Rift valley

Also called: Graben · Topic: Geomorphic Processes: Weathering, Mass Movement, Erosion and Soil Formation · NCERT: Class 11, Ch 2 "Structure and Physiography"; Class 11, Ch 3 "Drainage System"

Meaning

A rift valley (graben) is a long, narrow trough with steep walls. It forms when the Earth's crust is pulled apart (tension) and a strip of crust sinks between two parallel normal faults [4].

It matters because rift valleys decide which way some big rivers flow and what their mouths look like. In India, the Narmada and Tapi flow west through rift valleys and end in estuaries, not deltas. The Damodar valley, also a rift, holds much of India's Gondwana coal.

Explanation

How a rift valley forms (cause → process → result)

• Cause: tension. Heat inside the Earth moves the plates (large, rigid pieces of the crust and upper mantle). In some places this movement pulls the crust apart.
• Process: faulting, not folding.
• The crust is too brittle (hard and easy to break) to bend, so it cracks.
• A fault is a crack in the crust along which blocks of rock have moved.
• Under tension, normal faults (dip-slip faults) form. In a normal fault, the rock above the crack slides downward compared with the rock below it [4].

• Two such faults form side by side, running parallel.

• Result: a sunken strip.

• The strip of crust between the two faults drops down.

• This leaves a long, narrow trough with steep walls. The walls are the fault planes.

• Folding and faulting are the two main forms of diastrophism (all the processes that move, lift or build up the crust). They are endogenic forces (forces from inside the Earth) and act slowly, over millions of years.

Parts and related landforms

• Graben: German for "ditch". The word usually means a smaller rift valley [7].
• Horst: a raised block bounded by faults. It usually sits beside a sunken graben [5]. Horst = block mountain; graben = rift valley.
• Rift lake: a lake that fills the floor of a rift valley. Example: the Dead Sea [7].
• Failed rift: a rift where the crust began to pull apart but never split fully into an ocean. Example: the Narmada–Son Lineament [2][3].

Life cycle of a rift

• Active rift: the plates are still pulling apart, as in the East African Rift and the Dead Sea–Jordan rift [4][8].
• Rift → ocean: if the pulling continues, the trough widens and the sea floods in. The Red Sea shows this stage.
• Old, failed rift: the pulling stopped long ago. India's Narmada–Damodar rifts are of this kind [3].

What a river does in a rift valley

• Straight course, few meanders
• The river is held in a straight trough with hard rock on both sides.

• So it meanders (bends from side to side) very little.

• Little silt at the mouth

• The river flows fast down a narrow trough and carries little silt.

• So little sediment is dropped where it meets the sea.

• Estuary, not a large delta

• In the Gulf of Khambhat, strong tides wash the silt away.
• So an estuary forms (a funnel-shaped river mouth where fresh water mixes with sea water), not a large delta.

Where it occurs

World

• Rhine Rift Valley (Europe): about 300 km long [6].
• The Vosges and the Black Forest were once one block. A central strip sank and split them.
• Order from west to east: Vosges (France) → Rhine graben → Black Forest (Germany).

• The Vosges are a typical horst [5]. The Black Forest runs about 160 km north-east from Säckingen to Durlach [6].

• East African Rift System: the largest continental rift valleys. They run north to the Red Sea and east into the Indian Ocean [4].

• Dead Sea and Jordan Valley (West Asia)
• The Dead Sea rift is the land part of the Syrian–African Rift System. This rift runs along a plate boundary from the Indian Ocean through the Red Sea and the Gulf of Eilat (Aqaba) [8].
• The Jordan Valley follows the Jordan River from the Sea of Galilee to the Dead Sea and is about 10 km wide [9].
• The Dead Sea is a rift lake. Its surface is about 429 m below sea level, the lowest land on Earth [7].

