Cycle of erosion
- 1. GEOGRAPHICAL CYCLE OF EROSION • WILLIAM MORRIS DAVIS(1850-1934) • BASIC CONCEPT OF ‘ SEQUENTIAL CHANGE IN LANDFORMS THROUGH TIME LIKE EVOLUTION OF AN ORGANIC LIFE.’ • THREE STAGES: YOUTH, MATURE AND OLD • DEFINITION: GEOGRAPHICAL CYCLE IS THE PERIOD OF TIME DURING WHICH AN UPLIFTED LANDMASS UNDERGOES ITS TRANSFORMATION BY THE PROCESS OF LAND SCULPTURE ENDING INTO A LOW FEATURELESS PLAIN-A PENEPLANE. • ACCORDING TO DAVIS: THREE FACTORS – STRUCTURE, PROCESS AND TIME play important roles in origin and development of landforms of a particular place.
- 2. Trio of Davis • “ landscape is a function of structure, process and time.” • Stages of erosion
- 3. Stages of erosional cycle • cycle of erosion begins with the upliftment of landmass. There is a rapid rate of short-period upliftment of landmass of homogeneous structure. This phase of upliftment is not included in the cyclic time as this phase is, in fact, the preparatory stage of the cycle of erosion. • The above figure represents the model of geographical cycle wherein UC (upper curve) and LC (lower curve) denote the hill-tops or crests of water divides (absolute reliefs from mean sea level) and valley floors (lowest reliefs from mean sea level) respectively. • The horizontal line denotes time whereas vertical axis depicts altitude from sea level. AC represents maximum absolute relief whereas BC denotes initial average relief. Initial relief is defined as difference between upper curve (summits of water divides) and lower curve (valley floors) of a landmass. In other words, relief is defined as the difference between the highest and the lowest points of a landmass. ADG line denotes base level which represents sea level. No river can erode its valley beyond base level (below sea level). • Thus, base level represents the limit of maximum vertical erosion (valley deepening) by the rivers. The upliftment of the landmass
- 4. Youthful stage • Now erosion starts and the whole cycle pass through the following three stages: • Erosion starts after the completion of the upliftment of the landmass. The top surfaces or the summits of the water divides are not affected by erosion because the rivers are small and widely spaced. Small rivers and short tributaries are engaged in head-ward erosion due to which they extend their lengths. The process is called stream lengthening (increase in the lengths of the rivers). • Because of steep slope and steep channel gradient rivers actively deepen their valleys through vertical erosion aided by pothole drilling and thus there is gradual increase in the depth of river valleys. This process is called valley deepening. The valleys become deep and narrow characterized by steep valley side slopes of convex plan. • The youthful stage is characterized by rapid rate of vertical erosion and valley deepening because: • The channel gradient is very steep, • Steep channel gradient increases the velocity and kinetic energy of the river flow, • Increased channel gradient and flow velocity increases the transporting capacity of the rivers, • Increased transporting capacity of the rivers allows them to carry big boulders of high calibre (more angular boulders) which help in valley incision (valley deepening through vertical erosion) through pothole drilling.
- 5. Mature stage • The early mature stage is heralded by marked lateral erosion and well integrated drainage network. The graded conditions spread over larger area and most of the tributaries are graded to base level of erosion. Vertical erosion or valley deepening is remarkably reduced. The summits of water divides are also eroded and hence there is marked fall in upper curve (UC) i.e., there is marked lowering of absolute relief. • Thus, absolute relief and relative relief, both decrease. The lateral erosion leads to valley widening which transforms the V-shaped valleys of youthful stage into wide valleys with uniform or rectilinear valleys sides. The marked reduction in valley deepening (vertical erosion or valley incision) is because of substantial decrease in channel gradient, flow velocity and transporting capacity of the rivers.
- 6. Old stage • Old stage is characterized by almost total absence of valley incision but lateral erosion and valley widening is still active process. Water divides are more rapidly eroded. In fact, water divides are reduced in dimension by both, down- wasting and back-wasting. Thus, upper curve falls more rapidly, meaning thereby there is rapid rate of decrease in absolute height. Relative or available relief also decreases sharply because of active lateral erosion but no vertical erosion. Near absence of valley deepening is due to extremely low channel gradient and remarkably reduced kinetic energy and maximum entropy. • The valleys become almost flat with concave valley side slopes. The entire landscape is dominated by graded valley- sides and divide crests, broad, open and gently sloping valleys having extensive flood plains, well developed meanders, residual convexo-concave monadnocks and extensive undulating plain of extremely low relief. Thus, the entire landscape is transformed into peneplain. As revealed by fig. the duration of old stage is many times as long as youth and maturity combined together.
- 8. NORMAL CYCLE OF EROSION • The cycle of erosion which is carried by running water is known as normal cycle of erosion. This is because running water or fluvial processes are most widespread all over the world. It plays an important role in the regions of glacial and arid. • The normal cycle of erosion begins when a landmass gets uplifted in accordance with the sea level. As the land gets uplifted, the rivers get originated and erosional processes begin thereafter. After some duration of time the upliftment of the landmass stops resulting in erosional processes to be more active.
- 9. YOUTHFUL STAGE • This stage gets originated when the landmass gets uplifted. The streams are short in length and number. The master consequent streams get originated in very few numbers. • Several gullies and rills dominate the slopes. Through headward erosion gullies and rills lengthens their longitudinal profile. • The development of the tributaries from the master consequent streams, gives rise to dendritic drainage pattern. • The valleys are experienced through rapid rate of downcutting by the rivers. This is because the transporting capacity of rivers becomes high due to steep channel gradient. • The valleys become narrow and steep having convex side slopes. This result in the juvenile valleys to be V- shaped.
- 10. MATURE STAGE • This stage is characterized by a decrease in the valley deepening from the youthful stage, which results in a decrease in channel gradient and flow velocity. • Early maturity gets arrived which is indicated by a decrease in the transporting capacity, decrease in the channel gradient and a decrease in the river velocity. • Valley widening becomes dominant at this stage, through an active rate of lateral erosion. • Big boulders and sediments are deposited by the rivers at the foothill zones, due to the decrease in the transporting capacity of the rivers. The deposition of these materials results in the formation of alluvial fans and cones, which further results in the formation of piedmont plains due to their gradual expansion. • As there is decrease in the channel gradient, the rivers carve their courses resulting in the formation of several meanders and loops.
- 11. OLD STAGE • This stage is characterized by more decrease in the channel gradient and there is the total absence of valley deepening. • There is a decrease in the number of tributary streams. Moreover, the valleys become broad and flat comprising of concave slopes. • The transporting capacity of the rivers becomes low at this stage. Sedimentation and weathering become more active. • Rivers carves their courses to an extreme form and forms extensive deltas at the mouth of the rivers. • At this stage, the landscape gets transformed into extensive flat plains which are known as a peneplain. •
- 12. REJUVENATION • ACCELERATION OF EROSIVE POWER OF THE FLUVIAL PROCESS (RIVERS) CAUSED BY MANY FACTORS. • LENGTHENS THE PERIOD OF CYCLE OF EROSION • CAUSED EITHER DUE TO SUBSTANTIAL FALL IN SEA LEVEL OR DUE TO UPLIFTMENT OF LANDMASS • DRIVEN BACK TO YOUTHFUL STAGE CHARACTERIZED BY STEEP CHANNEL GRADIENT AND ACCELERATED VALLEY INCISION.