Live Talk Archive
Geysers Galore
by Park Ranger George Heinz
Presented Live Online in September 2008
Park Ranger George Heinz: My name is George Heinz. I’m a naturalist or interpretive ranger here in Yellowstone. A big question about Yellowstone is why do we have all of this stuff here? Why are there geysers? Why is there hot water pouring out of the ground? Why do we have it here when you probably don’t have that in your backyard? What’s different about Yellowstone?
Audience member: A volcano.
Ranger George: A volcano! So many people come here, and they spend a few days, and they go home, and they don’t realize they just spent their vacation on top of one of the world’s largest volcanoes. Is it active? I see hot water pouring out of the ground. I would bet you, it’s probably active. Yellowstone is one of, and maybe the, largest volcanoes on the planet. This is a massive volcano.
It’s a little different than most of the volcanoes we think of. When we think of volcanoes we think of that little shape, a strato-volcano. Yellowstone is a little different. Yellowstone is more like the Hawaiian volcanoes. But here in the continental United States where do we find most of our volcanoes? The west coast. Those volcanoes formed differently than the Yellowstone volcano.
This is very much simplified, but it really has to do with the weight of rocks, individual types of rocks. All up and down the west coast the Pacific Ocean floor is made up of basalt. Basalt is an extrusive igneous rock. It’s an igneous rock, liquid rock came to the surface and it flowed as lava. So it is extrusive. It made it to the outside of the planet. Basalt is heavy.
There are three main types of igneous extrusive rock; basalt, andesite, and rhyolite. We have a lot of rhyolite. The mountain behind me, the mountain in front of me, those are lava flows made of rhyolite. It’s rich in silica. It’s a very important ingredient for thermal features. But up and down the west coast, you have that heavy ocean floor and it’s banging into the continent. The continent is made up of intrusive igneous rock – granite.
Granite is igneous rock. It is liquid rock. It is trying to come to the surface and it never makes it. It starts to cool before it breaks the surface. So it’s intrusive. It’s inside the planet. It cools very slowly over hundreds and thousands and thousands of years. It cools so slowly that it forms little crystals and impurities and it is light. Granite is much lighter, intrusive rock, than the extrusive rock.
All up and down the west coast you have the continents floating around and you have this ocean floor banging into each other all up and down the west coast. The heavy ocean floor gets pushed below the continent. It’s called a subduction zone. So right up and down the west coast, when that basalt gets pushed down into the planet, it starts to melt. Volcanoes form right above that subduction zone. So you have Mt. Rainier, Mt. Saint Helens, Mt Hood, Mt. Baker, Mt. Shasta, Mt. Lassen – about 50 active volcanoes in North America. They are formed mostly by subduction zones.
Yellowstone’s volcano was formed by a hot spot, a lot like the Hawaiian volcanoes. Our continent is flowing over this hot spot. North America moves a couple inches a year in a southwest direction. So that raises some questions about the shape of the plume of liquid rock under us. So there are a bunch of geologic questions we don’t have the answer for, but we do know that 3 to 6 miles below you there is liquid rock; somewhere in there. So there’s magma right below us.
And this little diagram here – I’m gonna let that camera see it for a second – over here, this is the west, you see Yellowstone up here with these yellow circles. That is representing the hotspot below Yellowstone. Sixteen million years ago the hotspot was below the west coast. The continent has flowed over that hotspot. So right now, it’s below Yellowstone. Yellowstone has had 3 major volcanic eruptions in about the last 2.1 million years. The last one was 640,000 years ago. Geologically speaking, we might be due for a volcanic eruption. If we are, you might as well stay here and get a ride out of the deal because it is going to change the weather on the whole planet. It will throw so much stuff in the atmosphere that the temperatures around the planet will change. Growing seasons will change and things like that. But if you talk about geologic time, if it happens in ten thousand years, that’s pretty close, geologically speaking. It’s not going to affect us, right now. So that’s why we have them.
(Let me see the caldera there.) What happened here in Yellowstone is as that hot spot is underneath us, the ground above that hot spot started to bubble up. The analogy I use is that it’s a lot like a pie. You can tell I’m from Louisville, Kentucky when I say “pie”. It’s a lot like when your grandmother made a pie. She got a pan out. She put a crust in. She put some filling in and she put a crust over the top. But what did she do with that pie before she put it in the oven? She had to poke holes in it. We weren’t around here to poke holes in our big old pie so we got a big old eruption.
Eventually cracks and fissures started to form on the outside of what we call the caldera which is outlined in orange there. This is the map you got at the gate. It has your receipt stapled to it so it’s probably on your dashboard and you probably haven’t opened it. But it will help you get around Yellowstone if you look at it.