India

• Narmada–Son trough: a long trough in central India between the Vindhya and Satpura ranges [10]. Geologists call this line of weakness the Narmada–Son Lineament (a lineament is a long, straight line of weakness in the crust) [2].
• Narmada River (Madhya Pradesh, Maharashtra, Gujarat)
• Rises at Narmada Kund, Amarkantak, Madhya Pradesh, in the Maikala Range, about 1,080 m high [11].
• About 1,312 km long, with 41 tributaries [11].
• After Mandla it enters the valley between the Vindhyas and the Satpuras [11][12].
• It flows past the Marble Rocks gorge near Jabalpur and the Dhuandhar Falls, over Deccan Trap rocks (layers of old lava) [13].
• It reaches the Gulf of Khambhat (Cambay) through an estuary about 21 km wide, just below Bharuch [12].

• It is the largest west-flowing river in India. Madhya Pradesh has 86.9% of its basin [13].

• Tapi (Tapti) River

• Rises near Multai, Betul district (Madhya Pradesh).
• Flows west, south of the Satpuras and parallel to the Narmada, also through a rift.

• Reaches the Gulf of Khambhat near Surat.

• Damodar valley (Jharkhand–West Bengal)

• The Damodar flows east along a rift and joins the Hooghly.
• The Damodar and Koel valley basins run east–west along a tectonic trend and contain Gondwana rocks [3].
• Gondwana coal formed about 250 million years ago. Forests that grew in these sinking troughs were buried and pressed into coal.
• Coalfields: Jharia, Raniganj, Bokaro, Giridih, Karanpura.
• The Mahanadi, Godavari and Son valleys hold Gondwana coal in similar troughs.

Don't confuse with

• Horst (block mountain): a horst is a block that stands high between faults. A graben is a block that has sunk. Both are bounded by faults [5]. NCERT's Indian example of a block mountain is the Satpura range.
• Syncline: a syncline is a downfold made by compression. The rock layers bend into a "U" without breaking. A rift valley is made by tension, and the crust breaks along faults.
• Fault formed by compression: under squeezing, one block is pushed up over another. A rift valley needs normal faults formed under tension (pulling apart) [4].
• River-eroded valley: an ordinary river valley is cut by running water (an exogenic process). A rift valley is made by crust sinking (an endogenic process). The river only flows along it later.

Prelims Hooks

• A rift valley forms by tension along normal faults, not by compression [4].
• Order from north to south in central India: Vindhyas → Narmada rift → Satpuras → Tapi rift.
• Order from west to east: Vosges → Rhine rift valley → Black Forest. The Rhine rift is about 300 km long [6].
• Trap: the Narmada and Tapi form estuaries, not deltas. The Mahanadi, Godavari, Krishna and Kaveri form deltas on the Bay of Bengal.
• The Dead Sea, about 429 m below sea level, is a rift lake and the lowest land point on Earth [7].
• Damodar valley = rift (graben) containing Gondwana coal (Jharia, Raniganj). The Damodar–Koel basins run east–west [3].

Mains Points

• Tectonics shapes India's drainage (GS-I)
• The Narmada and Tapi follow fault troughs. So they flow west, against the general eastward slope of the Peninsula.
• Their channels are straight with few meanders, and they end in estuaries.

• This affects ports (Bharuch, Surat), tidal flooding risk, and dam sites such as Sardar Sarovar and Ukai.

• Rift valleys decide where resources lie (GS-III)

• Gondwana grabens (Damodar, Son, Mahanadi, Godavari) hold almost all of India's coal.
• Forests in these sinking troughs were buried and turned into coal about 250 million years ago.

• This explains why the coal and steel industry clusters in the Chota Nagpur region.

• Hazard angle (GS-III)

• The Narmada–Son Lineament is a failed rift. It was once a zone of tension. Today India keeps pushing into Asia, so the zone is being squeezed again [2].
• It is a weak zone in the crust, and earthquakes happen along it, as at Jabalpur (1997). So "stable" Peninsular India also has earthquake risk.
• Seismic zoning and building codes must allow for this. By contrast, the East African and Dead Sea–Jordan rifts are active plate boundaries that are still pulling apart [4][8].

Related concepts

• Folding
• Block mountain

Read more

• Folding and faulting: fold mountains, block mountains and rift valleys