The granite rocks that make up Mt. Rushmore crystallized from magma that formed 1.7 billion years ago. Click on image for full size (291 Kb) Courtesy of Peter Nabelek, University of Missouri

Earth's crust melts easier than previously thought, scientists have discovered.

In a paper published in this week's issue of the journal Nature, geologists report results of a study of how well rocks conduct heat at different temperatures. They found that as rocks get hotter in Earth’s crust, they become better insulators and poorer conductors.

The findings provide insights into how magmas are formed, the scientists say, and will lead to better models of continental collision and the formation of mountain belts.

"These results shed important light on a geologic question: how large bodies of granite magma can be formed in Earth's crust," said Sonia Esperanca, a program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.

"In the presence of external heat sources, rocks heat up more efficiently than previously thought," said geologist Alan Whittington of the University of Missouri. "We applied our findings to computer models that predict what happens to rocks when they get buried and heat up in mountain belts, such as the Himalayas today or the Black Hills in South Dakota in the geologic past.

"We found that strain heating, caused by tectonic movements during mountain belt formation, easily triggers crustal melting."

In the study, the researchers used a laser-based technique to determine how long it took heat to conduct through different rock samples. In all their samples, thermal diffusivity, or how well a material conducts heat, decreased rapidly with increasing temperatures.

The thermal diffusivity of hot rocks and magmas was half that of what had been previously assumed.

"Most crustal melting on Earth comes from intrusions of hot basaltic magma from the Earth's mantle," said Peter Nabelek, also a geologist at the University of Missouri. "The problem is that during continental collisions, we don't see intrusions of basaltic magma into continental crust."

These experiments suggest that because of low thermal diffusivity, strain heating is much faster and more efficient. Once rocks get heated, they stay hotter for much longer, Nabelek said.

The processes take millions of years to happen, and scientists can only simulate them on a computer. The new data will allow them to create computer models that more accurately represent processes that occur during continental collisions.

The Nature paper, "Temperature-dependent thermal diffusivity of the Earth’s crust and implications for magmatism," was co-authored by Whittington, Nabelek and Anne Hofmeister, a scientist at Washington University.

Text above is courtesy of the National Science Foundation

The granite rocks that make up Mt. Rushmore crystallized from magma that formed 1.7 billion years ago. Click on image for full size (291 Kb) Courtesy of Peter Nabelek, University of Missouri

Melting rocks doesn’t sound like an easy job. It usually takes the scorching heat found in the Earth’s interior to make it happen. But now scientists have discovered that Earth's crust melts more easily than they thought.

Geologists found that as rocks get hotter in Earth's crust, they become better insulators and poorer conductors. That means they hold onto heat and don’t easily transfer it to nearby rocks. This research is helping scientists understand a bit more about how rocks melt as plate tectonics causes continents to collide and mountains to form.

Usually, the rocks of the Earth’s crust melt when they come in contact with hot molten magma from deep in the mantle. But geologists know that crust rocks are able to melt without the help of magma from the mantle - they melt when continents collide and mountains are built too. Could the temperatures in the crust get hot enough for rocks to melt where continents are slowly smashing into each other?

To test this, the scientists heated different types of rocks with lasers. They measured how long it took heat to conduct through different types of rock.  The scientists noticed that as the heat was increased, the rocks lost some of their ability to conduct. They found that rocks heat up more efficiently than they thought.

In places where continents are colliding and mountains are forming, the strain heats the rocks. These experiments suggest that once rocks get heated, they stay hotter for much longer. It takes millions of years to happen, so scientists simulate the process with computer models.

"We applied our findings to computer models that predict what happens to rocks when they get buried and heat up in mountain belts, such as the Himalayas today or the Black Hills in South Dakota in the geologic past,” says geologist Alan Whittington.

The granite rocks that make up Mt. Rushmore crystallized from magma that formed 1.7 billion years ago. Click on image for full size (291 Kb) Courtesy of Peter Nabelek, University of Missouri

In places where continents are slowly smashing into each other and mountains are growing, could it get hot enough in the Earth’s crust for rocks to melt?

A team of scientists looked at this question. They discovered that the rocks of the Earth's crust melt more easily than they thought. They found that as rocks get hotter, they hold onto heat and don’t easily let it go.

We know that rocks melt when continents collide and mountains are built, because there is evidence preserved in the rocks. But scientists wanted to know how this worked. They heated rocks with lasers and measured how long it took heat to conduct through different types of rock.  The scientists noticed that as the heat was increased, the rocks lost some of their ability to conduct. The rocks held onto the heat.

In places where continents are colliding and mountains are forming, the strain of all that moving rock makes heat. This experiment suggests that once the rocks are heated, they stay hot for much longer.

The scientists put their findings into computer models that predict what happens to rocks when they get buried and heat up as mountains form. Hopefully this research will help make the models more accurate.