Geomorphic Cycles and their Interpretation | Geology Optional Notes for UPSC PDF Download

Concepts of Geomorphic Cycles and Landscape Development

The geomorphic cycle, also known as the geographic cycle or cycle of erosion, is a theory that explains the evolution of landforms over time. This concept was first proposed by American geographer William Morris Davis in 1899, who developed a model that showed the sequential changes in landforms through various stages of development, similar to the concept of biological evolution.

• According to Davis, the initial stage of landform development, known as the youthful stage, begins with an uplift that produces fold or block mountains. As streams dissect the area, it reaches maturity and is eventually reduced to a near-sea-level surface called a peneplain during the old-age stage. The cycle can be interrupted by uplift at any point, causing the landform to return to the youthful stage, a process known as rejuvenation.
• The geomorphic cycle can be applied to various landforms, including hills, valleys, mountains, and river drainage systems, and follows a predetermined framework based on the stage of development. The earth's surface is affected by two types of forces: endogenetic and exogenetic. Endogenetic forces create vertical irregularities on the earth's surface, forming various relief features, while exogenetic processes originating from the atmosphere (such as rivers, wind, glaciers, and sea waves) work to remove these irregularities, ultimately reducing the relief to a low, featureless plain called a peneplain.
• The entire period of relief feature creation by endogenetic processes and their destruction by exogenetic processes is called the cycle of erosion. Although initially proposed for humid temperate conditions, the Davisian model has been extended to other landscapes, such as arid, glacial, coastal, karst, and periglacial environments.
• Some modifications have been suggested to Davis's model, such as C.H. Crickmay's proposal of a more powerful process called panplanation, resulting in a panplain rather than a peneplain. L.C. King proposed a new cycle of erosion called the cycle of pediplanation, specifically for arid and savanna regions of Africa.
• In more recent years, some researchers have rejected the evolutionary concept of landform development and advanced alternative theories, such as the dynamic equilibrium model of landform development and the tectonogeomorphic model. Despite these alternative theories, many modern models of landform development can be seen as modified forms of Davis's original concept.

Davis Model of Cycle of Erosion

The Davis Model of Cycle of Erosion, also known as the "geographical cycle," was introduced by Davis in 1899 as a way to classify and describe landforms. The model suggests that landforms undergo an evolutionary sequence due to the effects of denudation (weathering and erosion) processes over time. This leads to an ever-changing landscape that moves from an initial to an ultimate form, with the height and slope of hills declining over time.

• The model is based on three main factors: structure, process, and stage. Structure refers to the composition and arrangement of underlying rocks, including their hardness, porosity, and the presence of folds and faults. Process encompasses the various weathering and erosion agents, such as rivers, wind, and glaciers, as well as mass movements. Stage represents the time duration during which these processes act on the landform.
• Davis used the life stages of living beings - youth, maturity, and old age - as an analogy for the evolutionary stages of landforms. He argued that landforms undergo sequential changes throughout their life cycle as they progress through these stages. However, the duration of each stage is not fixed and depends on various factors, such as the resistance of rocks to erosion.

The Davisian model of the geographical cycle is based on several key assumptions:

• Landforms are shaped by the interaction of internal (endogenetic) forces from within the Earth and external (exogenetic) forces from the atmosphere, such as weathering, erosion, and various erosional agents like rivers, wind, groundwater, sea waves, glaciers, and periglacial processes.
• Landforms evolve in an orderly manner, developing a systematic sequence of forms over time in response to environmental changes.
• Streams erode their valleys quickly until a stable, graded condition is achieved.
• The land mass experiences a rapid rate of uplift, though slower rates can also be considered in the model.
• Erosion does not begin until the uplift is complete, meaning that uplift and erosion do not occur simultaneously. This assumption has been a point of contention among critics of the cyclic concept.

Stages of the Cycle
The cycle of erosion begins with the upliftment of a landmass, where there is a rapid rate of short-period upliftment of a homogeneous structure. This phase is not included in the cyclic time as it is the preparatory stage of the cycle of erosion.
The model of the geographical cycle consists of an upper curve (UC) and a lower curve (LC), representing the hilltops or crests of water divides and valley floors, respectively. The horizontal line denotes time, while the vertical axis shows altitude from sea level. The cycle passes through three main stages: youthful, mature, and old.

