How Does Globe's Continental Crust Class? A New Bottom-Upwardly Theory

The McMillan Spires in North Cascades National Park have metamorphic rocks known as granulites that have equilibrated at pressure and temperature conditions typical of continental lower crust. Photo: John Scurlock/Jagged Ridge Imaging.

The McMillan Spires in Washington country take metamorphic rocks known as granulites that have equilibrated at pressure level and temperature conditions typical of continental lower crust. Photograph: John Scurlock/Jagged Ridge Imaging

Deep beneath Alaska's Aleutian Islands, down where the pressure and temperatures have become so high that rock starts to flow, new continental crust is existence born.

Scientists accept long believed that continental crust forms in volcanic arcs—they know the magma brought up in the arcs' volcanoes is geochemically very like to continental crust. The lingering question has been how exactly that happens. While the magma that reaches the surface is similar to continental crust, the lower crust below volcanic arcs is quite different from the lower one-half of continental crust.

A new study appearing in this calendar week's Nature Geoscience raises questions about one popular theory and provides new back up for another, in which arc lava from the surface and shallow "plutons"—magma that solidified without erupting—are pulled down into the Earth at subduction zones and and so rise up to accumulate at the bottom of the arc crust like steam on a kitchen ceiling. Scientists have found compelling testify to suggest that this could accept produced the vast majority of lower continental crust through Earth history.

Relamination of subducted sediment.

Relamination of subducted sediment.

The process, called relamination, starts at the edge of a continental plate, where an oceanic plate is diving under the continental plate and magma is ascent to class a volcanic arc. Equally the oceanic plate dives, information technology drags down sediment, lava and plutonic rock from the edge of the arc. Every bit arc material descends, minerals inside information technology become unstable with the rising pressure and heat, and they undergo chemical changes. New minerals form, and chunks of the rock and sediment can pause off. When those chunks are denser than the drape rock effectually them, they keep to sink. Merely when they are less dense, such every bit those that form silica-rich granulites, they become buoyant and float upward until they reach the lesser of the arc crust and accumulate there.

"Sediments are really well represented in continental lower crust, but how did they get on to the bottom of the continent? The easiest way is for that sediment to be pushed down a subduction zone and rise to accumulate at the base of the crust," said Peter Kelemen, a geochemist at Columbia University'due south Lamont-Doherty Earth Observatory and author of the paper with Mark Behn of Wood Hole Oceanographic Establishment.

Sampling the World's Chaff

To decide how arc crust could turn into continental crust, Kelemen and Behn examined the just two known sites where a complete section of arc lower crust is visible on land. One site, in Islamic republic of pakistan, had been caught in the aboriginal collision of tectonic plates between India and Asia, and was thrust upwardly into steep mountains. The other, the Talkeetna arc stretching from the Alaska Peninsula to Valdez, was pushed up at the edge of Northward America.

"We don't usually get to encounter the bottoms of arc lower crust, but in Alaska and Pakistan we tin can see correct downwards to the bottom. These old arcs formed, crashed into N America, turned on their sides, and were eroded over millions of years. Because they're tilted, we tin can walk correct downwards from the seafloor, past the base of the crust and into the mantle," Kelemen said.

Along the length of these areas of exposed arc chaff, the scientists took samples to see how the geochemical composition of the rock inverse with increasing depth in the crust. They were able to excerpt minerals that had recorded the pressure and temperature at the bespeak where the minerals crystalized deep underground, marking how deep the rock was at each signal.

The scientists plant meaning changes in the crustal composition about half way downwardly into the arc crust.

In the lower one-half of the arc crust, starting near xx kilometers below the original surface, the average concentration of "incompatible" trace elements—elements similar tantalum and potassium that prefer to remain in cook during crystallization—was much less than in lower continental crust at the aforementioned depth. It was only the upper 20 kilometers of the arc crust that had compositions like to lower continental chaff.

That becomes a problem for one leading theory of how continental chaff forms, Kelemen said. That theory suggests that the arc crust delaminates—dense bits of stone within the arc chaff slowly move downward and "founder" into the mantle until the arc chaff attains the limerick of continental crust. The new data suggests that for delamination to work would require removing much of the rock from a 20-kilometer thickness of chaff. Notwithstanding, delamination simply works below 35 to 40 km depth.

"And so, fifty-fifty after we remove a flake of dense stuff off the bottom, you're still going to end upward with lower chaff in the arcs that looks actually dissimilar from lower crust in the continents. The process isn't sufficient to brand continental lower chaff out of arc crust," Kelemen said. Delamination does take place, but for it to be the driving force would require a circuitous process of repeated crustal thickening and metamorphic events, he said.

Kelemen and Behn advise a simpler procedure.

The Aleutian Islands Test

The authors put their model to the test on the Aleutian Islands. In that volcanic arc, the lava and plutons are like to continental chaff, but the lower crust is highly depleted in elements that are abundant in lower continental chaff. To determine the potential for relamination to produce lower continental crust, the scientists calculated the density of the exposed lava and plutons at subduction zone pressures and temperatures.

About 44 percent of the Aleutian lavas and 78 percent of the plutons would be more buoyant than pall peridotite nether subduction zone conditions, they found. This suggests that if parts of the Aleutian arc are pulled downwards into the subduction zone, at a depth of 90 to 120 km, where temperatures exceed 700°C, the arc lavas and plutons would rising to accrue along the bottom of the crust. The composition of this accumulated cloth would look similar lower continental crust.

Intrigued by that finding, the scientists performed the same calculations for other arcs. They found that at the Alaska Talkeetna site, 48 percent of lavas and 37 percent of plutons would be buoyant. At Kohistan, the site in Pakistan, 36 per centum of lavas and 29 percent of plutons would be buoyant.

Relamination may be evident in Southern California'due south Pelona Schist where sections of lower continental crust are visible, Kelemen said. Clay rocks and blobs of mantle peridotite surrounded by more than buoyant materials tin can be found in the exposed, "underplated" chaff.

"We can come across young, volcanic sediments that were stuffed under older continental crust and are now role of the overall package. How did they go down there? It happened in Southern California, and I would argue information technology probably happens in a lot of places," Kelemen said.

Learn more near the piece of work underway at Lamont-Doherty Earth Observatory.