Hillslope evolution

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Hillslope evolution is the changes in the erosion rates, erosion styles and form of slopes of hills and mountains over time.

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Conceptual models

During most of the 20th century three models of hillslope evolution were widely diffused: slope decline, slope replacement and parallel slope retreat. Until the 1950s models of hillslope form evolution were central in geomorphology. The modern understanding is that the evolution of slopes is much more complex than the classical models of decline, replacement and retreat imply. [1]

Slope decline

Slope decline was proposed by William Morris Davis in his cycle of erosion theory. Davis' theory of slope decline originated from his essays, such as "The Convex Profile of Badland Divides" (1892), "The Grading of Mountain Slopes" (1898), "The Geographical Cycle" (1899), "Piedmont Bench Lands and Primarumpfe" (1932), and others. [2] The Slope Decline Theory consists of a gradual decrease in slope angle as stream incision slows down. This is accompaigned as slopes becomes more gentle they accumulate with fine-grained regolith stemming from weathering. [1]

Slope replacement

Slope replacement was first proposed by Walther Penck challenging Davis' ideas on slope development. Penck's morphological system aimed to determine the mechanism and origins of crustal movement by analyzing exogenetic processes and morphological attributes. In simpler terms, Penck sought to understand the geological evolution of areas based on the interpretation of their current landform features. [3] Slope replacement describes an evolution of slopes that is associated with decreasing rates of over-all erosion (denudation). It begins with a flattening of the lowermost slope that propagates upward and backward making the uppermost slope recede and decrease its angle while it remains steeper than the lower portions. [1] In Penck's own words: "The flattening of slopes always occurs from the bottom upward". [4]

Parallel slope retreat

Slopes will evolve by parallel retreat when a slopes rock mass strength remains constant and basal debris, like talus, is continuously removed. In reality, however, such uniform rock strength is rare. Rock strength is related to weathering and weathering to climate, so over large distances or over long time-spans slope retreat is unlikely to remain fully parallel in the absence of a structural control which can maintain parallel retreat. Such a structural control, however, is often found in areas where hard horizontal rock layers of basalt or hard sedimentary rock overlie softer rocks. Slopes influenced by the structural control of a durable cap rock tend to cease to evolve by parallel retreat only once overlying hard layers covering softer rock have been fully eroded. [1]

Parallel slope and scarp retreat, albeit proposed by early geomorphologists, was notably championed by Lester Charles King. [1] King considered scarp retreat and the coalescence of pediments into pediplains a dominant processes across the globe. Further he claimed that slope decline was a special case of slope development seen only in very weak rocks that could not maintain a scarp. [5] Slopes that are convex upslope and concave downslope and have no free face were held by King to be a form that became common in the late Tertiary. King argued that this was the result of more slowly acting surface wash caused by carpets of grass which in turn would have resulted in relatively more soil creep. [5] [6]

Unequal activity

The notion that slopes in an area do not develop all at the same time is known as unequal activity. Colin Hayter Crick, who coined the term, proposed that unequal activity may be regulated by removal of debris at the base of slopes. Following this thought erosion by the sea and lateral stream migration are of prime importance as these processes are effective in removing debris. [7] Unequal activity does also imply there are great disparities between stream erosion near stream channels and apparently unchanged uplands, and between headwaters with limited erosion and the more active middle and lower courses of streams. [8] From this it is derived that landscapes and slopes with limited river erosion may in many cases be considered as stagnant in their evolution. [8]

Numerical models

Contrary to early conceptual models that attempt to predict slope form a number of numerical models of erosion focus on describing what is happening at any given time, and are not concerned with changes in form.

Average erosion rates for a slope has been estimated using numerical models. [9] Using the heat transfer equation of Fourier as template W.E.H. Culling reasoned the flux of mass across the height gradient of a slope would could be described in a similar fashion as: [9] [10]

Equation (1) q̃ = −K∇z

On the left-hand side is sediment flux which is the volume of the mass that passes a line each time unit (L3/LT). K is a rate constant (L2/T), and ∇z the gradient or height difference between two points at a slope divided by their horizontal distance. This model imply sediment fluxes can be estimated from the slope angles (∇z). This has been shown to be true for low-angle slopes. For more steep slopes it is not possible to infer sediment fluxes. To address this reality the following model for high angle slopes can be applied: [9]

