Mesoplates
Encyclopedia
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
(Pilger, 2003). W. Jason Morgan
(1972), originally suggested that hotspots (inferred by J. Tuzo Wilson) beneath such active volcanic regions as Hawaii and Iceland
form a fixed "absolute" frame of reference for the motion of the overlying plates. However, the existence of a globally fixed reference frame for island-seamount
chains and aseismic ridges ("traces") that are inferred to have originated from hotspots was quickly discounted by the primitive plate reconstructions available in the mid-1970s (Molnar and Atwater, 1973). Further, paleomagnetic measurements imply that hotspots have moved relative to the magnetic pole
s of the Earth (the magnetic poles are further inferred to correspond with the rotational poles of the planet when averaged over thousands of years). Aside: the term "hotspot" is used herein without any genetic implications. The term "melting spot" might well be more applicable.
s have improved over the succeeding three decades since Morgan's original contribution, it is become apparent that the hotspots beneath the central North and South Atlantic
and Indian Ocean
s may form one, distinct frame of reference
, while those underlying the plates beneath the Pacific Ocean
form a separate reference frame. For convenience, the hotspots beneath the Pacific Ocean are referred to as the "Hawaiian set" after Hawaii, while those beneath much of the Atlantic and Indian Ocean are called the "Tristan set" after the island of Tristan da Cunha
(the Tristan hotspot
), one of the principal inferred hotspots of the set. Within a single hotspot set, the traces tied to their originating hotspot can be fit by plate reconstructions which imply only minor relative motion among the hotspots for perhaps the past 130 m.y. (million years) for the Tristan set and 80 m.y. for the Hawaiian set. However, the two hotspot sets are inconsistent with the hypothesis of a single hotspot reference frame; distinct motion between the two sets is apparent between 80 to 30 Ma (m.y. before Present; e.g., Raymond, et al., 2000).
It is important to acknowledge that radiometric dating
of volcanism
along hotspot traces may or may not accurately and precisely constrain the position of the plate above the underlying hotspot at the analytically produced age. However, reconstruction models for the Hawaiian set are constrained in age by the hotspot beneath Easter Island
and its traces on the Pacific
and Nazca plates
between approximately 50 and 30 Ma, as the hotspot was beneath the spreading center during that time interval, and resulting relative plate reconstructions constrain motion of the plates relative to the hotspot. Prior to 50 Ma and since 30 Ma, reconstructions can be determined that fit virtually all existing Hawaiian set traces; the actual ages have the greatest uncertainty. Similarly, plate reconstructions relative to the Tristan set are best constrained in age by relative plate reconstructions, a fortuitous consequence of spherical plate tectonics of three or more plates.
Lithospheric plates are recognized in terms of their lack of internal deformation. Thus two points on the same plate will not move relative to one another, even if the plate moves relative to another plate (or relative to the Earth's rotational poles). Plates are not explicitly defined in terms of their mechanical properties. In a sense, then, "plates" are a heuristic—rather like fitting a straight line through a set of points without a clear functional relationship. Analogously, the term "mesoplate" was introduced. Since the hotspots of the Hawaiian set appear to form a frame of reference (like points on a lithospheric plate, they don't appear to be moving at a very great rate relative to one another), the hotspots and that part of the upper mantle in which they are embedded is termed the "Hawaiian mesoplate". The "Tristan mesoplate" is similarly defined. A third mesoplate, "Icelandic", is inferred to underlie the northernmost Atlantic Ocean, the Arctic Ocean
, much of Eurasia to the north of the Alps and Himalayas; since the Iceland hotspot
trace is not consistent with either the Hawaiian or Tristan set.
Additional evidence for mesoplates comes from observations that intraplate stresses in stable continental interiors of North America and Africa are consistent with plate motions in the Tristan hotspot frame. This observation was first made for contemporary stresses (the maximum horizontal principal compressive stress - sigma-hx); and also appears to hold for paleostress
indicators between approximately 100 and 20 Ma (Pilger, 2003). This observation implies that the sublithospheric mantle over which the plates are moving comprises the same reference frame in which the hotspots are embedded.
