Nib asked
How do glaciers carve out valleys?
Ice is soft, but a glacier is a slow river of ice studded with stones: the stones grind the bed like sandpaper and the ice rips off blocks, and because ice fills the whole valley it widens a river's V into a U.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
Glacial valleys such as Yosemite Valley in California have flat floors and walls that rise almost straight up, hundreds of metres. During the ice ages a glacier filled the valley and shaped it. The cross-section is a U rather than the V of a river valley.
Glaciers leave wide U-shaped valleys with steep walls; rivers leave narrow V shapes.
2ice flows
A glacier is thick, heavy ice that slowly deforms and slides downhill under its own weight, typically a few metres to a few hundred metres a year. By itself ice is much softer than rock, so a smooth bulldozer blade of ice would not cut a valley; the cutting depends on what the ice carries.
A glacier is a slow river of ice that flows under its own weight, and ice alone is too soft to cut rock.
a common mix-upGlaciers do not carve like bulldozers: ice is softer than rock, and it cuts only with the stones it carries.
3river: V
A river flows in a narrow channel and cuts down into its bed. The valley walls above it slump and slide inward, so the cross-section is a V, narrow at the bottom and wide at the top.
A river cuts downward, so its valley is a narrow V.
4glacier: U
A glacier does not follow a narrow channel. It fills the valley, pressing on both walls and the floor, and grinds all of them back. The old V is widened and deepened into a U with steep walls and a flat floor.
Ice fills the whole valley and wears walls and floor, turning a V into a U.
Why does a glacier leave a U shape but a river a V?
Show the answer
A river cuts only along its narrow bed, while a glacier fills the valley and wears both walls and floor.
5ice sandpaper
Ice picks up rock fragments from the valley floor, and they freeze into the bottom of the glacier. As the glacier slides, these stones are pressed hard against the bedrock by the weight of the ice above, hundreds of tonnes on every square metre, so the base works like a huge sheet of sandpaper.
Stones frozen into the base of a flowing glacier scrape the rock bed like sandpaper.
6abrasion
This grinding is called abrasion. It cuts parallel scratches, called striations, into the bedrock and grinds some of the rock down to a fine powder called rock flour. Rock flour suspended in meltwater is what makes glacial lakes and streams look milky turquoise.
Abrasion scratches grooves into rock and grinds it to rock flour.
What makes a glacier able to scratch rock when ice itself is soft?
Show the answer
Stones frozen into the ice base do the scratching, pressed down by the weight of the ice.
7plucking
Plucking, also called quarrying, works on cracked rock. Meltwater trickles into joints in the bedrock and freezes, gripping the loose block. When the glacier moves, it pulls the block out and carries it along, adding more tools for abrasion. Together, plucking and abrasion deepen and widen the valley.
Plucking: meltwater freezes in cracks and the moving ice tears blocks from the bed.
How does plucking differ from abrasion?
Show the answer
Plucking tears whole blocks out of cracked rock; abrasion scratches and grinds the surface.
8the whole idea
The whole story: a glacier is a slow river of ice. Stones frozen into its base grind the bedrock (abrasion), meltwater freezing in cracks lets it tear out blocks (plucking), and because it fills the valley it wears the walls and floor alike, so over thousands of years a river's V becomes a glacier's U.
Abrasion and plucking by flowing ice turn a V-shaped river valley into a U over thousands of years.
Ice is soft, but a river of ice dragging stones is the slowest, strongest chisel on Earth.