Almost anywhere you go in the United States, you hear people talk about the glaciers and how they created the landscapes around us. Geologists and historians explore the landscape, pointing out river basins, hills, and valleys, commenting on the role the glaciers’ played in their formation. We read about them in books; pictures of great walls and rivers of ice unfurl their majesty across full-color pages. These giants contain most of the fresh water on Earth (Brown, et al.), are present on every continent except Australia, and their gradual change in size is a great indicator of changes in global climates (Post and LaChapelle).
But what is a glacier exactly, and how does one form? How can something so large—and which does not appear to be moving at all—create such dynamic landscapes?
A glacier is body of dense ice formed from long-term accumulation of snow. This snow builds up faster than it is able to melt or sublimate (evaporate back into the atmosphere without melting), and thus accumulates over many centuries, becoming heavier and heavier. It becomes so heavy that it moves under its own weight, slowly sliding downhill under the force of gravity, or across a thin layer of water created by water from ice melting at its base.
A glacier is body of dense ice formed from long-term accumulation of snow. This snow builds up faster than it is able to melt or sublimate (evaporate back into the atmosphere without melting), and thus accumulates over many centuries, becoming heavier and heavier. It becomes so heavy that it moves under its own weight, slowly sliding downhill under the force of gravity, or across a thin layer of water created by water from ice melting at its base.
As the glacier moves along the landscape, it pushes up rock from the terrain under it. It does this when some of the water melts, flows into the small cracks in the rock below, and then freezes again. When water freezes it expands (see the answer to “Why does ice float?”), and acts as a lever to pry up pieces of this rock, which then begin to move along with the glacier as it travels. This is called “plucking”, because the rock was “plucked” from the bedrock. Some pieces of rock may travel hundreds of miles!
As glaciers continue to move, carrying their load of plucked rocks and boulders, they also scratch and scrape the landscape underneath, grinding and polishing the bedrock. The result is called “rock flour”, and it is made up of tiny rock particles smaller in diameter than a human hair. This is called “abrasion”, and is the other major way in which the glaciers shape and define our landscapes. This abrasion also causes striations, or gouges in the landscape, along the path of the glacier, allowing geologists to map the progress of the glacier.
Through the plucking and abrasion of the bedrock below, the glaciers produce a variety of different landscape features. The various hill formations include moraines, drumlins, and roche moutonnée.
Moraines: Linear mounds of till, an assortment of rocks and boulders surrounded by fine rock flour. These may appear at the front of the glacier (terminal or end moraines), or at the sides of the glacier (lateral moraines).
Moraines: Linear mounds of till, an assortment of rocks and boulders surrounded by fine rock flour. These may appear at the front of the glacier (terminal or end moraines), or at the sides of the glacier (lateral moraines).
Drumlins: Elongated hills made of glacial till, resembling the shape of an elongated bullet or a thin, rounded wedge.
Roche moutonnée (or “sheep back”): A rock formation polished smooth, shallow slope on one side, and having a sharp cliff face on the other side. These are formed when a glacier passes over a rock formation, polishing it as it moves up one side of the formation. As the glacier tries to move down the other side of the formation, it plucks away rock, creating the sharp face.
Glaciers also change landscapes by moving through valleys, rounding, widening, deepening, and smoothing their features. Due to this polishing, a glacial valley will have pointed, triangular cliff faces from the sides of the adjacent mountains, called truncated spurs.
The points at which glaciers form prior to flowing into a valley are three-sided bowl-shaped cirques, where the snow begins to accumulate to form the valley glacier. Two or more glacial cirques may form side by side, forming a sharp arête in the middle. If many cirques encircle a mountain, they form sharp peaks at the top of the mountain, called horns.
The points at which glaciers form prior to flowing into a valley are three-sided bowl-shaped cirques, where the snow begins to accumulate to form the valley glacier. Two or more glacial cirques may form side by side, forming a sharp arête in the middle. If many cirques encircle a mountain, they form sharp peaks at the top of the mountain, called horns.
As temperatures rise, and the glaciers begin to melt, they eventually start to recede. The melting glacier will leave deposits of all the till and debris it has accumulated, leaving hills and mounds of sediments. Hills and mounds that form as a glacier recedes are called kames, while elongated deposits are called eskers.
TRY MAKING YOUR OWN GLACIER!*
*Procedures for this activity were provided by Ms. Betsy Watts from Hope, North Dakota. Thank you to Ms. Watts!
While we don’t have thousands of years to wait for a glacier to form, we can simulate one using a plastic cup, some sand, and a few rocks. We can simulate the landscape too, using a baking sheet and modeling clay!
Here’s what you’ll need:
1. 2-3 plastic cups
2. Sand, about half a cup or so
3. Pebbles of different sizes
4. 3-4 rocks, about 1 inch in diameter
5. A baking sheet (one you don’t want to use again!)
6. Enough modeling clay to cover the bottom of the baking sheet, preferably of a pale, muted color
7. Water from any faucet
Here’s what you need to do:
Put some sand, pebbles, and a few rocks in the bottoms of your cups. Add enough water to just cover the rocks—a little more than one inch in the bottom of the cup. Place these cups in the freezer overnight.