The mesoplate heuristic is very much a hypothetical construct. Several observations could discount it. It is conceivable that a missing plate boundary between the plates beneath the Pacific and those beneath the Atlantic and Indian Oceans might be hidden and responsible for the discrepancy between the two hotspot sets. However, progressive study of the most likely region for such a boundary has failed to find it.
The origin of hotspots, whether from deep mantle plumes, mid-mantle melting anomalies, or intraplate fractures, is constrained somewhat by the mesoplate hypothesis. The principal alternative models for the origin of hotspot traces, propagating fractures, are still actively advocated by many workers (see mantleplumes.org). Such a model does not explicitly recognize sublithospheric reference frames. However, it cannot completely explain all of the features of the most familiar hotspot traces (Pilger, 2007).
The mantle plume hypothesis for the origin of hotspots need not be inconsistent with mesoplates. However, it would need to be modified to recognize that the lack of motion between hotspots represents a kind of "embedding" of the "plume" in the upper mantle (shallow mesosphere) of the Earth. One of Morgan's rationales for plumes was the existence of an "absolute motion" reference frame. Numerical modeling now indicates that such a reference frame would be unlikely in the context of plume convection.
If continued research were to demonstrate the continued applicability of the mesoplate hypothesis, it would have important implications for the nature of convection
in the upper mantle: Convective motion beneath plates is almost entirely vertical within individual mesoplates; lateral motion in the mantle would be confined to mesoplate boundaries and to greater depths.
, and “tectonic plates
”.
” (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) 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).
Isacks, Oliver, and Sykes (1968) applied lithosphere and asthenosphere to their conception to the “New Global Tectonics” or what subsequently became known as plate tectonics
. In their conception, the base of the asthenosphere extended as deep as the deepest (650–700 km) earthquake
s in the inclined seismic zones where descending lithospheric plates
penetrate the upper mantle.
of plate tectonics
are so defined because they behave in a kinematically
rigid
manner. That is, any three points on the same plate do not move relative to one another, while the plate itself (and all points it contains) may more relative to other plates or other internal reference frame (e.g., the earth’s spin axis or geomagnetic pole
s). In other words, ideal lithospheric plates do not deform internally as they move.
A “mesoplate”, then behaves like lithospheric plates: empirical evidence (discussed above) indicates groups of melting anomalies (hotspots
) embedded in the shallow mesosphere do not move relative to one another, but collectively move relative to other hotspot groups and relative to overlying lithospheric plates.
Heuristic
Heuristic refers to experience-based techniques for problem solving, learning, and discovery. Heuristic methods are used to speed up the process of finding a satisfactory solution, where an exhaustive search is impractical...
for characterizing the motion of lithospheric plates relative to the sublithospheric source region of hotspot
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...
volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
(Pilger, 2003). W. Jason Morgan
W. Jason Morgan
William Jason Morgan is an American geophysicist who has made seminal contributions to the theory of plate tectonics and geodynamics...
(1972), originally suggested that hotspots (inferred by J. Tuzo Wilson) beneath such active volcanic regions as Hawaii and Iceland
Iceland
Iceland , described as the Republic of Iceland, is a Nordic and European island country in the North Atlantic Ocean, on the Mid-Atlantic Ridge. Iceland also refers to the main island of the country, which contains almost all the population and almost all the land area. The country has a population...
form a fixed "absolute" frame of reference for the motion of the overlying plates. However, the existence of a globally fixed reference frame for island-seamount
Seamount
A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface , and thus is not an island. These are typically formed from extinct volcanoes, that rise abruptly and are usually found rising from a seafloor of depth. They are defined by oceanographers as...
chains and aseismic ridges ("traces") that are inferred to have originated from hotspots was quickly discounted by the primitive plate reconstructions available in the mid-1970s (Molnar and Atwater, 1973). Further, paleomagnetic measurements imply that hotspots have moved relative to the magnetic pole
Magnetic pole
Magnetic pole may refer to:* One of the two ends of a magnet* The magnetic poles of astronomical bodies, a special case of magnets, two special cases of which are the Geomagnetic poles:...
s of the Earth (the magnetic poles are further inferred to correspond with the rotational poles of the planet when averaged over thousands of years). Aside: the term "hotspot" is used herein without any genetic implications. The term "melting spot" might well be more applicable.
