Tectonic Plates

Tectonic plates are the sections that divide the lithosphere (the Earth’s outer shell, including the crust and uppermost mantle). Tectonic plates are moving relative to each other and are responsible for many hazards such as volcanic activities, earthquakes and tsunamis.

How many tectonic plates are there?

There are seven major tectonic plates. These are: African, Antarctic, Eurasian, Indo-Australian, North American, Pacific and South American.

Fig. 1. - Principal tectonic plates

Why was the theory of tectonic plates proposed?

The theory of tectonic plates was proposed in the 1960s when seismographs recorded the vibrations of earthquakes. Seismographs were initially used in World War II to test for atomic bombs. They also found the epicentres of the earthquakes, making it possible to find the outline of tectonic plates. The theory of plate tectonics answers questions such as: why the earth’s geography changes, why certain locations are prone to certain hazards, and why some locations have mountain ranges.

Continental drift

In 1912, Alfred Wegener suggested that the continents of Earth had been separated from one large continent, called Pangaea. This process is called continental drift. He provided considerable evidence that the continents had drifted, but he was unable to find sufficient reasoning for it.

Some of this evidence includes:

• Coal found in the UK. Coal requires warmer and more humid environments to form.
• The fact that countries are shaped like puzzle pieces and could fit each other.

Fig. 2 - Continental drift

Seafloor spreading

The theory of tectonic plates is also supported by palaeomagnetism (the study of magnetic rocks and sediment to understand the Earth’s magnetic field). As rocks form and cool, the magnetic grains align in the direction based on the magnetic poles. The Earth’s poles switch periodically. Scientists analysed the rocks in the ocean ground and found that the magnetic signatures of some rocks were in opposite directions, even though they were side by side. In the 1940s, scientists theorised that magma fills the gap with rock with new magnetic alignment when the tectonic plates move apart. We call this seafloor spreading.

How do tectonic plates float on the mantle?

Tectonic plates are able to float on the mantle due to the composition of the rocks within the plates. This makes them less dense than the mantle. Continental crust is formed of granite rock which comprises quartz, feldspar and other relatively lightweight material mostly made of silicon and aluminium. The oceanic crust consists of basaltic rock and other materials predominantly of silicon and magnesium. The oceanic crust is much denser but significantly thinner in comparison to continental crust. The continental crust can have a thickness as great as 100km, whilst the oceanic crust is about 5km thick.

Why do tectonic plates move?

Tectonic plates move due to mantle convection, subduction and slab pull.

Mantle convection

To fully grasp the concept of mantle convection, it is important to understand the structure of the Earth’s inner core. The top layer of the Earth is the hard and brittle crust. Beneath the crust is the mantle, which makes up most of the Earth’s volume. It is made mostly of iron, magnesium and silicon. The temperature of the mantle varies between 1000°C near the crust and 3700°C near the core. The outer core is made up of liquid iron and nickel, while the inner core is solid, denser, hotter iron and nickel, reaching 5400°C.

Fig. 3 - The internal structure of the Earth

The process of mantle convection involves the heating of the liquid rock in the mantle by the core. This hot liquid rock rises to the crust because its density reduces. However, as it reaches the top, it cannot pass through the crust, therefore moving sideways along the crust. The plate then moves due to friction between the convection current and the crust. The liquid rock cools, sinks and the process is repeated.

Fig. 4 - Convection currents create movement through friction

Subduction and slab pull

Subduction is the process where two plates meet, and the denser oceanic crust is pushed under the other. The cool oceanic crust is denser than the hot mantle and eventually sinks due to the gravitational pull. This process is called slab pull. This causes tectonic movement as it drags the rest of the plate.

What are the impacts of tectonic plate movement?

The movement of tectonic plates relative to each other leads to tectonic processes, which are interactions between tectonic plates that impact the structure of the Earth’s crust. Tectonic processes can lead to tectonic hazards. They are responsible for the majority of earthquakes, volcanic activity and tsunamis. Tectonic hazards are then considered natural disasters when they cause significant damage to societies or communities (such as loss of life, injuries and damage to infrastructure), and they can no longer cope using their own resources.

What are the different types of tectonic plate boundaries?

The types of plate boundaries include divergent, convergent and conservative plate boundaries. A plate boundary is a location where two tectonic plates meet.

Divergent plate boundary

Fig. 5 - Divergent plate boundary separating

At divergent plate boundaries (also known as constructive plate boundaries), the plates are moving away from each other. This occurs as the convection current of the mantle pushes the plates apart, generating a gap in between, causing magma to fill the gap and producing a new crust. Most are located at ocean ridges and generate low magnitude earthquakes. Divergent boundaries between continental plates often form rift valleys.

Convergent plate boundary

Fig. 6 - Convergent plate boundaries are destructive

Convergent/destructive plate boundaries are where plates are moving towards each other. When an oceanic crust and a continental crust meet, the denser oceanic crust is pushed below the continental crust (also known as subduction). The plates slide on top of each other, and this process can lead to earthquakes and volcanic activity as the friction between the two plates increases and is released. The oceanic crust underneath is destroyed in the process. When an oceanic crust meets with another oceanic crust, subduction also occurs. Island arcs and oceanic trenches are often created. When continental plates collide, it can also cause either one or both of the plates to buckle up, consequently forming mountain ranges.

Conservative plate boundary

Fig. 7 - Conservative plate boundaries slide past each other

The regions where plates are sliding past each other in the horizontal direction are called the conservative plate boundaries or transform plate boundaries. Due to the irregularity of the surface of the plates caused by rocks, the friction and pressure build-up, and the plates eventually slide past each other, causing frequent earthquakes. The rocks from the plates are pulverised and often create fault valleys or undersea canyons.