Plate Tectonics

Remember on our tour of the Earth’s interior that I mentioned the earth’s lithosphere is made up of distinct plates? Let’s take a deeper look at that concept because it has a lot to do with how volcanoes work.

The Earth’s crust is made up of different “plates” similar to segments of eggshell on a cracked egg. They fit together like individual pieces of a jigsaw puzzle. There are 15 major plates and a bunch of minor ones.

Here’s the list:

African Plate
Antarctic Plate
Arabian Plate
Caribbean Plate
Cocos Plate
Eurasian Plate
Indian Plate
Indo-Australian Plate
Juan de Fuca Plate
Nazca Plate
North American Plate
Pacific Plate
Phillipine Sea Plate
Scotia Plate
South American Plate

Do better with a map? Me too. Check this one out:

Graphic created by USGS

But these plates don’t just take life sitting still – they like to move it, move it!

The convective currents down in the mantle act as a wheel of sorts. Imagine a conveyer belt. The belt sits on top of a series of wheels and when those wheels start spinning, the belt moves in one direction. There are many “wheels” in the mantle, and not all of them move in the same way. This causes the plates that are sitting on top of them to move in different directions and interact with each other in interesting ways.

The area along the edge of a plate is called a plate boundary, or plate margin. This is where most of the action happens as the plates come in contact with each other. Here are the 3 major ways that plates can interact with each other:

  • Convergent Margins
    Convergent margins are where two plates meet head on and collide with each other. One of two things are going to happen when they run into each other like this:
    1) The plates will both be stubborn as hell and will push against each other until they both buckle upward. This is what causes some of the more spectacular mountain ranges around the world, such as the Alps and the Himalayas. For example, Mt. Everest grows about 4 mm per year because of this constant pushing upward.

    2) One of the two plates will yeild, like in an arm wrestling match, and it’s usually the one that is the denser and heavier of the two. It will slide under the other plate – or subduct – in what is called a subduction zone. While it happens all around the world, the most notable place where this occurs is in the Pacific Ocean in an area called the Ring of Fire.  The Ring of Fire encircles the Pacific Ocean. Check out the map below.  The hot pink squiqqle shows the general area of this Ring of Fire.

    It starts down in New Zealand and arcs upward through Indonesia and Japan, circling up through Russia and Alaska, down the western coast of the United States and further down the coast of South America to Deception Island in Antarctica. This ring is the most active area on the planet when it comes to earthquakes and volcanoes. In fact, 75% of all the action happens right here.What makes it so active? The earthquakes come from the plate margins bumping into each other like they’re dancing at a rave. At the subduction zones, the rock that is being pushed under the plate begins to heat up and melt. The molten rock then shoots up through weak spots in the Earth’s crust and along the plate margins, creating volcanoes.
  • Divergent Margins
    Where you have plates crashing into each other, you also can have 2 plate boundaries that are moving away from each other. Perhaps the best and largest example of this is the Mid-Atlantic Ridge. This ridge runs right down the center of the Atlantic Ocean and the plates on either side of it are moving apart.  This action from the Mid-Atlantic ridge is actually inspired the theory of continental drift and plate tectonics. If you look on a globe at the shape of Africa and South America, sure, they’re not perfect, but they look like they could have fit together at one point like two puzzle pieces. This observation made one man, Alfred Wegener, curious. Were the two continents butted up against each other at one point? And, if so, why aren’t they now? Wegener founded the theory of continental drift and through years of research, scientists have substantiated many parts of this theory. We can now actually measure the rate at which those plates are spreading apart. South America does move further away from Africa every year. Granted, it’s a maddeningly slow process (a few inches a year, which is about the speed at which your fingernails grow), but it is As the plates continue to separate, the magma beneath is allowed through and it comes spurting to the surface, which in turn, creates new sections of crust. Iceland sits directly on top of this ridge, and its spunky volcanoes are a direct result of this divergent plate action.
  • Transform Margins
    Here you have two plates that move side by side, parallel to each other. Not much action going on here until one of the plates moves suddenly. Then you have a bit of rock and roll to dance to, and quite a bit of damage to contend with.

 While these plates can sometimes move rather freely, more often than not they get caught against each other. This creates a ton of pressure. It’s a bit like a rubber band being stretched. And stretched… Eventually it won’t take the pressure anymore and it will snap, releasing all that tension and energy that was built up. Plates do the same thing. They build up pressure and tension as they lock against each other. When it gets to be too much the plates slip, releasing all that built up pressure in the form of an earthquake. Some of the slips are so minor that we never even feel the earthquake, even if we do detect in on our seismographs. Others, however, can be so violent that it devastates the homes and landscapes nearby.

95% of all volcanic action can be found at these plate boundaries. Conditions here are perfect for volcanism to occur. The motion of the plates creates some fantastic rock melt in the lithosphere and asthenosphere, and there is ample opportunity for that magma to shoot right back up to the surface. This creates some awesome, and explosive, volcanoes.

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