Geography - Geomorphology - Continent – Continent Convergence November 19, 2017 In this post we will study about Continent – Continent Convergence. Understanding Continent – Continent Convergence is important to understand the Formation of the Himalayas, the Alps, the Urals and the Atlas mountains. We have studied in See Floor Spreading how convectional currents in the mantle drive the lithospheric plates. Rising vertical limbs of the convection currents in the mantle create a divergent plate boundary and falling limbs create a convergent plate boundary. In convergence there are sub-types namely: 1. Collision of oceanic plates or ocean – ocean convergence. 2. Collision of continental and oceanic plates or ocean – continent convergence. 3. Collision of continental plates or continent – continent convergence. 4. Collision of continent and arc or continent – arc convergence . In all types of convergence, denser plate subducts and the less denser plate is either up thrust or folded or both [up thrust and folded]. Continent – Continent Convergence or The Himalayan Convergence In ocean – ocean convergence and continent – ocean convergence, at least one of the plates is denser and hence the subduction zone is quite deep [few hundred kilometers]. At continental – continental convergent margins, due to lower density, both of the continental crustal plates are too light [too buoyant] to be carried downward (subduct) into a trench. In most cases, neither plate subducts or even if one of the plates subducts, the subduction zone will not go deeper than 40 – 50 km. The two plates converge, buckle up [The subduction of the continental crust is not possible beyond 40 km because of the normal buoyancy of the continental crust. Thus, the fragments of oceanic crust are plastered against the plates causing welding of two plates known as suture zone. Example: The- Indus-Tsangpo suture zone], fold, and fault. Geoclinal sediments are found along the continental margins. As the continental plates converge, the ocean basin (geosynclinical basin) is squeezed between the two converging plates. Huge slivers of rock, many kilometers wide are thrust on top of one another, forming a towering mountain ranges. With the building up of resistance, convergence comes to an end. The mountain belt erodes and this is followed by isostatic adjustment. As two massive continents weld, a single large continental mass joined by a mountain range is produced. Examples: The Himalayas, Alps, Urals, Appalachians and the Atlas mountains. Volcanism and Earthquakes in Continent – Continent Convergence Oceanic crust is only 5 – 30 km thick. But the continental crust is 50 – 70 km thick. Magma cannot penetrate this thick crust, so there are no volcanoes, although the magma stays in the crust. Metamorphic rocks are common because of the stress the continental crust experiences. With enormous slabs of crust smashing together, continent – continent collisions bring on numerous and large earthquakes. [Earth Quakes in Himalayan and North Indian Region] Convergent boundary = More deep focus earthquakes. Example: Kachchh region, Himalayan region Formation of Himalayans and Tibet The Himalayan mountains are also known as the Himadri, Himavan or Himachal. The Himalayas are a part of Alpine mountain Chain. The Himalayas are the youngest mountain chain in the world. Indo-Australian Plate Indo – Australian plate è Indian plate + Australian plate + Some parts of Indian Ocean. Indo – Australian Plate boundary North ==> Himalayas East ==> Purvanchal, Rakinyoma Mountains, Arakan coast, Andaman & Nicobar islands and Java Trench, South western Pacific plate. West ==> Suleiman and Kirthar ranges, Makrana coast, western margin of Red Sea rift, Spreading site between Indio – Australian plate and African plate South ==> Spreading site between Indio – Australian plate and Antarctic plate Explain the formation of Himalayas Himalayan mountains have come out of a great geosyncline called the Tethys Sea and that the uplift has taken place in different phases. During Permian Period (250) million years ago, there was a super continent. Its northern part consisted of the present day North America and Eurasia (Europe and Asia) which was called Laurasia or Angaraland or Laurentia. The southern part of Pangaea consisted of present day South America, Africa, South India, Australia and Antarctica. This landmass was called In between Laurasia and Gondwanaland, there was a long, narrow and shallow sea known as the Tethys Sea (All this was explained in detail in Continental Drift Theory). There were many rivers which were flowing into the Tethys Sea (Older than Himalayas. We will see this in detail while studying Antecedent and Subsequent Drainage). Sediments were brought by these rivers and were deposited on the floor of the Tethys Sea. These sediments were subjected to powerful compression due to the northward movement of the Indian Plate. This resulted in the folding of sediments. Once the Indian plate started plunging below the Eurasian plate, these sediments were further folded and raised. This process is still continuing (India is moving northwards at the rate of about five cm per year and crashing into rest of the Asia). And the folded sediments, after a lot of erosional activity, appear as present day Himalayas. Tibetan plateau was formed due to up thrusting of the Eurasian Plate. And the Indo-Gangetic plain was formed due to consolidation of alluvium brought down by the rivers flowing from Himalayas. The curved shape of the Himalayas convex to the south, is attributed to the maximum push offered at two ends of the Indian Peninsula during its northward drift. Himalayas do not comprise a single range but a series of at least three ranges running more or less parallel to one another. Therefore, the