NASA researchers have shown that the icy Greenland as we know it today wasn’t all that cold in the past by mapping the geothermal heat flux, or rate of heat escape, in Greenland. Their models, surprisingly, showed regional variations, plus a path of heat along a peculiar route from the northwest to the southeast of the island.
Dr. Yasmina M. Martos, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, mined publicly available magnetic field, gravity and other geologic information for clues about the amount and distribution of heat beneath the portion of the North American continent that is Greenland. The findings expose a thermal track beneath Greenland that records the movement of a continent through Earth’s history.
Scientists believe that Greenland wasn’t in its place as we know it today, but slowly moved over a mantle plume, a source of great heat, which left a diagonal scar of warm, dense rock below the surface as the tectonic plate shifted. Greenland moved from a more southern latitude toward the Arctic over 100 million years, a period when the supercontinent Pangaea was breaking up into the drifting continents of today. Eventually, the plume is thought to have formed Iceland above the surface of the ocean through countless volcanic eruptions – a visible trace of the plume’s existence, in contrast to Greenland’s hidden scar.
The team suggests the scar was created as the tectonic plate, which includes Greenland, moved through the millennia over a mantle plume that is active below the lithosphere. The lithosphere is Earth’s outer layer; it includes the crust and upper part of the mantle. This plume is a channel of hot rock that starts hundreds of kilometers below the surface. It rises through the mantle and reaches the bottom of the lithosphere. The heat is then transported up through the lithosphere and alters its chemical composition, which thickens the crust. Because the northwest region of Greenland moved off the plume earlier it appears in Martos’ models to be significantly cooler than the southeast. Though the southern region is slowly cooling off.
A similar plume formed the Hawaiian Islands and is currently fueling the K?lauea volcano eruptions. The Hawaiian chain of islands and seamounts that were created when the Pacific Plate moved over the plume in the middle of the Pacific Ocean is a visible representation of the type of scar that Martos found beneath Greenland.
Because Greenland is covered by an ice sheet that is up to 1.8 miles (3 kilometers) thick in the center, getting physical samples from the ground below the ice is nearly as difficult as getting them from the Moon. Remotely sensed data offers virtually the only window to Greenland’s subsurface dynamics. Martos’s team decided to look at magnetic field information collected by magnetometers, instruments flown by airplanes that measure the strength of the magnetic field of Earth. The data revealed anomalies in the magnetism of rocks below Greenland.
Magnetism is related to temperature, thus rocks heated to certain temperatures lose their magnetism. This typically happens deep inside Earth. Because magnetite is the most abundant magnetic mineral in the lower part of the crust, the researchers studied that mineral exclusively. Magnetite loses its ferromagnetic properties, or magnetism, when heated to 1,076 degrees Fahrenheit (580 degrees Celsius), a point known as the Curie temperature. Accounting for this temperature’s effect on magnetite allowed the team to find the base of magnetism in the crust of Greenland. From there, they observed the depth variations of the location of the Curie temperature for magnetite to map the heat released all over the island.
Along the plume’s path, the team found that the Curie temperature occurred closer to the surface. This offered evidence that the plume had heated the bottom of the lithosphere, and that the heat was still there. The team also used gravity data to model the features of the lithosphere and confirm the plume’s effect on crustal thickness.
In the central part of the island, the team estimated geothermal heat flux values around 60 to 70 milliwatts per meter squared, or up to 50 percent higher than the heat escaping parts of the island not affected by the plume. This is a tiny amount; a 100-watt light bulb, by comparison, generates three orders of magnitude – or 1,000 times – more heat.