Lesson 27

27

Ice Ages & the Power of Glaciers

How can ice reshape an entire continent?

Glaciers form from accumulated snow and slowly reshape the land as they move. The lesson covers glacial erosion and deposits, repeated Ice Ages, and the changing sea levels that exposed migration routes such as Beringia.

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Key Ideas

  • Glaciers form when snow accumulates faster than it melts, compresses into ice, and begins to flow, slowly grinding and reshaping everything beneath them.
  • Glaciers reshape land through erosion (carving U-shaped valleys, smoothing bedrock) and deposition (leaving moraines, drumlins, and glacial erratics).
  • During Ice Ages, sea levels dropped by up to 120 meters as water was locked in ice, exposing land bridges like Beringia that allowed animals and humans to migrate between continents.
  • The world we see today, with its Great Lakes, fjords, Alpine valleys, and island nations, was largely shaped by ice. The borders on our maps are where sea level happens to be right now.
  • Earth's Ice Ages are driven by predictable cycles in its orbit (Milankovitch cycles) and have repeated many times. The last glacial period ended about 11,700 years ago.

Spines

  • DK's Science as You've Never Seen it Before: Climate Change pg. 128-129
  • Mammoth Science: Climate pg. 146, Seasons pg. 144
  • Visual Timelines: Life on Earth pg. 120-121, 110-111
  • Alternatives: Britannica Student Glaciers
  • Alternatives: Britannica Ice Ages

Vocabulary

  • Ice Age: A period of long-term global cooling when ice sheets expand across large parts of Earth's surface.
  • Glacier: A large slow-moving mass of ice formed from compacted snow; glaciers carve valleys and move enormous rocks.
  • Erosion: The wearing away of land by water, wind, or ice; glaciers are among the most powerful agents of erosion.
  • Land Bridge: Dry land connecting two landmasses, exposed when sea levels drop due to water locked in glaciers.
  • Beringia: The land bridge between Asia and North America (now under the Bering Sea) used by humans and animals to migrate.
  • Moraine: A ridge of rocks, soil, and sediment deposited at the edges or end of a glacier as it moves and melts.
  • Sea Level: The average ocean surface level; drops during Ice Ages as water is locked in ice sheets, rising as they melt.

Discussion Questions

  1. How do glaciers physically reshape the land? Name at least two features they create.
    Sample Answer: Glaciers carve and grind the land (erosion), making features like U-shaped valleys, and they drop material as they melt (deposition), leaving moraines, drumlins, and glacial erratics.
  2. Why do land bridges form during Ice Ages, and what happened when the ice melted?
    Sample Answer: Water gets locked in ice, so sea levels drop and more land is exposed. When the ice melted, sea levels rose and the land bridges flooded.
  3. The Great Lakes hold 21% of Earth's surface fresh water, and ice carved their basins. What does that scale tell you about the Ice Age?
    Sample Answer: Ice carved basins that large, and glacier meltwater first filled them. That shows how enormous the Ice Age ice sheets were.
  4. Britain has been an island for only about 8,000 years. What does that tell you about how 'permanent' modern geography is?
    Sample Answer: Coastlines, borders, and islands are products of where sea level is right now, not permanent features of the planet. Before that, you could walk from England to France across dry land.

Digging Deeper

  1. Glacier Movement Demo: Fill a large tray with damp sand and small pebbles. Freeze water in a container to make an ice block. Slowly push the ice block across the sand. Observe scratches and grooves in the sand (erosion), rocks pushed ahead of the ice (like a moraine), and debris left when the ice melts. Add food coloring to show meltwater rivers. What landforms can you identify after the 'glacier' passes?
    Sample Answer: Answers will vary. You should see scratches and grooves in the sand, rocks pushed ahead of the ice like a moraine, and debris left behind when the ice melts.
  2. Milankovitch Cycles: Why Ice Ages Happen: Earth's ice ages follow predictable patterns driven by three cycles in Earth's orbit: (1) eccentricity: how elliptical the orbit is, cycling over ~100,000 years; (2) axial tilt: how much Earth tilts, cycling over ~41,000 years; (3) precession: the wobble of Earth's axis, cycling over ~26,000 years. Research and draw a diagram showing how each affects how much sunlight reaches different latitudes. How do these cycles explain why ice ages come and go?
    Sample Answer: Each cycle changes how much sunlight reaches different latitudes. Because the cycles follow predictable patterns in Earth's orbit, ice ages come and go in repeating patterns.
  3. Modern Glacier Watch: Use NASA Worldview (worldview.earthdata.nasa.gov) or Google Earth to find a glacier in Greenland, Alaska, or the Alps. Compare satellite images from different years. Can you see retreat? Look up 'glacier before and after' for Glacier National Park or Swiss glaciers. What do you notice? How does this connect to what you learned about sea level and land bridges?
    Sample Answer: Answers will vary.