Geomorphology Flashcards
Discuss the components of GW.
1) pore spaces of regolith and bed rocks, below the grnd surface
2) main source
3) aquifers: sands ideal; permeable sandstones
4) saturation of regoliths and rocks
5)saturated or phraeatic zone- GW table
6) unsaturated or vadose zone or aeration zone
7) aquiclude
8) confined water- artesian wells
9) perched water table
10) seasonal and annual fluctuations
Discuss the mechanism of geomorphic work of GW and the factors controlling the process.
1) erosion, limited transport and deposition
2) extremely slow
3) slumping, debris slides and fall and land slides
4) corrosion or solution; corrasion or abrasion;attrition and hydraulic action
5) RW mixed with atm and organic Co2 becomes active solvent agent
6) R.M. Garrels: seven variables that control limestone solution
7) amt of dissolution of carbonate rocks: T, Pco2, organic co2,carbonate rocks(limestone or dolomite, chalk), joints of rocks, nature and rate offlow of GW, contact time
What are Karst topography and where are they formed?
Landforms produced by chemical weathering or chemical erosion of carbonate rocks, mainly limestone and dolomites, by surface and subsurface are called KT
in areas where thick beds of massive limestones lie just below layer of superficial material
1) eastern margin of adriatic sea,in erstwhile yugoslavia
2) Causes region of southern france
3)Spanish andulasia
4) Northern Puerto Rico
5) Jamaica
6) Southern Indiana,Virginia, tennesse and central florida
7) other minor karst areas: Carlsbad area of USA, chalk area of England, chalk area of France, parts of Jura mts
8) India: non-existence of extensive thick limestone formations near the surface. Most of Vindhyan formations buried under thick covers of sandstones and shales. eg. Rohtas stage limestone are buried under 90 m thick cover of massive sandstones. few areas identified in Himalayas(J&K, SahasraDhara, Rovers cave), Panchmarhi,coastal area near Vishakhapatnam
What are the essential conditions favourable for development of karst topography?
1) massive, thickly bedded, hard and tenacious, well cemented and welljointed
2) Limestones should nt be porous
3) position should be above the GW table
4) should be widely distributed in both areal and vertical dimensions
5) rocks should be close to Grnd surface
6) should be highly faulted , folded or fractured
7) enough rf
various erosional and depositionl landforms of characterestic of Karst topography?
EROSIONAL LANDFORMS
1) Lapies: highly corrugated and rough surface of limestone lithology chracterized by low ridges and pinnacles, narrow clefts and numerous solution holes.
formed due to corrosion of limestones along their joints
2) Solution holes; sink holes (funnelshaped and cylindrical); compound sinks; swallow holes;ponores; dolines;collapsesinks; solutionpan (Lost river of Indiana); karst lakes; cockpits; karst window; uvalas and jamas; poljes(vertical side walls, flat alluvial floors, independent surface drainage system, irregular borders and central lake; Livno Polje of Balkan region of Europe)
(https://1drv.ms/u/s!AvN_8sA-Zf0djm5IToehaodk-lFs?e=zCs12e)
3) valleys of Karst region: sinking creek; blind valley; karst valley
4) caves and caverns: voids of large dimensio below the grnd surface; most significant erosional landforms of GW; vary in size (carlsbad and Mammoth caves of USA)
5) natural bridge: due to collapse of roofs or due to disappearance of surface streams as subterranean streams and formation of valleys below the grnd
DEPOSITIONAL LANDFORMS
Speleothems; calcite common constituent; travertines; tufa/calctufa; dripstones; stalactites and stalagmites; helictites and heligmites and globulites; flowstones; drapes or curtains
Give an account of various theories of Limestone caves formation?
1) most debatable among all karst landforms
2) Corrasion theory: Lapprent and Martonne; due to abrasion by GW in the vadose zone, above watertable
3) Two cycle theory: W.M.Davis; in 1st cycle, caves are formed by phreatic water i.e. water under hydrostatic pressure below water table and then in 2nd cycle, are is uplifted and caves come under vadose zone coz of lowering of water table and the cave becomes dry and thus forms the depositional Lf (speleotherms).
4) Water Table theory of A.C. Swinnerton: caves are formed by lateral flow of water in vadose zone or at water table level
5) Static water zone theory of Gardener: caves are formed due to solution of ls above water table
6) Invasion theory of C.A.Malott:caves are formed by subterranean streams
Explain the realtionship betn cave ceiling and formation of stalactites?
1) Stalactites (St) are formde by deposition of calcareous solutes carried by water dripping through cave ceilings in dry env.
2) incicle or needle like
3) shapes controlled by shape of cave ceiling
4) when cave ceiling is fault or is uniformly arched: St are uniform and their tapering lower ends are directly pointed towards cave floor
5) when cave ceiling is steeply inclined, inclined and elongated St are formed
6) when cave ceiling is flat but gently inclined, slightly inclined and elongated St are formed
Karst CoE: intro?
1) J.W. Beede(3 stage) and Jovan Cvijic(4 stage)
2) DAvis: special phase of normal fluvial cycle characterised by surface drainage, its disappearance undergrnd and reappearance of subterranean as surface drainage
3)simpler: uniformity of str and dominance of mono-process (solution)
4) Base level not precisely known, mostly believed to be Water table of GW
5) conditions for initiation:(i) exposure of thick limestone at Gnd surface (ii) overlain by thin veneer of NSR
6) Two types of str preferred: (i) Folded and (ii) faulted
Karst CoE: Youth?
1) surface drainage in regions of …
2) if overlain by NSR…
3) rainwater mixed with atm Co2–along joints—sink holes and swallow holes through soln; increase in size and no.
4)grnd surface develops in rough terrain due to dev of lapies
5) with enlargement, dolines are formed and disappear of surface drainage through sinks and blind valleys
6) undergrnd drainage starts formation of caves and caverns through soln and abrasion
7) termination mkd by complete disappearance of surface drainage
Karst CoE: Mature?
1) total disappearance-> grnd surfae dry
2) numerous sinking creeks
3) process of undergrnd soln and abrasion increases due to increase in vol
4) Gradual enlargement of caves, galleries and passages
5) Covering roofs- thinning-> uvalas, poljes and karst windows–> residual uplands betn uvalas become ridges
6) Late maturity characterised by destruc of most of solutional landforms
Karst coE: old stage?
1) cave roofs totally collapsed and ridges betn uvalas narrowed down and reduced in height
2) residual carbonate rocks that project above grnd surafce , called hums
3) Most of grnd surface is levelled and thus resemble peneplain
4) subterranean drainage again appears on grnd surface and nearly all of karst valleys and sinking creeks are removed.
Continental Drift: intro?
The idea of dynamic nature of earth crust was a complete departure from the then prevailing view of permanency of continents and ocean basins. Although scholars had proposed the idea of continental drift even before wegner but he was the first to mould this idea into a theoretical frame work.