Himalayas are supposed to have emerged out of the Himalayan Geosyncline i.e. the Tethys Sea in three different phases following one after the other. The first phase commenced about 50-40 million years ago, when the Great Himalayas were formed. The formation of the Great Himalayas was completed about 30 million years ago. The second phase took place about 25 to 30 million years ago when the Middle Himalayas were formed. The Shiwaliks were formed in the last phase of the Himalayan orogeny — say about two million to twenty million years ago. Some of the fossil formations found in the Shiwalik hills are also available in the Tibet plateau. It indicates that the past climate of the Tibet plateau was somewhat similar to the climate of the Shiwalik hills. There are evidences to show that the process of uplift of the Himalayas is not yet complete and they are still rising. [Recent studies have shown that convergence of the Indian plate and the Asian plate has caused a crustal shortening of about 500 km in the Himalayan region. This shortening has been compensated by sea floor spreading along the oceanic ridge in the Indian Ocean] Formation of Himalayas in Short Pangea’s breakup starts in Permian period [225 million years ago]. India started her northward journey about 200 million years It travelled some 6,000 kilometres before it finally collided with Asia. India collided with Asia about 40-50 million years ago. Convergent boundary gave rise to Himalayas 40 – 50 million years ago [Tertiary Period] [Formation of Deccan Traps began 70-60 million years ago] Scientists believe that the process is still continuing and the height of the Himalayas is rising even to this date. Evidences for the rising Himalayas Today’s satellites that use high precision atomic clocks can measure accurately even a small rise of one cm. The heights of various places as determined by satellites indicate that the Himalayas rise by few centimeters every year. The present rate of uplift of the Himalayas has been calculated at 5 to 10 cm per year. Due to uplifting, lakes in Tibet are desiccated (lose water) keeping the gravel terraces at much higher levels above the present water level. This could be possible only in the event of uplift of the region. The frequent tectonic activity (occurrence of earthquakes) in the Himalayan region shows that the Indian plate is moving further northwards and plunging into Eurasian plate. This means that the Himalayas are still being raised due to compression and have not yet attained isostatic equilibrium. The Himalayan rivers are in their youthful stage and have been rejuvenated [make or cause to appear younger or more vital] in recent times. This shows that the Himalayan Landmass is rising keep the rivers in youth stage since a long time. Formation of Alps, Urals, Appalachians and the Atlas mountains The formation of each of these mountains is similar to the formation of the Himalayas. Alps are young fold mountains which were formed due to collision between African Plate and the Eurasian Plate. Atlas mountains are also young folded mountains which are still in the process of formation. They are formed due to collision between African Plate and the North American, Eurasian Plates. Urals are very old fold mountains which were formed even before the breakup of Pangaea. They were formed due to collision between Europe and Asia. Appalachians are also very old fold mountains which were formed even before the breakup of Pangaea. They were formed due to collision between North America and Europe. Mains Question on Fold Mountains Why are the world’s fold mountain systems located along the margins of continents? Bring out the association between the global distribution of Fold Mountains and the earthquakes and volcanoes. Why fold mountains at continental margin? Fold mountains are formed due to convergence between two continental plates (Himalayas) or between an oceanic and a continental plate (Rockies. Explained in previous post). In Continent – Continent (C-C) convergence, oceanic sediments are squeezed and up thrust between the plates and these squeezed sediments appear as fold mountains along the plate margins. In Continent – Ocean (C-O) convergence, the continental volcanic arc formed along the continental plate margin is compressed and is uplifted by the colliding oceanic plate giving rise to fold mountains along the continental plate margin. Association In both C-C convergence and C-O convergence, there is formation of fold mountains and frequent occurrence of earthquakes. This is because of sudden release of friction between the subducting plate and up thrust plate. In C-C convergence, the denser plate pushes in to the less denser plate creating a fault zone along the margin. Further collision leads to sudden release of energy along this fault zone generating disastrous earthquakes (Himalayan Region). In C-O regions the subducting oceanic plate grinds against the surrounding denser medium producing mostly deep focus earthquakes. Volcanism is observed only in C-O convergence and is almost absent in C-C convergence. This is because of the thick continental crust in C-C convergence which prevents the outflow of magma. Magma lies stocked within the crust. In C-O convergence, metamorphosed sediments and melting of the subducting plate form magma which escapes to the surface through the less thicker continental crust. Continent – Arc Convergence or New Guinea Convergence New Guinea came into being about 20 million years ago as a result of continent – arc collision. The continental plate pushes the island arc towards the oceanic crust. The oceanic plate plunges under the island arc. A trench occurs on the ocean side of the island arc and, ultimately, the continental margin is firmly welded against the island arc.