The World's Foundations; or, Geology for Beginners · Ch. XXVI · Unit 4: Part III: Water at work today
29. Glaciers
Goal: Your child can explain how a glacier moves like a river, and show with ice in water how much of an iceberg is hidden below the surface.
You'll need
- The book, open to this chapter
- A tall clear glass of water and an ice cube (or a block of ice frozen in a cup)
- A tray, a squeeze bottle of honey or thick syrup, and a few sprinkles or seeds
- A marker
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter XXVI compares glaciers to rivers: faster in the middle and at the surface, bending round corners, with icefalls like waterfalls. It shows how much of an iceberg is under water, describes the glaciers of Mont Blanc and the Mer de Glace, the huge Greenland glaciers and the Humboldt glacier, icebergs carrying rocks, and the Greenland ice-foot.
The lesson, step by step
A question to start
SayWhen you see an iceberg, how much of it do you think is hidden under the water?
Write down the guess.
Read the chapter together
Read Chapter XXVI (pages 246 to 254). Look at the pictures of the floating iceberg and the Greenland glaciers. Stop on:
Whatever quantity of ice is seen to rise above the water in an iceberg, there is always about eight times as much below the water. You will understand this better if you float a small lump of ice in a basin, and notice what a small proportion of the whole lump remains out of the water.
Do the book's experiment
Float the ice in the glass of water. Mark the water line on the glass. How much sticks up? How much is below? Compare with the guess.
Middle and sides
Read:
In the ice of a glacier, as in the water of a river, the movement is quicker at the surface than down below, and quicker at the middle than at the sides.
Model it: squeeze a wide band of honey at the top of a tilted tray and drop a straight line of sprinkles across it. Watch for a few minutes. The line bends: the middle runs ahead, the edges drag. Ask why. (The sides rub on the "banks".)
Mont Blanc and Greenland
The book says ice from the top of the Mer de Glace takes about one hundred and twenty years to reach its end. Greenland is buried under ice, and its glaciers end in walls of ice in the sea, where huge pieces break off and become icebergs. Find Greenland and Mont Blanc on a map.
Icebergs carry rocks
An iceberg can carry rocks frozen into it far out to sea and drop them when it melts. The book adds:
The ocean-floor is not a mere flat plain, but consists, like the continents, of mountains, hills, table-lands, valleys, and lower levels.
Ask: If a boulder dropped on an undersea mountain were later lifted up, what would a geologist find?
Tell it back
Ask your child to explain, using the honey tray and the ice glass, two ways a glacier is like a river and one way it is different (it can even travel up a gentle slope).
What we know now
Glacier ice moves partly by slowly bending and flowing, like very stiff putty, and partly by sliding over a thin layer of meltwater at its base. Today most of the world's glaciers are shrinking, and the Mer de Glace is much shorter and thinner than when Giberne wrote. Her rule that about eight-ninths of an iceberg is under water is still right.
Afterwards
Notice it this week: watch ice cubes in a drink. Which part sticks up, and which part is hidden?
Tip
Honey is slow; give it several minutes, or warm the room. Syrup or dish soap works too.
Answers to the book's problems
About eight times as much ice is below the water as above (roughly eight-ninths of the whole). A glacier is like a river because it follows a valley, flows faster in the middle and at the surface, bends round corners and has icefalls; it is unlike a river because it can sometimes move up a gentle slope.
Worked out for this site from the scan. If a number in your copy differs, trust the book.