He propounded his concept on continental drift in the year 1912 but it could not come in light till 1922.
the continental drift theory of Wegener ‘grew out of the need of explaining the major variations of climate in the past’.Wegner being a climatologist was perplexed to find glacial deposits in the equatorial regions and evaporities in the sub polar regions. Such anomaly would only be explained eithen, rough shifting of climatic zones or continental shifting since climatic zones being governed primarily by the sun were least likely to shift, hence Wegner proposed the theoity of continental drift.
Thus, the main objective behind his ‘displacement hypothesis’ was to explain the global climatic changes which are reported to have taken place during the past earth history.
Continental Drift: basic axioms?
- Following Edward Suess, Wegener believed in three layers system of the earth e.g. outer layer of ‘sial’, intermediate layer of ‘sima’ and the lower layer of ‘nife’. Continents or sialic masses were floating on sima without any resistance. Oceanic crust represented the upper sima.
- During carboniferous period all the continents were united together in the form of one landmass, which he called ‘Pangaea’. Wenger does not describe conditions from pre-Carboniferous times.
- Pangaea was surrounded on all sides by an extensive water mass called ‘Panthalsa’.
- The northen part of ‘Pangaea’ Consisting of the present. America Europe and Asia was named Laurasia \Angaraland .The southern part of Range consisting of the present S America, Africa, Peninsular India, Australia and Antarctica was named “Gondwanaland”.
- South pole was located near present Durban at this time
- Pangea was disrupted during subsequent periods and broken landmasses drifted away from each other
Continental Drift: process of CD: headings?
- Forces responsible
- Process of Drifting of continents
- Creation of Oceans
- Mountain Building
- Origin of island arcs
Continental Drift: process of CD: Forces responsible?
- According to Wegener the continents after breaking away from the Panagaea moved (drifted) in two directions e.g. (i) equatorward movement and (ii) westward movement.
- The equatorward movement of sialic blocks (continental blocks) was caused by gravitational differential force and force of buoyancy.
- The westward movement of the continents was caused by the tidal force of the sun and the moon.
Continental Drift: process of CD: Process of Drifting of continents?
- The disruption, rifting and ultimately drifting of the continental blocks began in Carboniferous period.
- Pangaea was broken into two parts due to differential gravitational force and the force of buoyancy. The northern part became Lauratia (Angaraland) while the southern part was called by Wegener as Gondwanaland.
- The inter vening space between these two giant continental blocks was filled up with water and the resultant water body was called Tethys Sea. This phase of the disruption of Pangaea is called ‘Opening of Tethys’.
- Gondwanaland was disrupted during Cretaceous period and Indian peninsula, Madagascar, Australia and Antarctica broke away from Pangaea and drifted apart under the impact of tidal force of the sun and the moon. North America broke away from Angaraland and drifted westward due to tidal force. Similarly, South America broke away from Africa and moved westward under the impact of tidal force.
- The movement of the continental blocks away from the poles was dramatically called by Wegener as ‘the flight from the poles’.
Continental Drift: process of CD: Creation of oceans?
- Due to northward movement of Indian Peninsula Indian Ocean was formed while the Atlantic Ocean was formed due to westward movement of two Americas. It may be mentioned that North and South Americas were drifting westward at different rates and hence ‘S’ shape of the Atlantic Ocean could be possible.
- Arctic and North Sea were formed due to flight of the continental blocks from north pole.
- The size of the Panthalasa (primitive Pacific Ocean) was remarkably reduced because of the movement of continental blocks from all sides towards Panthalasa. Thus, the remaining portion of Panthalasa became the Pacific Ocean.
Continental Drift: process of CD: Mountain Building?
The frontal edges of westward drifting continental blocks of North and South Americas were crumpled and folded against the resistance of the rocks of the sea-floor (sima) and thus the western cordilleras of the two Americas (e.g. Rockies and Andes and other mountain chains associated with them) were formed.
Similarly, the Alpine ranges of Eurasia were folded due to equatorward movement of Eruasia and Africa to gether with Pennisular India (equator was passing thorough Tethys sea at that time).
Continental Drift: process of CD: Origin of island arcs?
Wegener has related the process of the origin of island arcs and festoons (of eastern Asia, West Indies and the arc of the southern Antilles between Tierra del Fugo and Antarctica) to the differential rates of continental drift.
When the Asiatic block (part of Angaraland) was moving westward, the eastern margin of this block could not keep pace with the westward mov ing major landmass, rather lagged behind, conse quently the island arcs and festoons consisting of Sakhalin, Kurile, Japan, Phillippines etc. were formed.
Similarly, some portions of North and South Americas, while they were moving westward, were left behind and the island arcs of West Indies and south ern Antilles were formed.
Continental Drift: Evidences in favour?
- Geographical similarity between the opposing coasts of the Atlantic ocean. The two sides of the Atlantic are such that they can be joined together i.e. eastern coast of South America can be fitted in to W.Coast of Africa, E.Coast of N. America can be fitted against W. Coast of Europe. This was called “Jigsawfit” of the opposing coasts of Atlantic ocean by Wegner.
- Geological Structure of the lands located on two coasts of Atlantic also exhibit remarkable similarity
(a)On both the coasts the structural and relief features are found transverse to the coasts.
(b) The Caledonian and Hercynian mountain system of both coasts of Atlantic are similar.
The best example provided by the Appalachian mountains of N. America which come right up to the coast and then continues their bend across the N. Atlantic ocean in the mountains of S.west Ireland, wales and central Europe.
Caledonian orogeny: https://1drv.ms/u/s!AvN_8sA-Zf0djnIfvuVcvb0NydKD?e=O8wz2o
Hercynian Orogeny: https://1drv.ms/u/s!AvN_8sA-Zf0djnOfsYxj7FAkNWbt?e=SzQA7s
So on both coasts, there is great Similarity in the structure and direction of Hercynian and Caledonian mts and their truncated end can be joined together.
The opposing coasts of S. Atlantic ocean in Africa and Brazil display even greater resemblance in their structure and rocks. The data relating to the Isotope ages of rocks in eastern south America and western regions of Africa has been plotted on maps and both the landmasses have been found to have rocks of the same ages and similar structure. This fact provides a strong proof of these two continents being joined together.
(c)The occurance of gold deposits in river alluvium in Ghana Coast (Africa) and absolute absence of source rocks in that regions. However across Atlantic ocean, the source rocks bearing gold are found in Belen sau in Brazil (S.America), This indicates that Africa and S. America existed together and the gold bearing sediments were transported down the slope from Brazil and deposited in Ghana.
- Paleoclimatic evidences :
The distribution of the carboniferous glacial deposit in Brazil, Falkland, S. Africa, Peninsular India, Australia and Antarctica Presents a powerful Proof of the fact that during permocarboniferous glaciations these land masses were assembled together. - Fossil evidences: - The fossil remains of Glossopteris fern and Mesozoje reptiles are found on all the pieces of Gondwanaland this indicates the unity of these pieces. In fact development of distinctive fauna in different parts of the world have been found after the Mesozoic era but before the Mesozoic era there is great similarity in fauna throughout the world. This is an important evidence in support of continental drift.