Equation (2) q̃ = −K∇z/ 1 − (|∇z|/Sc)2

Sc stands here for the critical gradient which at which erosion and sediment fluxes runs away. This model show that when ∇z is far from Sc it behaves like equation 1. On the contrary when ∇z approaches Sc erosion rates becomes extremely high. This last feature may represent the behavior of landslides in steep terrain. [9]

At low erosion rates increased stream or river incision may make gentle slopes evolve into convex forms. Convex forms can thus indirectly reflect accelerated crustal uplift and its associated river incision. [11] [12] [upper-alpha 1] As shown by equation 2 the angle of steep slopes changes very little even at very high increases of erosion rates, meaning that it is not possible to infer erosion rates from topography in steep slopes other than hinting they are much higher than for lower angle slopes. [9]

Parabolic hills

Beginning with the works of Grove Karl Gilbert (1909) and William Morris Davis (1892), soil-mantled convex or parabolic hills have long been held to reflect steady state equilibrium conditions of soil production and soil erosion. [9] [13] [14] Contrary to what an equilibrium between the erosion rates functions described above and the soil production function should imply soil depth can vary considerably in parabolic hills as result of stochastic bedrock weathering into soil. This means that the expected soil formation rates from the soil production function might vary greatly across a landscape in geomorphic equilibrium. [14]

Convex hills are often associated to tors. [15] Numerical modelling indicate that in periglacial settings broad low-angle convex hilltops can form in no less than millions of years. During the evolution of these slopes steeper initial slopes are calculated to result in the formation of numerous tors during the course of the lowering and broadening of the convex area. The presence of numerous tors would thus indicate that the original landscape was steeper and not flatter than present-day landscape. [16]

Notes

  1. ↑ Walther Penck is commonly but wrongly attributed the notion that accelerated uplift leads to the formation of convex slopes. [11]

Related Research Articles

<span class="mw-page-title-main">Erosion</span> Natural processes that remove soil and rock

Erosion is the action of surface processes 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 distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

<span class="mw-page-title-main">Geomorphology</span> Scientific study of landforms

Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform and terrain history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling. Geomorphologists work within disciplines such as physical geography, geology, geodesy, engineering geology, archaeology, climatology, and geotechnical engineering. This broad base of interests contributes to many research styles and interests within the field.

<span class="mw-page-title-main">Peneplain</span> Low-relief plain formed by protracted erosion

In geomorphology and geology, a peneplain is a low-relief plain formed by protracted erosion. This is the definition in the broadest of terms, albeit with frequency the usage of peneplain is meant to imply the representation of a near-final stage of fluvial erosion during times of extended tectonic stability. Peneplains are sometimes associated with the cycle of erosion theory of William Morris Davis, but Davis and other workers have also used the term in a purely descriptive manner without any theory or particular genesis attached.

<span class="mw-page-title-main">Terrain</span> Vertical and horizontal dimension and shape of land surface

Terrain or relief involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word terra means "earth."

<span class="mw-page-title-main">Walther Penck</span> German geologist (1888–1923)

Walther Penck was a geologist and geomorphologist known for his theories on landscape evolution. Penck is noted for criticizing key elements of the Davisian cycle of erosion, concluding that the process of uplift and denudation occur simultaneously, at gradual and continuous rates. Penck's idea of parallel slope retreat led to revisions of Davis's cycle of erosion.


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 interchangeably, erosion is the transport of soil and rocks from one location to another, and denudation is the sum of processes, including erosion, that result in the lowering of Earth's surface. Endogenous processes such as volcanoes, earthquakes, and tectonic uplift can expose continental crust to the exogenous processes of weathering, erosion, and mass wasting. The effects of denudation have been recorded for millennia but the mechanics behind it have been debated for the past 200 years and have only begun to be understood in the past few decades.

Drainage density is a quantity used to describe physical parameters of a drainage basin. First described by Robert E. Horton, drainage density is defined as the total length of channel in a drainage basin divided by the total area, represented by the following equation:

The geographic cycle, or cycle of erosion, is an idealized model that explains the development of relief in landscapes. The model starts with the erosion that follows uplift of land above a base level and ends, if conditions allow, in the formation of a peneplain. Landscapes that show evidence of more than one cycle of erosion are termed "polycyclical". The cycle of erosion and some of its associated concepts have, despite their popularity, been a subject of much criticism.

