Lesson 5

5

The Formation of Earth

How did a swirling cloud of rock and dust become the world we live on?

Earth formed about 4.5 billion years ago as smaller pieces of material collided and accumulated. This lesson follows its early heating and cooling, the formation of the Moon, and the separation of Earth's interior into layers.

Click here to hear the lesson video

Key Ideas

  • Earth formed from a cloud of gas and dust that clumped together through gravity. This is a process called accretion.
  • Early Earth was extremely hot and almost entirely molten from constant collisions and radioactive heating.
  • Our moon formed when a Mars-sized rock hit Earth 4.5 billion years ago.
  • As Earth cooled, heavier materials like iron sank to the center; lighter materials floated up.
  • This created Earth's layers: inner core, outer core, mantle, and crust, where we live.

Spines

Vocabulary

  • Accretion: The gradual build-up of a planet through collisions of smaller rocks and dust pulled together by gravity.
  • Molten: Melted into liquid form by extreme heat; early Earth was entirely molten.
  • Core: The innermost layer of Earth, made mostly of iron and nickel; the inner core is solid, the outer core is liquid.
  • Mantle: The thick layer of rock between Earth's core and crust; solid but flows very slowly over millions of years.
  • Crust: The thin, outermost layer of Earth where we live; made of cooled rock, broken into tectonic plates.
  • Differentiation: The process by which heavier materials sank to Earth's center and lighter materials floated to the surface as it cooled.

Discussion Questions

  1. What was early Earth like?
    Sample Answer: Early Earth was extremely hot and almost entirely molten from constant collisions and radioactive heating.
  2. Why did heavier materials sink to the center?
    Sample Answer: As Earth cooled, heavier materials like iron sank toward the center and lighter materials floated up.
  3. What are Earth's layers, from inside to outside?
    Sample Answer: Inner core, outer core, mantle, and crust.
  4. How did volcanoes help create the atmosphere and oceans?
    Sample Answer: Volcanoes released gases like water vapor and carbon dioxide, which helped form Earth's early atmosphere and oceans.

Digging Deeper

  1. Research seismic waves. How do scientists use them like X-rays to see inside the planet? What can the way a wave bends tell you about what it passed through?
    Sample Answer: Everything we know about Earth's layers comes from studying earthquake waves and how they bend, slow down, or bounce when they pass through different materials. That is how Inge Lehmann discovered the solid inner core in 1936 without ever leaving the surface.