(https://1drv.ms/u/s!AvN_8sA-Zf0djnQSYYFi6jyMDbRz?e=yindne) - Palaeomagnetism has provided the most reliable proof of continental drift and palaeomagnetic evidences indicate the existence of “Pangea” in some form at the end of the Paleozoic era. In fact the Paleomagnetic studies have not only confirmed continental drift but have also provided the evidence of sea floor spreading.
- It has been reported from geodetic evidences that Greenland is drifting westward at the rate of 20 cm per year. The evidences of sea floor spreading after 1960 have confirmed the movement of landmasses with respect to each other.
- The lemmings (small sized animals) of the northern part of Scandinavia have a tendency to run westward when their population is enormously increased but they are foundered in the sea water due to absence of any land beyond Norwagian coast.This behaviour suggests that landmasses were united in the ancient times and the animals used to migrate to far off places in the western direction.
Continental Drift: Criticism?
- The greatest criticism was levelled against the forces which, according to wegner, were responsible for causing the drift. The westward movement of the continents was ascribed to the tidal force of the moon and sun, but this movement is possible only if this force is ten thousand million times more than at present and if the tidal force would have been of this magnitude then it would have stopped the rotation of the earth within one year
The equator ward drift was attributed to the gravitational force exerted by the earth equatorial bulge, but this force is many million times feeble to drag the continents. - The mountain orogeny as explained by wegner invited strong criticism because he described the forceful resistance offered by Sima in the free movement of sialic continents in order to explain the origin of mountains, which is contrary to his basic premise, that sial floated on sima which offered no resistance.
- The jigsawit of the opposing coasts of the Atlantic ocean was found to be not so complete.
- Wegener has not elaborated the direction and chronological sequence of the displacement of the continents. He did not describe the situations of pre-Carboniferus times. Other questions like What kept Pangaea together till its disruption in Mesozoic era?’ Why did the process of continental drift not start before Mesozoic era? were not pondered over
Continental Drift: Conclusion?
Although in the absence of plausible driving mechanism his continental drift theory, invited criticism from diverse quarters and until about mid 20th century his theory remained,ghly controversial but after 1950’s his theory began to gain wide acceptance on account of the irrefutable evidences provided by palaeomagnetic studies as well as new theories of seafloor spreading and plate tectonics.
In fact the paleomagnetic and geological evidences have now enabled to reconstruct the history of continental drift back to Precambrian period.
This shows recurring episodes of continental rupture, drift and collision since earlier times each cycle lasting for a few hundred million years and has been called wilson cycle, named after J.T. wilson, one of the Pioneer of P.T.T.
Continental Drift: Diagrams?
https://1drv.ms/u/s!AvN_8sA-Zf0djnWHUW4nTnkGvw-6?e=BnHNEK
Plate Tectonics: intro?
Plate tectonics is a scientific theory that explains how major landforms are created as a result of Earth’s subterranean movements. The theory, which solidified in the 1960s, transformed the earth sciences by explaining many phenomena, including mountain building events, volcanoes, and earthquakes.
the theory of plate tectonics is not related to any individual scientist, rather a host of scientists of various scientific disciplines and research groups and expeditions have contributed in the development of this valuable concept of the second half of the 20th century. eg Mckenzie and Parker’s Paving stone hypothesis; in 1965, J.T Wilson become first to suggest that the earth’s lithosphere is made up of individual plates and identified relative motion of plates and by 1968 the concept of Continental drift and sea-floor spreading were united to create an all encompassing theory known as plate tectonic.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWDHYQK_lglKdl3d?e=JNC4O0
Plate Tectonics: Headings?
- Axioms
- Plates
- Plate Boundaries
- Evidence
4.1. Sea Floor Spreading
4.2 Paleomagnetism - PT and Continental Displacement
- PT and Creation of Oceans
- PT and Mountain Building
- PT and Vulcanicity
- PT and Earthquakes
- Criticism
- PT vs Continental Drift
Plate Tectonics: Axioms/Basic Postulates/ Assumptions?
Lithosphere is mobile and floating above denser but partially molten asthenosphere
Lithosphere is broken into amorphous pieces, larger and smaller known as major and minor plates
new material is generated by sea-floor spreading at the mid-ocean ridges
surface area is conserved, therefore plate material must be destroyed through another process
motion of plates is accomodated only along plate boundaries. The mobile plates are interacting along dynamic linear zones and geological processes are super active along such areas
Plate Tectonics: Plates?
The lithosphere, 50-100km thick, is rigid and is broken up into seven very large continental- and ocean-sized plates, six or seven medium-sized regional plates, and several small ones.
A tectonic plate may be a continental plate or an oceanic plate, depending on which of the two occupies the larger portion of the plate.
The Pacific plate is largely an oceanic plate whereas the Eurasian plate is a continental plate.
The lithosphere rests on and slides over an underlying partially molten (and thus weaker but generally denser) layer of plastic partially molten rock known as the asthenosphere
Plate movement is possible because the lithosphere-asthenosphere boundary is a zone of detachment.
The lithosphere itself includes all the crust as well as the upper part of the mantle (i.e., the region directly beneath the Moho), which is also rigid. However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity. This process begins at about 100 km (60 miles) below the surface. This change occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere. This upper portion of the mantle, which is known as the lithospheric mantle, has an average density of about 3.3 grams per cubic cm. The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3.4–4.4 grams per cubic cm
Plate Tectonics: Plates: major plates?
Major Plates:
- The Antarctic (and the surrounding oceanic) plate: continental core;
- The North American plate: mostly continental with oceanic slab (NW Atlantic); moving W; riding over Juan de Fuca minor plate
- The South American plate: mostly continental with oceanic slab of south Atlantic Ocean; moving W; riding over NAZCA plate
- The Pacific plate: It is entirely oceanic and moving in NW direction and is subducting below Eurasian and Indo-Australian Plate
- The India-Australia-New Zealand plate: largely oceanic; moving in N-NE direction and is subducting below Eurasian Landmass
- The Africa with the eastern Atlantic floor plate: mostly continental; moving NE and subducting below Eurasian Landmass
- Eurasia and the adjacent oceanic plate: moving in easterly direction; mostly continental though western part is oceanic
Young Fold Mountain ridges, oceanic trenches, and/or transform faults surround the major plates.
Diagram: https://1drv.ms/u/s!AvN_8sA-Zf0djVm5bQI8DuwufADr?e=7Gzulg
Diagram, self: https://1drv.ms/u/s!AvN_8sA-Zf0djW4HuhsDayHh5nNH?e=kWzwHR
Can also refer the video in folder abt how to draw
Plate Tectonics: Plates: continental and Oceanic plates properties?
