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Plate Tectonics: The Ends (and Beginnings) of the Earth, Part 1

Earth from Space

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?

Chicken or the eggWell, 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.

Earth's plate tectonics

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.

Earth's plate tectonics 2

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.

Continental Collisions

Plate tectonics, collisionThe 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.

Himalayan Mountain RangeTypical and totally average view on a hike through the Himalayas

Subduction Zones

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.

Location of mariana trench map 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. 

Marianas Trench depth reference

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!

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.

Oceanic volcanism subduction 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.

Seismic events - Earthquake in Japan Stay tuned for next week’s blog instalment – Plate Tectonics: the Ends (and Beginnings) of the Earth, Part 2.

Goodness, Gracious Great Balls of Ice!

Large hailstone damage

Some things on our planet are so ridiculous that when you really think about them, it’s enough to make you go biblical. Frogs falling from the sky, crop circles, giant swirling hurricanes, belching volcanoes, sulphur-based life forms and Paris Hilton’s immense wealth (and equally as immense lack of IQ). And then there’s hail. The fact that the updrafts within a thunderstorm can be strong enough to hold grapefruit-sized hail in suspension is nothing but ridiculous and wholly impressive.

Great balls of ice!

How Hail is Made 

Falling hail thunderstorms

Hail consists of balls of ice shockingly called “hailstones”. You may even say that hail is frozen rain, but it deserves a slightly more complex explanation than that…

Hail is made within powerful thunderstorms or cold fronts. Cold fronts tend to produce smaller hail that might inconvenience your dog’s plans to go do his business outside (thereby inconveniencing your plans to keep your house hygienic). The large hail responsible for denting cars, destroying crops and severely upsetting your heard of cows is typically associated with large thunderstorm systems that are well-endowed in the vertical and are sustained by powerful updrafts. These traits are especially exhibited by the “Big Daddy” of all small-scale tempests: supercell thunderstorms. These you will find all over the world, but most notoriously skipping across “Tornado Alley” during the northern hemisphere’s summer months.

Supercell thunderstorm
Supercell thunderstorm with rotating mesocyclone (*swoon!*). The presence of such large frozen water particles within the cloud selectively reflects light towards the lower energy (green) end of the color spectrum, which is why thunderstorms that produce large hail tend to make the sky appear a ghostly green.

What cold fronts and thunderstorms have in common is that they are both low pressure systems that suck in air and expell it out their rear. Thunderstorms pull in great volumes of warm and moist air, which rise, cool and condense to form towering cloudy behemoths. Yes, cumulonimbus clouds. The air, once cooled, loses its momentum and proceeds to sink towards the ground. Together, these two channels of air comprise the updraft and downdraft zones that sustain a thunderstorm: its lungs if you’ll indulge a bit of poetic licence.

Now, as you know, temperature decreases with height in the atmosphere. That’s why the tops of high mountains are frozen and it’s why you should always, ALWAYS go for a pee before sky diving. At a certain altitude within a thunderstorm, which can soar to as high as the interface between the troposphere and stratosphere at approximately 10 km above sea level, the temperature reaches zero degrees Celsuis – the temperature at which water freezes. Above this 0°C isotherm (an obnoxious way of saying “line of equal temperature”) all the water droplets in suspension are frozen.

The strong updrafts within a thunderstorm sweep water droplets above the 0°C isotherm where they freeze (consult the pretty diagram below). These pellets of ice then fall back down towards Earth in the downdraft zone, plummeting below the 0°C isotherm and defrosting into big globs of water. This is why thunderstorm rain gets you soaking wet in 10 seconds flat. Just like Channing Tatum in “Magic Mike”.

hail-formation-diagram

However, some of these falling frozen pellets of rain get caught up in the updraft zone again and are swept back up above the 0°C isotherm. Only, they’ve gained a layer of water, which they collected as condensation while chilling out below the 0°C isotherm. This additional layer of moisture freezes, forming a new layer of ice over the original ice pellet.

Concentric layers of ice_hailstone
Concentric layers of ice in a hailstone. Just kidding! It’s a microscopic image of a bacterium’s nipple.

This process can repeat itself several times and each time, the hailstone will grow larger and larger and larger as it collects more and more layers of ice. The next time you’re in the middle of a raging supercell storm, run outside, collect a couple of decent-sized hailstones, run back to the tornado shelter, bolt the trapdoor, watch your dad arm wrestle said trapdoor with an F5 tornado, watch your dad lose, resolve to become a hardcore white vest-wearing, tornado chasing sexpot with a serious deathwish. Oh! And remember those hailstones you collected? Cut them open to see those concentric circles of icy awesomeness.

When a hailstone finally gets too heavy for the thunderstorm’s updrafts to hold in suspension depends entirely on the strength of those updrafts. The stronger they are, the heavier the hailstones. This is why larger hailstones are associated with powerful thunderstorms, such as the Midwest supercells that are sustained by incredibly strong updraft zones.

And when hailstones get heavy, it’s time to run for cover.

large hailstone damage

 Sorry Boys… Size Really Does Matter

Farmers are more obsessed with size than that clutch of vacuous floozies and jockstraps in Jersey Shore. Considering their livelihood depends on it (and not their egos), this is easy to understand and empathize with. But, in no other aspect are they more obsessed with size than with hail. The happiness and health of their livestock and crops depend on it.

Some thunderstorms can create hailstones that are big enough to cave your head in. Even if you do have brains. The next time you’re at a party, scoop an ice cube out your rum and coke and toss it at your mate (preferably the one who’s hitting on your girlfriend). Listen to the dulcet sounds of squealing as it clobbers him in the noggin. Now imagine something easily ten times the size of that ice cube falling thousands of metres (or feet) from the heavens. Yup! Ouch.

Largest hailstone on record
Ermagherd! Ferkerng HUGE herlsterne!

On 23rd June 2010, the largest hailstone in recorded American meteorological history fell in Vivian, South Dakota (image above). This great ball of ice weighed in at 0.88 kg (1.93 lbs) and was a staggering (if it had hit you in the head) 20 cm (8 inches) in diameter.

That’s two inches longer than your average you-know-what, tee hee!

 Class Dismissed: Your Take-Home Message

hail_storms-on-road

Hailstones are physical evidence of the incredible air circulations going on inside a thunderstorm. Can you imagine how strong air must be to prevent something that weighs almost a kilogram from succumbing to gravity? I don’t know about you, but that blows my mind in the most delicious way. And so we see that thunderstorms are about so much more than just thunder and lightning and the occasional airborne cow. These bad tempered weather systems can also be that jerk at a party who throws ice at you.

But, then again, you were chatting up his cherry.

The Countdown Begins

An online course, a broken arm and some serious inertia have kept me from enlightening and entertaining you with Why? Because Science. Only one of those reasons is a decent excuse, as broken limbs tend to be. This very morning, I woke up at 03h30 with an idea burning inside my head and a terrible yearning in my chest. So, I pulled out the laptop and began to write. Furiously so.

On Monday 13th May, the beat drops dubstep style and I will be returning. Why? Because Science is back, bitches…

Crazy lady scientist