Meanwhile, press your clay into the bottom of your baking sheet. This clay will become your glacial landscape.
After sufficient time for the water to freeze, remove your cups from the freezer, and cut the plastic away to release the ice and debris. These are now your “glaciers”.
Place one of your glaciers, debris side down, onto the clay on one end of the baking sheet. Apply gentle pressure with your hand, and slowly push the glacier IN A SINGLE DIRECTION along the clay. You will see striations form in the clay from the debris, and you will push up ribbons of the clay along the front and sides of the glacier.
When you get to the other end of the baking sheet, allow the glacier to remain there and begin to melt. Repeat the process with the other glaciers you made. Because no two glaciers are exactly alike, you will make different formations each time.
After all your glaciers have melted, take a look at your new landscape and try to identify some of the formations. Where are the moraines? Drumlins? Look at the difference between where your glacier passed, and where it melted. Where are there more deposits—that is, where did your rocks and pebbles end up? REMEMBER TO WRITE DOWN ALL YOUR OBSERVATIONS IN A NOTEBOOK!
References for further research:
1) Brown, Molly Elizabeth; Ouyang, Hua; Habib, Shahid; Shrestha, Basanta; Shrestha, Mandira; Panday, Prajjwal; Tzortziou, Maria; Policelli, Frederick; Artan, Guleid; Giriraj, Amarnath; Bajracharya, Sagar R.; Racoviteanu, Adina. "HIMALA: Climate Impacts on Glaciers, Snow, and Hydrology in the Himalayan Region". Mountain Research and Development. International Mountain Society. Retrieved 16 September 2011
2) Post, Austin; LaChapelle, Edward R (2000). Glacier ice. Seattle, WA: University of Washington Press. ISBN 0-295-97910-0.
3) Glacier. Wikipedia. 2014, Feb 28. Retrieved 3-2-2014. (http://en.wikipedia.org/wiki/Glacier)
Licenses:
Wikipedia: Creative Commons Attribution-Share Alike 2.5 Unported license
Wikipedia: Creative Commons Attribution-Share Alike 3.0 Unported license
*Procedures for this activity were provided by Ms. Betsy Watts from Hope, North Dakota. Thank you to Ms. Watts!
While we don’t have thousands of years to wait for a glacier to form, we can simulate one using a plastic cup, some sand, and a few rocks. We can simulate the landscape too, using a baking sheet and modeling clay!
Here’s what you’ll need:
1. 2-3 plastic cups
2. Sand, about half a cup or so
3. Pebbles of different sizes
4. 3-4 rocks, about 1 inch in diameter
5. A baking sheet (one you don’t want to use again!)
6. Enough modeling clay to cover the bottom of the baking sheet, preferably of a pale, muted color
7. Water from any faucet
Here’s what you need to do:
Put some sand, pebbles, and a few rocks in the bottoms of your cups. Add enough water to just cover the rocks—a little more than one inch in the bottom of the cup. Place these cups in the freezer overnight.
Meanwhile, press your clay into the bottom of your baking sheet. This clay will become your glacial landscape.
After sufficient time for the water to freeze, remove your cups from the freezer, and cut the plastic away to release the ice and debris. These are now your “glaciers”.
Place one of your glaciers, debris side down, onto the clay on one end of the baking sheet. Apply gentle pressure with your hand, and slowly push the glacier IN A SINGLE DIRECTION along the clay. You will see striations form in the clay from the debris, and you will push up ribbons of the clay along the front and sides of the glacier.
When you get to the other end of the baking sheet, allow the glacier to remain there and begin to melt. Repeat the process with the other glaciers you made. Because no two glaciers are exactly alike, you will make different formations each time.
After all your glaciers have melted, take a look at your new landscape and try to identify some of the formations. Where are the moraines? Drumlins? Look at the difference between where your glacier passed, and where it melted. Where are there more deposits—that is, where did your rocks and pebbles end up? REMEMBER TO WRITE DOWN ALL YOUR OBSERVATIONS IN A NOTEBOOK!
References for further research:
1) Brown, Molly Elizabeth; Ouyang, Hua; Habib, Shahid; Shrestha, Basanta; Shrestha, Mandira; Panday, Prajjwal; Tzortziou, Maria; Policelli, Frederick; Artan, Guleid; Giriraj, Amarnath; Bajracharya, Sagar R.; Racoviteanu, Adina. "HIMALA: Climate Impacts on Glaciers, Snow, and Hydrology in the Himalayan Region". Mountain Research and Development. International Mountain Society. Retrieved 16 September 2011
2) Post, Austin; LaChapelle, Edward R (2000). Glacier ice. Seattle, WA: University of Washington Press. ISBN 0-295-97910-0.
3) Glacier. Wikipedia. 2014, Feb 28. Retrieved 3-2-2014. (http://en.wikipedia.org/wiki/Glacier)
Licenses:
Wikipedia: Creative Commons Attribution-Share Alike 2.5 Unported license
Wikipedia: Creative Commons Attribution-Share Alike 3.0 Unported license