- The continents have a crust that is broadly granitic in composition and, with a density of about 2.7 grams per cubic cm , is somewhat lighter than oceanic crust, which is basaltic (i.e., richer in iron and magnesium than granite) in composition and has a density of about 2.9 to 3 grams per cubic cm
- Continental crust is typically 40 km (25 miles) thick, while oceanic crust is much thinner, averaging about 6 km (4 miles) in thickness
- The distribution of these crustal types broadly coincides with the division into continents and ocean basins, although continental shelves, which are submerged, are underlain by continental crust.
- their behaviour is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west (the North American continent) and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.
These crustal rocks both sit on top of the mantle, which is ultramafic in composition (i.e., very rich in magnesium and iron-bearing silicate minerals). The boundary between the crust (continental or oceanic) and the underlying mantle is known as the Mohorovičić discontinuity (also called Moho)
The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle.
Plate Tectonics: Plates: minor plates? sub plates?
Minor Plates: Some important minor plates include:
- Cocos plate: Between Central America and Pacific plate
- Nazca plate: Between South America and Pacific plate
- Arabian plate: Mostly the Saudi Arabian landmass
- Philippine plate: Between the Asiatic and Pacific plate
- Caroline plate: Between the Philippine and Indian plate (North of New Guinea)
- Fuji plate: North-east of Australia
- Juan De Fuca plate: South-East of North American Plate
Sub plates: (In the process, of development as a plate)are the china sub plate, Somalian, Arabian, Persian Sub plate.
Plate Tectonics: Plates: Indian Plate?
The Indian plate includes Peninsular India and the Australian continental portions.
The Indian plate is one of the major plates whose northern boundary lies along the axis joining the northern flank of the peninsular plateau i.e. Rajmahal hills, Chota Nagpur, Baghelkhand, Bundelkhand, Malwa plateau and Aravalli hills. Its western boundary lies in the Arabian Sea by the Carlsberg Sea Ridge – 1752 m deep, the Indian Ocean by the Mid-Indian Sea Ridge – 2067 m deep and the southern boundary by the South-East Indian Sea Ridge in the Indian Ocean – 3017 m deep and the eastern boundary by the New Zealand Islands. And the Karmadic Ridge (located in the north-east of New Zealand) and the north-eastern border is located along the line joining New Guinea, Java, Sumatra and Andaman Islands
In the east, it extends through Rakim Yoma Mountains of Myanmar towards the island arc along the Java Trench. The Western margin follows Kirthar Mountain of Pakistan. It further extends along the Makrana coast of Balochistan and joins the spreading site from the Red Sea rift southeastward along the Chagos Archipelago.
The boundary between India and the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running in roughly West to East direction and merging into the spreading site, a little south of New Zealand.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWHsxhub0jBRk8qN?e=scduNI
After breaking away from gondwana, the Indian plate moved away @ 12cm/yr. As it approached the eurasian plate the sediments deposited in the Tethys sea was folded to form Himalaya mountains.
The Indian plate first collided with Eurasian plate near Ldakh which slowed down its motion.At the same time its rotation direction also changed. On hitting Ldakh the Indian plate started closing like a door. Today it’s flow speed has decreased to 5cm/yr.
Diag: https://tse2.mm.bing.net/th?id=OIP.YY5k-O1mxl4amfTAGhBcTgHaHj&pid=Api&P=0
Plate Tectonics: India-Madagascar rift?
According to American and Indian scientists of NASA, asteroids are the reason for the fission of the supercontinent Gondwanaland. A paper to this effect appeared in the January 1993 issue of the ‘Geology Journal’ of the United States, written by John R. Marshall and Hans Agarwal. The impact of a meteorite or asteroid falling on the Earth comes from the debris of asteroids found on the Earth’s crust, which is called Tillite.
The Madagascar plate experienced two major rifting events during the break-up of Gondwana. First, it separated from Africa about 160 Mya (million years ago), then from the Seychelles and India 66–90 Mya
According to scientists from the National Geophysics Research Institute (NGRI), Hyderabad, Madagascar was first physically associated with the Dharwad Protocontinent of India, which broke away from the Indian Plate about 140 million years ago. It reached its present position after about 60 million years after fragmentation. Until now, Madagascar has been considered a part of the continent of Africa. Before the fission, India and Madagascar were moving north at the rate of 4 cm per year. After fragmentation, the Indian plate collided with the Eurasian plate and formed the Karlsberg and Indian Ocean Ridges between the Madagascar minor plate and the Indian main plate, which are made of basalt on the composition plate edge. There are many evidences of the association of the Madagascar plate and the Indian plate to the Dharwad region in the past.
- Correspondence in Geo-Magnetism data obtained from geo-satellites and geologic and tectonic investigations from the eastern and western sides of Madagascar.
- Similarity in the geological structure of both.
- Equivalence in the remains of shallow seas located near both.
Prior to the study by the Department of Geophysical Survey of India, Bangalore, geologists believed that the Indian plate was stable in its place. The reason for the origin of the Himalayas is the southward flow of the Eurasian Plate or the Angarland, but the Geophysics Survey of India, Bangalore and the Chinese Geophysical Survey, Beijing have indisputably verified that the Indian plate itself moves in the north-east direction by 5.5 cm per year. rate is increasing.
Plate Tectonics: Geo-Environmental consequence of Indian Plate motion/ Evidence of Indian plate motion?
- The rate of elevation of the Himalayan ranges of 1.2 mm per annum is proof that the the northern edge of the plate is tetconically active and due to the drift in the north-east direction of the Indian plate, The Himalayan ranges are rising due to the pressure generated by the movement.
- Earthquakes of varying magnitude along the Himalayan region from the Pamir knot to the Arakanyoma confirm that the subducted portion of the northern edge of the Indian plate is reaching the asthenosphere and transforming into magma, which exerts pressure on the Himalayan structure. Due to which earthquakes occur on the Himalayas with the help of Main Boundary Thrust (MBT) and Main Central Thrust (MCT). The Garhwal earthquake is a vivid evidence of this.
- According to the data obtained from the aeromagnetic survey, the depth of the depression in the upper and middle Gangetic plains is 8000-6000 m, the thickness of the depression becomes more near the Himalayan mountain range and decreases towards the south plateau. This indicates that due to the north-easterly flow of the Indian plate, its northern edge submerged in the Himalayas, forming a trench between the Himalayan range and the northern edge of the plateau, the depth of which was greater in the north. as a result, The sediments brought by the rivers to the depression got deposited in greater thickness in the northern edge.
- Several landforms of the Konkan and Kannada coast indicate that they were formed as a result of uplift. This proves that the western coast of India (except for Kutch in Gujarat) has been uplifted in the geologic past.
