In the 1960s the unraveling of some mysterious features of the deep ocean floor led to the development of Plate Tectonics theory. In doing so it not only explained how continental drift occurred, but also helped us to understand the formation of many of our planet's most amazing and dramatic features such as volcanoes, earthquakes, ocean trenches and mountain ranges. Henry Ritson explains how...
The Story of a Revolution
In last month's article we showed how over the first half of the 20th century Alfred Wegener's 'Continental Drift' theory had helped prove that the earth's land masses were very slowly moving around the globe. This month we shall complete the story and show how developments since the early 60's have given us a much greater understanding of how the Earth works. To begin with it is best if we have a basic understanding of our planet's internal structure, which is arguably easiest done by analogy to a Scotch Egg!
In the center of the planet is the 'core', which is a bit like the egg part of the scotch egg, though in this case it is made out of Iron and Nickel. The main thing to remember about the core is that , at around 5500 degrees c, it is very very hot indeed.
The sausagemeat section of our scotch egg planet, around the core, is called the 'mantle' and is made of silicate rocks. The heat produced by the core is able to produce convection currents which move the Mantle rocks about in the same way that a gas burner will move the water in a pan as it boils (see fig1). However there is no need to start imagining the mantle as being all runny and liquid. The mantle rocks would appear fairly solid to you or I if we could see them, but they can deform under pressure and flow like an extremely viscous fluid in much the same way that glass, which we assume to be totally solid, will over a century or so start to flow down a window and thicken at the bottom.
Finally, the 'breadcrumbs' represent the 'crust', the relatively thin outer layer of the planet which floats on top of the mantle. This is the only section of the planet that humans have ever actually seen and makes up all the continents and all the ocean bed. It is very important to note that there are two totally different types of crust on planet earth.
The first type is relatively light and buoyant, made of predominantly granitic rocks, and can be up to 65km thick.
The second type, by contrast, is relatively dense, made of basaltic rocks and only between 6 and 10km in thickness.
Both types of crust float 'like rafts on a swimming pool' (Van Andel) on the mantle. However, the first type, being both more buoyant and thicker, always sits with its upper surface higher than the upper surface of the second type. We therefore are presented with a split level earth with two different heights of crust. Now, when we bring water into the equation we find that the second, lower type of crust is in fact entirely submerged by the waters of the oceans and so is known as 'oceanic crust'. In contrast the second, thicker, crust type usually protrudes above the ocean surface forming continents and is known as 'continental crust'. The few areas where continental crust gets covered by the ocean are known as continental shelves.
In summary there are two different types of crust on the earth, one of which is buoyant and thick and forms the continents and one of which is dense and thin and is covered by the oceans (see fig 1). However the problem up until the 1950's had essentially been that since we had very little ability to understand the deep ocean floor, we had only been looking at the continents for evidence. We did not fully understand continental drift because we had unknowingly only been considering half of the picture. However, in the years after the second world war great advances were made in understanding the sea bed, chiefly because the navy wanted to understand where its submarines were going. And as our knowledge grew, several mysteries presented themselves.
Firstly, why was it that when many continents are made up of rocks many billions of years old, no rocks of over 200 million years old could be found on the ocean bed? Indeed, most oceanic rocks were less that 100 million years old. It seemed that not only did we have a two level planet, but also a two age planet with young oceans and ancient continents.
Secondly, what were the strange ridges found in the middle of the ocean bed? The use of the precision depth recorder to map the sea bed using echo sounding allowed us for the first time to see what the deep ocean floor was like. The vast majority of it turned out to consist of mile after mile of tedious flat plain, perhaps with the occasional volcano. However, right along the centre of each ocean ran a vast ridge. These mid-oceanic ridges encircle the whole planet, straddling it like the seam on a tennis ball. The centre of each ridge was associated with volcanoes, tension-type earthquakes and young rocks. Rock age and deposited sediment depth increased away from the ridge, toward the continents.
Thirdly, why is it that the bedrock's pattern of magnetic pole reversal (as described last month) formed long stripes, running parallel to the ridge, which were symmetrical on either side of it?
Finally, in 1963 Fred Vine and Drummond Matthews realised what all this meant, and put together one of the most important theories in the history of earth sciences. What they said was this:
The mid-oceanic ridges are centres of sea floor spreading where new crust is formed as lava wells up to the surface, in-so-doing pushing the crust on either side further apart, thus causing the continents to move.
