How much of the Earth's surface is covered by the lithosphere? To understand the answer to this question, some basic geological vocabulary is needed. First, what is the lithosphere? The lithosphere is the outer layer of the Earth, composed of the crust and the uppermost part of the mantle. The lithosphere is one of three layers that are used to describe the mechanical properties of our planet; the other two are the asthenosphere, which is under the lithosphere, and the mesosphere, which is the lowermost layer. The thickness of the Earth's crust – and thus the lithosphere – varies from just a few kilometers under the ocean to nearly 100 kilometers under some mountain ranges on land.

The labels for the mechanical layers of the Earth are used to describe the concepts of plate tectonics and continental drift. Basically, the crust, which is part of the lithosphere, is the thin layer of the surface of the planet that is made of solid and relatively cool materials, rocks and soils. Along with the crust, the lithosphere is also composed of the uppermost part of the mantle, which is also relatively cool and solid.

The lithosphere essentially floats on the asthenosphere, which is composed of the lower level of the Earth's mantle. The asthenosphere is hotter and more viscous than the lithosphere, which allows the lithosphere to glide over it.

Tectonic plates are portions of the Earth's lithosphere. The movement of heat from the center of the Earth toward the crust creates convection currents, which cause the Earth's tectonic plates to move away from and toward one another. This movement is referred to as plate tectonics, and is an important concept to understand when studying the formation of geological features like mountains, mid-ocean ridges and seafloor spreading.

Since the lithosphere is composed of crust and part of the mantle, and the crust is the outermost layer of our planet, 100% of the Earth is covered by the lithosphere. The coverage is complete, but the thickness of the lithosphere compared to the distance from the crust to the inner core of the Earth varies with location.

We know that the thickness of the lithosphere varies, but for the sake of calculations, let's use an average lithosphere thickness of 100 km. According to NASA, the radius of the Earth at the equator – or the distance from the surface of the Earth to the center of the inner core – is 6,378 km. Therefore, the lithosphere is roughly 1.5% of the total radius of our planet – a very thin layer that supports all life on Earth.

It has been said that the habitable layer of our planet is comparable to one-half the thickness of one page of a thousand-page book. This comparison highlights the precious, unique and irreplaceable qualities of the planet we and all other living organisms call home.

Generally, the Earth's crust is thinner under oceans and thicker on land. The crust is only about 5 to 10 km under oceans, while it can be up to 60 km thick under mountain ranges. The thickness of the Earth's mantle also varies, from about 50 km to 100 km.

The average depth of the lithosphere is in the range of 70 to 100 km. Beneath this thin layer, which supports all life on Earth, the temperature of the planet is estimated to reach 1,832° Fahrenheit.

The difference in temperature between the lithosphere and the asthenosphere causes some of the most interesting and destructive geological events on our planet, including earthquakes, continental drift and volcanoes.