Physical Geography - Tectonic Plates

TECTONIC PLATES

A tectonic plate (lithospheric plate) is a massive, irregular shape of the solid rock slab. The concept of Tectonic Plates was first introduced in 1967.

• It is generally composed of both continental and oceanic lithosphere.

• The lithosphere includes the crust and top mantle with its thickness range varying between 5- 100 km in oceanic parts and about 200 km in the continental ranges.

• A tectonic plate maybe a continental plate or an oceanic plate, depending on which of the two occupies the larger portion of the table.

• Oceanic plate: Pacific plate

• Continental plate: Eurasian plate.

The Major & Minor Plates:

The Earth's lithosphere is divided into 7 (seven) major and some minor plates:

A.) Young Fold Mountain ridges, oceanic trenches, and transform faults surround the major plates. These includes

• The North American plate- with the western Atlantic floor separated from the South American plate along with the Caribbean islands.

• The South American plate- with the western Atlantic floor separated from the North American plate along with the Caribbean islands.

• Africa with the eastern Atlantic floor.

• The Antarctic and the surrounding oceanic plate.

• The Pacific plate.

• The India-Australia-New Zealand plate.

• Eurasia and the adjacent oceanic plate.

B.) Some Minor plates include

• Cocos plate- between Central America & Pacific plate.

• Nazca plate- between South America & Pacific plate.

• Juan de Fuca plate- Southeastern (SE) of North American plate.

• Philippine plate- between the Asiatic & Pacific plate.

• Caroline plate- between the Philippine & Indian plate (North of New Guinea)

• Fuji Plate- Northeastern (NE) part of Australia

• Arabian plate- Saudi Arabian landmass.

C.) Mountain Ridge: The chain of mountains or hills that form a continuous elevated crest for some distance.

D.) Ocean Trenches: The long, narrow depressions on the sea-floor. These are the deepest part of the ocean and some of the deepest natural spots on Earth. They are found in every ocean basin on the planet and the deepest ocean trenches ring the Pacific called the "Ring of Fire".

E.) Transform Fault: It is a type of fault in which 2 (two) tectonic plates slide past one another.

The Indian Plate:

• The Indian plate includes Peninsular India and the Australian continental portion.

• In the East, it extends through the Rakim Yoma Mountains (Myanmar) towards the island arc along with the Java Trench.

• In the West, it follows Kirthar Mountain (Pakistan). It further extends along the Makrana coast (Balochistan) and joins the spreading site from the Red Sea rift southeastward along with the Chagos Archipelago.

• The boundary between the Indian & the Antarctic plate is also marked by an oceanic ridge (divergent boundary) running roughly West to East direction and merging into the spreading site, a little south of New Zealand.

Asthenosphere:

• It is the zone of Earth's mantle that lies just beneath the lithosphere and is believed to be much hotter and more fluid than the lithosphere.

• The asthenosphere extends from about 100 km (62.13 miles) to about 700 km (434.96 miles) below Earth's surface.

Movement of Plates:

• The tectonic plates are not fixed but constantly move horizontally over the Asthenosphere as rigid units.

• Sometimes these plates collide, move apart, or slide next to each other which leads to Earthquakes or Volcanic Eruptions.

Rates of Movement of Plates: 

• The rate of movement of the tectonic plates is varied considerably.

• The Arctic Ridge has the slowest rate (less than 2.5 cm/ yr).

• The East Pacific, in the South Pacific, about 3,400 km west of Chile has the fastest rate (more than 15 cm/yr).

Force of Movement of Tectonic Plates: 

• The mobile rock beneath the rigid plates is believed to be moving circularly.

• The heated material rises to the surface, spreads & begins to cool, and then sinks back into deeper depths.

• This slow movement of the hot, softened mantle that lies below these rigid plates is the driving force behind the plate movement.

Subduction: 

• When tectonic plates shift, and one plate is pushed under another.

• This movement of the ocean floor produces a 'mineral transmutation', which leads to the melting & solidification of magma i.e., the formation of volcanoes.

• When a 'downgoing' oceanic plate is pushed into a hotter mantle plate, it heats up, volatile elements mix, and this produces the magma.

• The magma then rises up through the overlying plate and spurts out at the surface.

Boundaries of Plates:

The movement of the tectonic plates creates 3 (three) types of tectonic boundaries:

• Convergent- plates move into one another.

• Divergent- plates move apart.

• Transform- plates move sideways in relation to each other.

A.) Convergent Boundaries:

• It is formed when tectonic plates crash into each other, also known as destructive boundaries.

• These boundaries are often subduction zones, where the heavier plate slips under the lighter plate, creating a deep trench.

