The Earth and its plates

Introduction

This chapter covers the theory of plate tectonics and its connection to continental drift. It also gives an overview of an ancient super-continent, Gondwana, and its connection to modern continents.

The Earth's lands

The Earth can be divided into three main layers: the crust, the mantle and the core. The Earth's crust and the upper layer of the mantle make up the portion of the Earth called the lithosphere. The lithosphere is about 100 kilometres thick and it includes ocean basins and continents. Ocean basins are large underwater areas of the ocean. Large areas of land are called continents. The Earth's continents today are Australia, Africa, North America, South America, Europe-Asia and Antarctica. The Earth's surface did not always look the way it does today.

About 4600 million years ago the Earth was a hot, bubbling mass of melted and solid rocks. It took the hundreds of millions of years for the Earth to cool down and form a solid surface that we call the Earth's crust. The cooler surface sections of the Earth slowly floated on the deeper, hotter layers of liquid rock. As the Earth was cooling down, blocks of the cooled-down crust were breaking apart or joining together. Islands started to form. These islands were still quite mobile compared to modern lands. The cooler climate of the Earth led to volcanic steam condensation and the formation of the first ocean. The ocean sped up the cooling of the Earth. Larger, firmer, more stable islands started to form.

Gondwana

The cooled-down, floating blocks of land kept joining together, forming one big block of land. Eventually they were all connected in one super-continent that today we call Gondwana. Gondwana was made up of the lands that later became separate continents and countries like Australia, New Zealand, India, Africa and South America. According to modern scientific studies of different rocks, Gondwana was a rocky, volcanic land with rivers and lakes. When Gondwana was formed, the Earth's climate was much cooler, but it was still too hot for most living creatures. See image 1.

Pangaea

As time went by (millions of years) more islands joined Gondwana. An even bigger continent was formed. It was named Pangaea, meaning 'one land'. Pangaea had a single ocean, called the Tethys Ocean. According to fossils, Pangaea was covered with luscious green forests with rivers and lakes where different animals lived.

Formation of continents

The blocks of land that Pangaea was made of did not stop moving, they just slowed down. So after many millions of years these same blocks of land that formed Pangaea in the first place started pulling it apart. At first, narrow cracks started to form on the Earth's crust. These cracks grew wider and wider, creating new splits until Pangaea was no longer one large continent but a group of smaller blocks of land. These blocks of land kept moving apart further and further, changing as they moved until they became the modern continents which are: Africa, North America, South America, Antarctica, Europe and Asia.

If you take a map of the world and cut out all the continents you will notice that they all look like pieces of one giant puzzle because they all used to be one land. Modern continents still move very slowly today. They move roughly with the same speed as our fingernails grow. This slow movement of continents caused by plate tectonics is called continental drift. Continental drift can be tracked down by studying different rocks that formed in different times.

The plate tectonics theory

A tectonic plate is a large section of land made of the Earth's crust and the upper layer of the Earth's mantle. Tectonic plates vary in size and shape. See image 2.

The theory of plate tectonics was first formulated by the scientist Alfred Wegener at the beginning of the twentieth century. According to this theory, the Earth's crust is divided into about 28 large, slowly moving plates called tectonic plates. The movement of tectonic plates is called plate tectonics. All continents, oceans and islands sit on tectonic plates. Scientists are still arguing about the exact number of tectonic plates, so different sources might have different numbers. Tectonic plates are constantly moving and interacting with each other generating enormous and very powerful forces. These forces are the main and the strongest 'shapers' of the Earth's landforms. Plate tectonics make continents move and shake and volcanoes erupt.

Tectonic plate boundaries

As we have mentioned before, tectonic plates are always in motion. They push, break, fold, squash and pull each other constantly. That is the reason that the Earth's surface is so complex and multilayered. Events that 're-surface' the land, like earthquakes and volcanic eruptions, are caused by plate tectonics and usually happen at their boundaries. Folding of the Earth's crust leads to the formation of mountains. Valleys and ocean beds form as plates move away from, or on top of. each other.
There are four types of tectonic plate boundaries:

1. Divergent boundaries - areas where new crust is formed as the plates pull away from each other.
2. Convergent boundaries - areas where crust is destroyed as one plate slips under another.
3. Transform boundaries - areas where crust does not change as tectonic plates slide past each other.
4. Plate boundary zones - large areas in which the boundaries of plates are not well defined.

Tectonic motion

The Earth's crust does not get thicker because of the tectonic motion because when a new ocean crust is formed, the old one is dragged down into the Earth's mantle and destroyed there. This process is called 'subduction'. Tectonic motion around the world can be measured by different types of space technologies like satellite laser ranging (SLR), Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS).