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Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 5.972 × 10^24 kilograms and makes up 67% of the mass of Earth. It has a thickness of 2,885 kilometers, making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust. The mantle is the engine room of the planet, driving the movement of tectonic plates and shaping the surface we live on. Despite its solid state, the immense pressure and heat allow it to flow slowly over millions of years, creating a dynamic system that has been active since the Earth's formation 4.5 billion years ago. The mantle's behavior is a delicate balance between rigidity and fluidity, a state that allows for the slow, creeping motion that drives geological processes. This unique property is what makes the mantle the most significant layer of the Earth, influencing everything from the formation of mountains to the occurrence of earthquakes. The mantle's composition and structure are complex, with different layers and minerals that respond to the extreme conditions of depth and pressure. The study of the mantle has been a challenging endeavor, with scientists relying on indirect methods such as seismic waves and laboratory experiments to understand its properties. Despite the difficulties, the mantle remains one of the most fascinating and important layers of the Earth, holding the key to understanding the planet's history and future.
Layers of Depth
The Earth's mantle is divided into three major layers defined by sudden changes in seismic velocity: the upper mantle, the transition zone, and the lower mantle. The upper mantle starts at the Moho, or base of the crust, around 5 to 70 kilometers deep and extends to 410 kilometers. The transition zone, approximately 410 to 660 kilometers deep, is where wadsleyite and ringwoodite are stable. The lower mantle, approximately 660 to 2,885 kilometers deep, is where bridgmanite and post-perovskite are stable. The lower ~200 kilometers of the lower mantle constitutes the D'' region, a region with anomalous seismic properties. This region also 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, which was first noted by Andrija Mohorovičić in 1909; this boundary is now referred to as the Mohorovičić discontinuity or Moho. The upper mantle is dominantly peridotite, composed primarily of variable proportions of the minerals olivine, clinopyroxene, orthopyroxene, and an aluminous phase. The aluminous phase is plagioclase in the uppermost mantle, then spinel, and then garnet below 20 kilometers. Gradually through the upper mantle, pyroxenes become less stable and transform into majoritic garnet. At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite. Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This and other evidence has led to the hypothesis that the transition zone may host a large quantity of water. At the base of the transition zone, ringwoodite decomposes into bridgmanite (formerly called magnesium silicate perovskite), and ferropericlase. Garnet also becomes unstable at or slightly below the base of the transition zone. The lower mantle is composed primarily of bridgmanite and ferropericlase, with minor amounts of calcium perovskite, calcium-ferrite structured oxide, and stishovite. In the lowermost ~200 kilometers of the mantle, bridgmanite isochemically transforms into post-perovskite. The mantle's structure is a complex interplay of minerals and phases that respond to the extreme conditions of depth and pressure. The study of the mantle's structure has been a challenging endeavor, with scientists relying on indirect methods such as seismic waves and laboratory experiments to understand its properties. Despite the difficulties, the mantle remains one of the most fascinating and important layers of the Earth, holding the key to understanding the planet's history and future.
Earth's mantle has a mass of 5.972 × 10^24 kilograms. This mass represents 67% of the total mass of Earth.
When was the Mohorovičić discontinuity first identified?
The Mohorovičić discontinuity was first noted by Andrija Mohorovičić on the 19th of November 1909. This boundary marks the top of the mantle and is defined by a sudden increase in seismic velocity.
How deep does the lower mantle extend?
The lower mantle extends from approximately 660 kilometers to 2,885 kilometers deep. This region contains stable minerals such as bridgmanite and post-perovskite.
Where is the transition zone located within Earth's mantle?
The transition zone is located between 410 kilometers and 660 kilometers deep within Earth's mantle. This zone is where wadsleyite and ringwoodite are stable minerals.
What year did Project Mohole end?
Project Mohole was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration achieved during this project was approximately 183 meters.
How much water might be stored in Earth's mantle?
The transition zone may host a large quantity of water potentially containing more water than all the oceans combined. This water exists as hydroxyl groups bound within the crystal structure of minerals.
