In East Greenland – some way from the coast – the mountaintops are flat and at the same height. From the air it looks pretty spectacular, as if someone grabbed a knife and cut the top off of everything. Geologists have long debated how these tall but flat landscapes were created. Now, geologists from GEUS and Uppsala University have presented a new analysis of what happened to the landscape in East Greenland.
PhD Johan Bonow from Uppsala University and ScD (dr.scient.) Peter Japsen from GEUS have mapped the so-called peneplains (technical term for the flat-topped mountains) along the coast of East Greenland. They have mapped, analysed and reinterpreted the emergence of the landscape, using amongst other things old aerial photos and newer satellite data.
The conclusion is that the tall but flat landscape must have been created in several stages by erosion and uplifts.
”Some people will tell you that the high-lying plateau was created by glaciers but here we document that the wide expanses that span more than 1,000 kilometres from North to South, were created long before the Ice Ages as an erosion surface near sea level. After that the land was raised twice, just ten and five million years ago, so that the plateau reached its present height at around two kilometres above sea level,” says Peter Japsen.
In other words, the mountains of East Greenland are relatively new – geologically speaking. And as if that is not enough, the studies show that the crystalline basement is not as static as we once thought.
”In the field of geology, it has long been a dogma that crystalline basement areas are stable. Now we can show that they move vertically, up and down. And everyone can see it for themselves on the old aerial photos that we are presenting,” he says.
Land uplift and new rivers
After the uplift ten million years ago, the rivers were able to cut their way into the flat landscape, which was at sea level until then. Thereby a whole new system of river valleys was created.
”These river valleys were then lifted after the latest land uplift five million years ago, and as a result, these ’fossilised’ river valleys are now at a height of around one kilometre while the active rivers were able to cut even deeper into the landscape,” says Peter Japsen. (See river valley formation in right side of illustration below).
The dating of these different phases of uplift and erosion is based on an advanced technique called appatite fission-track analysis. The microscopic tracks are created by spontaneous fission of uranium, which is found in the mineral appatite. The length of these tracks depends on the temperature underground and can therefore be used to decide when erosion leads to lower underground temperature when isolating cover layers are removed.
Johan Bonow has mapped both the upper and the lower level of the landscape, the plateau and the ’fossilised’ river valleys respectively (in the illustrations marked with blue and red respectively). The mapping shows that rivers, not glaciers, dominated the landscape just five million years ago, before the current glacial age began 2.7 million years ago.
According to the researchers, the reason why the landscape could not have been made by glaciers is amongst other things that many of the smaller valleys terminating at the lower level are V-shaped. If the valleys had been cut out by glaciers, they would be more of a U-shape, says Peter Japsen. The V-shaped valleys can be seen clearly on the old aerial photos.