A glacial highway

Astronaut Mike Hopkins tweeted this image from the International Space Station of a glacier in Patagonia and added “wonder what causes the streaks?”

That’s a question for geologists to answer, so we’re on the case. Want to know the answer? You’ve come to the right place, because the full answer is found below. ……

Those black streaks are what we call “medial moraines”. They’re formed as a normal result of the growth of glaciers in mountain valleys.

Mountain glaciers don’t just appear in single channels. When a glacier forms in a mountain valley, snow and ice is supplied from a variety of locations. Think of a normal mountain range; it’s full of channels between peaks. Those channels are carved by water and they carry that water down slope towards larger drainages, streams, and rivers.

When a glacier forms on this terrain, ice forms in the same channels that water carved. Each small channel basically forms its own glacier, and those glaciers migrate down the slopes until they join together in the biggest channels. A large mountain glacier is sometimes made up of dozens of smaller glaciers which fed ice and snow into the valley.

Alongside the edge of a glacier, there is typically a pile of rock debris known as a lateral moraine. The edges of glaciers can pick up or erode loose rock from the bedrock below, and often rocks from the surrounding hills will fall down onto the edge of glaciers as avalanches. The edges of glaciers are dirty places.

So what happens when 2 glaciers come together in a valley? Their edges meet and press together. All the dirt and rock trapped at the edges of those glaciers suddenly finds itself sandwiched in the middle of a larger, moving mass of ice.

That’s how glaciers form medial moraines. 2 mountain glaciers meet and their dirty, rock-filled edges get sandwiched together. In fact, if you look closely, at the far left side of the image, you can see this process actually happening, where 2 glaciers join and a medial moraine line forms between them.

Each streak you see in the middle of this glacier is the remnant of 2 smaller glaciers coming together. How many different valley glaciers came together to form an ice mass like this one? Good luck counting those lines, I’m not going to try.

-JBBImage credit: https://mobile.twitter.com/AstroIllini/status/407977471162605568/photo/1