The Hidden Layers of Earth and Tectonic Plate Movements

In 1970, Russian geologists drilled a nine-inch wide hole into the Kola Peninsula’s Baltic Shield — a part of Earth’s crust with rock well over a billion years old — and began digging as deep as they could go.

After 20 years, what was called the Kola Superdeep Borehole reached a maximum depth of 40,230 feet, or nearly 7.62 miles, into Earth’s surface. Drilling stopped in 1992 because of equipment limitations with high heat, but the Kola Superdeep remains the deepest human-made hole in history.

And yet, amazingly, the project’s incredible depth was just 0.2 percent of the distance to Earth’s core, and not even halfway to what scientists refer to as the “upper mantle.” This particular section of our planet — comprised of two subsections known as the lithosphere and the asthenosphere — are where tectonic plates shift, causing earthquakes and volcanic eruptions, as well as the movement of the continents and the forming of mountains.

In essence, it’s where hot stuff happens.

As you might remember from science class, the Earth has three main layers: the crust, the mantle, and the core. Of these, the mantle and core are both identified as having two distinct parts: the upper and lower mantle, and the outer and inner core.

The mantle is, by far, the thickest part of Earth’s interior, extending for about 1,800 miles and comprising 84 percent of its total volume.

The mantle is mostly solid rock that’s comprised of silicates — silicon-oxygen compounds — found in most igneous rock, which form from the cooling of magma generated within the upper mantle. These rocks are often heavy in minerals like olivine and pyroxene.

Earth is thought to be the only planet in our solar system with a constantly active mantle. And ours isn’t just active and thick; it’s also hot. Its temperature likely varies from around 1,800 degrees Fahrenheit at the crust boundary to over 6,600 degrees Fahrenheit at the core boundary. The deeper one goes, the more heat and pressure they’ll find.

Read More: Drilling Deep: How Far Have We Gone Under Earth's Crust?

Fueling earthquakes, volcanic eruptions, and more with its heat and its pressure, the upper mantle extends from Earth’s crust to around 255 miles below Earth’s surface.

Of this 255-mile stretch, the first 62 miles are part of what’s called the lithosphere, which includes the crust and the uppermost portion of the mantle.

The lithosphere is the coolest, most rigid Earth layer, and it’s also a key player in the planet’s tectonic activity. In fact, the lithosphere is separated into several vast slabs, known as lithospheric or tectonic plates, that interact with one another, pushing together and pulling apart over time.

Within the lithosphere is what’s known as the Mohorovicic discontinuity, or Moho (which is obviously cooler and easier to say). The Moho is the boundary between the crust and the uppermost portion of the mantle. It exists at uneven depths due to isostasy — the gravitational equilibrium that allows the crust to basically “float” on top of the mantle. But in general, the Moho is found about five miles beneath the ocean floor and 20 miles beneath continents.

Below the lithosphere is the denser asthenosphere, stretching from the lithosphere-asthenosphere boundary (LAB) to about 255 miles beneath Earth’s surface. Here is where temperature and pressure cause rock to become semi-molten. As a result, the asthenosphere is considered more ductile — or malleable and able to stretch.

Read More: 3 Facts to Know About The Strange Spinning of Earth's Inner Core

Tectonic activity happens because lithospheric plates move on top of the asthenosphere. When these plates interact, earthquakes can occur and volcanoes can explode at their boundaries.

You can also think of these plates as cracked pieces of lithosphere. There are 15 plates in total, comprised of seven major plates and eight minor ones. They move together in all directions, as well as apart and underneath one another.

It’s important to note that scientists still aren’t sure why the plates move. Ideas range from the long-standing belief that heat-based convection currents in the mantle drive their movement, to more recent theory of “slab pull,” where slabs of oceanic lithosphere become denser with age and sink into the mantle at areas of “subduction,” pulling the surrounding lithosphere along with them.

There’s also the concept of “ridge push,” where plates of lithosphere formed atop “oceanic ridges” slide off those ridges after their formation, pushing the plates in front of them. That said, whatever the mechanisms of their movement, the interaction of these plates can trigger earthquakes and eruptions, which strike some areas more than others.

Read More: The Origins of Plate Tectonics May Stretch Further Back in Earth's History

About 90 percent of all earthquakes and about 75 percent of active volcanoes are located along the Ring of Fire, the roughly 25,000-mile-long line around the Pacific Ocean where many lithospheric plates meet.

Geographically, the Ring of Fire includes regions along the coasts of South America, North America, and extends across to Asia and Australasia, a subregion of Oceania.

The Ring of Fire also features many of Earth’s fault lines, which are found at plate boundaries, where stress builds up and causes plates to suddenly and dramatically slip. The San Andreas Fault is among the most famous of fault lines: The catastrophic 1906 San Francisco earthquake, which destroyed most of the city with falling debris and fire, was caused by the slippage of this fault.

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Some volcanic activity doesn’t take place along plate boundaries, but in areas known as hot spots, which sit on Earth’s crust over “mantle plumes,” or areas in the asthenosphere where the semi-molten rock is much hotter than it is in the rest of the mantle.

The heat around these plumes thins Earth’s crust above, creating cracks and sending out spurts of magma. This magma is thought to be liquefied rock from the asthenosphere, though it may originate in the core itself.

There are several dozen hot spots around the world, including those that are associated with the volcanoes in Iceland, Hawaii, and Yellowstone National Park.

Read More: What Causes Volcanic Eruptions: Can We Predict Them?

Scientists studying the upper mantle are still finding new information that can shape their thinking about the structure of Earth. Earlier this year, for example, researchers aboard the International Ocean Discovery Program’s JOIDES Resolution announced the successful retrieval of a nearly 3,300-foot-long rock core from seabed drilling in the Atlantic.

As Johan Lissenberg — an igneous petrologist aboard the ship — told Science in May, the core could help researchers understand how magma moves from the mantle to the crust, while also providing proof of the different composition of the two layers. Adding to its utility, radioactive elements in the core could improve understandings of how hot the mantle actually is and clarify the theory that convection currents could underpin plate tectonics.

The work is a reminder that the forces beneath us remain an exciting field of exploratory science, with much about the upper mantle yet to be revealed.

Read More: 5 of the Most Explosive Volcanic Eruptions