CYCLE OF EROSION (Denudation)

EROSION

The rock grains or angular fragments that result from the disintegration and decomposition of rocks do not remain lying at the foot of the hills. As water, ice or wind move over the them, some of their material is drawn away as part of the flow. The angular fragments or rock grains, which are now part of the flow, when brought in contact with the earth’s surface, their friction with the surface of the earth produces etching (design); cut and groove the surface over which they pass. This is known as Erosion. In other words we can say that Erosion is the break up and removal of rock by moving natural agents.
The chief agents of erosion are running water, underground water, moving ice, wind and wave. These agents attack the earth’s crust by various means and produce minor relief features, the landforms of the third order.
The friction is the basis of erosion and there can be no friction without transportation. Hence transportation is necessary for erosion.

INTRODUCTION

The concept of the erosion cycle, widely popularized by William Morris Davis (1850-1934) in the 19th century, dominated geologic thinking for nearly 50 years. Many doubts are now being raised about the validity of the cycle concept.

GENERAL DESCRIPTION

All types of landforms undergo an evolution, a development through various processes. These processes start from up-arching or elevation of large area of a continent by internal earth movement. This provides a new landmass, which is then attacked by various erosional agents. Process of erosion becomes very much advance as time passes and then erosional process will be stopped. All these different processes of erosion are generally described as “Cycle of Erosion”.

STAGES OF CYCLE OF EROSION

Cycle of Erosion consists of following stages:
i. Young or Initial Stage;
ii. Mature or Sequential Stage; and,
iii. Old or Ultimate Stage

DIFFERENT CYCLES FOR HUMID & ARID REGIONS

The stages for cycle of erosion may be different for arid and humid climates. A single cycle of erosion will not cover all occurrences. There are differences between the landforms developed in humid and arid climates. The landmasses developed in humid climate, at initial stage, are relatively smooth surfaced, representing an even sea floor, broadly up-arched by epeirogenic crustal movement. The landmasses in arid climates are mountainous, which develop because of breaking and bending of rock during orogeny. Hence, the cycle of erosion for humid and arid climates are discussed separately. (Strahler 420)

I. Cycle of Erosion in Humid Climate

a. Initial Stage
A landmass formed by up-arching of a relatively smoothed sea floor would have gentle slopes inclined seawards from high central area; this is said to be in initial stage. The overland flow upon the new surface could drain off in downslope direction and would soon develop initial streams. In this stage, the stream starts down-cutting to trench youthful, v-shaped valleys in the initial landmass. Marshes and lakes, occupying shallow depressions in the initial surface, would soon be drained.

b. Youth Stage
After the initial stage, the landmass down-cutting, because of the stream, is very rapid. The relief of the area is now increasing rapidly, whereas between the streams there remains relative flat portions of the initial land surface. This stage is known as “Early Youth Stage”.
Now with the passage of time as valleys deepen, they also widen, because the rock waste has swept down the valley sides into the streams. The unconsumed areas between the valleys are reduced and the steep valley slopes increase in extent. Small tributary valleys branch out from the larger streams, further cutting the landmass. Now the remnants of the landmass between the valley system shrink in area. The greater proportion of the region consists of steep valley slopes; this stage is termed as “Late Youth Stage”.
Relief has been steadily increasing as streams have been down-cutting. The conditions show remarkable change. When larger streams become graded and begin to form their flood-plain, the increase in relief is halted. The remaining flat remnants of the initial surface are finally consumed, and valley slope intersect in narrow divides.

c. Mature Stage
When the late youth stage is attended by the landmass and the relief has reached its maximum, then this stage is called as “Mature Stage”. From this time onwards, the valleys floors are lowered with extreme slowness, whereas the inter-stream divides are rapidly lowered. Thus, the relief of the region decreases rapidly. Slopes become progressively lower in angle, slope erosion and mass wasting no longer so active as in previous stages.

d. Old Stage
After a time period much longer than was required for maturity to be reached, the both down cutting and side cutting have ceased, the streams no longer grading their course and aggradation begins. The landscape is reduced to a low rolling surface and this stage of landmass is termed as “Old Stage”.
By this time, most of the streams have low gradients and extensive flood plains. The ultimate goal which would be reached is the reduction of the land to a surface coinciding with sea level projected in land. This imaginary surface is called “base-level” and is attainable only in theory.
The land surface produced at old stage in cycle of erosion is called as “Peneplain”. A peneplain is not perfectly flat but has gentle slopes. Because the streams are sluggish (slow) and the land slopes are low, further erosion is very slow.
It is not easy to set a figure for the number of years required for a region to pass from initial stage to old stage, because it depends upon how high the landmasses are elevated and how resistant the rocks are to weathering and erosion. Perhaps it would be safe to say that in known cases of geologic records, several million years have been required to reduce a mountain mass to a peneplain.
Sometimes the region which is progressing through cycle of erosion contains patches or zones of rocks which are more resistant to weathering or erosion. As the cycle progresses from maturity to old stage, these harder rocks are left standing as prominent hills or isolated mountains, which rise above the surrounding peneplains. These isolated hard rocks are called monadnocks.(Strahler 438)

CRITICISM

An erosion cycle takes so long that no one has ever seen a complete cycle. Geologists try to piece together the parts of the cycle by examining different regions in the modern world and by looking at the geologic record which is spread over several million years.
The idealized erosion cycle is based on the presumption that nothing interrupts the regime of erosion. Actually many interruptions can take place. Once formed, a peneplain is usually elevated again by crustal movement. Sometimes this uplift of land occurs concurrently as cycle progresses. The uplift has taken place either in youthful or during maturity. The landscape and the particular stage is modified to a very great extent.
The cycle of erosion may be modified by the factors of climate. The modification by temperature and rainfall is most dominant in humid tropics and temperate climates. In humid tropics, due to heavy rainfall and chemical decomposition of rocks, the landscape abounds (overflows) in conical hills with concave slopes.
The stages of youth and maturity can be illustrated by many modern examples, but the stage of old age is where the doubts about the Davis Cycle have been raised. The old age stage is supposed to be a surface of low relief across which streams meander (maze). Many alleged modern plains have proved to be alluvial plains that appear the same regardless of their ages. From the geologic record, we know that ancient peneplains did exist. But no peneplains have been proved in the modern world. Therefore, the details of the shift from the stage of maturity to that of old stage are conjectural (hypothetical) and controversial.

