwest Nevada that may indicate the presence of a former volcano and provide more information about other similar granite intrusions in the area.
The ancient blobs were formed during the time of the dinosaurs (250 million years ago to about 65 million years ago). At that time, there was essentially one continuous subduction zone along western North America. A subduction zone forms when one of Earth's tectonic plates is shoved underneath another one. The scraping of the two plates creates a seismically and volcanically active area around the subduction zone.
The ancient western North America subduction zone had an associated belt of volcanoes.
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Today, these volcanoes have mostly eroded, leaving only the deep roots of the volcanoes, where magma that never erupted froze into granitic rocks. The resulting belt of granitic intrusions, known as batholith, has been disrupted by more recent tectonics and faulting, but still exists along most of western North America.
It was not widely thought, though, that that this belt, known as the Sierra Nevada Batholith, stretched beyond the Sierra Nevada Mountains into northwest Nevada. But after Nicholas J. Van Buer, a Ph.D student at Stanford University, did some research on the Sahwave and Nightingale Ranges an hour north of Reno, Nevada, he found some similarities to the known intrusions.
"After kicking around for a couple of weeks, I realized the zonation of the granitic rocks formed a concentric pattern over a huge area, similar to more well-studied intrusions in the Sierra Nevada," Van Buer said. "I decided to map it out and analyze it in more detail, since none of the intrusions in that part of Nevada had even been carefully studied."
Bull's-eye find
The Sahwave Intrusive Suite, as the structure is called, had a similar pattern to intrusions found in the Sierra Nevada range. The structures are arranged in a bull's-eye pattern with successively younger and lighter-colored granitic rocks intruded into the middle.
"I collected rock samples to bring back to the lab to analyze the minerals and chemistry, to compare this intrusion with others in the Sierra Nevada, and to date the timing of this," Van Buer told OurAmazingPlanet in an email. The date of the rocks can be determined by measuring the radioactive decay of uranium into lead.
"It turns out that the Sahwave Intrusive Suite is quite similar to the giant intrusions in the Sierra Nevada in terms of its minerals and geochemistry, and was emplaced at about the same time, about 90 million years ago, and over a similar duration —s 4 million to 5 million years, a long time even for a geologist," Van Buer said.
The granite intrusions are the remnants of the magma from former volcanoes. The rock that is exposed today was around 3 to 6 miles (5 to 10 kilometers) deep when it was molten. Any volcanoes that existed then have been completely eroded away over the last 90 million years.
Ancient subduction
Today, we know Nevada as a very dry place, but 200 million years ago it was the site of a deep sea. Some scientists have hypothesized that the magma intrusions of the Sierra Nevada formed when continental crust was shoved under the volcanic arc from the east and melted, at the same time oceanic crust was subducting from the west. Van Buer thinks that the Sahwave Intrusive Suite was formed differently though.
"The Sahwave Intrusive Suite, which looks the same as the other intrusions, isn't in or near the right kind of easily meltable continental crust, so that can't be what causes these huge pulses of magma intrusion," he said. "Instead, they must be caused by something deeper."
Instead, Van Buer thinks that the subducting oceanic plate might have risen upward because it is thicker and more buoyant, an idea proposed by some scientists a few decades ago.
"I speculate this may have also allowed it to carry more water, which acts to help hot rock melt," Van Buer said. "These giant intrusions would have been emplaced in the time between when the thicker oceanic crust began to subduct and the time when the same crust floated up and blocked the supply of hot mantle rock."
Van Buer's next step is to determine the cooling history of the Sahwave Intrusive Suite, which may help provide the answer to where the eroded volcanoes went.
This article was provided by OurAmazingPlanet, a sister site of LiveScience.
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