This is our caldera so once that pie, little fissures and cracks started forming on the outside of that rim, lava started to flow out, and eventually a big portion of Yellowstone became hollow, the ground sank, and formed a caldera, which is a sunken volcanic crater. It is about 35 by 40 miles across. So it’s a massive thing.
One reason you can’t drive around Yellowstone and tell where the crater is, unless you know what to look for, is because lava flows continued until about 70,000 years ago. They filled in most of that caldera. The lava flow behind me is that ridge and that’s a lava flow in front of me. They’re from different dates but they’ve filled in this section of the caldera. You see that the geyser basin is squeezed right between two lava flows.
So that’s why we have them. We’re on top of this giant volcano. Yellowstone has about 10,000 thermal features and 300 of those are geysers. There are only 600 geysers in the world. So half the geysers in the world are here. And then there are 150 geysers in this geyser basin, the Upper Geyser Basin. So within walking distance of you are ¼ of the world’s geysers. This is an amazing place right here.
What happens is Yellowstone has a lot of earthquakes. We’ve already talked about its being a volcano, we have igneous rock here, well we have about 2000 earthquakes a year here. Those earthquakes keep shaking the ground and they form little cracks and fissures in the volcanic rock here. The water that is being heated by the heat source of the volcano – heat rises so once the water gets heated it wants to come back to the surface. So that hot water starts coming back to the surface and as it’s going through this rhyolite rock, which is rich in silica, it’s dissolving that rock, and it’s pulling minerals out of the rock.
As the water rises it starts to cool and there is less pressure. As it gets closer the pressure drops, the water gets cooler as it’s further from the heat source, those minerals start to precipitate back out because cooler water does not hold the same amount of minerals as the hotter water, so those minerals are coating those fissures that were formed by the earthquakes. They form plumbing systems. Eventually that mineral, the white rock you see out here, it’s a silica-based – call it siliceous sinter or geyserite – it’s just silica that’s being redeposited by the thermal water.
Every now and then a hot spring, if it’s hot enough, and the minerals are enough, somewhere in that plumbing system a constriction will form – a tight spot. This is the inside of Old Faithful. I’ll turn it in a minute. I’m gonna let that camera up there look at it. The top up here, that’s the surface of the ground. We stuck a little camera down in Old Faithful some years back, a little-bitty guy on a rope and we got it to almost 50 feet. The water was so hot down there. It was churning. We couldn’t go any further. But we noticed at 22 feet - you look right in here and I’ll show you this after the program if you want to look at it – right at 22 feet there’s a constriction, a tight spot. It’s only a little over 4 inches wide, 22 feet down. There’s going to be 8,000 gallons of water that are going to be pushed up through that 4-inch hole.
So what happens is there’s water up here. There’s a constriction. There’s water below it. The water below it is in a giant chamber and that water is getting hotter and hotter and hotter. It’s under extreme high pressure. It’s like a pressure cooker. So you can go way above the boiling point without boiling because of all the stuff above it; the water, the atmosphere and all that. We noticed, when we were waiting for Old Faithful to go off, you noticed it started to spit a little water. Everybody jumps up like it’s going to go, then they sit back down, you wait a few minutes, it spits a little water. Every time it spits a little water out of that upper chamber it’s dropping the pressure on the lower chamber. Spits a little more water, drops the pressure. Spits a little more water, drops the pressure. Eventually that water down below is able to boil and when it boils it expands about 1500 times. So you had this giant pocket of superheated water that’s flashing into steam and the only place for it to go is straight up. So it starts coming up through the plumbing system. All the water that’s above it gets thrown into the air. Then the steam pocket gets out, the geyser stops.
Old Faithful is not the biggest geyser here. It’s about number 5. You have Giant Geyser, Grand Geyser, Beehive Geyser, and Giantess Geyser. They’re all bigger than Old Faithful when they erupt. Grand just went a little while ago. Beehive went this morning. The other, you know, who knows when they go. Giantess is on Geyser Hill over here – that steam right before you get to the trees – goes once or twice a year, a great big geyser.
That’s how geysers work. They’re like a giant pressure cooker. Once that lower pocket of water is able to turn to steam it throws everything else out of its way. That’s why we have them and thank you for listening. In just a couple of minutes now, at quarter til, I’m going to do one more talk. This last talk is going to be on the personality and myths of some individual geysers, mostly on Geyser Hill over there. We’re going to start that in just a couple of minutes.
Thanks for watching at home.
APPLAUSE
Audience: Thank you.
Ranger George: You’re welcome. Thanks for listening.
