The geomorphic cycle is any cycle of events that leads to the formation of any relief (terrain) development in landscapes. It is a model explaining the formation of many different terrain structures such as hilltops, valleys, mountains and river drainage systems. It is also known as the geographic cycle or the cycle of erosion for the development of various landscapes. Hence, a geomorphic cycle is the theory of evolution of landforms that includes many distinct events of erosion and deposition due to a variety of reasons that will be discussed below in this article.
The Geomorphic Cycle
William M. Davis, first set the theory of the geomorphic cycle and laid down some concepts of geomorphic cycles and landscape development. According to the theory, there are commonly three stages of the cycle of erosion and the development of any landforms. The three periods are classified from youth to maturity and old age. These stages were considered to be gradually transitioning from one period to another. This model that explains the concepts of geomorphic cycles and landscape development is known as the Davis geomorphic cycle. Although certain aspects of the Davis geomorphic cycle are not currently accepted, it is still the primary and widely proclaimed theory of the geomorphic cycle that describes the various events leading to the formation of landscapes.
The initial stage as put forward by the Davis geomorphic cycle is the youthful stage of landscape development. This stage of terrain development begins with the upliftment of the landform. It includes either the uplifted or to-be-uplifted periods of the development process. During this stage due to the processes that favour the uplifting of the landform, significant folds are produced in the crust of the Earth. These folds are generally the mountains or the block mountains. This uplifting process of mountain formation is widely known and an accepted phenomenon. The folding or uplifting can occur due to a variety of phenomena such as the movement and clashing of the tectonic plates of which the Himalayan mountain range is an example. During the youthful stage, the rivers flowing through the uplifted landform would create another distinction between the uplands and valley bottoms. The differences between the uplands and the valley bottoms increase rapidly throughout this stage and even during the height of the youthful stage.
Following the youthful or the youth stage, comes the mature stage of the geographical landscape development. During this stage, due to the dissection of the streams or rivers, the ever-increasing difference in-between the valley bottoms and the uplands or the mountains reaches its height. These height differences are the greatest in the mature stage of the relief development. Another factor that plays a significant role in the mature stage is the slope decline. When the height difference between the uplands and the valley bottoms reaches the maximum, slope decline becomes an important phenomenon. The slope of the upland begins to decline faster than the incision or dissection by any river stream. This effectively leads to the decrease in the relief features of height and the difference between the uplands and the bottom of valleys starts to lessen and diminish gradually.
The stage following the mature stage is the ‘old age’ stage of relief development. In old age, the initially uplifted terrain feature gradually diminishes or reduces to a surface known as the peneplain. During the old age, which is the latest stage in the process, the erosion has been acting upon the terrain so long that although the terrain was at a significant height when uplifted, it is now reduced to a lowland which is known as the peneplain. The peneplain is usually at sea level and sometimes is reduced so much that it drowns around the nearest water bodies. Although some of the peneplains may be submerged, some may maintain the residual height from the initial upliftment.
Following old age, the second geomorphic cycle starts again with the upliftment of the terrain. One of the important factors according to this theory of Davis geomorphic cycle which explains the concepts of geomorphic cycles and landscape development, is the contribution of time in the process of erosion. Time plays an important role according to the theory in the entire cycle of erosion. But there are certain cases when during any of the stages of the geomorphic cycle the upliftment can start before completion of old age. This event when it occurs is known as rejuvenation. It may or may not be a common phenomenon for a particular relief structure. This is because according to Davis, the full geomorphic cycle was a rare or special case as there are continuous geological changes taking place throughout the world which can cause changes in the stages of the geomorphic cycle.
This theory of the evolution of landscapes applies to hilltops, valleys, mountains and river drainage systems. Based on this theory, it is assumed that once the stage of any landform is known, the history of the landform development can be known according to the established principles.
Drawbacks of the Common Theories of the Geomorphic Cycle
Although Davis had been able to acknowledge the factors affecting the geomorphic cycle as rock type, structure and processes of erosion, he emphasized the importance of time. But it is currently believed that time does play a significant role as Davis suggested and the contribution by it is the same as other factors in relief development. This theory of the cycle of erosion has been widely accepted although there is accumulating evidence that refutes the theory of the Geomorphic cycle. It is now usually considered that the initial conditions or the conditions surrounding the upliftment of the landform may not significantly guide the stages towards the end products. Instead of a gradual transition of the landforms through various stages, there is a dynamic equilibrium reached in-between the landforms and the processes that act upon them, which in turn leads to erasing of the physiographic history of the region.
Another drawback of the theory is that originally the theory intended to provide explanations for the development of temperate landscapes as the major focus was put on the erosive activities by the river streams on the uplands or uplifted regions. Also, the nature of surface processes was poorly represented by the model. It was mostly theoretical and deductive and did not take into account the complexity of tectonic movements and climate change. Nevertheless, the cycle of erosion has been accepted by extending and including certain modifications that involve the arid, glacial, karst, coastal and periglacial areas.