- Earlier India’s Dharwad (Karnataka), Madagascar and Africa were interconnected and India was on the equator in the south position than the present. After the fragmentation, Madagascar and Africa were separated from India and the land west of the India land was submerged, on which the water of the Indian Ocean spread which was called the Arabian Sea. It was during this fault that the Western Ghat Mountains of India were born and the equatorial forests of the submerged landmass were buried and converted into petroleum. The petroleum of ‘Bombay High’ situated in the Arabian Sea 80 km west of Bombay is the result of this.
- The main coal field of India or ‘Koylanchal’, located along the eastern and southern edges of the Chotanagpur plateau, is also evidence that this area was also covered with equatorial forest. During the tectonic event of the Meghalaya plateau separating it from the Chotanagpur plateau along the Dawki fault, these vegetation got buried and converted into coal.
- The study of fossilized coconut fruit found between the intertrap beds of Mohgaon kalan of Madhya Pradesh by palaeontologists by carbon-14 dating method shows that it is about 70 million years old of Tertiary age. This proves that at that time there were equatorial forests with coconut species in Madhya Pradesh, that is, Madhya Pradesh was on the equator. It is at its present latitudinal position due to plate tectonics.
- At the time of the eruption of the Rajmahal lava, about 100 million years ago, India was 2250 km south of the current position. In other words, it was at 20° south latitudinal position because 1° latitude = 111 km. Kanyakumari, which is today located at 8°4’ north latitude, was at that time about 12° south latitude.
- The shift in the geographic north and magnetic north of a place is called ‘magnetic flux’.
Due to the flow of the plate, the magnetic flux of each place of each plate keeps on changing. For example, in 1970, the magnetic field of Allahabad was 1/2° West, which was decreasing at the rate of 1’ (1 minute) annually - The Bay of Bengal on the north-eastern edge of the Indian plate is subducted by substantial sedimentation by the Ganges-Brahmaputra river system. The proof of this is the vertical roots of Sundari tree found in the Ganges Delta at a depth of 15 m to 150 m above sea level (Fig. 3)!
- The east coast of India is sinking down at a rate of 1.2 mm per year (Gardner GS - Physical Geogrophy page 443) due to the subsidence of the Bay of Bengal geodesic.
- Magma is coming to the earth’s surface through cracks created by the subduction of the Bay of Bengal. The volcanoes of Andaman and Nicobar Islands confirm this. 10 April 1991 The volcano erupted on Ko Baren Island (Middle Andaman) is a vivid example in this context.
- Before the origin of Shivalik, the Indobrahma river flowed from east to west along the southern mountain foot of the Himalayas and fell into the Arabian Sea. Due to plate tectonics, due to the rise of the Delhi water divide, the course of this river was changed which started falling in the Bay of Bengal and its abandoned part is still in the form of Ghaggar river in Haryana, whose water dries up in Talwara lake of Rajasthan during rainy season.
- A borewell carried out by the Natural Oil and Gas Commission in the Parvatpad region of Shivalik revealed a fresh water lake about 1700 feet deep, which is actually the valley of the Indobrahma river, in which sufficient water is also found in the sandy sediment. This valley was raised by the movement in the north-east direction of the Indian plate, which gave rise to the Shivalik range.
- When the landmass that is now the Indian subcontinent slammed into Asia about 50 million years ago, the oxygen in the world’s oceans increased, altering the conditions for life, scientists say (https://www.financialexpress.com/lifestyle/science/apart-from-rise-of-himalayas-what-happened-when-indian-plate-collided-with-eurasian-plate-in-pre-historic-era/1561513/)
due to plate tectonics, there will be another category in future from Terai to the south of Shivalik range. can come into existence. Rapid sedimentation is taking place in the Terai region by various rivers, where the sedimentation load is causing the downpour of the Terai region. Geologists have detected a fault at Devvand in Saharanpur district which is called Himalayan Front Fault. This is the result of the subsidence of the Terai belt.
Plate Tectonics: Plate Margins?
Three types:
1. constructive or Divergent plate boundaries
2. Destructive or Convergent Plate boundaries
3. Conservative or Shear or Transform plate Boundaries
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWIp-GbUTUG8Dtgw?e=uisU8L
Plate Tectonics: Plate Margins: Divergent Margins: aid to write answer?
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Plate Tectonics: Plate Margins: constructive or Divergent plate boundaries?
Along Such Boundaries, high energy flow from the interior has been found which is suggestive of the rising limb of convectional currents.
It is a linear mobile Zone of sea floor spreading.
Divergence takes place in continental as well as oceanic plates, in oceans MOR (mid oceanic Ridge) and on continents great rift valleys are formed.
As plates move apart at a divergent plate boundary, the release of pressure produces partial melting of the underlying mantle. This molten material, known as magma, is basaltic in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins
Divergence and creation of oceanic crust are accompanied by much volcanic activity and by many shallow earthquakes as the crust repeatedly rifts, heals, and rifts again. Brittle earthquake-prone rocks occur only in the shallow crust. Deep earthquakes, in contrast, occur less frequently, due to the high heat flow in the mantle rock. These regions of oceanic crust are swollen with heat and so are elevated by 2 to 3 km (1.2 to 1.9 miles) above the surrounding seafloor.
Divergent movement of plates results in (i) volcanic activity of fissure flow of basaltic magma, (ii) creation of new oceanic crusts, (iii) formation of submarine mountain ridges and rises, (iv) creation of transform faults, (v) occurrence of shallow focus earthquakes, (vi) drifting of oceanic plates etc.
On a global scale, these ridges form an interconnected system of undersea “mountains” that are about 65,000 km (40,000 miles) in length and are called oceanic ridges.
Plate Tectonics: Plate Margins: constructive or Divergent plate boundaries: Process?
Process of sea floor spreading i.e. when divergence is under oceanic crust:
- The convectional currents when rises, it pushes hot plume of magma from about 700 km of depth
- this hot magma and energy is involved in a process called intra crustal thinning which is simply the gradual but regular melting of crustal material.
- When the crust is weak enough, the lateral flow of convectional current beds crack into the oceanic slab. Eventually, the plates are horizontally displaced and sea-floor spreading begins.
- The best-known example of divergent boundaries is the Mid-Atlantic Ridge
- The fissure gradually widens and the magma pours out which gets deposited along trailing edges of the plate. The huge deposition creates mountainous ridges
- It is along the trailing slabs, where the magma clings on and newer plates are created, thus it is also called the constructive boundaries. The rift valley is found in the central parts of the ridge, which signifies the tensional forces.
- Since the magma comes from the asthenosphere it is peridotitic but when it comes on the surface due to magmatic differentiation becomes basaltic. Hence the new oceanic crust is basaltic.
Process of Continental Rifting i.e. when divergence is under continental crust
- Upwelling of magma causes the overlying lithosphere to uplift and stretch
- If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart. Settling of the continental blocks creates a rift valley, such as the present-day East African Rift Valley. If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass.
- As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift. Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.
Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries?
