I'm here with Dr. Simon Brocklehurst at the University of Manchester. He's a senior lecturer in Earth surface processes. So Simon, we're talking about glaciers and how they affect the Earth's surface. How does that work? What are the glaciers doing to change the structure of the Earth's surface? Glaciers erode very differently from rivers and have a very distinct signal in the landscape. So what we can see in this image here is the Tasman glacier in New Zealand, and in foreground there's a very conspicuous lake which is the Tasman lake. It's only about 50 years old. And along the side of the lake you can see the glacial moraine which is all material that the glacier has deposited within that or preceding that time frame. And that's this material all remaining right there? Exactly. We have moraines down the two sides and then you can see lots of material being transported on the top of the glacier in the background. So that's actually the glacier right here? Yes, it's extremely dirty. OK. In the far distance, you can see some of the more snow covered surface of the glacier but low down is transporting this material on top of the ice. That should look extremely grubby and somehow unimpressive. The key things to take from this is, the very short time scale involved. The last glacial maximum when ice was at its greatest extent, something like 15,000 years ago, this glacier was 40 kilometers longer extended far down valley. It's only been terminating in this area in the last 100 years or something like that. So these giant piles of debris have only accumulated in a relatively short time frame. And this debri, this rock and soil would have originated from up here in these peaks? Yes. It's a combination. Some of it is the material that has just fallen off the peaks to the side, and some of it is material that the glacier has picked up from below. And so the lake illustrates a key distinction between rivers and glaciers, so rivers always have to flow downhill. Glaciers can actually pick up material, and what's called over-deepen, at the bed of the glacier. They can actually scoop out and create a hole which rivers can't do. Tasman lake here is a fine example of a product of glacial erosion. A very distinct landscape from what we see when there are just rivers. And so, what does the shape then of the valley look like after it's undergone glaciation? Yes, the cross-section is one of the key distinctions between rivers and glaciers. Rivers tend to carve a V-shaped cross-section, and as you can see here, we have something approaching more of a U-shape. So, very broad in the bottom of the valley between the hillsides to either side. That's very distinctive of glacier landscape. Even though the ice is moving relatively slowly, so it may be only a few hundred meters a year, over time it has a really profound impact on the landscape. So, that's what these glaciers are doing locally to the terrain? If we look at it from a global perspective, there's this thing called the glacial buzzsaw hypothesis. What exactly is that and what does it say about the way the glaciers and ice sheets are controlling the shape, the evolution of the Earth's surface? Well, if we look at topography of glacier into mountains we see something very interesting. This figure here illustrates the topography as we go from the North Pole to the South Pole down the West side of the Americas. We have three lines on here. The red line is the modern snowline and the green line is the snowline of the Last Glacial Maximum which we discussed before. So snowline being the height of the zero degree Celsius isotherm? Yes. It's the lowest elevation that preserves snow through the winter. That's one good definition for that. It happens to coincide with why glaciers tend to erode most efficiently. What we see in the more wiggly line is topography along the same transect and hopefully you'll agree that there is some degree of coincidence between these lines which is a very intriguing pattern. It could be that's purely coincidence, our interview would be rather short and not interesting if that were the case. Alternatively, it could be that the topography in some way controls the snowline which is one way around, or it could be that the snowline through glacial erosion is controlling the topography. And that's the essence of this glacial buzzsaw hypothesis. The idea of a very efficient glacial erosion at the snowline means that a large proportion of the landscape is leveled off in these large U-shaped cross-section valleys. So a very high proportion of the landscape is somewhere like the snowline, having been lowered from somewhere higher by glacial erosion. And then these peaks are just poking above in a very narrow area or very small volume of boxes. Yes, exactly. We have broad valley floors, then we have steep hill slopes, and occasionally these rise all the way up to very sharp pointed mountains. So, this showing example here is Denali McKinley, the Alaska erode. Simon, as we look at this graph, we see this close association in general, but there are places for instance down here and up here where there are large sections of terrain that don't fit this model. What's going on there? At the equator, we just don't happen to have high mountains so there is no way for glaciers to form and the topography has just been no snowline. As we get towards the southernmost extent of the Andes, we reach a threshold in how the glaciers are behaving. It may sound a little surprising but most glaciers actually have liquid water at the bed. And that's extremely important for the dynamics of the ice. So this thin layer of water at the bed allows the ice to be lubricated and flow relatively rapidly although it's still only a few hundred meters a year and that allows efficient erosion. And that water comes from just the weight of the ice on the surface or? Yes. It's probably to do with the pressure of the ice and it's partly to do with this thick part of ice effectively insulating the bed of the glacier and so it retains geothermal heat better. For both of those reasons, we have this thin layer of water, the ice move relatively quickly and relatively efficiently. If we go to very high latitudes or high altitude, it's sufficiently cold that we don't have this liquid water present. We just have ice, it's frozen to the bed. The only definition of the ice is internal deformation and with the lack of slip at the bed, there is essentially no erosion by the ice. And so that's where we would not see the glacial buzzsaw hypothesis at the valley? Yes, exactly. This is what we're seeing in Patagonia. We're so far south at this point, for the ice is essentially frozen to the bed, not moving and when it's not moving it's not going to erode. And we see that a little bit here where we get about 60 degrees north. In the northern hemisphere we also see that as well. Yes. We see some are the same, and as I say also this exception of the very high peaks so this has always been an exception to the hypothesis. Every now and again just the geometry of different glaciers and that has allowed very tall mountains to form in between them. I think what is interesting about this is it shows perhaps a strong relationship between the lithosphere, the solid Earth, the cryosphere and in a way that perhaps we hadn't really considered before. I mean, all of the parts of the Earth's climate system are interacting and this is one way that the glaciers through the terrain and through the climate themselves will work in large ice sheets form are controlling the very landscape that they exist on. Yes, it's very striking. Our traditional view of how mountain ranges work is that, the mountain range is controlled by tectonics. So we have convergent, we have collision and we form mountain belts along these beds of collision, and the height of the mountains is governed by the rates of the tectonics. And traditionally, the GM of origin, the surface processes have largely been seen as a nice decoration on top creating some pretty landscapes but not really influencing the processes of the lithosphere as a whole. And what we see through the glacial buzzsaw, particularly if we go to places like the Himalayan chain, the rates of the tectonics are some of the fastest on earth, but even so the glaciers seem to control the topography. You can go across them, very striking gradients and uplift rate and its top peaks always the same because of the key climatic influence. So the climate doesn't change while the tectonics does, and actually the landscape doesn't change in response to the tectonics. What the glaciers are doing it's really controlling. So this concept then of the glacial buzzsaw has actually been around for maybe about 40 years or so it's relatively new hypothesis. It is. The figure, it was originally drafted in the '70s but I don't think the significance was fully appreciated until the late '90s. So that's when Nick Boroswitz, Doug Burbank and Andrew Meeks took similar concepts to the Himalayan chain and new digital topographic data convincingly showed that where the tectonics varied actually the landscape was very much similar and it's controlled more by the climate and the glacier erosion than it was by some of the most dramatic tectonic plates that we have on earth. It's fascinating. It sounds like we've got a lot to learn yet still about the way that glaciers interact with the terrain and vice versa. Yes. So this hypothesis has certainly gained a lot of attraction. And a lot of places around the world, it seems to work very well. As I say, there's some exceptions in terms of height peaks but other than that much of the landscape does seem to follow trends in the snowline. Excellent. Thanks so much for your time Simon. It's been a pleasure. Thank you.
