How do glaciers move? Could Antarctica be separated?

Question

Consider the example of the Larsen Ice Shelf. Ice platforms are attached to glaciers such that they prevent surrounding warm air from directly melting the glaciers. This is an important aspect of ice platforms.

What are the mechanisms of glacier movement?

When we talk about glacier movement, do we consider advance and retreat due to melting and snow deposition, or is the study of glacier movement more about the fluid sheet underneath the glacier that comes in contact with the rocks, which allows the glacier to move downhill?

With this second type of movement, glaciers can separate. So is it possible for Antarctica to separate and disappear?

Answer 1

Glaciology is a large field of research, and I cannot include all aspects in a short answer. I'll try to give some concepts that can help you in your understanding of the cryosphere.

Glaciers are, per definition, moving due to gravity towards lower altitudes. There are some exceptions, e.g. rock glaciers are the stagnant remain of a moving glacier.

Large bodies of ice are called either ice caps (if smaller than 50,000 km$^2$) or ice sheets (if larger). Today, we only have the Antarctic ice sheet and Greenland, but during times of larger glaciation, large parts of the Northern Hemisphere were covered. Within the ice sheets, fast flowing areas are referred to as glaciers. Where those areas are formed depends on many parameters such as the mass balance, the basal melting, the topography, etc.

If the glacier or ice sheet reaches the ocean, it becomes buoyant. For larger glaciers, the section near the coast remains bulky enough to remain in contact with the ground. The point of extent where the ocean reaches under the ice is called the grounding line - a subglacial beach, if you like. As it's floating, tidal variations moving the ice sheets help it to break up, but in some cases, depending on thickness and topography, it can float in a rather uniform shape which we call an ice-shelf.

The same way as rivers and lakes are formed as rainwater floats towards the oceans, ice sheets and ice caps are formed as snow falls in cold areas, where it doesn't get warm enough to melt during the summer. As the snow accumulates it forms firn and later ice. The ice starts moving as the gravity and shear forces make it unstable.

Eventually the ice will melt, either on the surface due to warm air, e.g. during summer months, or in the oceans as it calves off into icebergs and is warmed by the ocean. Glaciers can also melt on the base due to heat flux from the Earth beneath or due to pressure. In this way, subglacial lakes are formed under the ice.

Large parts of Antarctica are situated below sea level. Partly this is due to the intensive erosion of fast moving ice, but the main reason is that the weight of the ice pushes down the crust into the mantle. If the ice sheet melts, the mass decreases and the land under it will rebound. This happens almost on a human timeline. Some areas, such as Hudson Bay, Scandinavia, and the Antarctic Peninsula, have a fast uplift making sea into land.

Maps from Quantarctica. 1: Ice flow velocity. 2: The topography under the ice. 3: Geoid height (not so relevant for your question). 4: Snow accumulation and blue (=old, hard) ice.

As I've suggested before, I recommend you to download the Quantarctica package. It's rather big but free and easy to use. You can investigate how the ice flows are forming the ice sheet and how the bed map looks under.

The large chunk of Larsen C Ice Shelf that just broke off, doesn't directly reshape the Antarctic continent or cause any change in global sea level, as it was already floating. However, if the ice shelf becomes more unstable, it can affect the ice sheet left trailing it and the flow pattern of the glaciers, and that would have a large impact on most human activities - and beyond.

Answer 2

Glaciers move by two processes:

1. Internal deformation
2. Basal sliding

This figure (made for my candidacy exam), exemplify both processes as if we were following a single block of ice within a glacier:

Basal sliding is when the entire glacier slides together over the bedrock, aided by the lubrication of ice water between the glacier and ground. Internal deformation is when the glacier moves by spreading out like honey on a table, but it can still be fix and frozen to the bed.

But it is important to point out that the glaciers are still always moving in the same direction (downhill), caused by gravity. Therefore glaciers always advance and tend to remain together. (When we say that a glacier recedes, it means that the position of the front edge recedes, not the ice itself. And that happens when the ice at the front melts faster than it it replaced by the advancing movement.)

Out of the two processes mentioned above, basal sliding is the main one in large glaciers and antarctic ice streams, but it is unable to work in cold-based glaciers (i.e. glaciers where the base is not above the melting point, and therefore glaciers frozen all the way to the bed and stuck to it) which are more common to Antarctica itself.

The picture you are proposing of a glacier splitting in two is a very rare occurrence. Because glaciers are very ductile and have good cohesion, it can only happen in rare occasions on very steep mountain glaciers, where the driving stress and basal sliding can be strong enough to take a large fraction on the glacier and send it rushing downhill (like in this event).

In Antarctica the glaciers are so big, and the terrain so flat that you can be sure that will never happen. Ice there will evolve more like a viscoelastic material - you can picture that by imagining how honey will move if you pour a jar of it over a table.

When people talk about Antarctica splitting in two they don't mean that the land will move apart, nor that the glaciers themselves will move in different directions and fracture. Instead it is about the loss of the ice; as ice creeps to the edges of the continent and is lost as icebergs, the snowfall inside the continent is not enough to replenish the ice lost. Therefore, the ice sheet over the continent is thinning. The land of Antarctica (without any ice) is made up of two separate big landmasses (East and West Antarctica) and a multitude of islands.

As the ice sheet thins, at some point sea water will progress towards the large areas with bases currently below sea level, the ice start to float, and will eventually completely break up. If that were to happen, Antarctica would become split into two sub-continents, and the ice loss would generate a catastrophic sea level rise.