• This subduction changes the dense mantle material into buoyant magma, which rises through the crust to the Earth's surface.

• Over millions of years, the rising magma has been creating a series of active volcanoes known as a volcanic arc.

• Convergent plate boundaries also lead to mountain building and the formation of island arcs (Festoons). If both the convergent plates are oceanic, the volcanoes form a curved line of islands, known as an island arc, that is parallel to the trench.

There are 3 (three) ways in which convergence can occur:

• between an oceanic & continental plate

• between two oceanic plate

• between two continental plate

When continental & oceanic plates collide, the thinner & denser oceanic plate is overridden by the thicker and less dense continental plate.

Example:

• Oceanic-Continental convergent: the Washington-Oregon coastline of United States = Here, the Juan de Fuca oceanic plate is subducted beneath the North American continental plate.

• Oceanic-Oceanic convergent: Marina Trench, the deepest point of Earth. The mighty Pacific plate is subducted beneath the smaller, less-dense Philippine plate.

• Continental convergent: the Himalayan Mountain Range. India & Asia crashed about 55 million years ago, slowly giving rise to the Himalayas, the highest mountain system on Earth. Here, the Indian & Eurasia plates are currently in a collision.

Subduction Zones:

• It is the biggest crash scene on Earth. These boundaries mark the collision between two tectonic plates.

• When 2 (two) tectonic plates meet at a subduction zone, one bends and slides underneath the other, curving down into the mantle, the hotter layer under the crust.

B.) Divergent Boundaries:

• It is formed by tectonic plates pulling apart from each other, known as constructive boundaries.

• These are the site of seafloor spreading & rift valleys.

• At divergent boundaries in the oceans, magma from deep in the Earth's mantle rises toward the surface and pushes apart two or more plates. Mountain & volcanoes rise along the seam. The process renews the ocean floor and widens the giant basins.

• The best-known example is the Mid-Atlantic Ridge, where the American plates are separated from the Eurasian & African plates. The single mid-ocean ridge system connects the world's oceans, making the ridge the longest mountain range in the world ( 10,000 miles).

On Land, giant troughs such as the Great Rift Valley (in Africa) from where plates are tugged apart.

• If the plates there continue to diverge, millions of years from now eastern Africa will split from the continent to form a new landmass.

• A mid-ocean ridge would then mark the boundary between the plates.

Seafloor Spreading:

• It is the process of magma welling up in the rift as the old crust pulls itself in opposite directions.

• Cold seawater cools the magma, creating a new crust.

• The upward movement & eventual cooling of this magma has created high ridges on the ocean floor over millions of years.

• The East-Pacific Rise is a site of major seafloor spreading in the 'Ring of Fire'.

It is located on the divergent boundary of

• the Pacific plate

• the Cocos plate (west of Central America)

• the Nazca plate (west of South America)

• the North American plate & Antarctic plate.

Rift Valleys:

• It is a lowland region that forms where Earth's tectonic plates move apart or rift.

• It is found on land and at the bottom of the ocean, and are created by tectonic activity and not the process of erosion.

• The Great Rift Valley System, which stretches from the Middle East in the north to Mozambique in the south is a geologically active area. It features volcanoes, hot springs, geysers, and frequent earthquakes. 

C.) Transform Boundaries:

• It is formed as tectonic plates slide horizontally past each other but parts of these plates get stuck at the places where they touch.

• These boundaries are conservative because the plate interaction occurs without creating or destroying crust. Hence, they don't produce spectacular features like mountains or oceans, but the halting motion often triggers large earthquakes, such as the 1906 earthquake that devastated San Francisco.

• In this area of contact, stress is built which causes the rocks to break or slip, suddenly lurching the plates forward and causing earthquakes. These areas of breakage or slippage are called faults. The majority of Earth's faults can be found along transform boundaries in the 'Ring of Fire'.

• The San Andreas Fault (California) is an example of a transform boundary, where the Pacific plate moves northward past the North American plate. It is one of the most active faults on the 'Ring of Fire'.

Hotspots:

• Most of the volcanic activities are concentrated along or adjacent to plate boundaries, but there are some important exceptions, in which this activity occurs within plates, called the Hotspots.

• Hotspots exist over the Mantle Plumes. A mantle plume is an area under the crest of Earth, where magma is hotter than surrounding magma. The heat from this extra hot magma causes melting & thinning of the rocky crust, which leads to widespread volcanic activity on Earth's surface above the plume.

• The hotspots are stationary, unlike the tectonic plates where they are located.

• There are about 40 to 50 hot spots estimated to be around the world.

Major hot spots include:

• Iceland hotspot- in North America.

• Reunion hotspot- in the Indian Ocean.

• Afar hotspot- in Northeastern Ethiopia.