Lesson 7

7

Moving Continents: Plate Tectonics

How can something as solid as the ground beneath your feet actually move?

Earth's outer shell is divided into tectonic plates that move slowly over time. Their movement builds mountains, opens oceans, and causes earthquakes and volcanoes, while changing the positions of the continents.

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Key Ideas

  • Earth's crust is broken into large pieces called tectonic plates which slowly move on top of the softer mantle beneath them.
  • Plates can move apart (divergent), collide (convergent), or slide past each other (transform).
  • These movements cause earthquakes, volcanoes, mountains, and ocean trenches.
  • About 335 million years ago, all the continents were joined in a supercontinent called Pangaea.
  • The continents have been drifting apart ever since, and they're still moving today.

Spines

Vocabulary

  • Tectonic Plate: One of the large, slow-moving pieces of Earth's crust and upper mantle that carry continents and ocean floors.
  • Pangaea: The supercontinent that existed about 335 million years ago, when all of today's continents were joined together.
  • Earthquake: A sudden shaking of the ground caused when tectonic plates slip, grind, or collide along fault lines.
  • Volcano: An opening in Earth's crust through which magma, ash, and gases erupt; often forms at plate boundaries.
  • Continental Drift: The slow movement of Earth's continents across the surface over millions of years, driven by plate tectonics.
  • Magma: Molten rock beneath Earth's surface; when it reaches the surface and erupts, it is called lava.

Discussion Questions

  1. What was Pangaea?
    Sample Answer: It was a "supercontinent" from a long time ago when all the land on Earth was stuck together like one giant puzzle.
  2. How do we know the continents were once joined?
    Sample Answer: The shapes of the continents look like they fit together, and we find the same fossils on different sides of the ocean.
  3. What are the three types of plate boundaries?
    Sample Answer: Divergent (plates pulling apart), Convergent (plates crashing together), and Transform (plates sliding horizontally past each other).
  4. What can happen when two plates collide?
    Sample Answer: When two continental plates collide at a convergent boundary, neither plate is dense enough to sink, so the crust crumples and folds upward, creating massive mountain ranges like the Himalayas.

Digging Deeper

  1. How does plate tectonics explain the location of volcanoes and mountains?
    Sample Answer: Earthquakes and volcanoes often occur along plate boundaries, and many occur along the Pacific Ring of Fire. Mountains can form where continental plates collide, like the Himalayas. Volcanoes form when magma rises through cracks in Earth's crust, often at subduction zones, divergent boundaries, and hot spots.