Folding Understanding Earth's Structural Deformations
Discover the fascinating geological processes of folding, their causes, types, and impacts. Learn how tectonic forces and gravity gliding shape our landscapes, from Fold Mountains to sedimentary rock formations.
What is Folding?
Folding is the bending of rock layers due to compressional forces in the Earth's crust. It occurs when tectonic forces push sedimentary rock strata together, causing them to form wave-like structures known as folds.
When young sedimentary rock strata are compressed (pushed together), they form a series of wave-like undulations of synclines and anticlines known as folds. The forces that cause folds range from slight differences in pressure in the earth's crust, to large collisions of the crust's tectonic plates. As a result, a fold may be only a few centimeters in width, or it may cover several thousands of kilometers. As already noted, old sedimentary rock layers will break orfracture in response to these forces, in which case faulting occurs.
Folding may be produced in two ways:
1. Tectonic compression
2. Gravity gliding
Tectonic compression
In the Theory of Plate Tectonics in Chapter 2 it was noted that the earth's crust is composed of six major and minor plates. The major plates of the world include the Pacific plate, Australian plate, African plate, South American plate, North American plate and Philippine plate. Smaller plates include the Cocos plate, the Nazca plate, the Caribbean plate, and the Gorda plate.
Each plate may consist of either continental and oceanic crust materials or both. Some of these tectonic plates like the Pacific plate consist almost entirely of oceanic crust; others, such as the North American and Eurasian plates, are made up of mostly continental crust.
These plates are semi rigid and each has recognizable boundaries and moves as a unit. Plate boundaries are generally located in mid-ocean or close off-shore, but in a few places rise from the sea bottom and extend across dry land. The movement of the plates is due to a force originating from the interior of the earth's crust. The crust and upper mantle form what is called the lithosphere.
Under the lithosphere lies a fluid rock layer called the asthenosphere. The rocks in the asthenosphere move in a fluid (molten) manner because of the very hot temperatures of over 3,700° C and high pressures found there. The heat in this zone is generated from geochemical reactions and radioactivity. This results in the molten rocks to move in a form of convectional currents.
The convectional currents ascend along mid-ocean ridges before spreading out on both sides of the rifts and sinking back into the interior of the earth. The molten material rises from the very hot zones towards the cool earth's crust where it cools as it approaches the surface. As it cools, the material becomes denser and begins to sink again, forming a circular pattern of movement I called convectional currents. These large-scale convectional currents within the mantle and beneath the plates are part of what drives the movement of the plates in the earth's crust.
When plates collide or move against each other, they compress the earth's crust leading to the formation of compressional forces. The forces lead to the formation of folds with synclines and anticlines. The synclines form valleys while the anticlines form Fold Mountains.
One of the best examples of compression from plate collision is that of the African plate and the Eurasian plate, which resulted in the formation of the Mountains of the Alps. The Alps are a complex fold-mountain system. Sedimentary deposits of vast thickness, mainly limestone and dolomite, were laid down in the ancestral Tethys Sea during the Triassic and Jurassic periods. Subsequently, enormous pressure generated by a collision between the African and Eurasian plates thrust these rock strata upward and northward to form recumbent or horizontal folds, which in the process of movement were detached from their roots.
The Jura Mountains, which are clearly seen in Switzerland and France consists of a series of parallel folds in the strata, forming together a plateau about 320 km long and 32 to 56 km wide.
Another example of fold mountains are the Himalayas, a mountain system in Asia, forming a broad continuous are for nearly 2,600 km along the northern fringes of the Indian sub-continent with a width averaging 320 to 400 km. The Himalayan range was created from powerful earth movements that occurred as the Indian continental plate pressed against the Eurasian continental plate. The earth movements raised the deposits laid down in the ancient, shallow Tethys Sea to form the Himalayan ranges.
The Himalayas form the earth's highest mountain region, containing 9 of the 10 highest peaks in the world. Among these peaks is the world's highest mountain, Mount Everest at 8,848 meters above sea level. There are also more than 30 peaks towering 7,620 meters or more. Even today the mountains continue to develop and change, and earthquakes and tremors are frequent in the area.
Folded mountains are less prominent in Africa than in other continents, a reflection of the geological stability of its basement-complex rocks. The Atlas Mountains in north western Africa and the Cape ranges, including the Swartberg and Langeberg mountain ranges, in South Africa are the only main examples of folded mountains on the continent. The Atlas mountain system in north western Africa consists of several distinct ranges, extending between Tunisia and Morocco, a distance of 2,200 km. They are an extension of the Alpine system of Europe.
Gravity gliding
In gravity gliding, it is suggested that young sedimentary rock strata which is inclined, slides down the incline. Friction at the leading edge causes the sliding strata to fold.
The incline of the bedding plane of the rock strata may be caused by up-lift or subsidence. Uplift on one side of the strata causes the cline to tip on the other side. Subsidence may take place in geosynclines. Geosynclines begin as a belt of especially active sedimentation and eventually become trough, like. This is because the weight of the sediments in geosynclines causes the floor to subside, thus increasing the incline of the sides. Gravity gliding is best seen in the Otavi- Damara belts of south West Africa.
Folding in East Africa
In this region, folding is less prominent than in other parts of the world. Folding did not cause high mountains as the crust consists of mostly hard basement rocks which fractures when compressed or cannot glide. However minor folds are found in some areas of East Africa.
In Uganda, simple folding occurred in three main areas namely, the Karagwe-Ankole rock systems, the Buganda-Toro rock systems like along the Kampala-Mityana road, Kampala-Entebbe road and Mabira along Kampala-Jinja road, and in the gneiss complex of northern Uganda where simple folding led to the formation of Watian series with east-west axis as well as the Aruan series with steep axial planes in a north-south direction.
In Kenya and Tanzania, the Nyanzanian-Kavirondian, Bukoban and Mozambique belt experienced folding. The results of folding led to the formation of Ukamba folds in Kenya, as well as Usagaran and Dodoma folds in Tanzania. Most folds in East Africa result from gravity gliding.
Types of folds.
Folds are also classified according to their shape and angle.
Symmetrical fold
If the axial plane along which a fold occurs is vertical, the resulting fold is a symmetrical fold. This is the simplest type of fold. The rocks have not been turned upside down, and the crests of the waves are called anticlines and the troughs are called synclines.
Asymmetrical Fold
If one side of the fold is steeper than the other and tilted, the fold is termed asymmetrical. It means that the force causing the bend was stronger on one side.
Over Fold of overturn Fold
Sometimes the force on one side of the fold is so strong that the fold is pushed over on it’s side in an overturn.
Recumbent fold
With continued compression, the over fold turns into a recumbent fold.
