The word tectonic means deformation within the Earth’s crust and the geological structures produced by deformation, including folds, faults and development of mountain chains.
Plate tectonics is the theory or model to explain the distribution, evolution, and causes of the Earth’s crystal features. It proposes that the crust and part of the upper mantle called lithosphere comprise of separate rigid slabs termed plates which move independently forming part of a cycle in the creation and destruction of crust. These plates collide or move apart at boundaries (margins) and produce zones of volcanic activity and earthquakes.
Earth’s crust is divided into large segments ( fragments) separated along ridges. These segments are called plates (Lithospheric Plates). The term tectonic plates are used to describe the active mobile nature of these plates. These plates explain the drifting movement of the continents. They can collide and cause great destruction. The collision can result in volcanic activity, earthquake,s and mountain building. According to geologists, these are about twenty major plates out of which the following are the prominent:
The pacific plate (It covers most of the Pacific Ocean floor from South Alaska to Antarctic Plates).
The North American plate ( Covers North America)
The Eurasian Plate (It carrier all Eurasian Landmass North of Himalayas)
The African Plate (Africa and Madagascar)
The South American plate (Carries South American)
The Australian- India Plate ( Carries Australian and India).
The Antarctic Plate (Carries Antarctic Landmass)
The Nazca Plates ( Between South America and Pacific pacific Plate)
The Caribbean Plate ( Central America & Caribbean region)
The Cocos Plate ( Lies north of Nazca Plate).
The Rivera Plate ( Oceanic crust)
The Arabian Plate ( Arabian region)
The Philippine Plates ( Between Pacific & Eurasian Plate)
The Scotia Plates ( South of the Latin America)
The Juan de Fuca Plate ( Support Western Coast of North America near US-Canada border).
The Gorda Plates (South of Juan de Fuca Plates).
The theory of continental drift was first put forward by German geologist Alfred Wegener in 1915 in this book entitled “The Origin of Continents and Oceans”. According to this theory, in the beginning, all the landmasses of Earth were in the shape of a single combined landmass called PANGEA (All Earth). About 100 million years ago, the Pangea comprised of two major parts called Laurasia ( in the north) and Gondwana ( in the South). In other words, present-day Eurasia and North America were part of Laurasia whereas South American, Africa, Australia, and India formed Gondwana.
Late on, about 200 million years ago, the supercontinents Pangea broke up into various fragments as detailed above.
Wegener supports his theory with a lot of circumstantial evidence, fossil plants and animals from different regions, climatic environments (as indicated by sedimentary rocks), and the amazing jigsaw-like fit of the continents. However, Wegener could not give the proper mechanism of the Continental Drift. He suggested that Earth’s gravitational force which is slightly lower at the equator was very strong to pull the continents apart. Another geologist Arthur Holmes proposed in 1939 that gigantic heat sustained convection cells in the core of Earth could be responsible for dragging the continents along. Similarly, an American geologist F.B. Taylor also supported the theory of continental drift. However, majority of geologists did not agree with this theory because it could not explain the process of movements of contents.
Main points of Plates-Tectonics:
The main features of Plates- Tectonics are as given below.
1. Lithosphere compromised of about 20 lithosphere plates.
2. The plates carry continental as well as oceanic surfaces. For example, the 3. The arabian plate carries the Arabian Peninsula but the Pacific Plates carry most of the Pacific floor.
4. The thickness of the plates varies from 0 to 10 Km at the ridges to 100-150 Kms at other places.
5. The plates are large and small separated by faults.
7. The plates keep on moving continuously with different velocities ranging from 1 to 12 cm (0.4,8,4.7, inches) per year.
8. Special features such as oceanic ridges are seen where to plates diverge ( separate). As a result, the thin crust rift valley is formed.
9. When the plates spread outward they converge and collide in other zones resulting in subduction.
10. Moreover, earthquakes and volcanism also take place.
Causes of plates motion:
Major causes of the plate motion are the following:
1. Crystal formation at mid-oceanic ridges causes the movements of plates.
2. The difference between heat flows values between oceanic trenches and island areas causes the motion of plates.
3, The difference between rates of spreading at ridges and consumption of crust causes the movement of plates.
4. A difference in gravity is also a cause of the motion of plates.
5. The Lithosphere plates are very thick and strong. Thus there is no friction and their movement takes place.
6. Diverging convection current condition is the mantle is also a cause of the motion of plates.
Movements of Plates:
Lithospheric plates are constantly moving with different velocities ranging from 1 to 12 cm(0.4 to 0.7 inches) per year. In the 1980s, the geologists confirmed this theory by bouncing laser beams off new well-equipped satellites. As a result, surface distances were measured within an accuracy of 1cm. The term Tectonic plates refer to plate movements, which has been going on since 200 million years ago when the single great Landmass called Pangaea the beginning of its fragmentation into continents. Movements of plates result in most of Earth’s landscapes and landforms. The moving plates carry landmasses along and they push, drag, tilt, bend, warp, and fracture. Lava pours out of fissures and vents. Rocks are deformed in various ways. The African plate has moved eastward by the South American plate moves Westward. The new crust for both this plate is being created along the Mid –Atlantic Ridge.
The numbers given in the figure refer to the plate motion in centimeters per year. Number along the Mid-Oceanic Ridges are average velocities indicated by measurements, rocks magnetism. For example the figure of 11.7 shown for the North. Most segments of the East Pacific Rise means the distance between a point on the Cocas plates and a point as the pacific plate to the West increases by 11.7 Cm (4.6 Inches) Annually in the Direction of the arrow.
The term ocean floor spreading is also called crystal spreading. This phenomenon takes place as a result of the divergence or the separation of plates along the mid-oceanic ridges. In this activity, magma (lava) wells up from the asthenosphere, and new lithosphere is created. As a result, the lithosphere on opposing sides of mid-oceanic ridges is pushed apart. Thus thin crust right open. According to geologists, if tensional forces exist beneath a plate where a landmass occurs both the semantic crust above are pulled apart. Presently, a system of rift valleys occurs in eastern Africa. However, the Red Sea is in a more advanced stage of this process and it is likely to widen and become a new ocean in the future. It is so because the Arabian plates have separated from the African plates and now a basalt floored sea lies between them. In the beginning, when Pangaea was a supercontinent first fracture occurred as rift valleys. Later on, when the rifts widened and magma filled the collapsed water-filled trenches the spreading became seafloor.
Effects of Plate Tectonics:
The system of plate tectonic results in many changes in the shape and position of the crust of Earth. These changes are as under:
The plates tectonic is basically based on the theory of plate movement. It leads to plate divergence and plate convergence. Plate divergence leads to the formation of rift valleys and seafloor spreading. Plates tectonic explain the concept of continental drift as propounded by Alfred Wagner in 1915.
The collision of continental plates results in mountain building. Oceanic trenches are formed. Place convergence results in subduction and volcanic activity.
The term Isostasy is derived from the Greek works to Iso meaning the same and Stasy meaning to stand. Thus Isostasy means the condition of vertical equilibrium between the floating landmasses and the asthenosphere under them. This condition of sustained adjustment is maintained despite the forces that constantly operate to change the landmasses of the Earth.