The surface of the earth is not uniform but irregular and is marked by extreme contrasts. On one hand we have loy mountains while on the other hand wend deep trenches in the oceans. Broadly speaking landforms of the earth are divided into three categories viz. (i) Landforms of the first order (continents and oceans), (ii) Landforms of the second order (mountains, plateau and plains) and (iii) Landforms of the third order (valleys, gorges, waterfalls, cirques, sand dunes, beaches etc.)
A casual look at the globe shows that the earth’s surface has two broad areas namely land areas called continents and water areas called oceans. There are seven continents in all which cover about 29% of the surface area of the earth. These continents are (i) North America (ii) South America (iii) Europe (iv) Asia (v) Africa (vi) Australia (vii) Antarctica. The remaining 71% of the earth’s surface area is covered by water consisting of vast oceans. There are five major oceans which are named a s (i) Pacific Ocean (ii) Atlantic Ocean (iii) Indian Ocean (iv) Arctic Ocean and (v) Southern Ocean.
The origin of continents and oceans has been a subject of debate and several theories have been postulated to explain the present distribution of continents and oceans. However, two theories namely Wegener’s Continental Drift Theory and Plate Tectonic Theory are worth discussing.
Alfred Wegener was a German scholar who put forward his Continental Drift Theory in 1912 and revised it in 1924. Wegener observed surprising similarities on the eastern and western coasts of Atlantic Ocean. He explained that the Brazilian bulge would t in the Gulf of Guinea and North American coastline would closely t in the indentations of the coast of Scandinavia and Western Europe. Antarctica, Australia, Peninsular India and Madagascar were neatly nested together close to the southern tip of South Africa. Wegener called it ‘Jigsaw Fit’.
Wegener assumed that all the above-mentioned landmasses formed a single vast continent known as ‘Pangaea’ in the Upper Carboniferous Age. The northern part of Pangaea comprised of North America, Europe and Asia and was known as ‘Laurasia’. Similarly, the southern part of Pangaea comprised of South America, Africa, Peninsular India, Australia and Antarctica and was known as ‘Gondwanaland’. Both Laurasia and Gondwanaland were separated by a narrow and shallow sea known as Tethys Sea. The entire landmass of Pangaea was surrounded by a huge ocean known as Panthalasa.
Wegener assumed that some part of Pangaea dried westwards while some other part dried equator wards. This process started in the Upper Carboniferous Age about 250 million years ago. Laurasia and Gondwanaland came nearer to each other and Peninsular India started drifting towards the north-east. The size of Tethys Sea became smaller. The sediment deposition in Tethys Sea suffered folding and Himalayas and Alps were formed. About 180 million years ago in the mid-Jurassic age, Atlantic Ocean became 1,000 km wide due to westward drift of the Americas. The westward drift of the Americas also resulted in the wrinkling of their western edge and Rockies and Andes were formed. The continents assumed a shape somewhat similar to the present shape in the Pliestocene Age about 50-60 million years ago. Several factors favour the continental drift theory propounded by Wegener. These factors include jigsaw- jet, geological correlation between eastern end and western coasts of the Atlantic Ocean, building of young fold mountains, carboniferous glacierous, biological and polaeomagnetic evidences etc. However, this theory has been criticised on several accounts which include was in jigsaw-jet, direction of drift inadequacy of the forces causing drift folds etc.
It has been found that there are ridges in the middle of the oceans which have been emitting lava actively and similar rocks have been found on either side of the ridges. Younger rocks are formed near the ridges and they gradually become older away from the ridges towards the coasts. These facts coupled with the studies of magnetic properties of rocks on either sides of the mid-oceanic ridge prompted Hary Hess of the Princeton University to postulate the concept of ‘sea floor ‘reading’ in 1961. This concept was based on the research dings of numerous marine geologists, geochemists and geophysists. He argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crest and new lava wedges into it pushing the oceanic crust on either side. In this way the ocean or spreads and the concept of sea or spreading has become very popular. Hess further argued that the ocean or which gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed.
The development of concept of sea or spreading gave a new twist to the distribution of continents and oceans in form of plate tectonics. It was developed by Mc Kenzie and Parker and also by W.J. Morgan in 1967. The rigid lithospheric slabs or rigid and solid crystal layers are known as plates. e lithosphere is broken into several plates. It includes the crust and the top mantle with its thickness ranging between 5’100 km in the oceanic parts and about 200 km in the continental areas. Individual plates may have continental crust, or oceanic crust or both. All the plates move independently in different directions over the asthenosphere.
Three types of movements of lithospheric plates have been noticed as a result of which plates are said to have three types of boundaries.
There are some plates which are moving away from each other. These plates are called diverging plates and give rise to diverging boundaries. The sites where the plates move away from each other are called spreading sites. Most of the diverging boundaries are along the mid oceanic ridges. The best known example of divergent boundaries is the Mid-Atlantic Ridge where American Plates are separated from the Eurasian and African Plates.
Some plates come closer and collide with each other. The plates which move towards each other are called converging plates and the boundaries between two converging plates are known as converging boundaries. When an oceanic plate collides with continental plate, the denser oceanic plate is forced below the lighter continental plate. The overrun plate is subducted or thrust into the mantle and part of the plate is melted. The molten rock (magma) rises to the surface forming volcanic mountains along the continental edge. Alternately a trench may be formed. The Peru-Chile trench has been formed by collision of the Nazca Oceanic plate and the South American continental plate. When two continental plates converge, neither of the plates can be forced under the other. In such a situation the sediments are folded and fold mountains are formed. The Himalayas and the Alps have been formed in this way.
Two adjacent plates may slip horizontally past one another along a transform fault such as San Andreas fault in California. The boundaries along which such a movement of the plates takes place are called transform boundaries. Such boundaries are characterised by numerous earthquakes.
From the study of the above movements of the plates, it is clear that most of the geological events including earthquakes and volcanoes occur near edges of the plates.