Vector diagram indicating the Earth’s major tectonic plates, the boundaries between them and the direction of their motion. A vector is an arrow that physicists use to illustrate the direction AND relative speed of a moving body. I.e. the longer the arrow, the faster a plate moves.
Welcome back to this, the second instalment of our foray into the field of plate tectonics in which we seek to understand how the giant bumping and grinding shards of crust that make up the surface of our planet have helped to shape it, create it, destroy it and give Hollywood directors endless material for disaster movies. In Part 1, we began our journey with a look at convergent boundaries – where two tectonics plates come together causing a fender bender of such epic proportions that it has resulted in some of the highest (Himalayas) and deepest (Mariana’s Trench) topographical features on Earth.
We discussed the difference between continental collisions (where two continental plates crash into each other) and subduction zones (where one denser oceanic plate gets “pushed” underneath a lighter, crustier plate). Both are characterised by plates that are slowly, yet inexorably colliding into each other and both result in some totally awesome environmental features, such as soaring mountain ranges, plummeting ocean floors, city-shattering earthquakes and volcanoes with monstrous cases of indigestion.
In this week’s blog, we’ll take a look at the other boundary types and what kind of geological party one might expect to find there…
2. Divergent Boundaries: When Two Plates Pull Apart
At the opposite end of a plate’s convergent boundary, one tends to find a divergent boundary. Here, the prodigious convection currents in the Earth’s asthenosphere (the squishy onion layer beneath the crusty lithosphere) serve to wrench the two plates apart. This exposes the bubbly mess of searing molten rock beneath. For the same reason you want to sew your butt cheeks together when you have a really bad case of “Delhi Belly”, this runny mess of lithic indigestion explodes out from between the plates causing all sort of fun for the neighbouring wildlife.
Diagram illustrating what happens at a divergent pla…
Oh, Christ you’ve got eyes.
There are typically two geological features one finds at divergent plate boundaries and just as was the case with tectonic convergence, the resultant landscape depends very much on whether the plates pulling apart make up the continents or the ocean floor.
The Bold and Ridiculously defined Temporomandibularly Muscled
When the separation occurs between two oceanic plates, as is the case with the African and South American plate (in the southern Atlantic basin) and the Eurasian and North American plates (in the northern Atlantic basin), you get a mid-oceanic ridge, which doesn’t really look like Ronn Moss posing in the exquisite turquoise waters of some tropic paradise. No, mid-oceanic ridges are a lot bigger, a lot more ripped and far more complex, although perhaps not as emotionally so… and definitely not as annoyingly successful with the ladies.
Whoar! Now that’s what I’m talking about! The Mid-Atlantic ridge in all its ripped glory. More fault lines than hairs on a manly Portuguese chest, more complex than your girlfriend at her best time of the month and more broodingly seismic than her temper after that fight you had when you commented on her Portuguese heritage.
Two plates can’t get away with divorce without some serious repercussions. For one, the divergent motion of the plates releases a whole lot of pressure on the underlying asthenosphere. It subsequently melts in relief, releasing a surface-bound flood of molten rock known as magma, or at least until it actually reaches the Earth’s surface, at which point it becomes known as lava.
Don’t ask me why geologists have to make things so complicated.
This lava cools and solidifies upon contact with the atmosphere or, in the case of mid-ocean ridges, the overlying water, forming blocky solid structures of igneous rock. Over time, the release of magma from the divergent motion of the plates forms wave after wave of new ground in a process referred to as “seafloor spreading”. This all explains why the age of the rock closest to a plate boundary is younger than the rock as little as 100 metres away! Cool, huh?
Those bulbous rocky rocks are actually solidified magma (or lava) plumes, which have emerged from the depths of the central abyss.
Some of the attractions one might expect to see on a routine exploration of a mid-oceanic ridge include deep gorges and valleys and formidable submarine mountain ranges that are, in height, taller than Mount Everest. When you’re not “oohing” and “aahing” at the fantastic topography, you can “ugh” at the local wildlife.
This sexy sock-face with nipples for eyes is actually a Deep-sea Pompeii worm, which typically hangs out near the hydrothermal vent chimneys found along marine divergent boundaries. This large sea squishy enjoys black smokers, long walks along the trench and its ambient environment close to boiling point. Hydrothermal Vent Eelpout fish, Giant Tubeworm and the Hydrothermal Squat Lobster are more examples of wildlife that find boiling water totally amenable. In fact, there is a whole community of specialised critters that have become adapted to life in close proximity to blistering, incandescent volcanic vents.
