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4.12: Earth's Mantle

Difficulty Level: At Grade / Basic Created by: CK-12
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What is a diamond delivery system?

Some events happened when Earth was younger and hotter that do not happen any more. Kimberlite pipes shot up from deep in the mantle. These pipes are the most important source of diamonds, which form at very high pressure. Most kimberlites surfaced long ago.

Mantle

The two most important things about the mantle are: (1) it is made of solid rock, and (2) it is hot.

Solid Rock

Scientists know that the mantle is made of rock based on evidence from seismic waves, heat flow, and meteorites. The properties fit the ultramafic rock peridotite, which is made of the iron- and magnesium-rich silicate minerals (Figure below). Peridotite is rarely found at Earth's surface.

Peridotite is formed of crystals of olivine (green) and pyroxene (black).

Heat Flow

Scientists know that the mantle is extremely hot because of the heat flowing outward from it and because of its physical properties.

Heat flows in two different ways within the Earth:

  1. Conduction: Heat is transferred through rapid collisions of atoms, which can only happen if the material is solid. Heat flows from warmer to cooler places until all are the same temperature. The mantle is hot mostly because of heat conducted from the core.
  2. Convection: If a material is able to move, even if it moves very slowly, convection currents can form.

Convection in the mantle is the same as convection in a pot of water on a stove. Convection currents within Earth's mantle form as material near the core heats up. As the core heats the bottom layer of mantle material, particles move more rapidly, decreasing its density and causing it to rise. The rising material begins the convection current. When the warm material reaches the surface, it spreads horizontally. The material cools because it is no longer near the core. It eventually becomes cool and dense enough to sink back down into the mantle. At the bottom of the mantle, the material travels horizontally and is heated by the core. It reaches the location where warm mantle material rises, and the mantle convection cell is complete (Figure below).

Convection.

Vocabulary

  • conduction: The process in which energy moves from a location of higher temperature to a location of lower temperature as heat. The material does not move, just the heat.
  • convection: The movement of material due to differences in temperature.
  • convection cell: A circular pattern of warm material rising and cool material sinking.
  • kimberlite pipe: Volcanic rock shot up from between around 100 to 300 km depth that contains diamonds.
  • peridotite: Very dense, very mafic igneous rock that is the composition of the upper mantle.

Summary

  • The mantle is composed of solid peridotite.
  • Conduction from the core heats the lower mantle.
  • Mantle convection cells bring hot material up toward the surface and cooler material down toward the core.

Practice

Use these resources to answer the questions that follow.

1. What incorrect statement does this video make about the asthenosphere?

2. What causes plate movement?

3. What is convection?

http://www.youtube.com/watch?v=yt_K_bfKxTc

4. What happens to the denser material?

5. What happens to the less dense material?

6. Where are convection currents found?

http://www.learner.org/interactives/dynamicearth/structure.html

7. Explain the structure of the mantle.

Review

1. What is the composition of the mantle and how do scientists know this?

2. What is conduction?

3. How does convection work in the mantle?

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