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The interaction between erosion and tectonics has been a topic of debate since the early 1990s. While the tectonic effects on surface processes such as
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 distin ...
have long been recognized (for example, river formation as a result of
tectonic uplift Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thick ...
), the opposite (erosional effects on tectonic activity) has only recently been addressed. The primary questions surrounding this topic are what types of interactions exist between erosion and tectonics and what are the implications of these interactions. While this is still a matter of debate, one thing is clear, Earth's landscape is a product of two factors:
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
, which can create topography and maintain relief through surface and rock uplift, and
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
, which mediates the erosional processes that wear away upland areas over time. The interaction of these processes can form, modify, or destroy
geomorphic Geomorphology (from Ancient Greek: , ', "earth"; , ', "form"; and , ', "study") is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or n ...
features on
Earth's surface Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface ...
.


Tectonic processes

The term
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...
refers to the study of Earth's surface structure and the ways in which it changes over time. Tectonic processes typically occur at plate boundaries which are one of three types: convergent boundaries, divergent boundaries, or transform boundaries. These processes form and modify the topography of the Earth's surface, effectively increasing relief through the mechanisms of
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 ...
, crustal thickening, and
deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defo ...
in the form of
faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...
and
folding Fold, folding or foldable may refer to: Arts, entertainment, and media * ''Fold'' (album), the debut release by Australian rock band Epicure *Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot *Above ...
. Increased elevations, in relation to regional base levels, lead to steeper river channel gradients and an increase in orographically localized precipitation, ultimately resulting in drastically increased erosion rates. The topography, and general relief, of a given area determines the velocity at which surface runoff will flow, ultimately determining the potential erosive power of the runoff. Longer, steeper slopes are more prone to higher rates of erosion during periods of heavy rainfall than shorter, gradually sloping areas. Thus, large mountain ranges, and other areas of high relief, formed through tectonic uplift will have significantly higher rates of erosion. Additionally, tectonics can directly influence erosion rates on a short timescale, as is clear in the case of
earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
, which can trigger landslides and weaken surrounding rock through seismic disturbances. While
tectonic uplift Tectonic uplift is the geologic uplift of Earth's surface that is attributed to plate tectonics. While isostatic response is important, an increase in the mean elevation of a region can only occur in response to tectonic processes of crustal thick ...
in any case will lead to some form of increased elevation, thus higher rates of erosion, a primary focus is set on isostatic uplift as it provides a fundamental connection between the causes and effects of erosional-tectonic interactions.


Isostatic uplift

Understanding the principle of
isostasy Isostasy (Greek ''ísos'' "equal", ''stásis'' "standstill") or isostatic equilibrium is the state of gravitational equilibrium between Earth's crust (or lithosphere) and mantle such that the crust "floats" at an elevation that depends on its ...
is a key element to understanding the interactions and feedbacks shared between erosion and tectonics. The principle of isostasy states that when free to move vertically, lithosphere floats at an appropriate level in the asthenosphere so that the pressure at a depth of compensation in the asthenosphere well below the base of the lithosphere is the same. Isostatic uplift is both a cause and an effect of erosion. When deformation occurs in the form of crustal thickening an isostatic response is induced causing the thickened crust to sink, and surrounding thinner crust to uplift. The resulting surface uplift leads to enhanced elevations, which in turn induces erosion. Alternatively, when a large amount of material is eroded away from the Earth's surface uplift occurs in order to maintain isostatic equilibrium. Because of isostasy, high erosion rates over significant horizontal areas can effectively suck up material from the lower crust and/or
upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appro ...
. This process is known as ''isostatic rebound'' and is analogous to Earth's response following the removal of large glacial ice sheets. Isostatic uplift and corresponding erosion are responsible for the formation of regional-scale geologic features as well as localized structures. Two such examples include: * Continental shields – Generally large areas of low relief (<100 m) in
Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
where
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the P ...
crystalline
igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
and high-grade
metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
s are exposed. Shields are considered tectonically stable areas in comparison to the activity occurring at their margins and the boundaries between plates, but their formation required large amounts of tectonic activity and erosion. Shields, along with stable platforms, are the basic tectonic components of continents, therefore understanding their development is critical to understanding the development of other surface features on Earth. Initially, a mountain belt is formed at a convergent plate margin. Transformation of a mountain belt to a shield is majorly dependent on two factors: (1) erosion of the mountain belt by running water and (2) isostatic adjustment resulting from the removal of surface rock due to erosion. This process of erosion followed by isostatic adjustment continues until the system is at isostatic equilibrium. At this point large-scale erosion can no longer occur because the surface has eroded down to nearly sea-level and uplift ceases due to the system's state of equilibrium. * River anticlines – Geologic structures formed through the focused uplift of rock underlying confined areas of high erosion (''i.e.'', rivers). Isostatic rebound resulting from the rapid removal of overlying rock, via erosion, causes the weakened areas of crustal rock to uplift from the apex of the river. In order for the development of these structures to occur the erosion rate of the river must exceed both the average erosional rate of the area, and the rate of uplift of the orogen. The two factors influencing the development of these structures are stream power of the associated river and the
flexural rigidity Flexural rigidity is defined as the force couple required to bend a fixed non- rigid structure by one unit of curvature, or as the resistance offered by a structure while undergoing bending. Flexural rigidity of a beam Although the moment M(x) a ...
of the crust in the area. The combination of increased stream power with decreased flexural rigidity results in the system's progression from a transverse anticline to a river anticline.


