Movements of Continents and Plate Tectonics
The large landmasses on the surface of the earth are defined as the Continent. The earth is divided into seven continents- Asia, Africa, Europe, Australia, North America, South America, and Antarctica
CONTINENTAL DRIFT THEORY
- This theory explains that the Earth’s continents are not motionless but have changed their position and location over geologic time relative to each other. This is called “drifting of oceans” across the ocean bed.
- Abraham Ortelius(1596 AD)was the first person who put forward the idea of continental drift.
- However, Alfred Wegener(1912 AD) developed a Scientific theory on the Continental Drift. In 1922, his views were published in a book entitled ‘Die Enstenhung der Kontinente and Ozeane’ which was later translated in English in 1924 AD by the title ‘The Origin of the Continents and Oceans’.
- According to Continental Drift Theory, all the continents we seen today were united together in form of one landmass or supercontinent called Pangaea. An ocean called Panthalasa surrounded it. This super continent first broke into two parts- Gondwanaland and Laurasia. Gondwanaland was the southern part, consists of today’s South America, Africa, Antarctica, Australia and Peninsular India and Laurasia was the northern part consists of today’s North America, Greenland, Europe and Asia.
- During subsequent periods it was split into various landmasses and drifted away from each other forming the present position of the continents and ocean basins.
SEA FLOOR SPREADING THEORY
- This is another theory which explains the movement of landmasses on the Earth. An American geophysicist named Harry H. Hess presented this theory in 1960. Hess prepared a map of the ocean floor using sonar. He also discovered a mid-ocean ridge in the Atlantic Ocean. According to this theory, sea floor is constantly spreading by mid-ocean ridges, as the lava comes out from these ridges solidifies and push away to form oceanic crust. The old oceanic crust is pushed away toward Benioff zones where they are destroyed by going beneath the depth of the Earth. Thus new crust is made near mid-oceanic ridges and destroys near Benioff regions.
PLATES TECTONIC THEORY
Plates Tectonic Theory is an expansion of Continental Drift Theory and Sea Floor Spreading theory. According to this theory, the Earth’s crust is divided into numerous parts called plates. These plates which are around 100 km thick contain ocean and continents and are consistently moving or floating in different directions over asthenosphere. Plates moves due to the convection current generated from the Earth’s mantle. Plate tectonics cause the formation, movement, collision and destruction of lithosphere and ultimately to geological events like seismicity, volcanism, continental drift, and mountain building. Tectonic processes began on Earth between 3.3 and 3.5 billion years ago.
MAJOR AND MINOR TECTONIC PLATES
There are seven major plates and another eight or so minor plates.
- Major plates: The major plates are the African, Antarctic, Eurasian, North American, South American, India-Australian, and the Pacific plates. They are also known as the primary plate.
- Minor plates: they are also known as the secondary plate. Some important minor plates are the Arabian, Caribbean, Nazca, and Scotia plates.
Plate margins are the boundaries of the plates. They are characterised by a combination of tectonic and topographic features: oceanic ridges, Benioff zones, young fold mountains, and transform faults.
There are three types of plate margins:
- Constructive margins: It is also called Divergent Plate margin because two plates move away by these margins. The upwelling magma stretches the lithosphere, thus initiating a rift. This results in development of fractures that are invaded by the ascending magma.New crusts are created at oceanic ridges when two plates move away.
- Convergent margins: It is also called Destructive margins because two tectonic plates move toward each other which led to one plate to slide below the other. This process is known as subduction. Many folded mountains are found along with convergent margins, for example, the Himalayas (India, Nepal, China, Pakistan, and Afghanistan), Rocky Mountains (North America), Sierra Nevada (Pacific Coast), etc. This zone is also prone to Earthquakes.
- Conservative margins: It is also called Parallel Plate Margin or Transform Boundary. It occurs where plates slide past each other in opposite directions, or in the same direction but at different speeds.
OCEAN RIDGES AROUND THE WORLD
The Ocean Ridges are submarine mountain chain formed by plate tectonics. It set apart two plates. Volcanic activities occur in these zones which is responsible for creation of Ocean crust from the central valley, or axial trough.
|Ocean Ridges||Location||Important features|
|Aden Ridge||Gulf of Aden||It is divergent boundary between the Somalian and the Arabian plates.|
|Cocos Ridge||East Pacific Ocean , Ecuador||It is volcanic hotspot.|
|Explorer Ridge||Between West of Vancouver Island and British Columbia, Canada.||It is divergent tectonic plate boundary between Vancouver Island, British Columbia, Canada.|
|Gorda Ridge||Northern coast of California and southern Oregon.||It is tectonic plate spreading centre broken into three segments- the northern ridge, central ridge and the southern ridge.|
|Juan de Fuca Ridge||Pacific Northwest region of North America||It is a mid-ocean spreading centre and divergent plate boundary.It separates the Pacific Plate to the west and the Juan de Fuca Plate to the east.|
|American-Antarctic Ridge||between the South America and Antarctica||It is the tectonic spreading centre between the South American Plate and the Antarctic Plate.|
|Chile Rise||Peru-Chile Trench||It is a tectonic divergent plate boundary between the Nazca and Antarctic Plates.|
|East Pacific Rise||Pacific Ocean||It is a divergent tectonic plate that separates the Pacific Plate to the west from (north to south) the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate.|
|East Scotia Ridge||South Atlantic and Southern Ocean||It is the edge of the South Atlantic and Southern Ocean.|
|Gakkel Ridge (Mid-Arctic Ridge)||between Greenland and Siberia||It is a divergent tectonic plate boundary between the North American Plate and the Eurasian Plate.|
|Nazca Ridge||Southern Pacific Ocean.||Located in the Southern Pacific Ocean.|
|Pacific-Antarctic Ridge||South Pacific Ocean||It is a divergent tectonic plate that separates the Pacific Plate from the Antarctic Plate.|
|Central Indian Ridge||Western Indian Ocean.||It is a north-south-trending mid-ocean ridge.|
|Carlsberg Ridge||Central Indian Ridge||It is a divergent tectonic plate boundary between the African Plate and the Indo-Australian Plate, traversing the western regions of the Indian Ocean.|
|Southeast Indian Ridge||Southern Indian Ocean.||It is a divergent tectonic plate boundary stretching between the Rodrigues Triple Junction in the Indian Ocean and the Macquarie Triple Junction in the Pacific Ocean, the SEIR forms the plate boundary between the Australian and Antarctic plates.|
|Southwest Indian Ridge||Between south-west Indian Ocean and south-east Atlantic Ocean||It is a divergent tectonic plate boundary separating the African Plate to the north from the Antarctic Plate to the south.|
|Mid-Atlantic Ridge||Atlantic Ocean||Part of the longest mountain range in the world.It is a divergent plate or constructive plate.|
Ring of Fire
The Ring of Fire is a long arc-shaped belt located in the Pacific Ocean basin. It is earthquake-prone, volcanic active tectonic plate boundaries. Its length is about is 40,000-km. It follows chains of island arcs like the Indonesian archipelago, the Philippines, Japan, the Kuril Islands, and the Aleutians. The western coast of North America and the Andes Mountains are also parts of the Ring of Fire.