Cycle of Erosion/ Geomorphic Cycle:
Cycle of Erosion, which is also called geographic or geomorphic cycle 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 (a more or less level land surface produced by erosion over a long period, undisturbed by crustal movement). 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 its popularity, been a subject of much criticism. To understand the cycle of evolution of landscape, let us start with a relatively flat upland area in a humid region.
- During the youth stage, the area remains more or less flat. The stream valleys are generally narrow.
- In mature stage, the relief of the land increases and the landscape is changed into one consisting of hills and valleys. The “relief” of an area may be defined as the maximum difference in the elevation.
- In old stage, the streams will approach base level and the land will be reduced to a peneplain. The peneplain is an undulating plain which lies nearly at base level. Some mounds or hillocks of hard rocks still persist. These hillocks are called “monadnocks”.
Origin of the concept of cycle of Erosion:
The originator of the erosion cycle model, William Morris Davis, divided it into stages. The transition of these stages is gradual.
Youthful Stage: The beginning of the model starts from an uplifted landscape. The very first stage of the erosion is its youth stage, where river incision is the dominant process shaping the landscape. During the youthful stage the difference between ups and downs, and heights and valley bottoms increases rapidly.
Mature Stage: With the rapid change in the level of landscape the youthful stage enters into a mature stage in which the height differences between valley bottoms and uplands are at their greatest level. In this stage, slope decline becomes a more important phenomenon, and uplands lose height more rapidly than the rivers incising stage, effectively diminishing relief.
Old stage: The last stage is known is Old stage. In this stage, erosion has acted so long that the landscape, despite its original height, is reduced into a rolling lowland. That landscape of low relief is called a peneplain and may contain residual heights standing out from the general level. The peneplain can be uplifted, which starts a second erosion cycle.
Davis acknowledged that a full cycle was a special case and that initial uplift was not necessarily rapid or followed by a prolonged period of quiescence. However, as Walther Penck pointed out, Davis and his followers usually used a rapid uplift and quiescence approach to explain landscapes. This means that the model, as understood by most, assumes rapid and episodic tectonic uplift. Another characteristic of the model is that slopes evolve by decline, with initially-steep slopes worn out by erosion forming successively-gentler slopes. Weaknesses of the model are that it is mostly theoretical and deductive in nature and it does not take into account the complexity of tectonic movements or climate change. The nature of surface processes is also poorly represented by the model. The model in its original form is intended to explain relief development in temperate landscapes in which erosion by running water is assumed to be of prime importance. Nevertheless, the cycle of erosion has been extended, with modifications, into arid, glacial, coastal, karst and periglacial areas.
VARIANTS OF CYCLE OF EROSION ON VARIOUS TYPES OF LANDSCAPE
|Type of Landscape||Proposed By||Details|
|Arid Landscape||William Morris Devis||At the beginning of the cycle of erosion in arid climate there are numerous small basins to where material is washed during the scarce rainfall events. In the next stage (youthful stage) valleys are developed and highlands dissected by these. Gentle slopes and basins accumulated material derived from the highlands. In the mature stage drainage basins coalesce. At the end, a stage is reached in which the terrain has lost much of its relief and deflation hollows interfere with the drainage systems, breaking it up into local systems. During all stages sand and dust might be exported by wind to other landscapes.|
|Coastal Landscape||Douglas Wilson Johnson||Alternate models are proposed for shore profiles: one for shore profiles of emergence and another for shore profiles of submergence. A complementary model can be applied to shore lines where different parts might have undergone submergence and emergence.|
|Glacial Landscape||William Morris Devis||The glacial cycle of erosion deals with mountainous regions and lacks an old stage since Davis argues that nothing more developed than mature glacial landscapes exists at present. A glacial cycle of erosion begins with a pre-glacial landscape. Over time valley glaciers erode the underlying rock at different rates, creating valleys and glaciers that are more entrenched than others. As time passes the less-entrenched glaciers reach the same levels of entrenchment as the more entrenched ones, since the deeper a glacier erodes, its erosive power diminishes. In a mature stage, valleys form smooth-sided troughs.|
|Karst Landscape||Jovan Cvijic||The cycle of erosion in karst regions has three phases. At first, the upper parts of fractures are dissolved, enlarged and filled with water. Normal fluvial valleys develop on the surface, small poljes might exist. Subsequently, re-routing of water by the growth of a karst system disorganizes the fluvial drainage pattern, with valley bottoms developing large dolines and uvalas. Ridges between uvalas gradually disappear as those landforms coalesce. If the bedrock is underlain by insoluble rock, normal fluvial valleys will slowly re-appear once the underground river systems reach the insoluble rock. In the end, soluble rocks appear only as isolated hills. Contrary to Davis’s original cycle of erosion, the karst cycle does not end in the formation of a peneplain.|
|Periglacial Landscape||Peltier||The periglacial cycle of erosion begins with a non-periglaciated landscape. Once-periglaciated mass wasting of regolith exposes bedrock in the upper slopes. The outcrops are then subject to frost weathering that makes slopes retreat forming extensive blockfields at the base of the bedrock areas. At a later stage, solifluction wears down summits and fills in topographic lows.|