There is no doubt that, you have already heard how different people reacts on when it comes to some phenomena like Earthquakes or volcanic eruptions. Don't you think, there are some movements that cause these to happen? Yes, this is true, Just relax, today I want to clear this doubt, by writing this post to you on how plate tectonic theory gives us this understanding about our Earth's dynamics. Our Earth have passed through various geological processes over time that results into nowdays beautiful features such as mountains, oceanic basins, just to mention a few, that you can observe out there. Although these processes have to interact effectively , You see! Just let's see below how this can be explained by Plate tectonic theory.
Plate tectonic model involves the formation of new lithosphere at the ridge and its destruction at a subduction zones.
The theory explained how the Earth is active (dynamic). It proposed by 1960's. Most of geologists accept plate tectonic theory because it ties together many unrelated geologic features and natural phenomena.
Also it is through plate boundaries where some phenomenon such as Mountain building, Earthquake , Volcanic eruptions occurs.
There are major seven (7) Tectonic plates known, Africa, North America, South America, Pacific, Eurasian, Antarctic and Indian - Australian.
Also there are many small (micro) tectonic plates with size of about 10 - 1000 km some of them are Nazca plate, Caribbean plate, Fuji plate, Arabian plate, Turkish plate, Cocos plate.
Important assumptions for plate tectonics
i/ Generation of new plate occurs by sea floor spreading. Means it generated along active Mid Oceanic Ridge (MOR)
ii/ Destruction of plate occurs at subduction.
iii/ The Earth's surface remains constant. Means destruction is compasated by creation.
iv/ Plates are capable of transmitting stresses over a great horizontal distance without buckling.
PLATE BOUNDARIES
Plate Boundary: Refers to where one plate interacted with another plate. Interaction between plates can either be by converging, diverging or by conserving. It is also known as Plate Margins.
The terms Margin or Boundary always used interchangeably however Margin is used to refer to the end of the plate while Boundary is used to refer a place where two or more plates interacts either by meeting, diverging or sliding. Hence one plate can have the margin while no boundary.
Types of plate Boundaries
Based on how they interacted with each other there are three (3) types of plate boundaries,
1: Constructive Boundary
It occurs when two plates moves away from each other. It involves the formation of new oceanic crust.It involves upwelling of basaltic magma rich in Fe minerals at the spreading centre. These magma when cools and solidify lead to formation of Basalt rock at mid oceanic ridge called Mid - Oceanic Ridge Basalt (MORB). It is also known as Divergent plate margin.
Example Red sea , divergent between continental arabian plate (pennisula) and African Plate.
Mid Atlantic Ridge (MAR) , divergent between North and South America from Europe and Africa.
Due to high density of basaltic magma causes high Earth's gravity field over the ocean such as Positive gravity anomaly. However the ridge systems the gravity decreases towards the axis so that it become Negative gravity anomaly.
Why negative anomaly is superposed on the normally positive oceanic gravity anomaly?
This is because of local density structure under the ridge. Since at the ridge there is low density due to different mantle composition.
2: Destructive Boundary
It occurs when two plates moves towards each other.
It occurs at subduction zones where one plate with high density plunges beneath its neighbor to great depth and become destructed by partial melting in the mantle.
At subduction there is a long ,narrow, and steep sided depression on the oceanic floor known as Oceanic Trench. It is also known as Convergent plate margin
It can be divides into three (3) subparts based on types of plates involved
i/ Continental -Continental convergent boundary
This involves two continental crust moves towards each other. There is neither of the plate is subducted. Example Himalayas Mountains belt between India and Asia continents
ii/ Oceanic - Continental convergent boundary
This involves oceanic and continental crust where oceanic crust descends past continental crust due to its high density. Example Andes Mountain belt between South America and Pacific ocean.
iii/ Oceanic - Oceanic convergent boundary
It involves two oceanic crust. Example Aleutian island (island arc) and Japanese island
3: Conservative Boundary
It occurs when two plates moves (sliding) past one another.
It also known as Transform Boundary, due transform fault as a result of relative motion between neighboring plates. And this is the reason of why this margin is considered as most seismically active parts of a ridge system. The name conservative implies to the fact that neither new crust is formed nor destructed.
Example San Andreas Fault in Califonia. It separates between Pacific plate from the North American Plate.The fault is always parallel to the direction of movement of the plates, where the western half of California is moving North because it is part of the Pacific Plate, and the eastern half of California is moving South because it is part of the North American Plate.
However in some places, Transform Faults are marked by linear valleys along the boundary where rock has been ground up by the sliding. In other places, they are marked by features like stream beds, fences etc that have been split in half and the two halves have moved in opposite directions.
Forces driving the plates tectonics
- Slab pull force: This occurs at subduction zones as the down plunges plates pulls as it moves down as a result of force of gravity on its weight.
- Ridge push force: This occurs at spreading centre as the ridge pushes mantle away from spreading
- Convective flows: This is the results of mantle convection currents.
Plates tectonics Evidences
- Geophysical evidences
- Gravity: Negative gravity Anomalies at the ridge due to low density caused by mantle composition.
- Seismicity: Most areas along which plates sliding occurs and plates converged are most tectonic and Seismic active areas for Earthquakes . Example at Transform faults, and Ring of fire around the Pacific Ocean.
- Geothermal : Heat is very high at spreading centre due to upwelling of hot magma, while it decreases away from the centre.
- Magnetic: The preserved alternating stripes of magnetic patterns parallel along the ridge on either side of the plate, suggest that plates are moving apart.
- Petrological evidences: The Petrological studies of Ophiolites Suites (part of Oceanic crust with fossils) from Mountain ranges revealed that Oceanic crust was subducted during collision (converging ) but due to tectonic Uplift and erosion they became exposed.
- GEOLOGICAL EVIDENCES: Radiometric studies of oceanic rocks indicated that rocks are younger near the ridge centre while away from the centre they are old, this implies the spreading occurs.
All in all, it is totally very difficult to reveal how our Earth is dynamic without considering Plate tectonics theory.
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