Seeking the Holey Grail
Drilling all the way down to the Moho (the division between the Earth's crust and mantle) is an important scientific milestone, but despite decades of effort, nobody has yet succeeded. The United States and the Soviet Union both made great progress in the 20th century. A program called Project Mohole was begun by the U.S. in the 1950s, but it never reached a depth beyond about 183 meters (601 feet) under the seafloor.
In 2005, scientists with the Integrated Ocean Drilling Project drilled 1,416 meters (4,644 feet) below the North Atlantic seafloor and claimed to have come within just 1,000 feet of reaching the Moho.
The mantle is one of the three main layers of the Earth. It lies between the innermost layer, the core, and the thin outermost layer, the crust. The mantle consists of hot, dense, semisolid rock and is about 2,900 kilometers (1,802 miles) thick.
The mantle is divided into several layers.
Layers of the mantle
- Lithosphere. The thin outermost shell of the upper mantle is similar to the crust, though cooler and more rigid. Together with the crust, this layer is called the Earth’s lithosphere.
- Asthenosphere. The lithosphere is actually broken up into several large pieces, or plates. They “float” on a softer mantle layer called the asthenosphere. Their very slow motion is the cause of plate tectonics, a process associated with continental drift, earthquakes, volcanoes, and the formation of mountains.
- Upper mantle. Below the asthenosphere lies another layer, stronger and more solid than the asthenosphere. All layers below the crust down to a depth of about 670 kilometers (416 miles) are known as the upper mantle.
- Lower mantle. The rest of the mantle between the upper mantle and the core is known as the lower mantle. It is denser and hotter than the upper mantle.
Exploring the Deep Earth
Most of the Earth’s interior is much too deep for us to explore directly. Instead, scientists tell the mantle apart from the crust and core by measuring the spread of shock waves from earthquakes, called seismic waves. Two types of seismic waves pass through the Earth’s interior: P-waves, which represent vertical motion, and S-waves, which represent horizontal motion. Instruments placed around the world measure these waves as they arrive at different points on the Earth’s surface after an earthquake.
Seismic waves travel at different speeds and strengths through different material. For example, surface waves from a powerful earthquake near Northridge, California, in 1994 took 30 minutes to reach a point about 6,700 kilometers (4,163 miles) away, but it took P-waves only 10 minutes and S-waves just under 20 minutes to travel the same distance. Near the surface, P-waves travel about 6 kilometers per second through the ground. Below a certain point—an average depth of about 35 kilometers—they travel about 8 km/s, indicating that the waves have reached denser material at that point.
This abrupt divide between slower and faster speeds marks the boundary between the crust and the mantle. It is called the Mohorovicic discontinuity, or simply the Moho. At the base of the mantle, 2,900 kilometers (1,802 miles) below the surface, S-waves, which can’t continue in liquid, suddenly disappear, and P-waves are strongly refracted, or bent. This point, called the Gutenberg discontinuity, marks the end of the mantle and the beginning of the Earth’s liquid core.
Term Part of Speech Definition Encyclopedic Entry asthenosphere Noun
layer in Earth's mantle between the lithosphere (above) and the upper mantle (below).
line separating geographical areas.
Encyclopedic Entry: boundary continental drift Noun
the movement of continents resulting from the motion of tectonic plates.
Encyclopedic Entry: continental drift core Noun
the extremely hot center of Earth, another planet, or a star.
Encyclopedic Entry: core crust Noun
rocky outermost layer of Earth or other planet.
Encyclopedic Entry: crust dense Adjective
having parts or molecules that are packed closely together.
our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.
Encyclopedic Entry: Earth earthquake Noun
the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
Gutenberg discontinuity Noun
point between the Earth's mantle and the core below.
left-right direction or parallel to the Earth and the horizon.
outer, solid portion of the Earth. Also called the geosphere.
Encyclopedic Entry: lithosphere lower mantle Noun
bottom layer in Earth's mantle, closest to the core.
point between Earth's crust and the mantle below. Also called the Mohorovicic discontinuity.
Mohorovicic discontinuity Noun
point between Earth's crust and the mantle below. Also called the Moho.
landmass that forms as tectonic plates interact with each other.
plate tectonics Noun
movement and interaction of the Earth's plates.
seismic shock wave that represents vertical motion. Also called a primary wave.
to bend or change the path of something.
natural substance composed of solid mineral matter.
seismic shock wave Noun
vibration that travels from the focus of an earthquake in all directions.
seismic wave Noun
shock wave of force or pressure that travels through the Earth.
seismic shock wave that represents horizontal motion. Also called a secondary wave.
upper mantle Noun
layer in Earth's mantle between the asthenosphere (above) and the lower mantle (below).
up-down direction, or at a right angle to Earth and the horizon.
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
Encyclopedic Entry: volcano