ENVIRONMENTAL ASPECTS OF THE CYCLE

The environmental importance of initial stage in the cycle of erosion is very great. Regions in the initial stage of cycle of erosion are relatively flat plains, with poor drainage and marshy lands. Sandy beach deposits left by waves as the land surface emerge from beneath the sea usually produce infertile soils. The porous sands permit plant nutrients to be leached out of the soil. Not all regions in an initial stage have emerged from sea; some were built by aggrading streams and, although remarkably flat, are well-drained and do not have extensive marshes. The high plains posses a high productivity in wheat, not only because the soil and climate are favorable but also because the flatness of the land permits enormous fields to be cultivated and harvested by machines.
Still other areas of initial land surface are found by lava flows, poured out to inundate the previous topography and produce a high undulating lava patterns.
A region in youth of the cycle supports its population on the relatively flat areas between deep v-shaped valleys. Because these valleys are in a young stage, they have no flood plains; hence roads, railways, cities and farms are situated on uplands. A mature region, on the other hand, has no flat uplands remaining hence is not favorable to habitation, agriculture or transportation. Many of the world’s mountainous regions are in the stage of maturity of erosion cycle. Hence extremely great relief and steep slopes are the result of recent, high uplift of these portions of the earth’s crust. In some mature regions, the larger streams have already reached full maturity and have sizeable flood plains and the surrounding region is extremely rugged. Under such conditions, human activity is concentrated in the valley floors.
Regions with a humid climate in late mature or old stage of the cycle are usually favorable to agriculture. Slopes are moderate or low and are well-drained. Soils tend to be thick. Roads and railways cross the rolling surface without great difficulty or follow extensively developed flood plains.

II. CYCLE OF EROSION IN ARID CLIMATES
FEATURES OF ARID CLIMATIC REGION

In this cycle of erosion, we imagine a mountainous region formed by folding or fracturing of the earth’s crust and lying in an interior part of the continent. Relief is at the maximum in the initial stage and is reduced throughout successive stages. Many large depressions exist between mountain ranges. These are not filled up with water to form lakes as in humid climate but remain dry because of excessive evaporation in hot, dry climate. The flat central parts of such depressions provide the beds for temporary lakes known as Playas. Playa lakes are shallow and fluctuate considerably in level, often disappearing entirely for long periods. Because they have no outlets, playas lakes contain saline water, often more strongly saline than ocean water. (Strahler 445)

a. Initial Stage
The erosion takes place with heavy rains, which a particular locality of dry desert may experience once in several years, along with other weathering agents. When the rainfalls, the stream channels carry water and perform the same work as the constantly flowing streams of moist region.

b. Young Stage
Excess water runs off from the valley slopes into the streams washing down rock particles into the channel. Since the vegetation into the dry deserts is meager, hence the few small shrubs and herbs that survive offer little or no protection to bedrock. The swift downslope flow of water sweeps excessive quantities of coarse rock debris into the stream and in few minutes a dry channel is transformed into a raging flood, heavily charged with rock fragments.

c. Mature Stage
Throughout this erosion cycle, the intermountain depressions are filled with rock waste and alluvial fans are built out from adjoining mountain masses. When the basins are filled with alluvium and mountain masses are cut into an intricate (complex) set of canyons, the region is said to be in “Mature Stage”. As the maturity progresses, the mountains are worn lower, at the same time shrinking in size as the alluvium of the fans encroaches progressively inward upon the mountain base. (Strahler 445)

d. Old Stage
As the cycle progresses and further erosion takes place, then the mountains are further disintegrated and decomposed, and are represented by small island-like remnants. This is the “Old Stage” of the cycle. These remnants are compared to monadnocks on a peneplain but these are further eroded away and a vast plain remains. This surface is a type of peneplain, but it has not been developed with reference to sea level as a base because no streams drain out to the sea, therefore it may lie hundreds of feet above sea level. It contains shallow depressions occupied by playas rather than by flood plains of meandering rivers.
The old stage surface of the arid climatic landmass cycle, which consists in part of pediments is called as “Pedeplain”, which is equivalent to the term peneplain in humid climate landmass cycle. A pedeplain consists of alluvial fan and playa surfaces as well as pediments; it is thus partly erosional and partly depositional. (Strahler 447)

ENVIRONMENTAL ASPECTS OF THE CYCLE

Since the arid region includes rugged (uneven) mountains, broad sloping pediments, and perfectly Playa Lake floors, all these three landscapes have different environmental aspects.
The rugged mountains consist of valuable mineral deposits, which are an outstanding source of economic development.
The alluvial fan slopes or pediments are virtually worthless except in the few places where wells bring in a flow of water for the needs of isolated communities. The playas provide mineral wealth of different soil, including salts of calcium, sodium and potassium.