Scientists identify a deep anchor beneath the continents
What determines the level of the land? This is one of the fundamental questions about our planet, and Associate Professor Simon Lamb and his collaborators think they have found an answer.
Doing so will help scientists calculate how land levels will respond to the melting of ice sheets and sea level rise, as a consequence of global warming, as well as providing insights into changes in land area throughout the planet’s history.
“It has been long thought the key to this is the Earth’s crust,” says Associate Professor Lamb, from Te Herenga Waka—Victoria University of Wellington's School of Geography, Environment and Earth Science. “The crust floats on the underlying mantle like a rocky iceberg. And it has been thought that, like an iceberg with its underwater roots, the thicker the crust, the higher the land rises above sea level.
“But when we put together an accurate picture of the crust in the continents, we were surprised to see there is little relation between the average elevations of the continents and the thickness of the underlying crust.”
The reason lies in the fact the crust is just part of the Earth’s tectonic plates.
“We realised that beneath the crust the thick tectonic plates also have roots, and these act like an anchor, keeping the elevations relatively low even though the surface of the buoyant crust wants to rise up higher.”
Associate Professor Lamb’s international team—whose findings have just been published in the American journal G-cubed—made use of new measurements of the thickness of the tectonic plates. They also worked out the densities of the different rock layers from variations in the strength of gravity.
“It was clear the dense roots of the plates were capable of pulling down the surface of the Earth in exactly the way needed to explain the actual elevations,” says Associate Professor Lamb.
For example, Australia has a relatively thick crust and should be over a kilometre above sea level on average, but it is also part of a very thick tectonic plate. The vast weight of this plate counteracts the tendency for the thicker crust to rise up. The same principle applies to the other continents.
New Zealand is part of a young continent forming a thin tectonic plate and so most of it has hardly any ‘anchor’ to hold it down. With the same ‘anchor’ as Australia, New Zealand would be over 1km underwater.
The level of the land is determined by both the thickness of the crust and the tectonic plate, says Associate Professor Lamb. But over geological time, erosion wears away the crust, tending to keep the land near sea level, whatever the plate thickness.
“Antarctica, however, has been too cold for large rivers to significantly erode the landscape. This way, the crust has been ‘protected’ from the forces of erosion.
“Today, the weight of the vast East Antarctic ice sheet is pushing down the underlying bedrock. However, if all the ice melts—perhaps due to uncontrolled global warming—the surface of East Antarctica will bounce back over the following 10,000 years or so to form the highest continent of all, about 1km on average above sea level.”
The research is a collaboration between Te Herenga Waka—Victoria University of Wellington (Associate Professor Lamb and Professor Tim Stern, also from the School of Geography, Environment and Earth Sciences); Earth Observatory of Singapore, Nanyang Technological University of Singapore (Dr James D.P. Moore); and MARUM—Center for Marine Environmental Sciences, University of Bremen (Professor Marta Perez-Gussinye).
Associate Professor Lamb has written an article about the team’s research for The Conversation and it is available for free republication under Creative Commons.