<span class="mw-page-title-main">Knickpoint</span> Point on a streams profile where a sudden change in stream gradient occurs

In geomorphology, a knickpoint or nickpoint is part of a river or channel where there is a sharp change in channel slope, such as a waterfall or lake. Knickpoints reflect different conditions and processes on the river, often caused by previous erosion due to glaciation or variance in lithology. In the cycle of erosion model, knickpoints advance one cycle upstream, or inland, replacing an older cycle. A knickpoint that occurs at the head of a channel is called a headcut. Headcuts resulting in headward erosion are hallmarks of unstable expanding drainage features such as actively eroding gullies.

<span class="mw-page-title-main">Pediplain</span> Extensive plain formed by the coalescence of pediments

In geology and geomorphology a pediplain is an extensive plain formed by the coalescence of pediments. The processes through which pediplains forms is known as pediplanation. The concepts of pediplain and pediplanation were first developed by geologist Lester Charles King in his 1942 book South African Scenery. The concept gained notoriety as it was juxtaposed to peneplanation.

<span class="mw-page-title-main">Granite dome</span> Rounded hills of bare granite formed by exfoliation

Granite domes are domical hills composed of granite with bare rock exposed over most of the surface. Generally, domical features such as these are known as bornhardts. Bornhardts can form in any type of plutonic rock but are typically composed of granite and granitic gneiss. As granitic plutons cool kilometers below the Earth's surface, minerals in the rock crystallize under uniform confining pressure. Erosion brings the rock closer to Earth's surface and the pressure from above the rock decreases; as a result the rock fractures. These fractures are known as exfoliation joints, or sheet fractures, and form in onionlike patterns that are parallel to the land surface. These sheets of rock peel off the exposed surface and in certain conditions develop domical structures. Additional theories on the origin of granite domes involve scarp-retreat and tectonic uplift.

<span class="mw-page-title-main">Sediment transport</span> Movement of solid particles, typically by gravity and fluid entrainment

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

<span class="mw-page-title-main">Sheet erosion</span>

Sheet erosion or sheet wash is the even erosion of substrate along a wide area. It occurs in a wide range of settings such as coastal plains, hillslopes, floodplains, beaches, savanna plains and semi-arid plains. Water moving fairly uniformly with a similar thickness over a surface is called sheet flow, and is the cause of sheet erosion. Sheet erosion implies that any flow of water that causes the erosion is not canalized. If a hillslope surface contains many irregularities, sheet erosion may give way to erosion along small channels called rills, which can then converge forming gullies. However, sheet erosion may occur despite some limited unevenness in the sheet flow arising from clods of earth, rock fragments, or vegetation.

<span class="mw-page-title-main">Pediment (geology)</span> Very gently sloping inclined bedrock surface

A pediment, also known as a concave slope or waning slope, is a very gently sloping (0.5°-7°) inclined bedrock surface. It is typically a concave surface sloping down from the base of a steeper retreating desert cliff, escarpment, or surrounding a monadnock or inselberg, but may persist after the higher terrain has eroded away.

<span class="mw-page-title-main">Bank erosion</span> Marginal wear of a watercourse

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from erosion of the bed of the watercourse, which is referred to as scour.

<span class="mw-page-title-main">Scarp retreat</span>

Scarp retreat is a geological process through which the location of an escarpment changes over time. Typically the cliff is undermined, rocks fall and form a talus slope, the talus is chemically or mechanically weathered and then removed through water or wind erosion, and the process of undermining resumes. Scarps may retreat for tens of kilometers in this way over relatively short geological time spans, even in arid locations.

<span class="mw-page-title-main">Catena (soil)</span>

A catena in soil science (pedology) is a series of distinct but co-evolving soils arrayed down a slope. Each soil type or "facet" differs somewhat from its neighbours, but all occur in the same climate and on the same underlying parent material. A mature catena is in equilibrium as the processes of deposition and erosion are in balance.

A geomorphological system said to be in dynamic steady state has values that oscillate between maxima and minima around a central mean value.

<span class="mw-page-title-main">River incision</span>

River incision is the narrow erosion caused by a river or stream that is far from its base level. River incision is common after tectonic uplift of the landscape. Incision by multiple rivers result in a dissected landscape, for example a dissected plateau. River incision is the natural process by which a river cuts downward into its bed, deepening the active channel. Though it is a natural process, it can be accelerated rapidly by human factors including land use changes such as timber harvest, mining, agriculture, and road and dam construction. The rate of incision is a function of basal shear-stress. Shear stress is increased by factors such as sediment in the water, which increase its density. Shear stress is proportional to water mass, gravity, and WSS:

<span class="mw-page-title-main">Tillage erosion</span>

Tillage erosion is a form of soil erosion occurring in cultivated fields due to the movement of soil by tillage. There is growing evidence that tillage erosion is a major soil erosion process in agricultural lands, surpassing water and wind erosion in many fields all around the world, especially on sloping and hilly lands A signature spatial pattern of soil erosion shown in many water erosion handbooks and pamphlets, the eroded hilltops, is actually caused by tillage erosion as water erosion mainly causes soil losses in the midslope and lowerslope segments of a slope, not the hilltops. Tillage erosion results in soil degradation, which can lead to significant reduction in crop yield and, therefore, economic losses for the farm.

