What Is the Mohorovicic Discontinuity and Can Humans Ever Reach It?

Mohorovicic discontinuity (pronounced moh-haw-roh-vuh-chich), more commonly known as the "Moho," is the geological term that describes the boundary zone between Earth's crust and the lower-lying mantle. Its discovery was critical to scientists because it was the first time anyone conceived that there were multiple layers with varying compositions inside Earth.

Imagine cutting into a cake expecting fluffy vanilla and instead finding dense layers of chocolate and red velvet, too. That's what this was like.

Until the late 19th century, scientists believed that Earth's composition was the same from crust to core. As seismogram technology advanced, scientists like Mohorovicic could take more precise measurements — leading to unprecedented discoveries like the one he made Oct. 8, 1909, when that earthquake hit about 25 miles (40 kilometers) south of Mohorovicic's seismic station in Zagreb.

After the quake, the measurements Mohorovicic collected were bewildering. He called his colleagues in 40 other seismic stations around Croatia to confirm what he observed. There was a sudden increase of seismic waves as they passed from the crust to the mantle.

But if Earth's interior was made up of the same material all the way through, the waves should have been proportional to their distance. In other words, they should move gradually slower as they got further away from the quake's epicenter. But instead, Mohorovicic found they started traveling faster.

Mohorovicic knew that seismic waves are refracted faster through denser materials, a behavior explained by a natural law of refraction called Snell's Law. He hypothesized that those faster-moving waves must have encountered a denser layer of Earth.

At a very high level, you can imagine the wave refraction that Mohorovicic observed like this: There's a shark in the water, and you are swimming as fast as you can to get away. As soon as you hit the beach, you hit a sprint and escape much faster than you could in the water. Beyond the fact that sharks can't walk on land, once you hit a material you could move through faster — like air — you travel more quickly.

Certain seismic waves, like P-waves, travel through denser materials at a greater velocity. Mohorovicic knew this and hypothesized that a different material must be below Earth's crust that caused the refraction of the seismic waves.

He published his finding in a 1919 journal — the first person to introduce the idea that there is a sharp boundary made up of a different material that separates the superior crust from the mantle beneath it. It's now known as the Mohorovicic discontinuity.

On average, the Moho is 19.8 miles (32 kilometers) below continental surfaces, i.e. land, though beneath mountain ranges, it has depths up to 43.4 miles (70 kilometers). Beneath ocean basins, it's 4.9 miles (8 kilometers) deep, on average.

But the Moho isn't thought of as a straight uniform line. Because its depth differs depending on whether it's beneath the oceanic or continental crust or offset by thrust faults, it's considered a boundary zone where wave velocity increases quickly, about ~ 4.8 miles (7.7 kilometers) per second.

Because of its depth, the Moho has never been confirmed, which means scientists haven't drilled deep enough into Earth to reach the layer between the crust and mantle. Drilling that deep isn't cheap, and it's not the friendliest environment. Heat at that depth ranges from 1,832 to 6,692 degrees Fahrenheit (1,000 to 3,700 degrees Celsius) and pressures at that depth are enough to soften and melt rocks — and drilling equipment.

To date, the furthest humans have ever dug is 40,230 feet (12,262 meters). The hole is called the Kola Superdeep Borehole — a 20-year project initiated by the Soviets in the 1970s and was ceased when temperatures reached 356 degrees Fahrenheit (180 degrees Celsius), making it impossible to keep going.

Today, scientists continue to pursue the Moho. The International Ocean Discovery Program has begun the M2M or "Moho to Mantle" project to collect samples from Earth's mantle. If they succeed, it will be humankind's first (and deepest) step ever taken into Earth.