Distribution of Oceans and continents-Continental Drift and evidence
Theory of Origin of continents and ocean basins:
- Wegner gave the theory of continental drift in 1915.He stated that all the continents formed a single continental mass called PANGEA,which meant all earth and the pangea was surrounded by a mega ocean called PANTHALASA,meaning all water.
- During the Jurassic era,about 200 million years ago,the pangea started to split.It first broke into two continental masses-Laurasia and Gonwana,forming nothern and southern components respectively.
- Later,the continental masses began to split further to form the present day continents.
- Thus,he proposed that the continents are floating and constantly drifting,which became the basis of Plate Tectonics theory.
Evidences of movement of continents:
- Mathching of continents(Jig-saw fit)
- Geological evidence of identical mountains of eastern and western coasts of atlantic sea.
- Evidences of carboniferous glaciations show that the landmasses once had similar climatic conditions which is possible only if they were contiguous.
- Plate Tectonics:
It substantiates the concept of continental drift theory based on evidences of Sea floor spreading and Paleomagnetism. This theory explains various natural events such as mountain building, folding and faulting, continental drift, earthquakes and vulcanism.
It identifies 7major and 20 minor lithospheric plates. The major plates are-Eurasian, African, Indo-Australian, Pacific, North American, South American and Antarctic plates. Plate tectonics explains that it is the plates that are in motion and hence the continents that form a part of them also happen to move.
Plate margins or boundaries are divided into three types
a)Constructive or Divergent plate margins-pulling apart of plates which causes sea floor spreading.Shallow earthquakes are associated with it.
e.g-growth of atlantic ocean.
b)Destructive or Convergant plate margin-movement of two plates towards each other.This could be
i)ocean-continent collision-leads to formation of great mountain ranges.
ii)continent-continent collision-leads to crustal thickening,closing of ocean floor separating the two continents and folding causes formation of highlands.
iii)ocean-ocean collision-deep oceanic trenches,island arcs are found.
c)Transform or conservative plate movement-plates slide past each other along a vertical fracture called the tranform fault
Evidences for plate tectonics:
- Continuous eruption of lava and volcanoes along mid oceanic ridges.
- Rocks equidistant on either side of the crest of mid oceanic ridges show similarities in terms of period of formation,chemical and magnetic properties.
- Age of oceanic crust rocks are younger than continental rocks.
- Thin occeanic sediments.
- Deep oceanic trenches.