References

  1. 1 2 3 4 5 Summerfield, Michael A. (1991). "Exogenic processes and landforms". Global Geomorphology: An introduction to the study of landforms. Pearson Education. pp. 184–185. ISBN   0-582-30156-4.
  2. ↑ Paul, K (2023-06-28). "Slope Decline Theory by W.M. Davis". Studyprobe. Retrieved 2023-07-02.{{cite web}}: CS1 maint: url-status (link)
  3. ↑ "Slope Replacement Theory by Walther Penck". Studyprobe. 2023-06-29. Retrieved 2023-07-02.{{cite web}}: CS1 maint: url-status (link)
  4. ↑ Bremer, Hanna (1983). "Albrecht Penck (1858-1945) and Walther Penck (1888-1923), two german geomorphologists". Zeitschrift für Geomorphologie . 27 (2): 129.
  5. 1 2 Twidale, C.R. (1992), "King of the plains: Lester King's contributions to geomorphology", Geomorphology , 5 (6): 491–509, Bibcode:1992Geomo...5..491T, doi:10.1016/0169-555x(92)90021-f
  6. ↑ King, L.C. (1953). "Canons of landscape evolution". Geological Society of America Bulletin. 64 (7): 721–752. Bibcode:1953GSAB...64..721K. doi:10.1130/0016-7606(1953)64[721:cole]2.0.co;2.
  7. ↑ Huggett, p. 440
  8. 1 2 Twidale, C.R. (1993). "C.H. Crickmay, a Canadian rebel". Geomorphology . 6 (4): 357–372. Bibcode:1993Geomo...6..357T. doi:10.1016/0169-555x(93)90055-7.
  9. 1 2 3 4 5 6 Roering, Joshua J.; Kirchner, James W.; Dietrich, William E. (2001). "Hillslope evolution by nonlinear, slope-dependent transport: Steady state morphology and equilibrium adjustment timescales". Journal of Geophysical Research. 106 (B8): 16499–16513. Bibcode:2001JGR...10616499R. doi: 10.1029/2001jb000323 .
  10. ↑ Culling, W.E.H. (1960). "Analytical Theory of Erosion". The Journal of Geology . 68 (3): 336–344. Bibcode:1960JG.....68..336C. doi:10.1086/626663. S2CID   128740332.
  11. 1 2 Simons, Martin (1962), "The morphological analysis of landforms: A new review of the work of Walther Penck (1888-1923)", Transactions and Papers (Institute of British Geographers), 31: 1–14, doi:10.2307/621083, JSTOR   621083
  12. ↑ Chorley et al., p. 790
  13. ↑ Fernandes, Nelson F.; Dietrich, William E. (1997). "Hillslope evolution by diffusive processes: The timescale for equilibrium adjustments". Water Resources Research. 33 (6): 1307–1318. Bibcode:1997WRR....33.1307F. doi: 10.1029/97wr00534 .
  14. 1 2 Riggins, Susan G.; Anderson, Robert S.; Prestrud Anderson, Suzanne; Tye, Andrew M. (2011). "Solving a conundrum of a steady-state hilltop with variable soil depths and production rates, Bodmin Moor, UK". Geomorphology. 128 (1–2): 73–84. Bibcode:2011Geomo.128...73R. doi:10.1016/j.geomorph.2010.12.023.
  15. ↑ Linton, David L. (1955). "The problem of tors". The Geographical Journal . 121 (4): 470–487. doi:10.2307/1791756. JSTOR   1791756.
  16. ↑ Anderson, Robert S. (2002). "Modeling the tor-dotted crests, bedrock edges, and parabolic profiles of high alpine surfaces of the Wind River Range, Wyoming". Geomorphology. 46 (1–2): 35–58. Bibcode:2002Geomo..46...35A. doi:10.1016/s0169-555x(02)00053-3.
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