• Youthful stage: Erosion begins after the completion of the landmass upliftment. At this stage, the topsurfaces or summits of the water divides are not significantly affected by erosion as the rivers are small and widely spaced. The rivers and their tributaries experience headward erosion, extending their lengths. Due to the steep slope and channel gradient, rivers actively deepen their valleys through vertical erosion, which results in deep and narrow valleys with steep valley-side slopes.
  The youthful stage is characterized by a rapid rate of vertical erosion and valley deepening due to the steep channel gradient, increased velocity and kinetic energy of river flow, and increased transporting capacity of the rivers.
• Mature stage: As the cycle progresses to the early mature stage, lateral erosion becomes more significant, and the drainage network becomes well integrated. Vertical erosion decreases, and the summits of water divides start to erode, resulting in a decrease in absolute and relative relief. Lateral erosion leads to valley widening, transforming V-shaped valleys of the youthful stage into wide valleys with uniform or rectilinear valley sides.
• Old stage: During the old stage, valley incision is almost absent, but lateral erosion and valley widening continue. Water divides are reduced in dimension through both down-wasting and back-wasting, causing a rapid decrease in absolute height. Relative relief also decreases due to active lateral erosion and the absence of vertical erosion. Valleys become almost flat with concave valley-side slopes, transforming the landscape into a peneplain. This stage lasts much longer than the previous two stages combined.

Analysis of Davis Cycle of Erosion

• The Davis cycle of erosion, specifically the concept of peneplain introduced by Davis, has been met with varied opinions from different geomorphologists. Some consider the peneplain to be a theoretical landform because, for the Davis cycle to fully occur, the landform must remain stable for an extended period. This stability is unlikely in reality, where both endogenetic and exogenetic forces are constantly at work, potentially disrupting the smooth progression of the cycle.
• Additionally, critics argue that the concept of peneplain contradicts the principle of isostatic adjustment. As a river erodes its valley, the removal of the overlying load is compensated by the addition of more material to the landform's base. This process pushes the landform upward, continuing the process of upliftment indefinitely. Therefore, the attainment of the peneplain stage is questioned due to its conflict with the idea of isostatic adjustment.
• Many geomorphologists also believe that the sequential change of landforms proposed by Davis is an oversimplification of the actual process of landform evolution. In reality, the development of landforms is a much more intricate and complex process.

Criticism of Davis Model
The Davis Model has faced significant criticism for its concept of upliftment. Davis described a quick upliftment process of short duration; however, evidence from plate tectonics shows that upliftment is actually an extremely slow and continuous process.

• Additionally, the relationship between upliftment and erosion in the Davis Model is considered flawed. Davis claimed that erosion cannot begin until upliftment is complete, which contradicts the natural process where erosion starts as soon as the land begins to rise. He defended this by stating that he excluded erosion from the upliftment phase to simplify the model and because erosion is insignificant during upliftment.
• Moreover, the Davisian model requires a long period of crustal stability to complete the erosion cycle, but plate tectonics suggests that such a stable period is unlikely due to constant plate movements and frequent tectonic events. Davis acknowledged this objection, arguing that if the desired crustal stability is not possible, the erosion cycle would be interrupted and a new cycle might begin.
• Furthermore, Walther Penck criticized the Davis Model for its overemphasis on time. While the Davisian model presents a time-dependent series of landform development, Penck argued for a time-independent series. Penck believed that landforms do not undergo sequential changes over time and suggested removing 'time' from Davis' trio of 'structure, process, and time.'
• Several other researchers, including A.N. Strahler, J.T. Hack, and R.J. Chorley, have also rejected the Davisian concept of 'historical evolution' of landforms and instead proposed the dynamic equilibrium theory. However, the non-cyclic concept of 'dynamic equilibrium' and other open system, non-cyclic models have not generated much enthusiasm among modern geomorphologists.
• Despite these criticisms, the Davis cycle continues to be relevant in the study of slope evolution due to its widespread appeal and clear presentation.

Normal Cycle of Erosion

The normal cycle of erosion refers to the process of erosion driven primarily by running water, which is the most common type of erosion worldwide. It is particularly significant in glacial and arid regions. While W.M. Davis believed that humid temperate areas were the most typical case for fluvial cycles of erosion, this idea is still debated.

• In the normal cycle of erosion, a landmass is uplifted relative to sea level, leading to the formation of rivers and the beginning of erosional processes. Eventually, the uplift of the landmass ceases, allowing erosion to become more active. During this period of crustal stability, the landmass neither rises nor sinks, and river valleys develop in a sequence of stages: youthful, mature, and old. Ultimately, these processes result in a low, featureless plain called a peneplain.
• The youthful stage begins with the uplift of the landmass. At this point, streams are short and few in number, and the primary consequent streams are also limited. Gullies and rills dominate the slopes, and through headward erosion, they lengthen their profiles. The development of tributaries from the primary consequent streams leads to a dendritic drainage pattern. Rivers experience rapid downcutting in their valleys due to the high channel gradient, resulting in narrow, steep valleys with convex side slopes, forming V-shaped juvenile valleys.
• During the mature stage, valley deepening decreases compared to the youthful stage, leading to a reduction in channel gradient and flow velocity. This stage is marked by early maturity, which is characterized by decreased transporting capacity, channel gradient, and river velocity. Lateral erosion becomes more active, causing valleys to widen. As rivers lose some of their transporting capacity, they deposit large boulders and sediments at the foothills, forming alluvial fans and cones. These deposits eventually create piedmont plains as they expand. With the reduced channel gradient, rivers meander and form loops.
  Stages of Erosion
• In the old stage, the channel gradient decreases further, and valley deepening ceases altogether. The number of tributary streams also declines, and valleys become wide and flat, with concave slopes. Rivers have a lower transporting capacity, leading to increased sedimentation and weathering. The rivers carve their courses extensively, forming large deltas at their mouths. The landscape ultimately transforms into a vast, flat plain known as a peneplain.

