The earth’s crust is fragmented into vast pieces of rock, called tectonic plates. These slabs fit together like the parts of a huge jigsaw. Where the plates rise above the sea level they form continents and islands.
Some 250 million years ago the continents were joined together in the giant super-continent of Pangea (which is Greek and means, “all the earth”). About 200 million years ago Pangea slowly began to break up.
By 135 million years ago Pangea had split into two main land masses, known as Gondwandaland and Laurasia. North America and Europe split apart, and about 120 million years ago India began to drift north toward Asia.
Over the next 120 million years the continents drifted into their present-day positions. The Americas moved away from Europe and Africa; India joined on to Asia; and Australia and Antarctica split apart.
150 million years from now, the earth might look very different again. Africa will probably split in two, and the larger section will drift north to join Europe. Antarctica may join on to Australia. California will be crumpled up against Alaska.
The earth’s crust is made up of about 15 major plates. Plates that form the ocean floor are called oceanic plates. The plates that form the land masses are called the continental plates. Most of the earth’s plates are partly oceanic and partly continental. Scientists can locate the boundaries by monitoring earthquakes and volcanoes which tend to occur most frequently where different plates meet and collide.
- Europe and Africa combined would fit into Asia with room to spare
- Europe and the Americas currently drift about 4 cm (1.6 inches) further apart every year
- The African Rift Valley grows about 1 mm wider every year
- Fossils of tropical plants are fond as far north as Alaska, because the North American land mass was once to be found in the tropics
- Continental plates are up to 43 miles (72 km) thick, but the oceanic plates are only about 3 miles (5 km) thick
Of the total land mass of the earth’s non-oceanic surface areas, the proportions of the different continents can be expressed as relative percentages.
- Asia occupies 30% of the earth’s land surface
- Africa takes up 20% of the land surface of the earth
- North America covers 16% of the land masses
- South America boasts 12% of land coverage
- Antarctica occupies 9% of the continental earth
- Europe takes up 7 %
- and Australasia is the smallest, covering a mere 6% of the earth’s land surface
Plate tectonics is the theory of how and why the earth’s plates move. At their boundaries, the plates may be colliding, pulling apart, or sliding past each other. These different types of motion build mountains, cause earthquakes and volcanoes, and create deep-sea trenches.
To understand this better, we need to take a look at the following geophysical terms and what they mean:
- Subduction Zone
- Transform Faults
- Plate Convergence
- Mid-Ocean Ridges and Rift Valleys
Let’s explain each in turn.
When two plates collide, one plate sometimes rides over the other, forcing it down into the mantle.This type of boundary, called a “subduction zone”, often occurs at the edges of oceans where the thicker continental plate rides over the thinner oceanic plate. Deep ocean trenches form at these boundaries.
Transform faults are boundaries where two plates are sliding past each other. Earthquakes often occur at this type of boundary, as the plates slip and judder past each other. The San Andreas Fault in California, USA, is a transform fault.
When two continental plates collide, the earth’s crust often buckles and folds as they push past each other, forcing up great mountain ranges. The Himalayas and the Andes were formed by colliding plates.
Where two plates are pulling apart, molten rock from the mantle rises to fill the gap, crfeatring new crust. When this type of boundary occurs beneath the sea, mid-ocean ridges form. On land, these boundaries create steep-sided rift valleys.
There’s nothing scientists like more than a mystery. Scientists quickly lose interested in something that is completely understood and get excited about things we don’t fully understand yet. While we know that plate tectonics exist, that it’s a real thing that happens, we don’t yet know exactly how it all works. But there exist several theories of tectonic plate movement.
Scientists have yet to identify exactly what makes the earth’s tectonic plates shift around, but the three main theories involve convection, gravity, and the different weights of hot and cold rock.
Heat generated deep inside the earth creates convection currents in the mantle. These currents slowly push the overlaying plates around.
The plates are about 2 to 3 km (1 to 2 miles) higher at mid-ocean ridges than at ocean rims, so they could simply be sliding slowly downhill under the force of gravity.
Hot rock rising at mid-ocean ridges cools down as it moves further away from the ridge. As it cools it becomes heavier and sinks, pulling the rest of the plate down with it.
The tectonic plates move at different rates along their margins, and some plates move faster than others. The average rate of movement is approximately 2.5 cm (1 inch) every year. That’s about as fast as the average person’s fingernails grow!
Assuming that we humans don’t destroy it first by pollution, climate change, or a nuclear holocaust, the earth’s days are numbered by the life-cycle of the sun.
Our planet exists as it does because it hangs in a “sweet spot” not too close and not to far away from that giant sphere of heat and light. The sun is about 5 billion years old and another 5 billion years from now it will burn itself out and expand to become another kind of star called a Red Giant just before it dies. When that happens the earth and everything on it will be burned to a crisp and the story will be over.
So, we come to the end of our exploration of the earth’s continents, continental drift, and plate tectonics. Scientists around the world are making new discoveries each and every day about how the earth was formed and how it continues to change. Perhaps one day you’ll be a scientist yourself and help find out even more than we know today.