What’s a glacier and what makes them so cool? Questions I finally got answered in Alaska this summer.
I first saw a glacier on a holiday in France five years ago. Staring up at the huge icy tongue of Glacier Blanc, I nodded and smiled vacantly without really understanding the explanation on glacier formation.
How was the glacier constantly moving forward? It didn’t look like it was going anywhere to me! And if it was extending, how was it also shrinking?
I was face-to-face with all of that confusion again on my recent trip to Alaska, where glacier’s loom into view almost everywhere you turn. Luckily there was a great national park guide and bus tour driver on hand to explain my queries away, and now I’m taking this opportunity to share what I learnt.
Making of a glacier
A glacier isn’t normal ice. The key difference is that glacial ice is very heavy and very old – the heavy-weight grandfather of ice formation.
Old, because it never melted. Heavy, because over the hundreds of years it takes a glacier to form snow was compressed, squeezing out all of the air to become super dense ice. A little like when you vacuum pack clothes, remove the air and you get a densely packed set of clothes.
In the visitor center at Exit Glacier they demonstrate the dense-ness of glacial ice by getting you to feel the weight difference between three cubes of the same size. One represents snow, another normal freezer made ice and the third is glacial ice. The block representing glacial ice, feels like picking up a heavy dumbbell in the gym.
Squashing down that snow over hundreds of years doesn’t start off in the mountain valley’s we see the glaciers in. Glacier formation occurs way up in the mountain peaks.
In the beginning…snow falls at the top of a mountain. It’s 90% air at this point, light and fluffy. At high enough elevations the snow doesn’t melt and becomes ice. More snow falls on top and pushes it down, it is this repeated snow fall that squeezes out the air.
Up in the frosty climes of mountain ranges, compression of ice goes on for years. And years. And years. Until it turns into 100% thick, heavy glacial ice.
But that’s up top of the mountain, how does it form the glacier down in the valley?
Ice that moves
The sheer weight of the ice pushes it down off the mountain, to advance in-between peaks and form individual glaciers.
All the glaciers in Kenai Fjords National park in the southwest of Alaska started off in the mountains in an area called the Harding ice-field. A number of glacier tongues can be seen running off the ice-field, some all the way into the sea.
Shaping the world
The slow and heavy movement of glaciers have a powerful effect on the land. Rocks beneath the glacier are crushed and ground up – eroding the landscape. In the ice-ages huge areas of the world were covered in glacial ice, making glaciers crucial in shaping the earth as we know it.
A key sign of previous glaciers are U shaped valleys. Unlike V shaped valleys formed by rivers, U shaped valleys are rounded out by a glacier eroding the sides as much as the base.
Another sign of a glacial history include multiple mini-lakes, known as kettle ponds. These ponds are formed when a chunk of glacial ice is left behind by a retreating glacier, the ice creates a dent that becomes a pond.
Retreating glacier? I’ll save that for Part 2.
Science FYI 