Giant things that explode! It doesn't get better than this.
The Discovery Channel has a great article about the magma plume under the Yellowstone Caldera.
This is some way, way cool stuff for several reasons, but let's start with the basics. I love volcanoes. So far, they are my number one favorite thing in geology. Because, as said above, it just doesn't get any better than giant things that explode.
Most of us are used to imagining classic Composite volcanoes, like Mount St. Helens in Washington state or Mount Fuji of Japan, well known for its presence in a multitude of paintings and wood cuts. Composite volcanoes look like ordinary mountains, normally with a crater on top, and they go boom in a very explosive manner. I'll write more about composite volcanoes and why they like to explode at a later time.
The other really common type of volcano that you might be familiar with are shield volcanoes. Shield volcanoes are much, much bigger than composite volcanoes - in fact, they are the biggest volcanoes not just in the world, but in our solar system. (Olympus Mons on Mars is a shield volcano.) Instead of having steep slopes and that classic mountain shape, they're more like gentle domes. Imagine a round shield, like the kind you see in mosaics of Greek warriors. Lay that shield flat on the floor, and you get the general shape of these volcanoes. Shield volcanoes are another thing I'd like to write about some time. They don't explode like their smaller composite cousins, but they're very active. And, after all, the Hawaiian Islands are all shield volcanoes (though most of them are inactive now) so it's geology in action!
The kind of volcano I want to talk about now is in a category by itself, though. I've heard them called Caldera Volcanoes (which is a little misleading, and I'll explain why), Supercaldera Volcanoes, Super Volcanoes, and VEI-8 Volcanoes. Whatever you want to call them, they're massive. VEI-8 is probably the most scientifically correct way to classify them.
VEI stands for the "Volcanic Explosivity Index" - yes, it's a classification of how big the boom is when the volcano erupts - and 8 is as high as it goes. A VEI-8 volcano dumps out at least 1000 cubic kilometers of of gas, ash, and lava. Just to give you an idea of how big we're talking here, in 1980 when Mount St. Helens erupted (blowing one entire side of the mountain off in the process), its output was a mere 1.2 cubic kilometers. (Just as a note, 1980 eruption in question was VEI-5; VEI is an exponential scale, just like the Richter Scale is for earthquakes, which means VEI-8 is at least 1000 times more powerful than VEI-5.) This is the sort of eruption that will mess up the world climate for years to come, as well as devestate everything near it.
So what does this have to do with Yellowstone?
The big valley that makes up most of Yellowstone National Park is actually the caldera of one of these VEI-8/Super Volcanoes. There are actually several of these super massive (but thankfully defunct!) volcanoes in the United States. Yellowstone is the one we're interest in, however.
A caldera is formed when a volcano collapses in on itself. Most volcanoes have a crater at their summit - or even several, if the volcano is really big, like some of the shield volcanoes. A caldera is much, much larger than these craters. If you make a cone out of paper and cut just the very end off, that would be a volcanic crater. If you cut the cone in half, the new opening would be like a volcanic caldera.
Sometimes volcanoes collapse slowly, due to subsidence. But sometimes the collapse occurs during an eruption or series of eruptions. This makes the eruption far more explosive and dangerous as enormous amounts of solid rock go crashing down into the magma within the volcano. Imagine having a bowl filled entirely with water. If you dropped a handful of rocks into it, the water would explode out everywhere. It's the same idea, but even more powerful - because in the volcano, all of the magam is already under a lot of pressure.
So, when you come down to it, any volcano can have a caldera. All it has to do is collapse in on itself. But to give you an idea of relative size, a recent caldera formation occured at Mount Pinatubo in the Phillipines when it erupted (at VEI-6) in 1991. Mount Pinatubo's caldera is 2.5 kilometers wide. The Yellowstone Caldera is 55 kilometers by 72 kilometers.
This all adds up to a HUGE picture of Yellowstone.
Most volcanoes in the world sit on some kind of tectonic plate boundary. (Mount St. Helens sits where the Pacific plate is ground under the North American plate.) In some cases, though, aren't on these plate boundaries. They sit on top of mantle plumes, which feed them magma. A mantle plume is like a gigantic bubble of molten rock that rises up from the mantle, heading toward the surface. The Hawaiian Islands are fed by a mantle plume. The article that I linked to at the beginning of this is talking about the mantle plume that feeds Yellowstone. The heat from it feeds the geysers and hot springs.
The good news is that the last time the Yellowstone Caldera hasn't erupted with that big, VEI-8 boom in the last 640,000 years. Every 20,000 years ago, it'll have a little steam eruption (little being a relative term - the one that occured about 13,000 years ago left a crater in the caldera 5 kilometers across!), and it'll have non-explosive lava flows on occasion. The most recent of these occured 70,000 years ago. So don't worry, the giant that sleeps beneath Yellowstone shows no signs of waking.
I've never been to Yellowstone National Park, myself. The family vacations I went on never took us in that direction. Some day I want to go there, though, to camp and explore. And when I go, I'll be walking across the mouth of an ancient, sleeping volcano. That somehow makes it all the more beautiful.