The Earth’s mantle is a region of the planet that starts below the crust, and ends at the core. It accounts for the largest portion of the Earth’s volume, but what is the Earth’s mantle made of?
The Earth’s mantle is about 2,970 km thick, making up about 84% of the Earth’s total volume. You might think it’s a liquid, since volcanoes erupt lava from the mantle. But actually, the mantle is mostly solid rock. Magma is liquid rock that seeps out of the mantle and collects into pockets beneath the crust called magma chambers.
If you could take apart the rocks in the mantle, and separate them into their atomic elements, you would have 44.8% oxygen, 21.5% silicon, and 22.8% magnesium. There’s also iron, aluminum, calcium, sodium, and potassium. Of course, these elements are bound together into rocks. All of the rocks are various oxides. The most common is SiO2 at 48%, followed by MgO – 37.8%.
Geologists have broken up the Earth’s mantle into layered sections. The outermost layer is called the upper mantle, and goes down to a depth of 410 km. Below that is the the transition zone (660 km), then the lower mantle reaching a depth of 2,891 km. The various layers of the mantle have different types of rocks. Examples of rocks that you might find inside the mantle include: olivine, pyroxenes, spinel, and garnet.
You can read more on this question from the USGS.
We have also recorded a whole episode of Astronomy Cast just about the Earth. Listen to it here, Episode 51: Earth.