Drifting the time away

In the 1700's Sir Francis Bacon was one of the first to muse that Africa and South America look as if they are ``torn apart'' from a single continent. It took until 1912 for a German geologist, Alfred Wegener, to propose the theory of continental drift. He basically stated that there are massive `plates' that make up the Earth's crust and they are gradually moving both horizontally and vertically, causing the continents to ``drift''. Scientists now agree that the Earth's crust has 12 major plates.

The Earth's structure

The Earth is essentially made up of four layers. At its center, the Earth has a solid inner core made up of nickel and iron. Next is the liquid outer core, also made up of nickel and iron and is responsible for the Earth's magnetic field. After the outer core and the largest of the layers is the mantle. This is a liquid layer made of iron, magnesium, many other elements and liquid rock.

But don't be fooled by the word ``liquid'', because it is not a liquid that is similar to water. It is much more viscous than that and moves at such a slow rate you would think it is solid. Indeed, the top lateral speed between plates is typically only 0.66 to 8.50 centimeters per year.

Convection currents cause the heated mantle to rise. As it rises, it cools. This constant expansion and contraction causes continental drift by continually moving the Earth's top layer or crust.

The crust is the thinnest layer of the Earth. A good comparison would be the skin of an apple in comparison to the rest of the apple. The Earth's crust ranges from 5-10km thick underneath the oceans, which is predominantly basalt rock. The Earth's crust ranges from 10-70km thick in continental crust, or the land-based part of the Earth. It is mainly composed of granite. The crust of the Earth `floats' on top of the mantle and moves in many directions.

On the move

The various plates of the Earth's crust move in different directions. Plate divergence is where plates move away from each other. Two notable areas of plate divergence are the Mid-Atlantic Ridge and Africa, and the Middle East's Great Rift Valley.

The Mid-Atlantic Ridge is an example of sea-floor spreading. The ridge is in the middle of the Atlantic Ocean and runs between the Americas and Africa. There, the sea floor constantly spreads apart. The divergence causes magma to rise and fill the gap. As the magma cools, it forms brand new crust in an underwater mountain range. Because the sea floor is drifting apart, Africa and the Americas are also drifting apart by about 2.5cm per year. The Great Rift Valley, most noticeable in East Africa, is an example of plate divergence on continental crust. The plates are moving away from each other and gradually East Africa will no longer be a part of the mainland continent. Characteristics of plate divergence are high areas of volcanic activity and deep lakes, of which Lake Victoria and Mt Kilimanjaro are examples.

Convergence

Convergent plate movement is when two plates collide as they move towards each other. (See the graphics below and below right.) If two oceanic plates move towards each other, sometimes one slides underneath the other. This is called subduction. Subduction causes deep oceanic trenches, such as the Mariana Trench in the Pacific Ocean. At 11km deep, it is the deepest part of the ocean. Another example of subduction is when oceanic and continental crusts collide. The oceanic crust is generally forced under the continental crust and becomes part of the Earth's mantle.

When two continental crusts converge, the rock between the plates often has nowhere to go but up, resulting in a phenomenon called folding. Folding creates mountain ranges. The highest mountain range on Earth, the Himalayas, was formed in this way.

Recent evidence and theories suggest that the Andes - the longest mountain range, runs down most of the western coast of South America - was born from the convergence of continental and oceanic crusts.

Transform boundaries

When tectonic plates slide or, perhaps more accurately, grind past each other along transform faults, that is called lateral movement. If intense friction causes the plates to stick, a tremendous amount of energy is built up. Eventually they will give way, and this leads to a sudden movement of the plates, resulting in an earthquake.

The most famous example of lateral plate movement is the San Andreas Fault in the US state of California. There the slip of the plates is clearly seen in crop fields and aerial photographs. Earthquakes are a frequent result of this type of tectonic activity, which is why California has so many. Australia, which sits on its own plate, doesn't have volcanoes, whereas Indonesia, the Philippines and the Japanese island arc are all close to or on fault lines. Indeed, Japan's tallest peak, Mount Fuji, is a volcano. Areas along fault lines are frequented by volcanoes, earthquakes and tsunamis.

Past, present and future

The now-popular Continental Drift hypothesis asserts that 200 million years ago all of the land above sea level was joined together in a massive landmass known as Pangaea. As the tectonic plates moved, two super continents formed. Laurasia to the north, and the enormous Gondwana, comprising South America, Africa, Madagascar, India, Antarctica and Australia, to the south.

One reason the theory has been so readily accepted by scientists is that it easily explains the once unexplainable question: ``How can rocks that are clearly marine in origin exist thousands of meters above sea level?''

As time progressed, the continents gradually drifted apart, separating first was India, then Africa and Madagascar. Eventually, South America, Antarctica and Australia split off about 40 million years ago, forming the continents as we know them today. The relatively recent split of the last three continents are the reason why South America has marsupials, as does Australia, and underneath the ice of Antarctica are the remains of plants that are also found in Australia.

But what's for the future? In a hundred million years or so, the Antarctic Ocean may be bigger than the Pacific Ocean as the Mid-Atlantic Ridge keeps growing. Australia is moving north at about 7cm a year and will eventually be Japan's neighbor, and the world map will be almost unrecognizable. So the next time you are at Lumpini park or on Samui island when you should be studying, relax because you really are drifting the time away!

David Canavan has an MSc in Behavioral Ecology and teaches science, math and ICT at Garden International School. David is fascinated by science and loves animals, especially the dangerous kind; the more dangerous the better. You may contact David at davidc@gardenbangkok.com .