Mountains, Plateaus, Plains and its Different Types
In general, geomorphic processes that originate within Earth, called endogenic processes result in an increase in surface relief, while the exogenic processes, those that originate at Earth’s surface, tend to decrease relief. Igneous and tectonic processes constitute the endogenic geomorphic processes.
Exogenic processes consist of various means of rock breakdown, collectively known as weathering, and the removal movement, and relocation of those weathered rock products in the continuum of processes known as erosion, transportation and deposition. These processes decide the landforms on earth.
COMPRESSIONAL TECTONIC FORCES
Compressional Tectonic Forces are divided into two types. They are
- Folding and
Folding, which is a bending or wrinkling of rock layers, occurs when compressional forces are applied to rocks that are ductile (bendable), as opposed to brittle.
- Rocks that lie deep within the crust and that are therefore under high pressure are generally ductile and particularly susceptible to behaving plastically that is, deforming without breaking.
- As a result rocks deep within the crust typically fold rather than break in response to compressional forces.
- As elements of rock structure, unfolds are called anticlines, and downfolds are called synclines.
- The rock layers that form the flanks of anticlinal crests and synclinal troughs are the fold limbs.
- Faulting is the slippage or displacement of rocks along a fracture surface, and the fracture surface along which movement has occurred is a fault.
- When compressional forces cause faulting either one mass of rock is pushed up along a steep-angled fault relative to the other or one mass of rock slides along a shallow, low-angle fault over the other.
- The steep, high-angle fault resulting from compressional forces is termed a reverse fault.
- Where compression pushes rocks along a low-angle fault so that they override rocks on the other side of the fault, the fracture surface is called a thrust fault, and the shallow displacement is an overthrust.
- In both, reverse and thrust faults, one block of crustal rocks is wedged up relative to the other.
- Reverse or thrust faulting can also result from compressional forces that are applied rapidly and in some cases to rocks that have already responded to the force by folding.
- In the latter case, the upper part of a fold breaks, sliding over the lower rock layers along a thrust fault forming an overthrust.
- Together recumbent folds and overthrusts are important to rock structures that have formed in complex mountain ranges such as the Andes, Alps, and the Himalayas.
Types of Mountains
Based on their mode of formation, four main types of mountains can be distinguished.
- Fold Mountains
- Block Mountains
- Volcanic Mountains
- Residual Mountains
- They are caused by large-scale earth movements when stresses are set up in the earth’s crust.
- When such stresses are initiated, the rocks are subjected to compressive forces produce wrinkling or folding along the lines of weakness.
- The unfolded waves are called anticlines and the troughs or downfolds are synclines.
- Due to the complexity of the compression forces, the folds developed much more complicated forms.
- When the crest of a fold is pushed too far, and over fold is formed. If it is pushed still further, it becomes a recumbent fold.
- In extreme cases, fractures may occur in the crust, so that the upper part recumbent fold slides forward over the lower part of a thrust plane, forming an overthrust fold. The over-riding portion of the thrust fold is termed a nappy.
- When the earth’s crust bends folding occurs, but when it cracks, faulting takes place.
- Faulting may be caused by tension or compression, forces which lengthen or shorten the COMPRESSION earth’s crust, causing a section of it to subside or to rise above the RIFT VALLE surrounding level.
- Examples are the Huns Ruck Mountains, the Vosges and Black forest of the Rhineland.
- These are in fact, volcanoes which are built up from material ejected from fissures in the earth’s crust.
- The materials include molten lava, volcanic bombs, cinders, ashes, dust and liquid mud.
- They fall around the vent in successive layers, building up a characteristic volcanic cone.
- Volcanic mountains are often called mountains of accumulation.
These are mountains evolved by denudation. Where the general level of the land has been lowered by the agents of denudation some very resistant areas may remain and these form residual mountains.
Plateaux are elevated uplands with extensive level surfaces and usually descend steeply to the surrounding lowland. They are sometimes referred to as tablelands.
Types of Plateaus
There are three types of Plateaus. They are
- Tectonics Plateaus
- Volcanic Plateaus
- Dissected Plateaus
- These are formed by earth movements which cause uplift, and are normally of a considerable size, and fairly uniform altitude.
- They include continental blocks like the Deccan plateau in India. When plateaux are enclosed by fold mountains, they are known as intermont plateaux.
- Examples are the Tibetan plateau between the Himalayas and the Kunlun, and the Bolivian Plateau between two ranges of the Andes.
- Molten lava may issue from the earth’s crust and spread over its surface to form successive sheets of basaltic lava. These soldify to form a lava plateau.
- Some of the better known volcanic plateaux are the Antrim plateau of Northern Ireland and the north-western part of the Deccan plateau.
- The most remarkable plateau built by lava is the Colombia-snake plateau which covers an area almost twice as big as Malaysia.
- Through the continual process of weathering and erosion by running water, ice and winds, high and extensive plateaux are gradually worn down, and their surfaces made irregular.
- In the humid highlands, stream action and sometimes glaciation cut deep, narrow valleys in the plateaux, which are then described as dissected plateaux.
- An example is the Scottish Highlands. In drier countries, vertical corrosion by rivers and abrasion by winds will dissect the plateau into steep-sided tabular masses termed as mesas and buttes, intersected by deep canyons.
- This is a common feature of arid and semi-arid areas, e.g; in the south-western U.S.A.
A plain is an area of lowland, either level or undulating.
Types of plains
There are three types of plains. They are
- Structural Plains
- Depositional Plains
- Erosion Plains.
- These are the structurally depressed areas of the world that make up some of the most extensive natural lowlands on the earth’s surface.
- They are formed by horizontally bedded rocks, relatively undisturbed by the crustal movements of the earth.
- These are plains formed by the deposition of materials brought by various agents of transportation.
- They are comparatively level but rise gently towards adjacent highlands. Some of the largest depositional plains are due to deposition by large rivers.
- They are called alluvial plains, flood plains and deltaic plains. They form the most productive agricultural plains of the world, intensively tilled and very densely populated.
- The Nile delta of Egypt is noted for rice and cotton cultivation, the Ganges delta for rice and jute growing, while the plain of North China, where the Hwang Ho has spread out a thick mental of alluvium, supports a wide range of crops.
- Glaciers and ice-sheets may deposit a widespread mantle of unsupported fluvial glacial sands and gravels in the out plain or may drop boulder clay, a mixture of various sizes of boulders and clay, to form a till plain or drift plain.
- Outwash plains are usually barren lands, e.g. Some parts of Holland and northern Germany, but boulder clay may be very valuable farming land e.g. the Mid-West of the U.S.A. and East Anglia in England.
- In coastal regions, waves and winds often drive beach materials, mud, sand or shingle, landwards and deposit them on the coastal plain to form marine swamps, mud-flats, tidal and estuarine lowlands.
- Winds may blow Aeolian deposits very fine particles known as loess-from interior deserts or barren surfaces and deposit them upon hills, valleys or plains forming a loess plateau as in north-west China, or loess plains, as in the Pampas of Argentina.
- The loess helps to level an undulating plain by filling up grooves and depressions.
- These plains are carved by the agents of erosions. Such plains of denudation are described as pen plains.
- Mechanical weathering in arid and semi-arid areas wears back the mountains slope to leave a gently sloping pediments or Pedi plains.