Diastrophism

Diastrophism

diastrophism, also called tectonism, large-scale deformation of Earth’s crust by natural processes, which leads to the formation of continents and ocean basins, mountain systems, plateaus, rift valleys, and other features by mechanisms such as lithospheric plate movement (that is, plate tectonics), volcanic loading, or folding.

The study of diastrophism encompasses the varying responses of the crust to tectonic stresses. These responses include linear or torsional horizontal movements (such as continental drift) and vertical subsidence and uplift of the lithosphere (strain) in response to natural stresses on Earth’s surface such as the weight of mountains, lakes, and glaciers. Subsurface conditions also cause subsidence or uplift, known as epeirogeny, over large areas of Earth’s surface without deforming rock strata. Such changes include the thickening of the lithosphere by overthrusting, changes in rock density of the lithosphere caused by metamorphism or thermal expansion and contraction, increases in the volume of the asthenosphere (part of the upper mantle supporting the lithosphere) caused by hydration of olivine, and orogenic, or mountain-building, movements.

Orogenic
it is marked by deformation of the earth's crust ,including radial and slow deformation. orogeny, mountain-building event, generally one that occurs in geosynclinal areas. In contrast to epeirogeny, an orogeny tends to occur during a relatively short time in linear belts and results in intensive deformation. Orogeny is usually accompanied by folding and faulting of strata, development of angular unconformities (interruptions in the normal deposition of sedimentary rock), and the deposition of clastic wedges of sediments in areas adjacent to the orogenic belt. Regional metamorphism and magmatic activity are often associated with an orogenic event as well. Orogenies may result from subduction, terrane accretion (landmass expansion due to its collision with other landmasses.

This diagram depicts some common fold types

Fault Types







Epeirogenic regional up lift of the crust which result in large scale deformation .

-involves vertical movement of crust .
-not involves severe deformation of the crust .
-causes submergence of cratons by several thousand of meters

Epeirogenic movement can be permanent or transient. Transient uplift can occur over a thermal anomaly due to convecting anomalously hot mantle, and disappears when convection wanes. Permanent uplift can occur when igneous material is injected into the crust, and circular or elliptical structural uplift (that is, without folding) over a large radius (tens to thousands of km) is one characteristic of a mantle plume.

Epeirogenic movement has caused the southern Rocky Mountain region to be uplifted from 1300 to 2000 m since the Eocene.This followed and is distinct from the creation of the Rocky Mountains during the Laramide Orogeny during the Late Cretaceous–early Cenozoic. The uplift is interpreted as due to lithospheric heating resulting from thinning and the intrusion of widespread middle Tertiary batholiths of relatively low density.

In contrast to epeirogenic movement, orogenic movement is a more complicated deformation of the Earth's crust, associated with crustal thickening, notably associated with the convergence of tectonic plates. Such plate convergence forms orogenic belts that are characterized by “the folding and faulting of layers of rock, by the intrusion of magma, and by volcanism

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RELIEF FEATURES

RELIEF FEATURES

Various type of landform of the Earth’s crust classified under three orders of magnitude:

The first order comprises continents and Oceans which are the largest features on the Earth. The configurations of continents & ocean basins haven't remain static such as sea floor spreading and faults. Many ancient features have understandably dissolved in the mists of time. However, geoscientists have pieced together evidences which show that parts of the earth’s crust had been elevated and depressed, extensive stretched of dry lands of the present day were under water, violent episodes of volcanic activity and impact of extra-terrestrial bodies has scared the earth’s face and mountain ranges had been heaved up and worn down.

The second order comprises mountains and plains which have resulted by the action of the internal forces of the earth. Such action of forces emanating from deep within the bowels of the earth includes both orogenic and epeirogenic movements.

The third order features result by the action destructional force and give rise to residual features of peaks, erosional features including valleys and canyons and depositional features like deltas. Also, weathering, streams, waves, wind and glaciers produce relief features of the third order.

First order landforms:

The slope element of the Earth:

-deep sea platform of the ocean

- Continental slope

- Continental shelf

- Continental platform 

The earth's surface is inhomogeneous:

1. 70.8 % of the earth's surface are under water.

2. 29.2 % of the land continents

3. Land and sea are mostly antipodal arranged. Only1.5 % of the surface has land antipodal to land.

4. About two-third of land is in northern hemisphere

5. The deepest parts of oceans aren't always far out from the land and are often located clothe to mountain ranges as in Island arcs

Major topographic element:

1- Ocean ridges:

Wide oceans has brought to light traversal fractured linear ridges extending over a distance of about 64000 K m with width of 2000-4000 Km and rise from 1-3 Km from the ocean floor.

Examples:

The Atlantic & Indian ocean are irregular.
The east pacific ridge is smooth arch.
The mide Atlantic ridge is characterized by 25 – 50 Km wide axial rift valleys.

The Carlsberg ridge is a branch of Indian ocean ridge, branches off towards the north, enters the Gulf of Aden and thence the red sea.
Southerly branch runs through the rift valley system of East Africa.
The East pacific ridge isn't marked by central rift valleys; it’s bordered by faulted steps, ridges & troughs.
Ridges are tectonically unstable marked by shallow earthquakes, high heat flow & volcanic activity

2-Ocean basins:

The ocean basins flanking the mid – ocean ridges are abyssal plains which characterized by hills and sea mounts.
The abyssal plains are tectonically inactive and have gentle gradients of less than 1:1000.They are known to be up to 1000 Km in width below water columns measuring 3-6 Km in depth. They are found to increase in thickness toward the continental slope and shelf.
The sea mount in the bed are spectacular features with width 2-100 Km and rising to dizzy heights of more than 1000 m from the abyssal plain
Sea mounts have sharply pointed and flat tops and they are in the form of hills whose tops are in some cases below a water column of 200 meters.
Guyots: are steep-sided seamounts (12o- 35o) appearing to have ware leved platforms whose submergence is attributed to sea-floor subsidence and rise in water level in post –glacial time.

