From Glacier Erosion to Glacier Bazzsaw

Before this paper, people already know glaciers are a highly efficient erosion agent, is able to curve thousands meters of fjords. This paper extend it further to buzzsaw hypothesis, which is powerful erosion at ELA, and is strong enough to limit the long-term tectonic uplift.

In order to prove this, Egholm provided a statistic evidence of terrain, which is called hypsometric distribution, and a result from numerical modelling.

What is Hypsometric

If we look into this plot, this elevation distribution(a) at two different regions. The one is fluvial landscape, the one is glacier landscape. The value that appears most often is hypsometric maxima. Fluvial landscape has very low hypsometric maxima, because eventually fluvial erosion will result plains close to sea level. For glacier landscape, the maximum elevation cannot over hypsometric maxima too much, because above maxima the slope is very steep.

What’s the Relationship between Hypsometric Maxima and Snowline?

If we plot hypsometric maxima over the world, we get the plot (b) in right. This color means the data from different continents.

This black line is modern snow line, and the dash line is LGM snowline. Basically this curve is a function of latitude and local elevation, which is not very accurate. For these areas still have glaciers, the red line from glaciers inventory is real snow line.

Please note, in between area is the area that may be covered by glaciers in history, which is also the workspace of the glacier buzzsaw.

This means, globally, the lower snowline, the lower hypsometric maxima. In other words, the hypsometric maxima can not exceed the snowline due to glacier buzzsaw, Egholm said this is a result independent to the rate of tectonic uplift, and lithology.

The Relationship between Elevation Maximum with Snowline?

The paper further appears that most summit elevations are confined to altitudes 1,500 m above the local snowline.

This suggests that glaciers concentrate surface area just below the snowline, which thereby acts like a local basiss level for glacial erosion, leaving only a limited amount of topography (horns and aretes) rising above this level.

Numerical Modelling Reproduced the Glacier Erosion Processes

Settings

  • Warm-based glacier erosion modelling.
  • Choose a study area in Spain, as an initial landscape, because it was little affected by Quaternary glaciation. Basically this is a fluvial landscape area, with only few cirque valleys near the highest summit.
  • In order to isolate the effects, the model do not include uplift, fluvial erosion and periglacial processes, So this just glacier erosion but include slope diffusion to avoid unrealistic slopes.

Modelling Results

After 500 kyr years, firstly glacier erosion significantly changed the elevation distribution. And the hypsometic maxima developed just below the snowline.

  • Above snowline: only steep ridges remain.
  • Below snowline: widened and deepened valley
  • It also produced typical glacier landscapes, aretes, cirque, hanging valleys.

Egholm also mentioned, his model also results in the low relief area, this is veryinterestingg, later the paper two will elaborate this more.

Zoom in and Look into the Slope Development

These typical glacier landforms, have suffered hundred to thousands meters erosion and around 100 m isostic uplift and left some relative less steep slopes.

  • This red line is original, the black line is after glacier erosion. The snowline is around 2000 meters.

The development of the slope is supported by another paper by Pedersen. He use global DEM to plot the elevation distribution in glacier landscape area, but also plot the slope. He concluded that landscapes near the local snowline altitude are indeed characterized by relatively low average slopes.

Short Summary

Main Findings:

The evidence and modelling indicate that glacial buzzsaw is capable of keeping pace with any modern tectonic uplift rate. 

Therefore, the relationship between tectonism and climate is such that climate and latitude ultimately control the height to which tectonic processes can drive topography.

Interesting Details:

Small glaciers have the most effective glacial buzzsaw at ELA, and further limit the maximum altitude. The large trunk glaciers reach far below the snowline cause deep incised valley instead. This is so called the variable influence of topography on ice flow.

Secondly, the modelling found the concentration of low relief surface occurs in the temperature window of 0 Celsius degree where periglacial processes are most effective. But, this modelling do include periglacial processes. So Egholm think, if frost weathering continue working on surface, this short-wavelength topography will plane off into smooth paleic surface.