Development of the concept
As plate reconstructionPlate reconstruction
Plate reconstruction is the process of reconstructing the positions of tectonic plates relative to each other or to other reference frames, such as the earth's magnetic field or groups of hotspots, in the geological past...
s have improved over the succeeding three decades since Morgan's original contribution, it is become apparent that the hotspots beneath the central North and South Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
and Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
s may form one, distinct frame of reference
Frame of reference
A frame of reference in physics, may refer to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an observer.It may also refer to both an...
, while those underlying the plates beneath the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
form a separate reference frame. For convenience, the hotspots beneath the Pacific Ocean are referred to as the "Hawaiian set" after Hawaii, while those beneath much of the Atlantic and Indian Ocean are called the "Tristan set" after the island of Tristan da Cunha
Tristan da Cunha
Tristan da Cunha is a remote volcanic group of islands in the south Atlantic Ocean and the main island of that group. It is the most remote inhabited archipelago in the world, lying from the nearest land, South Africa, and from South America...
(the Tristan hotspot
Tristan hotspot
The Tristan hotspot is a volcanic hotspot which is responsible for the volcanic activity which forms the volcanoes in the southern Atlantic Ocean. It is thought to have formed the island of Tristan da Cunha and the Walvis Ridge on the African Plate....
), one of the principal inferred hotspots of the set. Within a single hotspot set, the traces tied to their originating hotspot can be fit by plate reconstructions which imply only minor relative motion among the hotspots for perhaps the past 130 m.y. (million years) for the Tristan set and 80 m.y. for the Hawaiian set. However, the two hotspot sets are inconsistent with the hypothesis of a single hotspot reference frame; distinct motion between the two sets is apparent between 80 to 30 Ma (m.y. before Present; e.g., Raymond, et al., 2000).
It is important to acknowledge that radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
of volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
along hotspot traces may or may not accurately and precisely constrain the position of the plate above the underlying hotspot at the analytically produced age. However, reconstruction models for the Hawaiian set are constrained in age by the hotspot beneath Easter Island
Easter Island
Easter Island is a Polynesian island in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian triangle. A special territory of Chile that was annexed in 1888, Easter Island is famous for its 887 extant monumental statues, called moai, created by the early Rapanui people...
and its traces on the Pacific
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
and Nazca plates
Nazca Plate
]The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction along the Peru-Chile Trench of the Nazca Plate under the South American Plate is largely responsible for the...
between approximately 50 and 30 Ma, as the hotspot was beneath the spreading center during that time interval, and resulting relative plate reconstructions constrain motion of the plates relative to the hotspot. Prior to 50 Ma and since 30 Ma, reconstructions can be determined that fit virtually all existing Hawaiian set traces; the actual ages have the greatest uncertainty. Similarly, plate reconstructions relative to the Tristan set are best constrained in age by relative plate reconstructions, a fortuitous consequence of spherical plate tectonics of three or more plates.
Lithospheric plates are recognized in terms of their lack of internal deformation. Thus two points on the same plate will not move relative to one another, even if the plate moves relative to another plate (or relative to the Earth's rotational poles). Plates are not explicitly defined in terms of their mechanical properties. In a sense, then, "plates" are a heuristic—rather like fitting a straight line through a set of points without a clear functional relationship. Analogously, the term "mesoplate" was introduced. Since the hotspots of the Hawaiian set appear to form a frame of reference (like points on a lithospheric plate, they don't appear to be moving at a very great rate relative to one another), the hotspots and that part of the upper mantle in which they are embedded is termed the "Hawaiian mesoplate". The "Tristan mesoplate" is similarly defined. A third mesoplate, "Icelandic", is inferred to underlie the northernmost Atlantic Ocean, the Arctic Ocean
Arctic Ocean
The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...
, much of Eurasia to the north of the Alps and Himalayas; since the Iceland hotspot
Iceland hotspot
The Iceland hotspot is a hotspot which is partly responsible for the high volcanic activity which has formed the island of Iceland.-Description:...
trace is not consistent with either the Hawaiian or Tristan set.