7  Major Geomorphic Theories of Landform Development

1. Davisian Theory

• American geomorphologist William Morris Davis proposed the widely accepted theory of landform development, known as the geographical cycle or cycle of erosion. This theory categorizes and systematically describes how landforms transform over time.
• According to Davis, the geographical cycle represents a period during which an uplifted landmass undergoes a gradual transformation through land sculpting processes. The ultimate result is a low and featureless plain called a peneplain, a term coined by Davis.
• Davis developed his theory during the late 19th century, presenting several key models, including the "complete cycle of river life" in his essay on The Rivers and Valleys of Pennsylvania in 1889. In 1899, he introduced the concept of the "geographical cycle" and "slope evolution."
• The "complete cycle of river life" model proposed by Davis explained the progressive development of erosional stream valleys. On the other hand, the "geographical cycle" provided a sequential description of how landforms evolve over time.
• Contrary to some beliefs, Professor Savindra Singh argued that Davis's general theory of landform development is not solely the "geographical cycle." Instead, Davis's theory can be summarized as follows: "Landforms undergo sequential changes over time, progressing through youth, maturity, and old age stages, ultimately leading to the development of a well-defined end product, the peneplain."

2. Penck’s Theory

• Walther Penck proposed that the physical features of a region are shaped by the interplay between tectonic activity and erosion processes, rather than progressing through a fixed sequence as suggested by Davis.
• Penck's model asserted that landforms are a reflection of the balance between the rates of uplift and degradation in a continuous manner, helping us understand the causes of tectonic movements based on morphology.
• Penck's approach did not rely on a time-dependent progression for landscape development.
• The shape of hillslopes is determined by the relative rates of river valley incision and debris removal from the hillslope.
• Penck identified three crustal states: a stable state with no active displacement, initial domed uplift followed by widespread uplift, and extensive crustal uplift.
• Penck emphasized that uplift and erosion processes always coexist.
• He described three states of adjustment between crustal movement and valley deepening: balance when uplift and erosion rates are equal, a state where uplift is faster than valley deepening, and a state where valley deepening exceeds uplift.
• Despite criticism, particularly in the United States, regarding Penck's concepts of continuous uplift and crustal movements, his ideas about slope development and weathering processes remain significant in the field of geomorphology.

3. Gilbert’s Theory

• Grove Karl Gilbert's research on landforms and their formation in the United States led to the development of important geomorphic principles.
• His ideas formed the foundation for the dynamic equilibrium theory in geomorphology, which revolutionized the field's methodology.
• Gilbert proposed that landscapes result from two opposing tendencies: variability (when driving forces exceed resisting forces) and uniformity (when they are equal).
• He suggested that landscapes remain in equilibrium, but their history involves rhythmic changes and their forms show frictional rhythms due to driving and resisting forces.
• Three main components of Gilbert's geomorphic principles are quantification, time, and equilibrium.
• Gilbert's concept of equilibrium, known as the principle of least force, implies that the sum of forces on a final landform equals zero.
• Two types of forces are driving forces and resisting forces, and he applied this concept to explain the formation of laccoliths resulting from volcanic activity.
• Gilbert argued that the growth of a laccolith depends on the balance between the driving force (magma rising) and the resisting force (overlying pressure from the rock above).
• As long as the rising magma's driving force exceeds the resisting force, the laccolith continues to grow, but when these forces balance, equilibrium is reached, and growth stops.
• At this point, the principle of least work becomes relevant, where the sum of driving and resisting forces is zero.

4. Theory of L.C. King

L.C. King's theory of landform development is based on his research in arid, semi-arid, and savanna regions of South Africa. He created several cyclic models and believed they were applicable globally. He argued that landforms developed uniformly across different environments and were minimally affected by climate. Major landscapes on all continents, he claimed, were shaped by rhythmic global tectonic events. King emphasized the continuous retreat of hillslopes in a parallel manner.