This is also known ‘Subduction zone’ or the ‘Zone of collision’ or ‘The Benioff Zone’
it is the Zone of maximum and most complex geological processes.
Events like volcanism, Earthquake, fold mts, deep trenches, are associated with these of boundaries.
Process:
- When two plates collide, the denser plate is subducted below the lighter one because lesser the density greater the buoyancy.
If the density is similar the plate with greater velocity will Be subducted. If the plates have similar velocity and density, then the older and the rigid plate subducts. - Because oceanic crust cools as it ages, it eventually becomes denser than the younger oceanic crust or continental crust, and so it has a tendency to subduct, or dive under, adjacent continental plates or younger sections of oceanic crust.
- The plate subducts at an angle of 45 degrees, when it enters the asthenosphere (the margin of subducting plate)
- At a depth of 100 km to 300 km, the partial melting begins and differentiation of magma takes place but silica has upward mobility and the differentiated magma begins to rise. This is known as volcanic outpour.
- The rising magma creates a vent or a fissure and appears on the surface as volcanic arc.
- As the two plates further converges and the sedimentary material (only in case of C-C or O-C collisions) and the relief features of the subducting plate is scratched and gets deposited in the trench. Later on when the compressional force is increased these materials are squeezed, compressed, and folded leading to orgenesis. The mountain thus raised is known as tectonic arc.
Convergent movement of plates results in (i) occurrence of explosive type of volcanic eruptions, (ii) deep focii earthquakes, (iii) formation of folded moun- tains, island arcs and festoons, oceanic trenches etc.
Convergent plate Boundaries can be of three types
Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries: three types: O-O?
- Ocean-Ocean collision:
(i) older oceanic crust having relatively denser material is subducted into upper mantle.
(ii) Such collision and subduction occurs along east Asia. The mighty Pacific plate subducts beneath the smaller, less-dense Philippine plate.
(iii) the resultant tectonic expression of plate collision and subduction includes deformation in crustal area, vulcanism, metamorphism, formation of oceanic trenches, island arcs and festoons etc., and occurrence of earthquakes
(iv) Trench indicate the zone of subduction. eg. Kuril trench, Japan trench, Philipines trench.
(v) volcanic arc is represented by Kuril is., Japanese Is., Philippines
(v) Back arc basin (explained in later f/c): eg. sea of Okhotsk, Japan sea, East China sea, South China sea etc.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXAYcrd6A8zITKVd?e=ZtgZVf
Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries: three types: O-C ?
Ocean-Continent collision:
(i) the greater buoyancy of continental crust prevents it from sinking, and the oceanic plate is preferentially subducted.
(ii) Continents are preferentially preserved in this manner relative to oceanic crust, which is continuously recycled into the mantle. This explains why ocean floor rocks are generally less than 200 million years old whereas the oldest continental rocks are more than 4 billion years old.
(iii) resultant tectonic expressions are deformation of crustal rocks, metamorphism, volcanic eruptions, formation of folded mountains and occurrence of deep-focus earthquakes.
(iv) Collision of American and Pacific plates is a typical example of this category and formation of majestic western cordillera of N. America and Andes of S. America is significant resultant tectonic expression of such situation.
(v) It may be mentioned that one of the manifestions of continent-oceanic plate collision is the exposure of deep ocean rocks through their thrusting in resultant mountain masses. This process is called obduction which is opposite to subduction
(vi) further the sediments also get accumulated in trench and later folded to form tectonic arcs (folded mts). eg. O-C collision is represented by Nazca plate and S. American plate. The trench is represented by Peru-Chile trench. The tectonic arc is represented by Andes mts. The volcanic arc is represented by Ojas Del Salado volcano on Argentina-Chile border, Cotopaxi and Chimborazo volcano in Ecuador etc.
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Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries: three types: C-C?
Continent-Continent Collision:
(i) Two plates carrying continental crust collide when the oceanic lithosphere between them has been eliminated.
(ii) responsible for creation of folded mountains and occurrences of earthquakes of varying magnitudes.
(iii)The collision of Asiatic-Indian plates, and European-African plates is typical example of such situation and the formations of Alpine and Himalayan mountainous chains are major manifestions
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djXKT3xMohYhPxS6a?e=Xmd64Q
Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries: Why no volcanic arcs are found in case of C-C collision?
A few possible factors:
- First of all, volcanic and igneous rocks are found along Himalayas and Alps (refer Mid Continental belt volcanoes). These however were formed because of collision between Eurasian plate and oceanic crust below the Tethys sea at the forefront of the Indian Plate and Eurasian plate. Volcanic arcs must have existed as the oceanic crust under Tethys sea subducted under Eurasian plate, but as the Indian plate continued onward, some time later it collided with Eurasian plate and folded the sediments into fold mt that destroyed these volcanic arcs. Nevertheless, part of that volcanic arc indeed is visible now in the surface. No volcanic edifice have survived the cataclysm that give rise to the Himalayas, but the rocks that formed the arc are now known as the Dras Volcanics, a band of volcanic basalts and dacites outcrops that are part of the Indus suture zone. And that are thought to originate in the Mesozoic (from about 252 to 66 Ma ago) volcanic arc that existed in the Eurasia plate as a consequence of the subduction of the oceanic crust under the Tethys sea.
- under C-C collision, both plates are buoyed in asthenosphere and neither subducts rather they fuse together along ‘suture zone’ like the one present in Himalayas. Since neither plate is subducted, no magma is formed, no upwelling and no volcanic arcs.
- even if one of the continental plate is subducted, it does not have the moisture of the oceanic crust. Moisture of the oceanic crust is necessary as it is the release of this moisture at the depths of ~100km that decreases the melting point of the lithosphere subducted. Since continental plate lacks this moisture, its melting point does not decrease enough and it does not melt.
- Even if some magma is formed, and it rises up, it rarely reaches the surface and cools down while subterranean forming volcanic intrinsic structures and igneous rock deposits. Remember that continental lithosphere is thicker than oceanic.
Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries: Subduction zone?
The subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithosphere about 100 km (60 miles) thick that carries a relatively thin cap of oceanic sediments.
The path of descent is defined by numerous earthquakes along a plane that is typically inclined between 30° and 60° into the mantle and is called the Wadati-Benioff zone. The factors that govern the dip of the subduction zone are not fully understood, but they probably include the age and thickness of the subducting oceanic lithosphere and the rate of plate convergence.
EQs and the seismic activity extends 300 to 700 km (200 to 400 miles) below the surface, implying that the subducted crust retains some rigidity to this depth. At greater depths the subducted plate is partially recycled into the mantle.
The site of subduction is marked by a deep trench, between 5 and 11 km (3 and 7 miles) deep, that is produced by frictional drag between the plates as the descending plate bends before it subducts.
Plate Tectonics: Plate Margins: destructive or Convergent plate boundaries: back arc basins? sea anchor?
back-arc basin, are submarine basin that forms behind an island arc.
