Mesosphere (mantle)
Encyclopedia
The mesosphere refers to the mantle
in the region between the asthenosphere
and the outer core
. The upper boundary is defined as the sharp increase in seismic wave
velocities and density
at a depth of 660 km. As depth increases, pressure builds and forces atoms into a denser, more rigid structure; thus the difference between mesosphere and asthenosphere is likely due to density and rigidity differences, that is, physical factors, and not to any difference in chemical composition. At a depth of 660 km, ringwoodite
(gamma-(Mg,Fe)2SiO4) decomposes into Mg-Si perovskite
and magnesiowustite
. This reaction marks the boundary between upper mantle and lower mantle. This measurement is estimated from seismic data and high pressure laboratory experiments.
The base of the mesosphere includes theD zone which lies just above the mantle-core boundary
at approximately 2700 to 2890 km. The base of the lower mantle is at about 2700 km.
“Mesosphere” (not to be confused with mesosphere
, a layer of the atmosphere
) is derived from “mesospheric shell”, coined by Reginald Aldworth Daly
, a Harvard University
geology
professor. In the pre-plate tectonics
era, Daly (1940)2 inferred three spherical
layers comprise the outer earth
: lithosphere
(including the crust
), asthenosphere
, and mesospheric shell. Daly’s hypothetical depths to the lithosphere–asthenosphere boundary ranged from 80 to 100 km and the top of the mesospheric shell (base of the asthenosphere) from 200 to 480 km. Thus, Daly’s asthenosphere was inferred to be 120 to 400 km thick. According to Daly, the base of the solid Earth mesosphere could extend to the base of the mantle (and, thus, to the top of the core).
A derivative term, mesoplates
, was introduced as a heuristic, based on a combination of "mesosphere" and "plate", for postulated reference frames in which mantle hotspots
apparently exist.
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
in the region between the asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
and the outer core
Outer core
The outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies beneath the Earth's surface...
. The upper boundary is defined as the sharp increase in seismic wave
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...
velocities and density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
at a depth of 660 km. As depth increases, pressure builds and forces atoms into a denser, more rigid structure; thus the difference between mesosphere and asthenosphere is likely due to density and rigidity differences, that is, physical factors, and not to any difference in chemical composition. At a depth of 660 km, ringwoodite
Ringwoodite
Ringwoodite is a high-pressure polymorph of olivine, and it is stable at high temperatures and pressures like those in the Earth's mantle near 600 km depth. This mineral was first identified in the Tenham Meteorites in 1969, and it is inferred to be present in large quantity in the earth’s...
(gamma-(Mg,Fe)2SiO4) decomposes into Mg-Si perovskite
Perovskite
A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
and magnesiowustite
Ferropericlase
Ferropericlase or Magnesiowüstite is a magnesium/iron oxide that is interpreted to be one of the main constituents of the Earth's lower mantle together with a magnesium /iron silicate with a perovskite structure. Ferropericlase has been found as inclusions in a few natural diamonds...
. This reaction marks the boundary between upper mantle and lower mantle. This measurement is estimated from seismic data and high pressure laboratory experiments.
The base of the mesosphere includes the
Core–mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...
at approximately 2700 to 2890 km. The base of the lower mantle is at about 2700 km.
“Mesosphere” (not to be confused with mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...
, a layer of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
) is derived from “mesospheric shell”, coined by Reginald Aldworth Daly
Reginald Aldworth Daly
-External links:* Daly′s Biography, American Geophysical Union* GSA Today, vol. 16, no. 2, 2006* National Academy of Sciences, Washington, DC, 36 pp., 1960...
, a Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
professor. In the pre-plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
era, Daly (1940)2 inferred three spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
layers comprise the outer earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
: lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
(including the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
), asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
, and mesospheric shell. Daly’s hypothetical depths to the lithosphere–asthenosphere boundary ranged from 80 to 100 km and the top of the mesospheric shell (base of the asthenosphere) from 200 to 480 km. Thus, Daly’s asthenosphere was inferred to be 120 to 400 km thick. According to Daly, the base of the solid Earth mesosphere could extend to the base of the mantle (and, thus, to the top of the core).
A derivative term, mesoplates
Mesoplates
The concept of "mesoplates" was introduced as a heuristic for characterizing the motion of lithospheric plates relative to the sublithospheric source region of hotspot volcanism . W. Jason Morgan , originally suggested that hotspots The concept of "mesoplates" was introduced as a heuristic for...
, was introduced as a heuristic, based on a combination of "mesosphere" and "plate", for postulated reference frames in which mantle hotspots
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
apparently exist.