Continental slope and rise:

The present shoreline of ocean does not limit the extent of continental rocks. The outer edge of the continental shelf, approximately located 0.135 kilometers below the sea level delimits the continental rock.

Continental slope is the part leads into the deep sea, from its outer edge descends at slope up to 6o to depth of two kilometers. Continental sediments in the form of coalescing fans and aprons mark the base of continental slope. In seaward extension of large rivers, submarine canyons mark the continental shelf and slope.

Continental rise is a wide, gentle incline from an ocean basin to a continental slope. A continental rise consists mainly of silts, muds, and sand, and can be several hundreds of miles wide. Although it usually has a smooth surface, it is sometimes crosscut by submarine canyons.

3 - Ocean Trenches, deeps or troughs:
Oceans trenches define the deepest parts of ocean floor. They are known to be variable in length (300 – 5000 Km) and 30 – 100 Km in width, with slopes of 10o – 16o in their deeper parts. The trenches run parallel to island arcs or younger volcanic zones on their seaward side.

4- Island Arcs:

Most ocean trenches on their landward side are marked by parallel accurate festoons of islands. In certain cases, they are topographically and structurally continuous with continental belts of young folded mountains. They are tectonically active zones with profound seismicity.

5- Marginal Sea Basins:

They occur between island arcs and continents. Some of them are 500 to 1000 Km wide and have rugged bottoms with faults, undulations and small sea-mounts characterizing complex histories and different sediment sources. Both tectonically active and inactive basins are known

6- Folded Mountains

It is formed in sediments under the impact of compression. Folding, thrusting and uplift are thrown up as curvilinear mountain chains which may be associated with volcanic activity, deep igneous emplacement and metamorphism.
Broadly, folded mountains may be divided into older and younger groups. The older groups have medium scale elevations and are tectonically more stable than younger group. The younger groups include mountains of highest terrestrial elevations like ALPS and Himalayas.

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Basic concepts and significance of geomorphology

Basic concepts and significance of geomorphology
Geomorphology means a discourse on earth forms. The Geomorphological studies encompass landforms of the continents, their margins and the sea floor.

Landforms are studied from three different points of view:

‐The geologist looks into the geological controls in the evolution of landforms. In the study of landforms by geologists, understanding the historical and dynamic elements of the process that change the geological materials and structure is very important.
‐The geographer concerns with the adjustment of human activities in charging landforms.
‐The engineer assesses the terrain from the point of view of engineering construction and availability of materials for construction.

Sources and time of geomorphological activities:

All geological and geomorphological activities of the earth are due to endogenetic and exogenetic sources of energy. Example of endogenitic sources is the convection currents in the mantel that may produce earthquake, volcanoes and plate movements. Exogenetic sources may be represented by the solar radiation,
gravitational attraction and biological processes.

Landform units are studied not only with the reference to their magnitude, but also to the time it is taken to be formed. The time of the evoluti

on of geomorphological landforms ranges from few minutes (ex. Ripple marks) to long period of millions of years (ex. Mountainous chains).

Concepts of geomorphology

During the fifteens and sixteen evolution of landforms was based on the philosophy of catastrophism until James Hutton (1726 – 1797) has changed it to scientific though with his principle of uniformitarianism. This leads to the

main concepts of geomorphology which are as follows:

‐First concept depends on the principle of uniformitarianism that conveys that the present is the key of the past. James Hutton applied this principle rigidly and stated that geological processes have been active at the same level of intensity throughout the geological time. IT IS NOW RECOGNISED THAT IT IS NOT TRUEthat the physical processes and laws which are in operation during the present time were active in the geological past, but notnecessarily with the same intensity as now.

‐Second concept: that in the evaluation of landforms, structure such as joint, folds, fault and permeability plays a crucial role and is reflected in the landforms. Geomorphic features developed on rocks are in general much younger to the structural features of the rocks.

‐Third concept is that geomorphic process operates not on a uniform rate but at differential rates resulting in the evolution of relief features. The processes are affected by many factors as altitude, temperature, moisture, type of vegetation and microclimatic condition.

‐Fourth concept that geomorphic processes produce distinctive imprints on landforms and characteristic assemblage of landforms evolve as a result to the operation of different geomorphic processes. For example, deltas and alluvial fans result by stream action where caves in limestone terrain are attributed to ground water

‐Fifth concept states that according to W. Davis (1850‐1934) thought that landforms display distinctive characteristic depending on the stage of their development. So that the evolution of landforms is through youth, maturity and old age stages. Though many geomorphologiste are not convinced of this
concept, because an orderly sequence of landforms come into being by the action of different erosional agents on the earth’s surface.

‐Sixth concept is that geomorphic evolution is one of complexity then simplicity. In one geomorphic process or cycle, remnants of landforms not related to the current cycle of erosion are recognized. Five categories of landforms including simple, compound, monocyclic, multicyclic and exhumed have been recognized.

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