• King's ideal hillslope profile includes four elements: summit, scarp, debris slope, and pediments. These hillslopes develop in regions with adequate relief, where fluvial processes are the primary erosive agents.
• King introduced a new cyclic model called "pediplanation" in the 1940s to explain unique landscapes in arid and semi-arid parts of Africa. He described three basic elements in the African landscape: rock pediments along river valleys, scarps bounding uplands, and steep-sided residual hills called inselbergs. The size and shape of inselbergs depend on erosion magnitude and underlying structure.
  
• King's concept of uplift and crustal stability resembled Davis's ideas. The pediplanation cycle involves scarp retreat and pedimentation, starting with rapid uplift followed by tectonic stability. The cycle progresses through youth, maturity, and old age, similar to Davis's cycle of erosion. However, there are differences: Davis's peneplain forms due to downwasting, while King's pediplain results from the integration of multiple pediments formed through parallel scarp retreat. King's pediplains continue to grow headward, with new scarps forming and old ones retreating.
  
• King's pediplains are comparable to Penck's piedmont treppen, consisting of intersecting pediplains extending headward.
  

Critiques of King's Model
(a) Geographic Limitation: King's model primarily focuses on African landscapes, limiting its applicability to other regions.
(b) Uniformity Questioned: The idea of uniform landform development across different environments has been questioned.
(c) Antique Pediplanation: King's concept of "antique pediplanation" remains debatable.

5. Theory of J.T. Hack

• J.T. Hack aimed to address the conceptual gaps left by the criticisms of Davisian evolutionary models and Penck's morphological system.
• Hack argued that landscapes with multiple levels and features (polycyclic relief) couldn't be explained by the idea of multiple erosion cycles, as proposed by Davis, but rather through the concept of dynamic equilibrium.
• In Hack's view, the geomorphic system is open, and as long as the energy within the system remains constant, the landscape will remain in a steady state, even as denudational processes lower the land's elevation.
• Hack's model suggests that landscapes develop independently of time. In other words, landforms reflect the balance between the resistance of underlying materials to erosion and the erosive energy of active processes.

The main assumptions of Hack's model include:

• A balance exists between denudational processes and the resistance of rocks.
• All components of landscapes experience uniform downwasting rates.
• Differences in landform shapes are explained by the spatial relationships influenced by geological patterns.
• Present denudational processes have shaped the earth's surface landscapes.
• There is a lithologic adjustment to landforms.

6. Theory of Morisawa

• Morisawa developed a geomorphic model based on tectonic movements and changes, drawing from various studies on erosion and landforms conducted by different researchers worldwide.
• She found that landmasses experiencing uplift tend to erode quickly because the energy needed for erosion increases with greater elevation, resulting in higher kinetic energy from faster channel flow velocities, ultimately accelerating erosion.
• Morisawa established a strong positive correlation between denudation rates and basin relief, with 90% of differences in erosion rates across different drainage basins attributed to their average relief.

The key principles of Morisawa's model are:

• Landforms are shaped by variations in force, resistance, or a combination of both.
• Differences in landforms arise from variations in the rates of exogenetic (external) processes acting on different geological materials and rates of endogenetic (internal) processes.
• Nature seeks equilibrium between process forces and geological material resistance, though this balance is often disrupted due to Earth's dynamic and unstable nature, including isostatic feedback influencing uplift, erosion, deposition, and subsidence.
• Present-day landforms result from differing ratios of endogenetic and exogenetic processes.
• Newly uplifted land undergoes rapid transformation through exogenetic (denudational) processes, and the pace of change depends on the interplay between force and resistance.
• Some geological features can be explained by plate tectonics.

7. Theory of S.A. Schumm

• S.A. Schumm's theory is centered on the episodic erosion model, which challenges the idea that denudation is a continuous, gradual process. Instead, he proposes that landscape development involves cycles of rapid erosion followed by extended periods of deposition.
• Schumm's theory introduces the concepts of geomorphic thresholds and complex response to explain the complexity of landform evolution.
• Geomorphic thresholds suggest that changes in fluvial systems are primarily influenced by inherent geomorphic controls rather than external factors. For instance, sediment deposition can lead to instability when a critical slope threshold is reached, causing increased channel flow velocity and erosion of the deposited sediments.
• Complex response, according to Schumm, occurs when a fluvial system experiences rejuvenation. This response involves reaching a new equilibrium through processes like down cutting, aggradation, and renewed erosion, which is a response to internal geomorphic controls.
• When external variables such as isostatic upliftment are combined with geomorphic thresholds and complex response, the geomorphic cycle of erosion is not strictly progressive, but rather characterized by periods of episodic erosion interspersed with periods of stability.
• The resultant landscape complexity arises from the fact that events in one segment of a river do not have an immediate impact on the entire channel length, leading to a complex and evolving landscape.
• While Schumm's ideas were not universally accepted by all geomorphologists, modern studies of thresholds and complex response aim to integrate Davisian cyclic decay models and Gilbert's steady-state model into a more comprehensive understanding of landform evolution.