Introduction: The Moving Giants

When you look at a photo of a glacier, it looks like a still, frozen statue. But don't be fooled! Glaciers are often called "rivers of ice" because they are constantly on the move. Gravity pulls them downhill, and as they move, they reshape the entire landscape. In this chapter, we will explore how they move and why some glaciers are speed demons while others move at a snail's pace.

Note: To understand this, it helps to remember "Mass Balance" from our previous chapter (2A.4). Glaciers move to transport ice from the top (accumulation zone) to the bottom (ablation zone).

1. Polar vs. Temperate Glaciers

Before we look at the "how," we need to know that not all glaciers are made equal. Their temperature determines how they behave. Geographers split them into two main types:

A. Temperate (Warm-based) Glaciers

These are found in "warmer" mountainous regions like the Alps or the Rockies.

  • The ice temperature is at or very close to the Pressure Melting Point (PMP).
  • What is PMP? It’s the temperature at which ice melts under pressure. Because the ice is so heavy, the bottom of the glacier can actually melt even if the temperature is slightly below \(0^\circ C\).
  • Result: There is a layer of liquid meltwater at the bottom, which acts like grease on a slide!

B. Polar (Cold-based) Glaciers

These are found in the extreme cold of Antarctica or Greenland.

  • The ice is well below the Pressure Melting Point.
  • The glacier is literally frozen to the bedrock.
  • Result: There is no meltwater at the base. These glaciers move much more slowly.

Quick Review: Think of a Temperate glacier as a bar of soap on a wet floor (slippery), and a Polar glacier as a block of ice frozen to a wooden table (stuck!).

2. How Do Glaciers Move?

There are two main "engines" that drive glacier movement: Basal Sliding and Internal Deformation.

A. Basal Sliding (The "Slip and Slide")

This is the primary way temperate glaciers move. Because there is meltwater at the base, the entire block of ice slides over the rock. This can happen in a few ways:

  • Slippage: The ice simply slides on the water film.
  • Regelation: When ice hits a small bump in the rock, pressure increases on the "upstream" side. This extra pressure melts the ice (reaching PMP). The water flows around the bump to the "downstream" side where pressure is lower, and it freezes again. This helps the glacier "inch" over obstacles.

B. Internal Deformation (The "Slow Ooze")

This happens in all glaciers, but it is the only way polar glaciers move. Since polar glaciers are frozen to the ground, they can't slide. Instead, the ice itself changes shape.

  • Under the massive weight of the ice, individual ice crystals begin to slide over each other or "creep."
  • Imagine a deck of cards. If you push the top cards, they slide over the bottom ones. The bottom card stays still (frozen to the rock), but the top ones move forward.

Did you know? Even though the bottom of a polar glacier is stuck, the top of the glacier can still move forward through internal deformation!

3. Rates of Movement: What Makes a Glacier Fast?

Glacier speeds vary wildly. Some move only a few centimetres a day, while others can "surge" several metres in a single day. Here are the factors that control the speed:

1. Altitude and Slope

Gravity is the driver! Steeper slopes mean faster movement. Glaciers on high, steep mountains move faster than those on flat plains.

2. Lithology (The Rock Beneath)

If the bedrock is "soft" or permeable, it might absorb water, slowing movement. If it is hard and smooth, the glacier slides more easily. If the rock is weak, it can even deform under the ice, helping the glacier move along.

3. Ice Thickness

Thicker ice is heavier. This creates more vertical pressure, which helps reach the Pressure Melting Point and increases internal deformation. Thick glaciers generally move faster than thin ones.

4. Temperature

As we saw earlier, warm-based glaciers move much faster because of basal sliding. Cold-based glaciers are the slowcoaches of the geography world.

5. Mass Balance

If a glacier has a very positive mass balance (lots of snow at the top), the extra weight pushes the ice down the valley faster.

Summary Table: Polar vs. Temperate

Feature Temperate (Warm-based) Polar (Cold-based)
Location Low latitudes (e.g., Alps) High latitudes (e.g., Antarctica)
Basal Temperature At or near PMP (\(0^\circ C\)) Below PMP (e.g., \(-20^\circ C\))
Main Movement Basal Sliding Internal Deformation
Speed Relatively Fast Very Slow

Common Mistakes to Avoid

  • Mistake: Thinking polar glaciers don't move at all because they are frozen to the rock.
    Correction: They still move via internal deformation; it's just very slow!
  • Mistake: Thinking ice melts at \(0^\circ C\) everywhere in a glacier.
    Correction: High pressure lowers the melting point. At the bottom of a deep glacier, ice can melt at temperatures like \(-1^\circ C\) or \(-2^\circ C\).

Key Takeaways

1. Basal sliding requires meltwater and is common in warm-based glaciers.
2. Internal deformation involves ice crystals shifting and happens in all glaciers.
3. The Pressure Melting Point (PMP) is the "magic number" that decides if there is water at the base to help the glacier slide.
4. Rates of movement depend on slope, thickness, temperature, and the type of rock underneath.