Let’s face it, mountains are awesome. They’re awesome to look at and even more awesome to climb. There are few things greater in life than climbing to the top of a mountain and looking down at the vast openness knowing you just matched the mountain’s awesomeness by climbing it. But what forces are responsible for controlling the elevation of mountains?
In a recent study published in Nature, scientists have come up with a new classification scheme for mountain belts that uses just a single number to describe whether the elevation of the mountain belt is controlled mainly by weathering and erosion or by properties of the Earth's crust, i.e., the lithospheric strength. This new classification scheme is known as the “Beaumont number” or Bm, and is named after Chris Beaumont, a scientist who, together with his team, developed coupled models of surface processes and tectonic forces.
This new classification states that a Bm number between 0.4 and 0.5 means that the mountains are in a so-called flux steady state in which the controlling factors of mountain growth are tectonic forces and the lithospheric strength, balanced by weathering processes as, for example, in Taiwan. With a Bm value lower than 0.4, mountains are also in a flux steady state but with erosion as controlling factor like the Southern Alps of New Zealand. A Beaumont number above 0.5 means that the mountains still grow (non-steady state) with lithospheric strength controlling the process. Examples for this type are the Himalaya-Tibet mountains and the Central Andes.
This classification is resolving a long-standing question whether tectonic forces and strength of the Earth's crust are the controlling factors of mountain elevation or weathering processes. The new study says it can be one or the other -- depending on geographic location, climate, and underground properties.
The team of scientists led by Sebastian G. Wolf of Bergen University in Norway used a new coupled surface process and mantle-scale tectonic model for their study by combining the thermomechanical tectonic model FANTOM with the landscape evolution model FastScape. Thus, they were able to reconcile high erosion rates in some active orogens with long-term survival of mountain belts for hundreds of millions of years.
Putting observations into numbers helps us understand the world better, and this new classification scheme will undoubtedly help us understand mountains and their geological processes that much better, as well. Will we be able to use this type of classification scheme on other worlds? Only time will tell, and this is why we science!
As always, keep doing science & keep looking up!