At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite. Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This and other evidence has led to the hypothesis that the transition zone may host a large quantity of water. The presence of water in the mantle has profound implications for the Earth's geological processes, including the formation of magma and the movement of tectonic plates. The water stored in the mantle's minerals may be a significant reservoir, potentially containing more water than all the oceans combined. The discovery of water in the mantle has been a groundbreaking development in the field of geology, challenging previous assumptions about the Earth's interior. The water in the mantle is not in the form of liquid or ice, but rather as hydroxyl groups bound within the crystal structure of minerals. This form of water is stable under the extreme conditions of the mantle, allowing it to be stored for billions of years. The release of water from the mantle can trigger volcanic activity and influence the Earth's climate over geological time scales. The study of water in the mantle has been a challenging endeavor, with scientists relying on indirect methods such as seismic waves and laboratory experiments to understand its properties. Despite the difficulties, the mantle remains one of the most fascinating and important layers of the Earth, holding the key to understanding the planet's history and future. The presence of water in the mantle has also led to new theories about the origin of Earth's oceans and the evolution of the planet's surface. The water in the mantle may have been delivered to the Earth by asteroids and comets during the early stages of the planet's formation, or it may have been present since the Earth's formation. The study of water in the mantle is an active area of research, with scientists working to understand the role of water in the Earth's geological processes and the evolution of the planet's surface.
The Moon's Shadow
Seismic images of Earth's interior have revealed in the lowermost mantle two continent-sized anomalies with low seismic velocities. These zones are denser and likely compositionally different from the surrounding mantle. These anomalies may represent buried relics of Theia mantle material remaining after the Moon-forming event proposed in the Giant-impact hypothesis. The Giant-impact hypothesis suggests that the Moon was formed from the debris of a collision between the early Earth and a Mars-sized body called Theia. The collision would have been so violent that it would have melted the Earth's surface and created a disk of debris that eventually coalesced to form the Moon. The remnants of Theia's mantle may have sunk to the bottom of the Earth's mantle, where they have remained for billions of years. The discovery of these anomalies has been a groundbreaking development in the field of geology, providing new insights into the Earth's early history and the formation of the Moon. The anomalies are located in the lowermost mantle, near the core-mantle boundary, and are characterized by low seismic velocities and high density. The presence of these anomalies suggests that the Earth's mantle is not homogeneous, but rather contains distinct regions with different compositions and properties. The study of these anomalies has been a challenging endeavor, with scientists relying on indirect methods such as seismic waves and laboratory experiments to understand their properties. Despite the difficulties, the anomalies remain one of the most fascinating and important discoveries in the field of geology, holding the key to understanding the Earth's early history and the formation of the Moon. The anomalies may also provide clues about the Earth's early atmosphere and the evolution of the planet's surface. The study of the anomalies is an active area of research, with scientists working to understand the role of Theia's mantle in the Earth's geological processes and the evolution of the planet's surface.
