Original Or Tectonic Mountains

Fold Mountains (the Himalayas, the Rockies, the Andes)
Block Mountains (Vosges mountains in France, the Black Forest in Germany, Vindhya and Satpura in India)
Volcanic Mountains (Cascade Range in the USA, Mount Kenya, Mount Kilimanjaro, Mount Fujiyama).

Fold Mountains

A fold is an undulating structure (wave-like) that forms when rocks or a part of the earth’s crust is folded (deformed by bending) under compressional stress. The folds are made up of multiple strata (rock layers). The folds that are upwardly convex are called as anticlines. The core (centre) of an anticline fold consists of the older strata, and the strata are progressively younger outwards. In contrast, the folds that are downwardly convex are called synclines. The core of a syncline fold consists of the younger strata, and the strata are progressively older outwards.

How are Fold mountains formed?

Fold mountains are created where two or more of Earth’s tectonic plates are pushed together. At these colliding, compressing boundaries, rocks and debris are warped and folded into rocky outcrops, hills, mountains, and entire mountain ranges. Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones.

Convergent plate boundaries are sites of collisions, where tectonic plates crash into each other. Compression describes a set of stresses directed at one point in a rock or rock formation. At a compression zone, tectonic activity forces crustal compression at the leading edge of the crust formation. For this reason, most fold mountains are found on the edge or former edge of continental plate boundaries. Rocks on the edge of continental crust are often weaker and less stable than rocks found in the continental interior. This can make them more susceptible to folding and warping.

Most fold mountains are composed primarily of sedimentary rock and metamorphic rock formed under high pressure and relatively low temperatures. Many fold mountains are also formed where an underlying layer of ductile minerals, such as salt, is present. Some examples of Fold mountains are The Himalayas, the Rockies, The Alps, the Aravallis, etc.

Types of Folds

A symmetrical foldis one in which the axial plane is vertical.
An asymmetrical foldis one in which the axial plane is inclined.
An overturned foldhas a highly inclined axial plane such that the strata on one limb are overturned.
A recumbent foldhas an essentially horizontal axial plane. An isoclinal foldhas limbs that are essentially parallel to each other and thus approximately parallel to the axial plane.

Characteristics of Fold Mountains

Fold mountains are formed when sedimentary rock strata in geosynclines are subjected to compressive forces.
They are the loftiest mountains, and they are generally concentrated along continental margins.
Fold mountains belong to the group of youngest mountains of the earth. The presence of fossils suggests that the sedimentary rocks of these folded mountains were formed after accumulation and consolidation of silts and sediments in a marine environment.
Fold mountains extend for great lengths whereas their width is considerably small. Generally, fold mountains have a concave slope on one side and a convex slope on the other. Fold mountains are mostly found along continental margins facing oceans (C-O Convergence).
Fold mountains are characterized by granite intrusions (formed when magma crystallises and solidifies underground to form intrusions) on a massive scale. Recurrent seismicity is a common feature in folded mountain belts.
High heat flow often finds expression in volcanic activity (Himalayas is an exception, because of C-C convergence). These mountains are by far the most widespread and also the most important.
They also contain rich mineral resources such as tin, copper, gold etc.

Block Mountains

W. M. Davis coined the term “block mountain,” while D. W. Johnson in 1903 introduced the alternative “fault-block mountain.” Strahler (1946) referred to them as “fault blocks.” These terms describe the initial land forms, or mountains, formed by crustal fracturing when geotectonically positive. A simple, symmetrically bounded positive fault block is called a horst, and Geikie in 1914 referred to it as a “horst mountain.” However, the term “block mountain” can also encompass tilted fault blocks and complex faulted uplands.

What are Block Mountains?

Block mountains are like the Earth’s rugged backbone, rising proudly between two faults or flanking the sides of a rift valley or graben. These mountains come into existence because of the Earth’s internal forces playing tug-of-war, pulling and pushing in different directions. Imagine these uplifted blocks as the proud horsts standing tall, and the lowered blocks as the humble graben, creating a dynamic landscape. It’s like a geological dance orchestrated by Mother Nature. Take a journey to the Great African Rift Valley, where the valley floor is a graben, or explore the charming Rhine Valley with its graben charm. Venture into the Vosges mountain, proudly standing as a horst in Europe. These natural wonders tell a story written in the language of Earth’s movements.

Types of Block Mountains

Block mountains are generally of two basic types:

Tilted block mountains having one steep side represented by fault scarp and one gentle side.
Lifted block mountains represent real horst and are characterized by flattened summit of tabular shape and very steep side slopes represented by two boundary fault scarps.

What are Faults?

Faults are fractures in Earth’s crust where rocks on either side of the crack have slid past each other. Sometimes the cracks are tiny, as thin as hair, with barely noticeable movement between the rock layers. But faults can also be hundreds of miles long, such as the San Andreas Fault in California and the Anatolian Fault in Turkey, both of which are visible from space. There are three kinds of faults: strike-slip, normal and thrust (reverse) faults. Each type is the outcome of different forces pushing or pulling on the crust, causing rocks to slide up, down or past each other. Each describes a different kind of relative motion.