Additional evidence for mesoplates comes from observations that intraplate stresses in stable continental interiors of North America and Africa are consistent with plate motions in the Tristan hotspot frame. This observation was first made for contemporary stresses (the maximum horizontal principal compressive stress - sigma-hx); and also appears to hold for paleostress
Paleostress
Paleostress is a subset of mechanical stress within geology. Variations in stress fields within the Earth's crust can result in a variety of mechanical responses:*Microscopic:*Crystal deformation, including twinning,*pressure solution*Microfractures,...
indicators between approximately 100 and 20 Ma (Pilger, 2003). This observation implies that the sublithospheric mantle over which the plates are moving comprises the same reference frame in which the hotspots are embedded.
The mesoplate heuristic is very much a hypothetical construct. Several observations could discount it. It is conceivable that a missing plate boundary between the plates beneath the Pacific and those beneath the Atlantic and Indian Oceans might be hidden and responsible for the discrepancy between the two hotspot sets. However, progressive study of the most likely region for such a boundary has failed to find it.
The origin of hotspots, whether from deep mantle plumes, mid-mantle melting anomalies, or intraplate fractures, is constrained somewhat by the mesoplate hypothesis. The principal alternative models for the origin of hotspot traces, propagating fractures, are still actively advocated by many workers (see mantleplumes.org). Such a model does not explicitly recognize sublithospheric reference frames. However, it cannot completely explain all of the features of the most familiar hotspot traces (Pilger, 2007).
The mantle plume hypothesis for the origin of hotspots need not be inconsistent with mesoplates. However, it would need to be modified to recognize that the lack of motion between hotspots represents a kind of "embedding" of the "plume" in the upper mantle (shallow mesosphere) of the Earth. One of Morgan's rationales for plumes was the existence of an "absolute motion" reference frame. Numerical modeling now indicates that such a reference frame would be unlikely in the context of plume convection.
If continued research were to demonstrate the continued applicability of the mesoplate hypothesis, it would have important implications for the nature of convection
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...
in the upper mantle: Convective motion beneath plates is almost entirely vertical within individual mesoplates; lateral motion in the mantle would be confined to mesoplate boundaries and to greater depths.
Origin of the term
“Mesoplates” is a combination and contraction of two terms: “mesosphere”, as applied to the solid earthEarth
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...
, and “tectonic plates
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
”.
Mesosphere (solid earth)
“MesosphereMesosphere (mantle)
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...
” (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) 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
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....
(and, thus, to the top of the core).
Isacks, Oliver, and Sykes (1968) applied lithosphere and asthenosphere to their conception to the “New Global Tectonics” or what subsequently became known as plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
. In their conception, the base of the asthenosphere extended as deep as the deepest (650–700 km) earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s in the inclined seismic zones where descending lithospheric plates
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 :...
penetrate the upper mantle.
Lithospheric (tectonic) plate
The (spherical) lithospheric platesLithosphere
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 :...
of plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
are so defined because they behave in a kinematically
Kinematics
Kinematics is the branch of classical mechanics that describes the motion of bodies and systems without consideration of the forces that cause the motion....
rigid
Rigid
In mathematics, a rigid collection C of mathematical objects is one in which every c ∈ C is uniquely determined by less information about c than one would expect....
manner. That is, any three points on the same plate do not move relative to one another, while the plate itself (and all points it contains) may more relative to other plates or other internal reference frame (e.g., the earth’s spin axis or geomagnetic pole
Geomagnetic pole
The geomagnetic poles are antipodal points where the axis of a theoretical dipole intersects the Earth's surface. This dipole is equivalent to a powerful bar magnet at the center of the Earth, and it is the dipole that comes closest to accounting for the magnetic field observed at the Earth's...
s). In other words, ideal lithospheric plates do not deform internally as they move.
A “mesoplate”, then behaves like lithospheric plates: empirical evidence (discussed above) indicates groups of melting anomalies (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...
) embedded in the shallow mesosphere do not move relative to one another, but collectively move relative to other hotspot groups and relative to overlying lithospheric plates.