Mohorovičić discontinuity

The Mohorovičić discontinuity, often called the Moho, is an important place inside our planet. It shows where the Earth's outer layer, called the crust, meets the layer beneath it, called the mantle.

We can find this place 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 know where the Moho is.

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.

This boundary was first discovered in 1909 by a Croatian seismologist named Andrija Mohorovičić. He noticed that seismic waves from earthquakes traveled in two different ways. This helped scientists learn more about the structure of our planet.

Nature and seismology

The Moho is 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 important about Earth in 1909. He was studying an earthquake in Zagreb when he noticed two kinds of waves moving away from the earthquake. He saw that these waves move faster through denser materials. This helped him guess that there is a layer inside Earth where the material suddenly gets denser. By studying the waves, he found 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 change when they hit the Moho, scientists made new ideas about Earth's structure. This work helped start the modern study of seismology. Later, in the 1960s, a big science project called Project Mohole tried to drill down to the Moho from the ocean. They made some progress but faced many problems, and the project stopped in 1966.

Exploration

Scientists want to drill to reach the Moho. This is still an important goal. Soviet scientists tried 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 with a heavy tungsten needle to travel to the Moho. 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. 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.