In this article we will discuss about the meaning and causes of interruptions in the cycle of erosion.
Meaning of Interruptions in the Cycle of Erosion:
Davisian cyclic concept envisages gradual sequential changes in uplifted landmass through the stages of youth, mature and old culminating into the development of low featureless plain called as peneplain. The completion of such generalized and ideal form of cycle of erosion depends on tectonic stability of longer period of time, which is seldom possible in the nature as the earth is unstable.
Thus, the cycle of erosion is liable to frequent interruptions which cause inequilibrium condition in the cyclic model. Any sort of obstacle in the normal functioning of cycle of erosion is called interruption of cycle. The basic causes of interruption may be climatic or tectonic or both. Tectonic factors are related to positive and negative movement of base levels of erosion.
The interruptions in cycle of erosion caused by positive movement of base level (due to subsidence of landmass or rise in sea level) shortens the cyclic time as it advances forward e.g., if the cycle is in youth stage, it may advance (due to positive movement of base level) to mature stage or if it is in the mature stage it may advance to old stage.
Conversely, negative change in base level of erosion (due to upliftment of landmass or fall in sea level) lengthens the cyclic time as the cycle is pushed backward e.g., if the cycle is running through late mature stage and there is either sudden upliftment of landmass or fall in sea level, the cycle goes back to youth stage characterized by renewed vigour of valley incision known as rejuvenation.
Thus, the cycles punctuated by interruptions are called as interrupted cycles which lead to occurrences of several cycles in a region. Such cycles are called polycycles. If the polycycles (multi-cycles) occur in succession, they are called successive cycles of erosion and the landscapes resulting therefrom are called polyclic or multi-cyclic landscapes. Applachian regions of the U.S.A. and Chotonagpur region of India present typical examples of polycyclic landscapes where several cycles have been completed.
Paired terraces, valley in valley topography, and incised meanders of the Damodar Valley at Rajroppa in Hazaribagh (Jharkhand) and valley in valley topography and paired terraces, Dhunwadhar falls and incised meander of the Narmada River at Bheraghat near Jabalpur (M.P.) are indicative of rejuvenation and polycyclic reliefs.
It may be pointed out that the interruption of cycle of erosion is generally divided into two categories i.e., if the interruption is such that the period of cycle is either lengthened (due to rejuvenation) or shortened it is simply called as interruption of cycles. It may also be called as base level change interruption.
If the interruption is such that the chapter of ongoing cycle is closed and a new cycle is initiated after a gap of long time, such interruption is called as accident which is caused either due to climatic changes or volcanic eruptions.
Suppose, a fluvial cycle of erosion is running through mature stage, and if there is a widespread volcanic fissure flow leading to pouring of immense volume of lava resulting in the obliteration of all the existing streams, then the chapter of cycle of erosion would be closed. New set of streams would appear on new surface after the lava is cooled and solidified and a fresh cycle of erosion would start.
Causes of Interruption in the Cycle of Erosion:
1. Interruptions due to vulcanicity and climatic changes (accidents):
The chapter of any cycle of erosion is closed due to permanent interruptions caused by volcanic eruption and climatic changes. The widespread volcanic fissure flows cause upwelling and pouring of immense volume of basaltic lava which covers larger areas and obliterates surface drainage and reliefs which results in the permanent interruption and closure of the existing chapter of existing fluvial cycle of erosion.
The fresh cycle of erosion may start only when the fissure flow ceases, lavas are cooled and solidified, new surface is formed and new sets of streams are initiated. It may be pointed out that such interruptions in the fluvial cycle occurred over Indian peninsula during Cretaceous period when Deccan lava flows covered vast areas of the Deccan plateau including Chotanagpur plateau and even southern parts of the Vindhyan upland causing closure of the Jurassic cycle of erosion. The new Tertiary cycle was initiated only after the Deccan trappean lavas were cooled and solidified and monsoon climate set in.
Climatic interruptions (accidents) occur due to major changes in the climate of the concerned region, for example, if the fluvial cycle of erosion in a humid region is passing through mature stage, and there is sudden climatic change leading to onset of either extreme dry conditions or extreme cold conditions, then the cycle of erosion is interrupted to such an extent that the chapter of current cycle of erosion is closed and another set of cycle either arid cycle (if the climate becomes extreme hot and arid) or glacial or periglacial cycle (if the climate becomes glacial/periglacial or subglacial) sets in.
It may be mentioned that if there are minor changes in the climate, then the chapter is not closed, rather it (cycle) is either augemented or slowed down. Suppose, if the climate becomes more humid leading to increased amount of rainfall, then the surface runoff and stream discharge will automatically increase which would cause local interruption in the cycle by accelerating the rate of erosion (case of rejuvenation), the effects of which may spread over larger areas.
2. Interruptions due to base level changes:
Any change in the base level of erosion (determined by the sea level) causes interruptions in the cycle of erosion or may initiate new cycle whether the previous cycle is complete or not. Changes in base level of erosion are basically of two types i.e., positive change (due to rise in sea level) and negative change (due to fall in sea level).
The base level changes caused due to sea level changes are eustatic (wide spread). Base level of erosion also changes due to tectonic factors i.e. subsidence of landmass causes positive change whereas upliftment of landmass is responsible for negative change. It may be mentioned that such type of subsidence or upliftment of landmass may be or may not be related to sea-level.
Positive change of base level (or rise in base level) shortens the cyclic time. Secondly, positive change of base level of erosion is indicative of accelerated alluviation (deposition) whereas negative change causes accelerated erosion (rejuvenation). It is evident that rejuvenation is the most important factor of interruption in the fluvial cycle of erosion and genesis of polycyclic (multi-cyclic) reliefs and hence it needs detailed discussion.