Such basins are typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. Examples of back-arc basins include the Sea of Japan, the Kuril Basin in the Sea of Okhotsk, the Mariana Trough in the Philippine Sea, and the South Fiji Basin.
Back-arc basins are sites of significant hydrothermal activity, and the deep-sea vents that occur in these regions often harbour diverse biological communities.
Back-arc basins were initially an unexpected phenomenon in plate tectonics, as convergent boundaries were expected to universally be zones of compression
Process:
A back-arc basin is formed by the process of back-arc spreading, which begins when one tectonic plate subducts under (underthrusts) another. Subduction creates a trench between the two plates and melts the mantle IN THE OVERLYING PLATE, which causes magma to rise toward the surface.
Rising magma increases the pressure at the top of the overlying plate that creates rifts in the crust above and causes the volcanoes on the island arc to erupt. As additional magma breaks through the cracks in the crust, one or more spreading centres develop, which widen the seafloor and expand the section of the overlying plate behind the trench. (Spreading centres that form in back-arc basins are much shorter than those found along oceanic ridges, however.)
As the basin expands, the leading edge of the overlying plate may be forced oceanward, causing the trench to “roll back” (The backward motion of the subduction zone relative to the motion of the plate which is being subducted is called trench rollback ) over the subducting plate, or it may serve as a “sea anchor” by remaining fixed in place relative to the top of the subducting plate. In the latter case, the enlargement of the basin forces the trailing part of the overlying plate to move in the opposite direction.
Plate Tectonics: Plate Margins: Conservative or Shear or Transform plate Boundaries?
where two plates pass or slide past each other along transform faults. These are called conservative because crust is neither created nor destroyed.
The significant tectonic expression of such situation is the creation of transform faults which move, on an average, parallel to the direction of plate motion. Transform faults offset mid-oceanic ridges.
Besides oceanic transform faults, there are also continental transform faults e.g. San Andreas fault (California, USA), Alpine fault (Af rica) etc. It may be mentioned that San Andreas fault ‘is ridge to ridge transform fault.’
they don’t produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes, such as the 1906 earthquake that devastated San Francisco.
In these areas of contact, stress is built which causes the rocks to break or slip, suddenly lurching the plates forward and causing earthquakes.
These areas of breakage or slippage are called faults. The border between the Pacific and North American Plates, a stretch of the Ring of Fire is a transform boundary.
The San Andreas Fault in California is an example of a transform boundary, where the Pacific Plate moves northward past the North American Plate.
It is one of the most active faults on the Ring of Fire.
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Plate Tectonics: Sea Floor Spreading?
first propounded by professor Hary Hess of the Princeton University in the year 1960.
When the data of magnetic anomalies obtained during the many surveys of MOR were displayed on a chart, there emerged well defined patterns of stripes. Based on these information Hary Hess propounded that the mid-oceanic ridges were situated on the rising thermal convection currents coming up from the mantle
The seafloor is spreading in a ‘conveyor belt’ fashion.
The oceanic crust moves in opposite directions from mid-oceanic ridges, from which molten lava oozes out. These molten lavas cool down and solidify to form new crust along the trailing ends of divergent plates (oceanic crust). Thus, there is continuous creation of new crust along the mid-oceanic ridges and the expanding crusts (plates) are destroyed along the oceanic trenches.
Below the oceanic ridges the rising limb of convectional currents are found. Huge amount of magma is gradually moving up with the rising convectional current. The convectional current spreads laterally on reaching the base of the crust and the downward limb is found below the oceanic trenches.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWOOMaXgAqjr1Zoy?e=qmbJdE
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Along the ridges, the oceanic crust is new and along the trenches, the oceanic crust is older.
Thus, the rates of spreading (drifting) of different oceans have been determined. The Atlantic and Indian Oceans are spreading (expanding) very sluggishly i.e. at the rate of 1.0 to 1.5 cm per year while the Pacific Ocean is expanding at the rate of 6.0 cm per year. (only one side is measures which means actual expansion will be double).
Note However, that despite the higher rate of expansion of Pacific Ocean through its MOR, it is overall sinking because it is losing at margins at a higher rate.
Evidences in Support of Sea floor spreading:
1. The existence of the alternative strips of formal and reverse polarity, which lay as mirror images across the ocean ridges was the strongest evidence in support of Sea- floor spreading.
2. In line with Sea-floor spreading there is no sediment on the mid Atlantic ridge and the thickness of the sedimentary deposits increases with distance from the ridge.
3. No where ocean crust’s found older that 200 million years
4. Occurrence of earthquakes near the mid oceanic ridges. Association of active volcanic islands with Mid-Atlantic ridge.
Plate Tectonics: Paleomagnetism?
Palaeomagnetism refers to the preservation of magnetic properties in the older rocks of the earth. The hot and liquid lava and magma with high ferromagnesian contents, when cooled and solidified to form igneous rocks, get magnetised, the records of which are preserved in the rocks. Such magnetism preserved (frozen) in the rocks are called remanent or palaeomagnetism. It is to be remembered that the newly formed rocks are magnetised in the direction of existing geomagnetic field, and thus the magnetic inclination/dip of newly formed rocks is the same as that of the geomagnetic field at the time of the formation of said igneous rocks.
Magnetism of Earth:
1. 1600 A.D. when William Gilbert, the physician of Queen Elizabeth, postulated that the earth behaved like a giant magnet and magnetism of the earth was produced in the inner part of the earth.
2. magnetic field of the earth is like a giant bar magnet of dipoles, located in the centre (core) of the earth and is aligned approximately along the axis of rotation of the earth.
3. The origin of geomagnetic field is in no case related to mantle rather it is related to the outer core of the earth because of the fact that there is gradual westward migration of geomagnetic field at the rate of 0.18° per year which proves that the rotation of geomagnetic field is slower than the rotation of the earth. This indirectly proves that the core of the earth rotates at slower rate than the overlying mantle.
4. generation of electrical currents is possible only in metallic liquid materials and such situation is found in the outer core of the earth which functions as self exciting dy namo. Thus, the energy coming out of the core is transformed into electrical currents which in asso ciation with metallic liquid substances produce geo centric dipole magnetic field.
Findings of Reconstruction of Paleomagnetism of rocks from various continents:
1. revealed the fact, ‘that magnetic poles have changed their positions and there has been considerable wandering in the position of poles.’ Based on the calculations derived from magnetic dip observed in remnant magnetiism of rocks from that period, the positions of poles were determined in Japan, Italy, France etc. on the basis of palaeomagnetic reconstruction of Cenozoic lavas.
2. Established the fact that Cotinents are drifting and not stationary. Polar wandering curves are prepared for dif ferent continents on the basis of data derived through palaeomagnetic reconstruction. As per rule if there has not been continental drift, then the polar wan dering curves of different continents at a certain time period (same time for all the continents) shall be the same, but if the continental drift has occurred then these polar wandering curves would be different for each continent. Findnigs were of different polar wandering curves for each continent for the same time.
