The plates that make up the Earth

The words 'the Earth's Plates' may have occasionally brushed past your ears, and may have even confused you. If you remember, they appeared in our last Geo Facts article as well, (the one about the Pacific Ring of Fire.) If you go down memory lane, although it is not pleasant to think about, you will certainly remember the tsunami that devastated a large portion of Sri Lanka's coastal belt.

This was also a result of some variations in the plates of the Earth. So, today, let us explore this plate theory.

Plate tectonics

The concept of the Earth's surface being made up as a set of plates is quite an interesting one. This is known as 'Plate Tectonics'. The theory of plate tectonics was developed in the 1960s, and explains the movement of the Earth's plates, and also the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation and many other geological phenomena (happennings).

However, before trying to understand what this is all about, we must have an insight into some special terms that appear in geology.

First is the term 'lithosphere'. The lithosphere is a part of the outer layer of the Earth. This contains the Earth's crust and the stiff topmost part of the mantle. The other part of the Earth's outer layer, which lies below this lithosphere, is the 'asthenosphere'.

The asthenosphere has relatively low viscosity (thickness) and can flow like a liquid, but extremely slowly, moving at rates of a few centimetres per year.

The layer below the asthenosphere is more rigid again, which is not due to cooler temperatures but due to high pressure. The whole theory of the Earth's plates has been made up, taking these two layers in to consideration.

The lithosphere of the Earth is broken up into what are called 'tectonic plates'. As identified to date, there are seven major and many minor plates. As you would understand by now, the lithosphere plates reside on the asthenosphere.

These plates move in relation to one another due to the flowing nature of the asthenosphere.

The sideways movement of the plates is typically at speeds of about one to nine centimetres per year. You might be surprised to know that the maximum speed given here is roughly similar to the speed at which human nails grow.

Major and minor plates

The main plates are
* African Plate covering Africa
* Antarctic Plate covering Antarctica
* Australian Plate covering Australia
* Eurasian Plate covering Asia and Europe
* North American Plate covering North America and north-east Siberia
* South American Plate covering South America
* Pacific Plate covering the Pacific Ocean

Important minor plates include the Indian Plate, Arabian Plate, Caribbean Plate, Juan de Fuca Plate, Nazca Plate, Philippine Plate and Scotia Plate.

Our country is situated on the Indian Plate. Since Sri Lanka is situated in the middle of the plate and not on a boundary, we have so far been safe from disastrous earthquakes and volcanoes.

Plate boundaries

Now that you know about the existence of plates, you would like to know about the boundaries of these plates, wouldn't you? Earthquakes, volcanic activity, mountain-building and oceanic trench formation occur along plate boundaries.

Plate boundaries can be divided into three types - transform boundaries, divergent boundaries and convergent boundaries - depending on the way two plates would move relative to each other. Let's look at these three types in detail.

* Transform boundaries

Also known as 'conservative boundaries', transform boundaries occur where plates slide or, in fact grind past each other along transform faults (break in layers of rock).

When two plates move sideways against each other, there is a large amount of friction which makes the movement jumpy.

The plates slip, then stick as the friction and pressure build up to very high levels. Earthquakes occur when the pressure is released suddenly, and the plates suddenly jerk apart.

* Divergent boundaries

Sometimes plates move away from each other, resulting in the formation of new oceanic crust from magma that comes from within the Earth's mantle. An area where the oceanic plates are moving away from each other is called a zone of divergence.

* Convergent boundaries

When two plates collide while moving towards each other, some crust is destroyed on impact and the plates become smaller. The plates that collide maybe oceanic plates or continental plates, and the result varies depending on what kinds of plates collide.

When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate. This phenomenon is called subduction.

When two oceanic plates collide, one may be pushed under the other and magma from the mantle rises, creating volcanoes in the surrounding area. When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upwards.

Aravinda Dassanayake