The ocean floor, it transpires, is something like a giant conveyor belt. Lava is being extruded all the time forming new crust at the centre of the ridge. On either side of this the crust is being pushed apart by the new material at a rate of 2 to 20 cm per year. This mechanism has meant that the whole sea bed is slowly being pushed along. At the edge of an oceanic crust plate where it meets a continent, the continent can also get pushed along by this process producing continental drift. The ultimate driving force for this mechanism is not yet fully understood, but is thought to involve the convection currents set up within the mantle by the heat of the core.
This is why America and Europe/Africa have been moving further apart since the break up of Pangaea (see last article). They are being slowly pushed apart by new crust erupting in the mid atlantic ridge, and we can actually measure this happening these days using the satellite based global positioning system. Not to put too fine a point on it, since the first part of this article was published last month, your house has moved about 2mm further away from the Statue of Liberty.
If we produce a map of all the major earthquakes in the world over a period of time, such as is shown in fig 2, we find that they are not at all randomly distributed but follow very clear zones and bands. For a start, they follow all of the mid oceanic ridges and they also follow the edges of some continents.
What is shown to be happening is that the outer surface of the earth is divided into a number of separate plates (there are about 12 major ones, see fig 2) made up of the crust plus a bit of the mantle below. These plates, tectonic plates as they are known, are able to move independently of one another, being pushed up by the sea floor spreading. At the edges of the plates, where they rub against each other, earthquakes occur due to the friction.
There are logically three ways that plate margins can move relative to one another.
i) Moving away from one another.
This results in new oceanic crust being formed as lava fills the gap between the plates. This is known as a constructive margin and is what occurs at a mid oceanic ridge.
ii) Moving towards each other.
These margins are called "destructive margins" since crust gets destroyed as the plates collide. If two continental plates collide then the crust ruptures and crumples up forming a mountain range such as the Himalayas (which are forming as the Indian plate slowly crashes into the Eurasian plate.) Alternatively if an oceanic plate collides with a continental plate then the continental crust, being more buoyant, rides over the top of the oceanic plate. The oceanic plate is subducted back into the mantle, thus destroying oceanic crust, to balance the crust being produced at the mid oceanic ridges.
This is why all oceanic crust is much younger than the continental crust; it is constantly being recycled. Even though new oceanic crust is always being formed, old crust is always being destroyed, and so there is no very ancient oceanic rock around. If this didn't happen, the world would have to be constanly expanding to make way for the extra crust being formed!
As the oceanic plate gets pushed down into the mantle, a vast ocean trench is formed by the drastic lowering of the sea bed. These trenches are by far the deepest areas of the worlds' ocean and are home to some of the planet's most extraordinary wildlife. Sometimes some of the subducted oceanic plate, once melted into magma within the mantle, begins to rise and push up through the continental plate on the other side, forming volcanoes, and ultimately, a mountain range such as the Andes (caused by the Nazca plate sinking below the South American plate).
iii) Sliding past each other.
Tectonic plates are also able to slide in opposite directions whilst lying next to one another. As crustal material is neither destroyed nor created in this procedure these are known as conservative margins. However the edges of the plates are rough and cause friction. This means that rather than sliding smoothly past each other they tend to jam and stick in one place until the pressure builds up to be so great that it has to give. At that point the plates move suddenly, causing an earthquake. For this reason the fault lines along conservative plate margins tend to often be the most dangerous earthquake zones in the world. For example, the San Andreas Fault forms a junction between the North American and Pacific Plates. Although both plates are moving northwest, the pacific plate moves faster, giving the illusion that they are moving in opposite directions at a rate of about 6cm per year. When sufficient pressure has built up, the pacific plate suddenly jerks forwards, resulting in massive earthquakes in California. If this process continues, Los Angeles will eventually end up as an island off the Canadian coast!
So there you have it! The world's surface is made up of Tectonic Plates formed from continental or oceanic crust and the top of the mantle. Convection in the mantle results in new crust being formed at the mid-oceanic ridges, which pushes the plates apart and drives the movement of the continents. Volcanoes, earthquakes, mountains and trenches are a few of the key features this produces. And, of course, the world is like a scotch egg...
First published in Amateur Astronomy & Earth Sciences, December 1995.