Essay on the Plate Tectonics Theory | Earth | Geography

After reading this essay you will learn about the plate tectonics theory.

Plate Tectonic theory is based on an earth model characterized by a small number of lithospheric plates, 70 to 250 km (40 to 150 mi) thick, that float on a viscous under-layer called the asthenosphere. These plates, which cover the entire surface of the earth and contain both the continents and seafloor, move relative to each other at rates of up to ten cm/year (several inches/year).

The region where two plates come in contact is called a plate boundary, and the way in which one plate moves relative to another determines the type of boundary- spreading, where the two plates move away from each other; subduction, where the two plates move toward each other and one slides beneath the other; and transform, where the two plates slide horizontally past each other. Subduction zones are char­acterized by deep ocean trenches, and the volcanic islands or volcanic mountain chains associated with the many subduction zones around the Pacific rim are sometimes called the Ring of Fire.

According to the plate tectonic model, the surface of the Earth consists of a series of relatively thin, but rigid, plates which are in constant motion. The surface layer of each plate is composed of oceanic crust, continental crust or a combination of both. The lower part consists of the rigid upper layer of the Earth’s mantle.

The rigid plates pass gradually downwards into the plastic (soft) layer of the mantle, the asthenosphere. The plates may be up to 70 km thick if composed of oceanic crust or 150 km incorporating continental crust. Plates move at different velocities, The African plate moves about 25 mm per year, whereas the Australian plate moves about 60 mm per year.

Most of the Earth’s tectonic, seismic and volcanic activity occurs at the boundaries of neighbouring plates.

There are three types of plate boundaries:

Divergent, convergent and transform boundaries.

1. Divergent plate margins:

At this type of boundary new oceanic crust is formed in the gap between two diverging plates. Plate area is increased as the plates move apart. Plate move­ment takes place laterally away from the plate boundary, which is normally marked by a rise or a ridge. The ridge or rise may be offset by a transform fault. Presently, most divergent margins occur along the central zone of the world’s major ocean basins. The process by which the plates move apart is referred to as sea floor spreading.

The Mid-Atlantic Ridge and East Pacific Rise provide good examples of this type of plate margin.

The rate at which each plate moves apart from a divergent margin varies from less than 50 mm per year to over 90 mm per year and can be determined from the pattern of magnetic anomalies either side of a spreading ridge.

Either side of a spreading centre, weak magnetic anomalies 5-50 km wide and hundreds of kilometres long can be identified. Molten rock cools between diverging plates the magnetic minerals present align themselves with the orienta­tion of the Earth’s magnetic field at that time. The polarity of the Earth has changed at regular intervals throughout geological time.

Magnetic north has alternated between the Arctic (normal polarity) and the Antarctic (reversed polarity). As a result of this, sections of crust formed during a period of normal polarity have a paleomagnetic remnance which is ori­ented towards today’s magnetic north, while a section of crust formed during a period of reversed polarity does not. These long linear strips of magnetic anomalies form a symmetrical pattern either side of a spreading centre.

A record of the changes in the Earth’s magnetic polarity has been established and dated for the Cenozoic and is the basis for magnetostratigraphy. This record, in conjunction with the magnetic stripes found either side of a spreading ridge, allows the rate and pattern of sea floor spreading to be examined. Magnetostratigraphy uses records of changes in polarity of the geomagnetic field preserved in sedimentary sequences to correlate between wells and to date the sediment. Individual normal and reverse polarity intervals (“Chrons”) typically range from ∼10 thousand to 10 million years in duration.

For example:

Since geomagnetic polarity reversals are globally synchronous, their records represent “ab­solute” time planes in sedimentary sequences which can provide a robust stratigraphic correlation framework. This is especially useful in biostratigraphically-barren sequences. Furthermore, sets of reversals often carry distinctive “fingerprints”, which can be matched with appropriate parts of the standard.

2. Convergent plate boundaries:

At a convergent boundary two plates are in relative motion towards each other. One of the two plates slides down below the other at an angle of around 45 degrees and is incorporated into the Earth’s mantle along a subduction zone. The path of this descending plate can be found from analysis of deep earthquakes and the initial point of descent is marked on the surface by a deep ocean trench. Plate area is reduced along the subduction zone. When two plates of oceanic crust collide a volcanic island arc may form. As one of the plates is subducted beneath the other it begins to melt at a depth of between 90 and 150 km and the resulting magma rises to the surface above the subduction zone to form a chain or arc of volcanoes. The edge of the plate which is not descending is therefore marked by a chain of volcanic islands.

3. Conservative or transform margins:

Transform plate boundaries, where plates slide hori­zontally against each other, neither create nor destroy lithosphere. However, at these bound­aries, or transform faults, powerful earthquakes can occur. The San Andreas fault system is the most famous example of this type of boundary. Here two plates move laterally past each other and oceanic crust is neither created nor destroyed.

What causes plates to move?

This question has yet to be fully resolved.

Four main hypotheses have been put forward to explain this:

1. Convection currents:

This hypothesis suggests that flow in the mantle is induced by convec­tion currents which drag and move the lithospheric plates above the asthenosphere. Convec­tion currents rise and spread below divergent plate boundaries and converges and descend along convergent.

Three sources of heat produce the convection currents:

a. Cooling of the Earth’s core

b. Radioactivity within the mantle and crust

c. Cooling of the mantle.

The convection hypothesis has been proposed in several different forms throughout the last 60 years. Convective models of plate evolution clearly show how important convective heat transport is to the modern Earth, over length scales as small as 100 km and times of 60 million years. Earth is a spendthrift, living on its inherited capital of primaeval heat, not on its radiogenic modern income.

2. Magma injection:

This hypothesis invokes the injection of magma at a spreading centre pushing plates apart and thereby causing plate movement.

3. Gravity:

Oceanic lithosphere thickens as it moves away from a spreading centre and cools, a configuration which might tend to induce plates to slide under the force of gravity, from a divergent margin towards a convergent margin.

4. Descending plates:

This hypothesis suggests that a cold dense plate descending into the mantle at a subduction zone may pull the rest of the plate with it and thus cause plate motion.