Internal Energy Sources Heat Transfer And Flow From Deep In The Earth
in geology crustal heat flow is a measure of the amount of heat energy leaving the Earth from internal energy sources, measured in calories per square centimeter per second. Typical heat-flow values are about 1.5 microcalories per centimeter squared per second, commonly stated as 1.5 heat flow units. most crustal heat flow is due to heat production in the crust by radioactive decay of uranium, thorium, and potassium. heat flow shows a linear relationship with heat production in granitic rocks. some crustal heat flow, however, comes from deeper in the Earth, beneath the crust.
The Earth exhibits a huge variation in temperature, from several thousand degrees in the core to essentially zero degrees Celsius at the surface. The Earth's heat and internal energy were acquired by several mechanisms, including these:
• heat from accretion as potential energy of falling meteorites was converted to heat energy
• heat released during core formation, with gravitational potential energy converted to heat as heavy metallic iron and other elements segregated and sank to form the core soon after accretion
• heat production by decay of radioactive elements
• and heat added by late-impacting meteorites and asteroids, some of which were extremely large in early Earth history heat produced by these various mechanisms gradually flows to the surface by conduction, convection, or advection, and accounts for the component of crustal heat flow that comes from deeper than the crust.
heat flow by conduction involves internal thermal energy flowing from warm to cooler regions, with the heat flux being proportional to the temperature difference, and a proportionality constant k, known as thermal conductivity, related to the material properties. The thermal conductivity of most rocks is low, about one-hundredth that of copper wire.
Advection involves the transfer of heat by the motion of material, such as transport or heat in a magma, in hot water through fractures or pore spaces, and, more important, on a global scale, by the large-scale rising of heated, relatively low-density buoyant material and the complementary sinking of cooled, relatively high-density material in the mantle. The large-scale motion of the mantle, with hot material rising in some places and colder material sinking in other places, is known as convection, an advective heat-transfer mechanism. For convection to occur in the mantle, the buoyancy forces of the heated material must be strong enough to overcome the rock's resistance to flow, known as viscosity. Additionally, the buoyancy forces must overcome the tendency of the rock to lose heat by conduction, since this would cool the rock and decrease its buoyancy. The balance between all of these forces is measured by a quantity called the Raleigh number. Convection in Earth materials occurs above a critical value of the Raleigh number, but below this critical value heat transfer is by conductive processes. Well-developed convection cells in the mantle are very efficient at transporting heat from depth to surface and are the main driving force for plate tectonics.
Heat transfer in the mantle is dominated by convection (advective heat transfer), except in the lower mantle near the boundary with the inner core (the D" region), along the top of the mantle, and in the crust (in the lithosphere), where conductive and hydrothermal (also advective) processes dominate. The zones where conduction dominates the heat transfer are known as conductive boundary layers, and the lithosphere may be thought of as a convecting, con-ductively cooling boundary layer.
The main heat-transfer mechanism that takes internal energy from deep in the Earth's mantle to the near-surface region is convection. It is a thermally driven process where heating at depth causes material to expand and become less dense, causing it to rise while being replaced by complementary cool material that sinks. This moves heat from depth to surface in an efficient cycle since the material that rises gives off heat as it rises and cools, and the material that sinks gets heated only to rise again eventually. Convection is the most important mechanism by which the Earth releases heat, with other mechanisms including conduction, radiation, and advection. However, many of these mechanisms work together in the plate tectonic cycle. Mantle convection brings heat from deep in the mantle to the surface, where the heat released forms magmas that generate the oceanic crust. The midocean ridge axis is the site of active hydrothermal circulation and heat loss, forming black smoker chimneys and other vents. As the crust and lithosphere move away from the midocean ridges, they cool by conduction, gradually subsiding (according the square root of their age) from about 1.5-2.5 miles (2.5-4.0 km) below sea level. Heat loss by mantle convection is therefore the main driving mechanism of plate tectonics, and the moving plates can be thought of as the conduc-tively cooling boundary layer for large-scale mantle convection systems.
The heat transferred to the surface by convection is produced by decay of radioactive heat-producing isotopes such as uranium 235, Thorium 232, and Potassium 40, remnant heat from early heat-producing isotopes such as I 129, remnant heat from accretion of the Earth, heat released during core formation, and heat released during impacts of meteorites and asteroids. During the early history of the planet at least part of the mantle was molten, and the Earth has been cooling by convection ever since. Estimating how much the mantle has cooled with time is difficult, but reasonable estimates suggest that the mantle may have been up to a couple of hundred degrees hotter in the earliest Archean.
The rate of mantle convection is dependent on the ability of the material to flow. The resistance to flow is a quantity measured as viscosity, defined as the ratio of shear stress to strain rate. Fluids with high viscosity are more resistant to flow than materials with low viscosity. The present viscosity of the mantle is estimated to be 1020-1021 Pascal seconds (Pa/s) in the upper mantle and 1021-1023 Pa/s in the lower mantle, viscosities sufficient to allow the mantle to convect and complete an overturn cycle once every 100 million years. The viscosity of the mantle is temperature-dependent, so it is possible that in early Earth history the mantle may have been able to flow and overturn convectively much more quickly, making convection an even more efficient process and speeding the rate of plate tectonic processes.
There is currently on ongoing debate and research relating to the style of mantle convection in the Earth. The relatively heterogeneous upper mantle extends to a depth of 416 miles (670 km), where there is a pronounced increase in seismic velocities. The more homogeneous lower mantle extends to the D" region at 1,678 miles (2,700 km), marking the transition into the liquid outer core. One school of mantle convection thought suggests that the entire mantle, including both the upper and the lower parts, is convecting as one unit. Another school posits that the mantle convection is divided into two layers, with the lower mantle convecting separately from the upper mantle. A variety of these models, presently held by the majority of geophysicists, is that there is two-layer convection, but that subducting slabs can penetrate the 415-mile (670-km) discontinuity from above, and that mantle plumes that rise from the D" region can penetrate the 415-mile (670-km) discontinuity from below.
The shapes taken by mantle convection cells include many possible forms reflected to a first order by the distribution of subduction zones and midocean ridge systems. The subduction zones mark regions of downwelling, whereas the ridge system marks broad regions of upwelling. Material is upwelling in a broad cell beneath the Atlantic and Indian Oceans and downwelling in the circum-Pacific subduction zones. There is thought to be a large plumelike "superswell" beneath part of the Pacific that feeds the East Pacific rise. Mantle plumes that originate from the deep mantle punctuate this broad pattern of upper-mantle convection, and their plume tails are distorted by flow in the convecting upper mantle.
The pattern of mantle convection and transfer of internal energy to the surface deep in geological time is uncertain. Some periods such as the Cretaceous seem to have had much more rigorous mantle convection and surface volcanism. More or different types or rates of mantle convection may have helped to allow the early Earth to lose heat more efficiently. some computer models allow periods of convection dominated by plumes, and others dominated by overturning planiform cells similar to the present Earth. some models suggest cyclic relationships, with slabs pooling at the 425-mile (670-km) discontinuity, then suddenly all sinking into the lower mantle, causing a huge mantle overturn event. Further research is needed on linking the preserved record of mantle convection in the deformed continents to help interpret the past history of convection.
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