geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

and

geophysics Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' so ...

, thermal subsidence is a mechanism of

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

in which

conductive In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gene ...

cooling of the

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

thickens the

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...

and causes it to decrease in elevation. This is because of

thermal contraction Negative thermal expansion (NTE) is an unusual physicochemical process in which some materials contract upon heating, rather than expand as most other materials do. The most well-known material with NTE is water at 0~4 °C. Water's NTE is the r ...

: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes denser than the surrounding material. Additional material added to the lithosphere thickens it and further causes a

buoyant Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...

decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming a

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsiden ...

Causes

Thermal subsidence can occur anywhere in which a temperature differential exists between a section of the lithosphere and its surroundings. There are a variety of contributing factors that can initiate thermal subsidence or affect the process as it is ongoing.

Delamination

endogenous Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, ...

and

exogenous In a variety of contexts, exogeny or exogeneity () is the fact of an action or object originating externally. It contrasts with endogeneity or endogeny, the fact of being influenced within a system. Economics In an economic model, an exogen ...

processes cause

denudation Denudation is the geological processes in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interc ...

of the earth's surface, lower, warmer sections of the lithosphere are exposed to relative differences in weight and density. This relative difference creates

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

Isostatic uplift Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...

can then further expose the lithosphere to conductive cooling, causing a “rise and fall” phenomenon as warmer, less dense rock layers are pushed or buoyed up, then cooled, causing it to contract and sink back down.

Conduction

The conditions to create thermal subsidence can be initiated by various forms of uplift and denudation, but the actual process of thermal subsidence is governed by the loss of heat via thermal

conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured free improvisation ...

. Contact with surrounding rock or the surface causes heat to leach out of a section of the lithosphere. As the lithosphere cools, it causes the rock to contract.

Isostasy

When conduction causes a section of the lithosphere to contract and increase in density, it does not directly add mass to the rock. Instead, it causes the volume to decrease, increasing the mass of the section for a given area. The lithosphere is isostatic with the

; its weight is supported by the relative density of the surrounding rock. When a section cools and its density increases, it sinks, causing the relative elevation to decrease. This can create a basin in which sediments are deposited, which adds weight on top of the sinking section of the lithosphere and increase the total mass of the section per unit area, causing it to sink further.

Effects

Thermal subsidence can have an effect on island formation.

can be balanced with thermal subsidence in response to erosion on islands without barrier reefs, which sink only when subjected to wave

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

. However,

volcanic islands Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...

and

seamounts A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abru ...

with barrier reefs are shielded from wave and stream erosion, and thus the countervailing isostatic uplift is eliminated, causing them to subside and create an

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gro ...

Metamorphism

Thermal subsidence can drive metamorphism in rocks. The conduction of heat out of a section of lithosphere causes the rock to thicken and become more insulated to heat flowing in from the mantle; as this thicker section is buried by the descending column of the lithosphere, it descends into surrounding rock layers with a higher relative

geothermal gradient Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate bo ...

. This gradient can cause metamorphism in rocks, as seen in South Australia.

Eustasy

Eustasy The eustatic sea level is the distance from the center of the earth to the sea surface. An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or more mid-oceanic ridges. ...

refers to a change in the relative sea level. It can have effects on the thermal subsidence during the formation of geological features such as

mountain ranges A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arise ...

. Sea level often changes in response to the formation of

glaciers A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as ...

on land; the weight of these glaciers or the absence thereof can influence the overall rate of thermal subsidence.

Petroleum formation

As the lithosphere cools and subsides, a

can be formed on top of the subsiding mass. The characteristics of the basement sediments can produce conditions conducive to the conversion of

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

into

petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...

. The gigantic

Wilmington Oil Field The Wilmington Oil Field is a prolific petroleum field in Los Angeles County in southern California in the United States. Discovered in 1932, it is the third largest oil field in the United States in terms of cumulative oil production. The field ...

in the

Los Angeles Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the largest city in the state of California and the second most populous city in the United States after New York City, as well as one of the world ...

basin was formed as a result of this process.Turcotte, D. L., D. C. McAdoo. “Thermal subsidence and petroleum generation in the southwestern block of the Los Angeles basin, California.” Journal of Geophysical Research, vol. 84, issue B7, p. 3460-3464. July 1979.

References

{{reflist Geodynamics