The Deep Sea Quest
Exploration of the mantle is generally conducted at the seabed rather than on land because of the relative thinness of the oceanic crust as compared to the significantly thicker continental crust. The first attempt at mantle exploration, known as Project Mohole, was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration was approximately 183 meters. In 2005 an oceanic borehole reached 1,268 meters below the sea floor from the ocean drilling vessel JOIDES Resolution. More successful was the Deep Sea Drilling Project (DSDP) that operated from 1968 to 1983. Coordinated by Scripps Institution of Oceanography at the University of California, San Diego, DSDP provided crucial data to support the seafloor spreading hypothesis and helped to prove the theory of plate tectonics. Glomar Challenger conducted the drilling operations. DSDP was the first of three international scientific ocean drilling programs that have operated over more than 40 years. Scientific planning was conducted under the auspices of the Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES), whose advisory group consisted of 250 distinguished scientists from academic institutions, government agencies, and private industry from all over the world. The Ocean Drilling Program (ODP) continued exploration from 1985 to 2003 when it was replaced by the Integrated Ocean Drilling Program (IODP). On the 5th of March 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, midway between the Cape Verde Islands and the Caribbean Sea. The exposed site lies approximately three kilometers beneath the ocean surface and covers thousands of square kilometers. A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007. The Chikyu Hakken mission attempted to use the Japanese vessel Chikyū to drill up to 7,000 meters below the seabed. This is nearly three times as deep as preceding oceanic drillings. A novel method of exploring the uppermost few hundred kilometers of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe which melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks. The probe consists of an outer sphere of tungsten about one meter in diameter with a cobalt-60 interior acting as a radioactive heat source. It was calculated that such a probe will reach the oceanic Moho in less than 6 months and attain minimum depths of well over 10 kilometers in a few decades beneath both oceanic and continental lithosphere. Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago. In 2023, JOIDES Resolution recovered cores of what appeared to be rock from the upper mantle after drilling only a few hundred meters into the Atlantis Massif. The borehole reached a maximum depth of 1,268 meters and recovered 886 meters of rock samples consisting of primarily peridotite. There is debate over the extent to which the samples represent the upper mantle with some arguing the effects of seawater on the samples situates them as examples of deep lower crust. However, the samples offer a much closer analogue to mantle rock than magmatic xenoliths as the sampled rock never melted into magma or recrystallized. The exploration of the mantle has been a challenging endeavor, with scientists relying on indirect methods such as seismic waves and laboratory experiments to understand its properties. Despite the difficulties, the mantle remains one of the most fascinating and important layers of the Earth, holding the key to understanding the planet's history and future.
The Flowing Rock
Because of the temperature difference between the Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material rises (in a mantle plume) while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones. Locations on the surface that lie over plumes are predicted to have high elevation (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot volcanism. The volcanism often attributed to deep mantle plumes is alternatively explained by passive extension of the crust, permitting magma to leak to the surface: the plate hypothesis. The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to be an integral part of the motion of plates. Plate motion should not be confused with continental drift which applies purely to the movement of the crustal components of the continents. The movements of the lithosphere and the underlying mantle are coupled since descending lithosphere is an essential component of convection in the mantle. The observed continental drift is a complicated relationship between the forces causing oceanic lithosphere to sink and the movements within Earth's mantle. Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core. The mantle within about 200 kilometers above the core-mantle boundary appears to have distinctly different seismic properties than the mantle at slightly shallower depths; this unusual mantle region just above the core is called D'' (D double-prime), a nomenclature introduced over 50 years ago by the geophysicist Keith Bullen. D'' may consist of material from subducted slabs that descended and came to rest at the core-mantle boundary or from a new mineral polymorph discovered in perovskite called post-perovskite. Earthquakes at shallow depths are a result of faulting; however, below about 70 kilometers the hot, high pressure conditions ought to inhibit further seismicity. The mantle is considered to be viscous and incapable of brittle faulting. However, in subduction zones, earthquakes are observed down to 670 kilometers. A number of mechanisms have been proposed to explain this phenomenon, including dehydration, thermal runaway, and phase change. The geothermal gradient can be lowered where cool material from the surface sinks downward, increasing the strength of the surrounding mantle, and allowing earthquakes to occur down to a depth of between 410 and 670 kilometers. The pressure at the bottom of the mantle is approximately 136 gigapascals. Pressure increases as depth increases, since the material beneath has to support the weight of all the material above it. The entire mantle, however, is thought to deform like a fluid on long timescales, with permanent plastic deformation accommodated by the movement of point, line, and/or planar defects through the solid crystals composing the mantle. Estimates for the viscosity of the upper mantle range between 10^19 and 10^21 pascal seconds (Pa·s) depending on depth, temperature, composition, state of stress, and numerous other factors. Thus, the upper mantle can only flow very slowly. However, when large forces are applied to the uppermost mantle it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries. The flow of the mantle is a complex process that has been active since the Earth's formation 4.5 billion years ago. The study of the mantle's flow has been a challenging endeavor, with scientists relying on indirect methods such as seismic waves and laboratory experiments to understand its properties. Despite the difficulties, the mantle remains one of the most fascinating and important layers of the Earth, holding the key to understanding the planet's history and future.