What is the difference between basal slip and internal plastic flow?

Glacial Movement: It’s Not Just Ice, It’s a Whole Lot of Slipping and Sliding

Glaciers: those majestic rivers of ice that carve out our landscapes. We tend to think of them as static, frozen giants, but the truth is, they’re constantly on the move. This movement isn’t just one thing; it’s a combination of different processes, with basal slip and internal plastic flow being the big players. But what exactly are these processes, and how do they work? Let’s break it down in a way that makes sense, even if you’re not a glaciologist.

Internal Plastic Flow: The Inner Creep

Think of internal plastic flow as the glacier’s inner workings. It’s how the ice itself deforms and moves. Imagine a stack of pancakes, each representing a layer of ice crystals. Now, put a massive weight on top. The pancakes start to squish and slide against each other, right? That’s essentially what’s happening inside a glacier.

As snow piles up and compacts into ice, the pressure builds. And I mean serious pressure. When it hits a certain point, the ice crystals start to get all twisted and start sliding. It’s like they’re trying to find the path of least resistance, allowing the ice to flow, albeit slowly. We’re talking about a creep, not a sprint.

This internal deformation is more common in colder glaciers, the kind that are frozen solid to the bedrock below. In these “cold-based” glaciers, there’s not much sliding happening at the base, so the internal flow does most of the work. How fast does it go? Well, that depends on a few things: how thick the ice is, how warm it is (relatively speaking, of course), and whether there are any weak spots in the ice. Thicker, warmer ice tends to deform faster, but even then, we’re usually talking about movement measured in meters per year. Slow and steady wins the race, I guess.

Basal Slip: Sliding on a Waterbed

Now, let’s talk about basal slip. This is where the glacier gets a little help from a layer of meltwater at its base. Think of it as sliding on a giant waterbed. This meltwater comes from a couple of sources. First, there’s pressure melting. The sheer weight of the ice actually lowers the melting point of water, causing some of the ice at the bottom to turn into liquid. Second, there’s friction. As the ice grinds against the bedrock, it creates heat, which also melts some ice.

This thin layer of water acts like a lubricant, making it much easier for the glacier to slide over the rock beneath. The amount of meltwater, the slope of the ground, and how rough the bedrock is all play a role in how fast the glacier slides. A rougher bed can actually increase meltwater production (more friction!), but too much roughness can slow things down. It’s a delicate balance.

Basal slip is the main mode of transport for “temperate” or “warm-based” glaciers, where the ice at the bottom is right at the melting point. These glaciers can really move, sometimes several meters per day. That’s a huge difference compared to the slow creep of internal flow.

The Nitty-Gritty: Key Differences

Okay, let’s put it all together in a handy table:

FeatureBasal SlipInternal Plastic FlowWhat it isGlacier sliding on meltwaterIce crystals deforming and movingTemperatureTemperate glaciers (warm base)Cold-based glaciers (frozen to bedrock)Water’s RoleMeltwater is essentialWater doesn’t matter muchSpeedGenerally fasterGenerally slowerWhere it happensAt the base of the glacierThroughout the glacier