Is it possible for something that’s spherical to have a physical end or beginning? A ball just keeps going on and on and on and on. No matter how many times you turn it, you never get to any definitive beginning or end. Where does an egg start and where does it end? With the chicken or the egg or the chicken or the egg or the chicken?
Well, in spite of its spherical shape, planet Earth has many beginnings and endings and they are found at the boundaries of the colossal shifting plates that comprise its surface! Plate tectonics account for many of the soaring and plummeting landscapes on our planet and it explains a host of our most frightening natural disasters, from spewing volcanoes to shuddering earthquakes. It builds beautiful fertile islands in the middle of vast ocean expanses while ripping the ocean floor apart elsewhere, forming trenches in excess of 10 kilometres deep. Understanding plate tectonics is key to understanding our planet and its dynamic surface, which, as stable as it seems under our feet, is in reality anything but.
The Earth’s Surface is Divided into Plates
The Earth’s outermost crusty layer is known as the lithosphere (lithos meaning “stone” in Greek) and it can be likened to a giant shell that has been broken into large, rigid interlocking pieces (refer to the image below). These pieces sit upon the warmer and more malleable asthenosphere and basically bumble their time away by colliding into each other, pulling apart and rubbing against each other. They also, you know, support the entire biodiversity of planet Earth in their spare time.
Meet Planet Earth’s tectonic plates: Americans and Canadians get the North American Plate, Europeans and Asians get the Eurasian Plate and the penguins get the Antarctic Plate… EVERYONE gets a plate!
The asthenosphere, which is fluid-like and warmer and more pliable than the outer crusty lithosphere, promotes the migration of the Earth’s tectonic plates. Prodigious convention currents of heat and molten magma travel from the bowels of the planet to its surface, compelling these giant puzzle pieces to move. Just like Tree Ents from “the Lord of the Rings” and the cogs in your brain after a heavy night out, these motions are frightfully slow. Some plate boundaries, such as the Mid-Atlantic Ridge, move as fast as your fingernails grow, which is approximately 1 to 4 cm per year. Doesn’t exactly make for riveting viewing, does it?
But over time, patience wins out against the resistance of solid rock and the results are as creative as they are destructive.
The Three Plate Boundary Types
All of the plates that make up the lithosphere are in constant motion thanks to the giant hot and moist “visco-elastic” asthenosphere upon which they sit. Hot and moist. If you’ll refer back to the map above, you’ll notice that every plate fits snugly into another, much like a giant jigsaw puzzle. Since each plate is in constant motion, one can definitely assume that it’s where they meet – at the plate boundaries – where the party’s at.
The picture below shows us the direction of motion of each of Earth’s tectonic plates. At any given time, one periphery of a plate is wrenching away from another. At the opposite end of the plate, there is a violent collision going on, while the sides are causing iniquitous mayhem as they rub lasciviously against each other. And as the more, erm, experienced will know… friction leads to all sorts of seismic events.
Map indicating the direction of motion of Earth’s tectonic plates. The red ‘teeth’ indicate where two plates are colliding, which, as we shall find out momentarily, has resulted in the formation of the magnificent Himalayan mountain range (continental collision) and Mariana’s trench (subduction zone). The first is home to the highest viewpoint on Earth (although you might kill yourself getting there) and the second, the very deepest point in Earth’s crust (although, again, you might kill yourself getting there).
1. Convergent Boundaries: When Two Plates Collide
If you drove your car at the rate of fingernail growth into a brick wall, you would have no idea what would happen because you would have gotten out long ago to use the toilet and get married (probably in that order). But hypothetically speaking, in the absence of arseholes to use and arseholes to marry, you’d probably discover that nothing very much would happen in a collision between a brick wall and your car moving at the rate of fingernail growth. Why? Because you’re going too slowly!
BUT! Substitute your car with a billion tonne megalith and that brick wall would be cement dust in… oh a few million years or so!
The convergent boundaries of Earth’s plates result in the formation all sorts of interesting topographical features. Two colliding plates can either become a subduction zone (where one plate – usually the denser one – plummets beneath the other one), or it can become a collision zone. The plate boundaries that are home to continental soil tend to opt for the latter, while the plate boundaries that are home to ocean soil, the former.
The coolest example of a continental fender bender on Earth has got to be the Himalayan mountain range, which is home to the world’s highest, most hostile and most abundantly body-strewn slopes. This formidable mountain range is the product of two continental tectonic plates (the Indian and Eurasian plate) crashing together and forcing each other to crumple and buckle into soaring mountain peaks and plummeting mountain valleys. There are more than 100 mountain peaks in the Himalayas that smash the 7,000 m (23,000 ft.) altitude mark. Mount Everest, the range’s and world’s largest mountain, comes in at 8,848 m… a staggering 29,029 ft. above sea level.
When two plates collide and the one happens to be heavier and denser than the other, it typically gets forced beneath the less dense plate. Imagine Paris Hilton gets into a fight with Natalie Portman. Who would come out on top? My vote would be on the substantially less dense (and Harvard degree-wielding) Miss Portman.
This kind of active plate boundary is known as a subduction zone and it can form deep-sea trenches that plunge for kilometres into the ocean floor, as well as yawningly vast abyssal plains that are home to a plethora of deep-sea squishies, only a fraction of which have had the pleasure of joining our taxonomy system. The remaining majority have not yet been discovered or named, although one did feature very briefly in the Pixar animated film, Finding Nemo.
The vertical antithesis of the Himalayas is Mariana’s Trench, a deep gash in Earth’s crust in the Mid-Pacific, directly east of Southeast Asia (refer to the map above). Here, the Pacific plate smashes into the Philippine Sea Plate and the former, which is composed of denser, more metal-rich rock than the crusty, silty continental latter, gets forced downwards. There are examples of mid-ocean trenches all over the world, but at 11,000 m (36,070 ft.), Mariana’s Trench is the very deepest. Not even an inverted Mount Everest could fill this gash.
That is a huge gash.
But wait, there’s more! One plate does not simply get sucked underneath another without the appropriate ceremony! Deep-sea trenches are very good and all, but we want fire and brimstone!
I’m so glad you asked…
The Ring of FIRE!
When one tectonic plate plummets beneath another, it faces the fiery wrath of the Earth’s immensely pressured mantle. This heat causes the hydrous (water-containing) minerals within the plate’s rock to release their moisture. Since water acts to lower the melting temperature, the mantle overlying the subducting plate melts (surprise!), sending plumes of magma towards the Earth’s surface.
These pockets of molten rock tend to become trapped underneath the crusty rock making up the lithosphere, where the pressure builds up. Eventually, all hell breaks loose and you get a volcanic eruption. This can occur either on the ocean floor or on land surface. Sub-aquatic volcanism tends to result in the formation of fiery, volcano-strewn islands, such as the Pacific Ring of Fire. Terrestrial volcanism tends to result in Pierce Brosnan being a hero and other awesome feats such as pyroclastic flows, earthquakes and village-bound lava lakes.
As long as the Earth’s tectonic plates are mobile, subduction will remain an ongoing process. The denser plate is continuously consumed by the continental plate, sending plume after plume of magma to the Earth’s surface, fuelling the ingoing wrath of these lithic pimples.
Stay Tuned for Part Two!
Want to find out what happens when a billion billion tonne slab of rock rubs against another billion billion tonne slab of rock? Things get seismic.