Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass that makes up 67% of the mass of Earth. The layer has a thickness making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but behaves as a viscous fluid on geologic time scales. Scientists sometimes describe its consistency as having the texture of caramel. Partial melting of this material at mid-ocean ridges produces oceanic crust. Partial melting at subduction zones creates continental crust.
Andrija Mohorovičić first noted a sudden increase in seismic velocity in 1909. This boundary is now referred to as the Mohorovičić discontinuity or Moho. The upper mantle starts at the Moho around 35 kilometers down to 410 kilometers. The transition zone spans approximately 410 to 660 kilometers. Wadsleyite and ringwoodite are stable within this middle section. The lower mantle extends from 660 kilometers to roughly 2,900 kilometers. Bridgmanite and post-perovskite remain stable in this deepest region. The lower 200 kilometers of the lower mantle constitutes the D double-prime region. This area contains large low-shear-velocity provinces and ultra-low velocity zones.
The top of the mantle is defined by a sudden increase in seismic velocity. The upper mantle is dominantly peridotite composed primarily of olivine. Variable proportions of clinopyroxene and orthopyroxene also exist there. An aluminous phase appears as plagioclase in the uppermost mantle. Spinel forms below that depth before garnet takes over near 70 kilometers. Olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite at the top of the transition zone. These high-pressure polymorphs have a large capacity to store water in their crystal structure. Ringwoodite decomposes into bridgmanite and ferropericlase at the base of the transition zone. The lower mantle is composed primarily of bridgmanite and ferropericlase with minor amounts of calcium perovskite.
Temperatures range from approximately 500 kelvin at the upper boundary with the crust. Heat reaches approximately 4,200 kelvin at the core-mantle boundary. The enormous lithostatic pressure prevents melting despite these extreme temperatures. The solidus increases with pressure so the mantle remains almost exclusively solid. Hot material rises in a mantle plume while cooler heavier material sinks downward. Downward motion occurs at convergent plate boundaries called subduction zones. Locations on the surface lying over plumes exhibit hot spot volcanism. Estimates for the viscosity of the upper mantle range between 10^20 and 10^23 pascal seconds depending on depth. The entire mantle deforms like a fluid on long timescales.
The first attempt at mantle exploration was known as Project Mohole. It was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration reached approximately 183 meters. In 2005 an oceanic borehole reached 3,700 meters below the sea floor from the JOIDES Resolution vessel. More successful was the Deep Sea Drilling Project that operated from 1968 to 1983. Glomar Challenger conducted the drilling operations for this program. On the 5th of March 2007 scientists aboard the RRS James Cook embarked on a voyage to an area where the mantle lies exposed without any crust covering. This site lies approximately three kilometers beneath the ocean surface midway between the Cape Verde Islands and the Caribbean Sea. A Chikyu Hakken mission attempted to drill up to 7,000 meters below the seabed later in 2007.
Seismic images of Earth's interior have revealed two continent-sized anomalies with low seismic velocities. These zones are denser and likely compositionally different from the surrounding mantle. They may represent buried relics of Theia mantle material remaining after the Moon-forming event. Yuan Li Desch and colleagues published findings on this hypothesis in Nature in 2023. Large low-shear-velocity provinces exist within the lowermost mantle. Ultra-low velocity zones also appear in this region. These anomalies suggest remnants of the impactor that formed the Moon still reside deep inside our planet.
Common questions
What is Earth's mantle and how much of the planet does it make up?
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass that makes up 67% of the mass of Earth and occupies about 84% of Earth's volume.
When did Andrija Mohorovičić discover the boundary now known as the Moho?
Andrija Mohorovičić first noted a sudden increase in seismic velocity on the 2nd of May 1909. This discovery led to the naming of the boundary as the Mohorovičić discontinuity or Moho.
How deep do scientists drill into Earth's mantle during exploration projects?
The deepest penetration reached approximately 183 meters during Project Mohole which was abandoned in 1966. An oceanic borehole later reached 3,700 meters below the sea floor from the JOIDES Resolution vessel in 2005.
What minerals are stable within the transition zone of Earth's mantle?
Wadsleyite and ringwoodite are stable within the middle section of the mantle spanning approximately 410 to 660 kilometers. Ringwoodite decomposes into bridgmanite and ferropericlase at the base of this transition zone.
Where can scientists find exposed mantle material without any crust covering it?
Scientists aboard the RRS James Cook embarked on a voyage on the 5th of March 2007 to an area where the mantle lies exposed. This site lies approximately three kilometers beneath the ocean surface midway between the Cape Verde Islands and the Caribbean Sea.