If you were to embark on a journey to the center of the Earth in the style of the classic Jules Verne sci-fi novel, you wouldn’t stumble upon a herd of behemoths or a hidden sea where the plesiosaur swims free. You would come across such high temperatures and such intense pressure that you would die long before you reached your goal: In fact, the the deepest we’ve never drilled on Earth is only about 7.6 miles away.
But thanks in large part to seismic waves transmitted during earthquakes, scientists were able to map the unexplored interior of the Earth. The planet is divided into four layers: the crust, the mantle, the outer core and the inner core. The top layer – the crust– is the thinnest, with an average thickness of about 19 miles underground and only about 3 miles thick under the ocean. That said, it can reach nearly 50 miles deep on land in some places.
The lower parts of the crust combine with the upper layer of the upper coat to form what is called the lithosphere, where the tectonic plates are found. Beneath this is the mantle asthenosphere, then the lower mantle, then another combination layer between the lower mantle and the outer core that scientists call D” (pronounced “D double prime”). In total, the mantle is Earth’s thickest layer, with an average radius of around 1800 miles, although again this varies by location.
The mantle consists of mostly solid rocks, and most of these rocks are silicon-oxygen compounds known as silicates. Olivine and garnet are two major players in this category. Non-silicate elements include iron, potassium, calcium, etc. It’s also extremely hot in the mantle, with temperatures ranging from around 1830°F to nearly 6700°F.
Beneath the mantle is the outer about 1370 miles thick heart, composed mainly of liquid nickel and iron. And, finally, the inner core: a sphere of solid iron (among other things) with a radius of nearly 760 miles. At over 9300°F, the inner core is certainly hot enough to melt that iron, but it’s not. Scientists believe that the pressure exerted on the core by all the layers above (and the atmosphere) can keep the iron in solid form.
If you add the inner and outer core mileage together, it’s technically more than the mantle. But since the two are generally considered separate layers, the mantle is most often cited as Earth’s thickest slice.