Epeirogenic Earth Movements

Eperiogenic movements are the ones which operate vertically from the centre of the earth to its surface caused by radial forces (Convection Currents). It causes regional upliftment but on a large scale; therefore, not much noticeable deformation of the surface takes place.

In Greek, ‘epeiroes’ means a continent. They are very wide and large-scale movements, spreading over the continental platform or the stable block of land. Therefore these processes are also known as a continental formation. They characterize large land areas by broad, gentle and widespread warping, subsidence, upliftment, emergence, and submergence. These movements are so slow and widespread that no obvious folding and fracturing can be seen in the rocks.

Bloom has defined epeirogeny as a continental vertical tectonic movement of low amplitude relative to its wavelength, not within an orogenic belt, that does not deform rocks or the land surface to the extent that is measurable within a single exposure.

This broad regional tectonic movement with no local deformation can be either a positive movement like upliftment or a negative movement like subsidence. Upliftment movement causes the upliftment of the continental masses. Either the whole continent or the part of it. It also causes the upliftment of the coastal land of the continents. Such a type of upliftment is called emergence.

Subsidence of the continental masses again happens in two ways. For example, one is subsidence of the land area called subsidence; alternately, the land near the sea coast is moved downward or subsided below sea level and is thus submerged under seawater. Such movements are called submergence.

Glacial isostasy is one more kind of epeirogeny in which the reason for the regional subsidence is the weight of an ice sheet. Deglaciation due to the removal of this load and postglacial recovery has resulted in its discovery.

Did You Know?

These large-scale movements were prevalent in all continents of the earth’s surface, including Antarctica. Various pieces of evidence have proven both upward and downward movements since Precambrian times. For example, the presence of marine sedimentary rocks of Palaeozoic, Mesozoic and Cenozoic age, which lie on all continents, upon the much eroded older rocks.

Epeirogenic Landforms

Prevailing isostatic equilibrium has maintained the average continental crustal “freeboard” within less than 100 meters of sea level throughout Phanerozoic geologic history. The “freeboard” of a continent is the amount exposed above sea level. These freeboards are supported by isostatic buoyancy.

If a continent has a crustal thickness of just 30 km and a density of 2.8 g/cm3, it will not be having any freeboards but will be at isostatic equilibrium just at sea level. Only at the orogenic belts where the crust more thickness, otherwise continental crust is typically not thicker than 33 km., therefore even minor changes in the sea level owing to the epeirogenic movements can lead to the flooding of extensive areas of the continent and deposition of the layers of sediments. There are many examples to prove such kind of Subsidence.

Over a very large continent area, one can find sedimentary rocks of 1000 to 2000 m in total thickness. These undeformed marine sedimentary rocks overlay over the continental crust’s igneous and high-grade metamorphic rocks. They are as old as the early Paleozoic Era. These sedimentary rocks are like platforms separated by disconformities with only minor relief. They were never high enough above sea level to be subjected to deep fluvial dissection. These platform sedimentary rocks, which are almost in their initial depositional attitude, are almost completely eroded from their continental landscape before any significant relief feature can emerge.

In such landscapes, erosional features like broad structural domes cover crust and basins with epeirogenic tectonic relief of a few hundred meters over distances of hundreds of kilometres are common. Whereas the submergence of strata is very less and undetectable, usually less than 1 per cent.

At the central region, topographic basins are formed when these epeirogenic domes are subjected to erosion. The rate of tectonic upliftment is extremely slow and less compared to the rate of erosion. The result is topographic inversion by erosion on epeirogenic structures.

Movements of greater magnitude are to be seen by the sedimentary successions built upon the continental shelves within the continent. Vast depression is recorded towards the end of the Cretaceous period, with a series of transgressed sediments deposited in epicontinental seas. On the stable blocks of the continents, one can notice the various kinds of sedimentary deposits. They could include Aeolian deposits, lacustrine deposits, coal, evaporates and tillities also, the arkores formed out of decayed uplifted mountains.

A sequence of such long continuous deposits can be seen in the interior of low and stable continental margins of the Karroo sediments of South Africa. Lake Victoria and Lake Kyogo in E Africa are other examples illustrating the shallow basin with sinuous outlines, indicating drowning of land.

Cratons, also known as continental cratons are the extensive continental crust or shields. They are a complex of deeply eroded accreted orogenic terrains and can be ranged to various ages. It has crystalline shields or platforms of thin, old sedimentary rocks, which are monotonous, low altitude and low relief not far above sea level, and can date back centuries.

About one-third of the sub-aerial landscape is eroded from exposed ancient igneous and metamorphic continental crust rocks without any sedimentary cover. They are exposed rocks which must have solidified from magma or metamorphosed at depths of 10 to 20 km or more.

For example, present relief on the Canadian Shield continues as an unconformity under the Paleozoic and Mesozoic sedimentary rocks on its periphery. The Australian craton is a region of exceptionally low relief and tectonic stability, with landforms alleged to be 10⁷ to 10⁵ years old. Fig.

If Cratons are such large low-lying platforms of thin, old sedimentary rocks, then what tectonic processes could cause the epeirogenic uplift of other large continental areas? A very characteristic behaviour of craton is the long continuous rise of a localized plateau. The local circulation causes elevation. These elevations are caused by mantle plumes deep beneath the areas of continental crust or due to the collision of two continental plates. This elevation can be as fast as 8 mm per year.

The up-arching of the continental crust by the hypothetical plume head may also have caused the surface rifting and volcanism in the Columbia Plateau and Snake River Plain to the north. This plum migrated to the west with its tail fixed in the mantle, therefore, it appears to have migrated eastward from the Columbian plateau to its present position under Yellowstone volcanic plateau.

Did You Know?

The west of Yellow Stone National Park covers about 8000 sq km. The area is rising slowly at the rate of about 3 to 5 mm per year. From 1955-73 the Central Adirondacks in New York State rose by 40 mm, and its northern margins subsided to about 50 mm. The rate at which the Rhine Massif rises is about 0.35mm per year. The Colorado Plateau has risen for about 550 mm at the rate of 0.1mm per year in the late Cenozoic, and the Deccan Plateau in India has uplifted in the Tertiary and Quaternary and continues at present at the rate of 0.36mm per year.

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