The Mantle – Earth’s Fascinating Layers, Part 2

If you missed Part 1 of our Earth’s Fascinating Layers series, The Crust, click here.

So, what did you think of the Moho? Pretty neat area, huh? Now that you dug through the Moho you are in the mantle. The mantle is approximately 1,800 miles thick! Are you tired yet? Keep going because this is a remarkably interesting area of the Earth. The solid mantle material is made of materials rich in iron, aluminum, magnesium, oxygen, and silicon. The temperature in the mantle is over 1,000 degrees Celsius and increases its temperature with depth to roughly 4,000 degrees Celsius so you will start to sweat from the heat as you dig.

Plate Tectonics and the Mantle

When we talked about the crust, you will remember that we went through the lithosphere, which is the crust and the very upper portion of the mantle. The rigid plates of the lithosphere move around the Earth on a soft, fluid (also called plastic) part of the mantle, called the asthenosphere, which is present at a depth of roughly 30 to 90 miles below the land surface. The depth to the top of the asthenosphere is greater on continents than below the ocean and the layer extends to depths of approximately 155 miles below the land surface. Below the asthenosphere, is the mesosphere portion of the mantle.

Within the mantle at the border of the lithosphere and asthenosphere is another seismic transition zone. This zone is the Gutenberg low-velocity zone, named for the American geologist Beno Gutenberg who lived from 1889 to 1960. Mr. Gutenberg noticed that at this depth, seismic waves are absorbed more strongly than at any other depth within the Earth. This clue led scientists to determine that below the Gutenberg low-velocity zone, the Earth’s material is partially melted and is constantly moving over the Earth. This partially melted, plastic asthenosphere, is the material that helps the continental plates to move around the Earth. As the plates move around on the soft, partially melted asthenosphere, they bump into one another causing earthquakes, some volcanic activity, and other geologic events. This plate movement is called “plate tectonics.”

What Rocks are in the Mantle Layer?

The upper mantle is composed of the igneous rock peridotite (olivine, pyroxene, and garnet) and the igneous/metamorphic rock eclogite (pyroxene and garnet). The high heat and pressure form in this upper portion of the mantle allows the formation of diamonds. In some areas of the mantle, the rock will melt and produce volcanic magma. The material in the lower mantle is composed of iron and magnesium silicate minerals.

I hope you had fun digging through the mantle. Maybe you found a nice diamond or garnet along the way!

Geology Trivia

See if you can answer this mantle trivia question. The answer is at the bottom of the page.

Mantle Trivia:  The word “asthenosphere” is derived from the Greek word “asthenes.” What does “asthenes” mean?

Up next: Part 3 of our Earth’s Layers series – The Outer Core


Mantle Trivia Answer:
   Weak

Images Courtesy USGS
Sources:
Crystal at the Center of the Earth; Ronald Cohen and Lars Stixrude; Carnegie Institute of Washington
Earth’s Interior; J. Louie; University of Nevada Reno; 1996
Structure of the Interior of the Earth; Lisa Gardiner; National Earth Science Teacher Association; 2010
The Interior of the Earth; Eugene C. Robertson; USGS

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