Strike-slip faults indicate rocks are sliding past each other horizontally, with little to no vertical movement. Both the San Andreas and Anatolian Faults are strike-slip.
Normal faults create space. Two blocks of crust pull apart, stretching the crust into a valley. The Basin and Range Province in North America and the East African Rift Zone are two well-known regions where normal faults are spreading apart Earth’s crust.
Reverse faults, also called thrust faults, slide one block of crust on top of another. These faults are commonly found in collisions zones, where tectonic plates push up mountain ranges such as the Himalayas and the Rocky Mountains.
Strike-slip faults are usually vertical, while normal and reverse faults are often at an angle to the surface of the Earth. The different styles of faulting can also combine in a single event, with one fault moving in both a vertical and strike-slip motion during an earthquake.

How are Block Mountains formed?

Most of the geologists are of the opinion that block mountains are formed due to faulting. Block mountains are formed in a number of ways:

Block mountains are formed due to upward movement of middle block between two normal faults . The up-thrown block is also called as horst. The submittal area of such block mountain is of flat surface but the side slopes are very steep.
Block mountains may be formed when the side blocks of two faults move downward whereas the middle block remains stable at its place. It is apparent that the middle block projects above the surrounding surface because of downward movement of side blocks. Such block mountains are generally formed in high plateaux or broad domes.
Block mountains may be formed when the middle block between two normal faults moves downward. Thus, the side blocks become horsts and block mountains. Such mountains are associated with the formation of rift valleys.

Volcanic Mountains

A Volcanic Mountains starts out as a simple crack in the Earth called a volcanic vent. Magma erupts out of the ground as lava flows, clouds of ash, and explosions of rock. This material falls back to Earth around the vent, and piles up around it. Over time (and sometimes quite quickly) a volcanic mountain builds up, with the familiar cone shape.

Types of Volcanic Mountains

On the basis of activity:

Active Volcano: An active volcano erupts continuously in the present time. Example: Barren Island volcano at Andaman Nicobar Islands in India is the only active volcano of India. There are around 700 active volcanoes in the world, many under the sea. Stromboli in Italy or the ‘Lighthouse of the Mediterranean’ is one of the most active volcanoes on Earth.

Dormant or sleeping volcanoes: These are the most dangerous volcanoes in the world. Sleeping volcanoes do not have any fixed time of eruption , they can erupt any time. Example-Mt. Kilimanjaro ( Tanzania).

Extinct Volcano: An extinct volcano is the one which has not erupted for the last 10,000 years and is a dead volcano according to scientists. An extinct volcano has no lava supply in its magma chamber. Example- Mt. Thielson in Oregon USA is a dead volcano because it had last erupted 2,50,000 years ago.

On the basis of structure:

Shield Volcanoes: Imagine a volcano shaped like a warrior’s shield. These volcanoes slowly build up from fluid lava flows spreading out in all directions.The Hawaiian Islands, like Kilauea and Mauna Loa, are examples of shield volcanoes. They’re huge, with Mauna Loa being the world’s largest active volcano.

Cinder Volcanoes: Cinder cones are the simplest volcanoes. Picture a circular or oval cone formed by blobs of lava thrown into the air, which then solidify and fall back. Parícutin Volcano in Mexico is a cinder cone, rising about 1,200 feet above its surroundings.

Composite Volcanoes: These are like the grand mountains you see, such as Mount Fuji in Japan or Mount St. Helens in Washington. They’re steep and symmetrical, made of layers of lava, ash, and rocks. Most have a crater at the top with a central vent or group of vents. Composite volcanoes build up as materials like lava and ash are added to their slopes.

Calderas: Think of calderas as giant bowls formed when a volcano has a massive eruption. The eruption can be so powerful that it empties the magma chamber beneath the volcano. Mount Tambora in Indonesia is an example of a volcano that lost its summit during a caldera-forming eruption. Calderas can be massive, stretching tens of miles across, with steep walls.

Understanding these different types of volcanoes helps us grasp the amazing diversity and power of these geological features.

Characteristics of Volcanic Mountains

A magma chamber is a hollow space inside the earth surface. The molten rocks are called magma. It is present just below the vent of the volcano and is a source of lava.
Magma chamber contains molten rocks and gases under high pressure. Conduit is a pipe-like structure through which magma comes out and it opens into the main vent. The depth of the magma chamber is usually 1km to 10 km.
A vent is a hole in the earth’s surface through which the lava comes out with great pressure. Generally vent is a weak point on the crust of the earth. There could be more than one vents of a volcanic mountain.
Lava is the molten rock or magma that comes through a volcanic vent. It generally consists of silicates or silica rocks and is in liquidor semi-solid form. The temperature of lava is anywhere between 700 C to 120.
Lava cools and hardens after coming into contact with the air. This cooled lava forms a volcanic mountain-like shape. Craters are the circular depressions around the main vent. Craters are developed due to a large explosion in the volcano. These are formed when the magma chamber is present near the vent. The volcano becomes active due to the creation of high pressure in the magma chamber.
Generally, the magma chamber contains a large amount of water which is continuously converting into steam because of the geothermal energy. When the pressure becomes so high, it comes out as an explosion causing a volcanic eruption. A pyroclastic flow is the flow of molten semi-liquid lava away from the volcanic mountain, downhill.