Channel flow

Channel flow describes the process through which hot,
viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inter ...
crustal material flows horizontally between the upper crust and lithospheric mantle, and is eventually pushed to the surface. This model aims to explain features common to metamorphic
hinterland Hinterland is a German word meaning "the land behind" (a city, a port, or similar). Its use in English was first documented by the geographer George Chisholm in his ''Handbook of Commercial Geography'' (1888). Originally the term was associated ...
s of some collisional orogens, most notably the
Himalaya The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
Tibetan Plateau The Tibetan Plateau (, also known as the Qinghai–Tibet Plateau or the Qing–Zang Plateau () or as the Himalayan Plateau in India, is a vast elevated plateau located at the intersection of Central, South and East Asia covering most of the Ti ...
system. In mountainous areas with heavy rainfall (thus, high erosion rates) deeply incising rivers will form. As these rivers wear away the Earth's surface two things occur: (1) pressure is reduced on the underlying rocks effectively making them weaker and (2) the underlying material moves closer to the surface. This reduction of crustal strength, coupled with the erosional
exhumation Burial, also known as interment or inhumation, is a method of final disposition whereby a dead body is placed into the ground, sometimes with objects. This is usually accomplished by excavating a pit or trench, placing the deceased and objec ...
, allows for the diversion of the underlying channel flow toward Earth's surface.


Erosional processes

The term
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 distin ...
refers to the group of natural processes, including
weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs ''in situ'' (on site, with little or no movement), ...
, dissolution, abrasion, corrosion, and transportation, by which material is worn away from Earth's surface to be transported and deposited in other locations. * Differential erosion – Erosion that occurs at irregular or varying rates, caused by the differences in the resistance and hardness of surface materials; softer and weaker rocks are rapidly worn away, whereas harder and more resistant rocks remain to form ridges, hills, or mountains. Differential erosion, along with the tectonic setting, are two of the most important controls on the evolution of continental landscapes on Earth. The feedback of erosion on tectonics is given by the transportation of surface, or near-surface, mass (rock, soil, sand,
regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
, etc.) to a new location. This redistribution of material can have profound effects on the state of gravitational stresses in the area, dependent on the magnitude of mass transported. Because tectonic processes are highly dependent on the current state of gravitational stresses, redistribution of surface material can lead to tectonic activity. While erosion in all of its forms, by definition, wears away material from the Earth's surface, the process of mass wasting as a product of deep
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviogla ...
incision has the highest tectonic implications.


Mass wasting

Mass wasting is the geomorphic process by which surface material move downslope typically as a mass, largely under the force of gravity As rivers flow down steeply sloping mountains, deep channel incision occurs as the river's flow wears away the underlying rock. Large channel incision progressively decreases the amount of gravitational force needed for a slope failure event to occur, eventually resulting in mass wasting. Removal of large amounts of surface mass in this fashion will induce an isostatic response resulting in uplift until equilibrium is reached.


Effects on structural evolution

Recent studies have shown that erosional and tectonic processes have an effect on the structural evolution of some geologic features, most notably orogenic wedges. Highly useful sand box models, in which horizontal layers of sand are slowly pressed against a backstop, have shown that the geometries, structures, and kinematics of orogenic wedge formation with and without erosion and sedimentation are significantly different. Numerical models also show that the evolution of orogens, their final tectonic structure, and the potential development of a high plateau, all are sensitive to the long term climate over the mountains, for example, the concentration of precipitation in one side of the orogen due to
orographic lift Orographic lift occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and cr ...
under a dominant wind direction.Garcia-Castellanos, D., 2007. The role of climate in high plateau formation. Insights from numerical experiments. Earth Planet. Sci. Lett. 257, 372–390, doi:10.1016/j.epsl.2007.02.03

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See also

*
Dynamic topography The term dynamic topography is used in geodynamics to refer the elevation differences caused by the flow within Earth's mantle. Definition In geodynamics, ''dynamic topography'' refers to topography generated by the motion of zones of differing ...
* Glacial buzzsaw * Orogenic collapse *
Post-glacial rebound 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 a ...


References

{{River morphology Geophysics Geomorphology