Mohorovičić discontinuity

The Mohorovičić discontinuity, often called the Moho, is an important boundary inside our planet. It marks the place where the Earth's outer layer, called the crust, meets the layer beneath it, called the mantle. We can find this boundary by watching how seismic waves—waves created by earthquakes—travel through the Earth. These waves move faster when they go through the mantle compared to the crust, which helps scientists discover where the Moho is located.

The Moho is usually about 5 to 10 kilometers below the ocean floor and between 20 to 90 kilometers deep under the continents, with an average depth of around 35 kilometers. It lies inside a strong outer layer of the Earth known as the lithosphere, except under mid-ocean ridges where it marks a different important boundary.

This boundary was first discovered in 1909 by a Croatian seismologist named Andrija Mohorovičić. He noticed that seismic waves from shallow earthquakes traveled in two different ways—one close to the Earth's surface and another that bent through a faster layer below. This discovery helped scientists understand more about the structure of our planet.

Nature and seismology

The Moho is the place where the Earth's outer layer, called the crust, meets the layer below it, called the mantle. Scientists know this boundary exists because seismic waves, which are like the waves we feel during an earthquake, change speed when they go through different kinds of rock. Above the Moho, these waves move at speeds similar to rocks called basalt, while below, they move faster, like through rocks called peridotite or dunite. This change in speed shows that the rocks have changed.

Starting in the 1980s, scientists learned that the Moho isn't always exactly where the crust ends and the mantle begins. They found that in some places, special rocks brought up by volcanoes and data from seismic waves show that the boundary can be more complicated, with layers of different rocks making it thicker in some areas. Also, certain changes in the rocks near underwater mountain ranges can make the Moho appear deeper than expected.

History

Croatian scientist Andrija Mohorovičić discovered something very important about our planet in 1909. While studying an earthquake in Zagreb, he noticed two different kinds of waves moving away from where the earthquake started. He realized these waves travel faster through denser materials. This led him to guess that there is a clear layer inside Earth where the material suddenly becomes denser. By looking closely at the waves, he figured out this layer, called the Moho, was about 54 kilometers deep.

The Moho has helped scientists learn more about Earth for over 100 years. By watching how earthquake waves bend and change speed when they hit the Moho, scientists made new ideas about what Earth is made of. This work helped start the modern study of seismology. Later, in the 1960s, there was a big science project called Project Mohole that tried to drill down to the Moho from the ocean. Even though they made some progress, the project faced many problems and was stopped in 1966.

Exploration

Scientists want to reach the Moho by drilling, which is still an important goal. Soviet scientists worked on this from 1970 to 1992 at the Kola Superdeep Borehole and reached a depth of 12,260 metres before stopping. One idea is to use a special capsule powered by radioactive material with a heavy tungsten needle to travel to the Moho and explore the area.

There were also plans for the drill-ship JOIDES Resolution to travel from Colombo in Sri Lanka to the Atlantis Bank in the Indian Ocean in 2015. They tried to drill to a depth of about 1.5 kilometres but only reached about 1.3 kilometres. Researchers hope to try again in the future.