According to A.G. Wegener all the continents were joined together in the form of Pangaea till late Permian period. If this was so, then there should be only one palaeomagnetic pole for all the continents during Palaeozoic era. This inference became true when the palaeomagnetic polar wan dering curve was prepared for Palaeozoic Pangaea by joining all the present day continents together so as to conceive the situation in Palaeozoic era.
- Reversal of polarity: The available data of palaeomagnetism reveals the fact that about 50 per cent of the rocks of the crust have got magnetized in opposite direction to the geomagnetic field. Most of the scien tists are of the opinion that terrestrial rocks are magnetized always in the direction of geomagnetic field, but there is reversal in the direction of geomagnetic field,
Scientists have measured magnetic polarity of rocks upto 4.5 million years
there are four polarity epochs wherein two epochs (e.g. Gauss and Bruhnes) are of normal polarity while two epochs (e.g. Gilbert and Matuyama) are of reverse polarity.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWS8UGWFCmCB6LEc?e=qOiC8k
- Corroborated the Theory of Sea Floor Spreading:
Plate Tectonics: Continental Displacement?
Though the sequence of events of Continental Displacement based on evidence of Paleomagnetism and sea-floor spreading is available only for the last 200 million years but on the basis of general mechanism of plate tectonics and the evidences from the continents the sequence of earlier events may be reconstructed.
Acc to Valentine and Moors (1970) and Hallam (1972),
- About 700 million years ago all the landmasses were united together in the form of one single giant landmass known as ‘Pangaea l’
- About 600-500 million years before present first Pangaea was broken because of thermal convective currents coming from within the earth, most probably from the mantle and different landmasses drifted apart.
- These landmasses were again united together due to plate motions in one land mass known as ‘Pangaea II’ about 300-200 million years before present.
- Second Pangaea began to break during early Jurassic period and N.W. Africa broke away from N. America and drifted away. The zone of sea-floor spreading continued to extend towards north and south. The separation of South America and Africa was accom plished during middle Cretaceous period, and North America and Europe began to move away from each other
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWVGrnId8E4peVW1?e=AeOVws
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWbM0U1fFHVd1VEq?e=uDcpAS
Plate Tectonics: Rift valleys?
A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift.
Rift valleys are found both on land and at the bottom of the ocean and are created by the process of seafloor spreading.
Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion.
The Great Rift Valley System which stretches from the Middle East in the north to Mozambique in the south is a geologically active area.
It features volcanoes, hot springs, geysers, and frequent earthquakes.
Plate Tectonics: Creation of Oceans?
The opening of North Atlantic was accom plished in many phases.
1. Formation of new ocean basins 700 million years ago.
2. Deposition of miogeocline and eugeocline on the margins about 500 million years ago.
3. Closing of the Atlantic Ocean and the formation of part of the Applachians due to convergence of Eurasian and Ameri can plates about 400 million years ago.
4. Atlantic closed completely and the formation of the Applachians of North America and Hercynian mountains of Europe was completed about 300 million years ago.
5. Reopening of the Atlantic along with Pangea break-up abt 180 mya. Both were initiated by the eruption of the Central Atlantic Magmatic Province (CAMP), one of the most extensive and voluminous large igneous provinces in Earth’s history
6. break-up of Pangaea began in the Central Atlantic, between North America and Northwest Africa, where rift basins opened during the Late Triassic and Early Jurassic. This period also saw the first stages of the uplift of the Atlas Mountains. While Northern AO developed with development of Labrador sea, separation of Eurasia-North America and Greenland and then finally separation of Eurasia and N. America, South America developed from Cretaceous period onwards.
7. Presently AO is still expanding. However, An embryonic subduction margin is potentially developing west of Gibraltar. The Gibraltar Arc in the western Mediterranean is migrating westward into the Central Atlantic where it joins the converging African and Eurasian plates.
Indian Ocean : The origin and evolution of the Indian Ocean is the most complicated of the three major oceans. Its formation is a consequence of the breakup, which began about 180 million years ago, of the southern supercontinent Gondwana (or Gondwanaland); by the movement to the northeast of the Indian subcontinent (beginning about 125 million years ago), which began colliding with Eurasia about 50 million years ago; and by the western movement of Africa and separation of Australia from Antarctica some 53 million years ago. By 36 million years ago the Indian Ocean had taken on its present configuration. Although it first opened some 140 million years ago, almost all of the Indian Ocean basin is less than 80 million years old.
With expansion of AO and IO, PO ceded space.
Plate Tectonics: Red Sea and Gulf of Aden?
- Red Sea is an example of axial trough which is located between Africa and Arabian peninsula
- The surveyed magnetic anomalies in this area show, that both Red Sea and Gulf of Aden are spreading at the rate of 1centimetre per year (total spreading 2 cm/year) since the past 3-4 million years.
- The Red Sea and the Gulf of Aden are located at the junction of three plates viz. Nubian plate, Somali plate and Arabian plate
- Nubian and Somali plates are separated by Ethiopian fault.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWfzHmKHn5INdzbd?e=sEBumw
Plate Tectonics: Gulf of California?
Pacific Ocean is a waning ocean because it is continuously being contracted in its size because of gradual encroachment of westward moving American plates.
It is believed that like mid-Atlantic ridge there might have been a mid-oceanic ridge in the Pacific Ocean but it has now been remarkably deformed due to plate move ment.
The magnetic survey of the Gulf of California revealed the presence of stripped magnetic anomaly. This situation validates two facts viz.
(i) East Pacific Rise (ridge) is also located in the Gulf of California and there has been continuous spreading of the gulf along the ridge since the past four million years and
(ii) Baja, the Californian peninsula, was previously united with the mainland of North America but later on it broke away from the continent due to spreading of sea floor.
Diag: https://1drv.ms/u/s!AvN_8sA-Zf0djWlHWTP6oCvX6Uiu?e=QiVVr6
Plate Tectonics: Mediterranean Sea evolution?
The geologic history of the Mediterranean Sea is complex. Underlain by oceanic crust, the sea basin was once thought to be a tectonic remnant of the ancient Tethys Ocean; it is now known to be a structurally younger basin, called the Neotethys, which was first formed by the convergence of the African and Eurasian plates during the Late Triassic and Early Jurassic.
The Messinian salinity crisis started about six million years ago (mya) when the Mediterranean became landlocked, and then essentially dried up.
Scientists estimate that the sea was last filled about 5.3 million years ago (mya) in less than two years by the Zanclean flood. Water poured in from the Atlantic Ocean through a newly breached gateway now called the Strait of Gibraltar at an estimated rate of about three orders of magnitude (one thousand times) larger than the current flow of the Amazon River
Plate Tectonics: Mountain Building: different process?
Plate Collision and subduction. Can be of three types depending on the plates i.e.
- O-O
2 O-C
3 C-C