When tectonic divergence occurs between two continental plates, rift valleys can form. East Africa provides us with a beautiful example of this in the shockingly named “East Africa Rift Valley.” I mean, how left field can you get? Here, the splitting apart of the Somalia and Arabian portion of the African plate has caused the ground to sink in a complex series of fault lines. The resultant synclines (fancy geology speak for “valley” or “dip”) can become filled with water, as is the case with Lake Malawi, Lake Tanganyika and Lake Victoria… some of the oldest, deepest and largest lakes in the world.
A bird eye’s view of Lake Malawi: just one of the major bodies of water formed by the divergent motion of two Africa plates causing a physical cleft in the landscape.
“Hold on,” you say. You’ve referred back to the map of the world’s major tectonic plates and there isn’t a plate boundary anywhere near East Africa.
“How observant you are!” I exclaim saccharinely…
The Africa plate is in the process of splitting into two, like a giant amoeba or your mother’s personality when she drinks too much gin. The plate to the east of the Rift Valley is the Somali Plate and the one to the west is the Nubian or Arabian Plate (check out the diagram below). These two crusty offspring are referred to as “protoplates” or “subplates”.
What other exciting attractions do rift valleys have to offer us other than very old, very large and very deep lakes? Seismic activity of course, which includes all manner of fire, brimstone, earthquakes and highly specialized organisms that have adapted to the heat and the strange chemical environment found around aquatic volcanic vents.
3. Transform Boundaries: Where Two Plates Rub Together
We’ve looked at convergent and divergent plate boundaries, but what happens along the peripheries of the plate if the “front” is having a head-on collision and the “back” is being torn asunder?
If your guess was a great idea for a blue film, I commend you on your filthy mind. However, “transform fault” was more along the lines of what I looking for.
Transform boundaries are characterised by two plates grinding past each other. Since jagged rock rarely slides easily past jagged rock, this fault line tends to be the source of much rocking and rolling in the Earth’s crust. Every now and then – which is painfully slowly in geological time – one plate gets snagged on the other and they are brought to a strained halt. The pressure mounts as the one plate tries in vain to move on, but is held back emotionally by the other, until, in a sudden Earth-shattering shudder, they become unsnagged, sending the plates shooting past each other.
This is precisely why transform faults are notorious for causing earthquakes. One of the best-known examples of such a boundary is California’s San Andreas Fault (image below), which is currently – as we speak – being torn asunder by the divergent motion of the North American and Pacific plate.
San Andreas transform fault, California
San Andreas fault is also testament to just how stupid humans can be… building a massive city on a fundamentally unstable Earth foundation is a disaster movie begging to be scripted and cast with slack-jawed hunky men and big-breasted, blue-eyed blondes. Although, if you are a film director and find yourself being inspired by this, please consider casting me as the clip-board wielding, surprisingly young, yet double PhD-educated science floozy! I may not have blonde hair, but you know what they say…
You can easily sleep with a blonde, but a brunette will keep you up all night long.
Mila Kunis is scientific evidence of this fact.
The disturbing reality about San Andreas fault is that it’s been 107 years since a major earthquake has occurred, which means that all these long years, the pressure between the plates has been building. Sure, there has been a smattering of decent earthquakes in between the 1906 San Francisco event and the present day – the most recent being the 6.0 magnitude Parkfield earthquake of 2004.
Don’t get me wrong, a 6.0 magnitude will leave your martini shaken and not stirred, but according to the latest Uniform California Earthquake Rupture Forecast (kind of like a weather forecast, but for earthquakes), California has a 99.7% chance of experiencing a larger than 6.7 magnitude earthquake in the next 30 years! I.e. you can bank on it.
It gets worse: the chance that this earthquake could achieve a magnitude of 7.5 or more is a frightening 46%. This may seem like a paltry percentage at first, but if your tandem buddy had to suddenly turned to you on a sky dive and tell you there was a 46% chance the parachute wouldn’t unfurl, you’d most definitely soil your undergarments. You can bank on that, too.
Could the next “Big One” finally send San Francisco into sliding into the sea? Is “Frisco” about to become the next city of Atlanta?
Who can say? Only time… and the underlying tectonic plates. Not Enya.
Class Dismissed: Your Take-Home Message
Plate tectonics play an incredible large-scale role in shaping the surface of our planet. Of course there is a myriad of smaller scale (both spatially and temporally speaking) factors that mould the mountains you climb over, the oceans you swim across and the valleys you… bungee jump across?… to be with the one you love.
But, plate tectonics are the daddy of global scale change and transformation.
Be in awe!
So very in awe…