Plate Tectonics: Definition, Theory & Map

In the 1960s, the theory of plate tectonics revolutionized earth science. This made it possible for the first time to explain different phenomena both on the surface of the earth and in the interior of the earth. But what exactly is plate tectonics? Why is it so important? And what are all these phenomena that affects them?

Plate tectonics – definition of terms

The lithosphere is divided into seven larger and some smaller plates. These continental blocks are designated as follows:

• Pacific and Antarctic plates
• North and South American Plate
• African and Eurasian plates
• Australian plate

The smaller plates include the Cocos plate, the Indian plate, the Scotia plate and the Philippine plate. There are also a few microplates, but little is known about how they differ from other plates.

The theory of plate tectonics

When looking at the continents, it is striking that South America and Africa look like two separate pieces of the puzzle.

The polar explorer and geologist Alfred Wegener (1880–1930) also noticed this. He asked himself whether these two continents once belonged together in the past.

Figure 1: The outlines of South America and Africa Source: canva.com

For this purpose, he examined fossil finds on both continents. To this end, he undertook geological investigations by, among other things, collecting a large number of pieces of evidence and developing his theory of plate tectonics from them. In his opinion, the continents are drifting across the globe, constantly changing their position. However, he could not explain what forces should drive this movement, which is why his theory was not generally accepted.

Figure 2: Alfred Wegener Source: planet-wissen.de

Seabed exploration began in the 1950s and 1960s. Among other things, that was also echo sounder deployed to the surface shape to measure the sea floor.

During the measurements, the researchers found that the bottom of the sea has a ridged structure. In addition to deep ditches, there are also high mountains and a large number of volcanoes.

Studying the magnetic field found regular patterns that looked the same on either side of the mid-ocean ridge. Some areas were found magnetized in the direction of the Earth’s magnetic field, while others were magnetized in the opposite direction.

From this, the researchers concluded that that the Earth’s magnetic field must have completely reversed polarity several times over the course of time. They also found that the rock gets older the further it is from the mid-ocean ridge. From this they concluded that the sea floor is spreading.

But if the sea floor spreads out and the continents are connected to it by solid rocks, then they must inevitably move and collide somewhere else. Thus, the theory of Alfred Wegener was considered again.

Figure 3: Mountains in the seaSource: scinexx.de

Plate tectonics – movement of the plates

The various continental plates are always in motion. Therefore, any photograph of the earth can only be viewed as a snapshot. Each plate performs a different movement to an adjacent plate.

There are three types of plate motion:

• divergence
• subduction
• transformation flow

divergence

Divergence occurs at mid-ocean ridges and deep-sea lairs. New magma constantly rises to the surface and solidifies as lava to form new soil. In this process, the seabed is pushed apart; one also speaks of diverging plate boundaries or Sea Floor Spreading. Strong volcanism also occurs at these plate boundaries.

Figure 4: The divergence of two plates in a graph Source: planet-schule.de

subduction

Since the oceanic plates cannot grow indefinitely, they submerge at other plate boundaries. The significantly heavier oceanic plate slides under the continental plate and sinks into the mantle. This process is called subduction. As it sinks, the rock begins to melt again. The water content in the rock can lead to volcanic activity.

If these plates slide under each other, it can happen that they get stuck with each other. This can trigger earthquakes or seaquakes. As a result, tsunamis can also form. As a result of the subduction, large trenches are formed. The 1100 m deep Mariana Trench is one of those ditches.

When two continental plates meet, they begin to compress. Such a process created the Himalayas and the Alps, among others.

Figure 5: The subduction of the Nazca Plate under the South American PlateSource: planet-schule.de

transformation disorder

Another type of plate movement is the transformation disorder or also conservative plate boundary called. The two plates slide horizontally past each other. In this area the lithosphere is neither newly formed nor subducted. As the plates move against each other, there is high tension. This is discharged by sudden displacement jumps. When the plates move suddenly, severe earthquakes occur.

Figure 6: The San Andreas GrabenSource: scinexx.de

Causes of plate tectonics

The continental drift itself is hardly doubted among geoscientists, but there is still uncertainty about which processes in the Earth’s interior are responsible for the movements. There are two theories that complement each other:

It is assumed that there is one between the mantle and the core of the earth heat transfer in the form of slow convection currents, the so-called mantle convection. The earth’s mantle is slowly heated up from below. The energy for this process can come from the time the earth was formed on the one hand and from radioactive decay processes on the other. As warm, liquid rock is transported upward, the cold, solid rock of the Earth’s crust is pulled down from the upper mantle to the core. Another theory suggests the possibility of convection centers before: One is said to lie under the continent of Africa and one under the Pacific Plate.

The Pacific Plate is steadily shrinking in size, which is why it is assumed that all of the continental plates in its area will unite to form another supercontinent. When this process is complete, the ratio will reverse again and the continents will drift apart again. This is also called Wilson cycle designated.

continental drift

In this case, the plates do not rest passively on the earth’s mantle, but are actively in motion. While the plate grows on one side, it becomes heavier on the other side and is slowly pulled down again by this weight. Due to the high pressure on the sinking side, there is a kind of plate tension, while there is back pressure on the newly emerging side. There is a tension between the two sides. This affects the speed of movement of the plate. This process is also known as continental drift.

Figure 7: Simple representation of the plate tectonic principle Source: wikipedia.de