Geology of the National Parks

GeoMations and GeoClips


There is one GeoMation and there are four GeoClips available for your viewing in this unit. Hopefully, you'll find this helpful in understanding Unit 5 just a little bit better.


Click Here for Transcript of Redwoods Video

So out here there's the great Pacific Ocean. And the wind comes trucking in from the Pacific Ocean. And it's just going great guns except at some point it runs into the giant mountain range of the Sierra, which we know goes up over the top and down the other side and down to Death Valley.

And so when the air runs into that, the air has to rise. And you may know that when air has to rise, it expands. And when you have air expanding, whether it be out here on the Pacific or from a bicycle tire, it cools. And when the air cools that makes nice clouds. And when that makes clouds that makes rain that comes dripping down. And so sitting underneath that, as you might imagine, you have really wonderful trees called the Redwoods because it rains like crazy on them. And they're really happy with that.

Now the air is cooling and the cooling rate is something vaguely about three degrees Fahrenheit for each 1,000 feet that the air goes up. It should be five, five is the thermodynamics. But when the cooling causes condensation that makes the rain that we see, condensing actually gives up a little heat. And so you only get a cooling of about three degrees Fahrenheit per 1,000 feet going up.

Now when the air comes over the top and starts coming down the other side, there's no water in it to evaporate. There's no water there, it's dry. And when air is coming down like that and being squeezed, it ends up warming. And that warming is about five degrees Fahrenheit per 1,000 feet that it comes down.

And so it cools going up at about three degrees Fahrenheit per 1,000 feet. It warms coming down five degrees Fahrenheit per 1,000 feet. And that in turn means that because the mountains are really high, that if the air comes trucking in at something like 70 degrees Fahrenheit, by the time it goes over 15,000 foot high, it's almost 15,000 feet to the top of the Sierra, and the air has to get over, why when it comes back down here to Death Valley, it is 100 degrees Fahrenheit. And you really would be wiser to go visit in the winter rather than in the summer.

Credit: Dr. Richard Alley


We can measure the uplift of mountains, which may occur slowly, or suddenly in earthquakes, and we can watch volcanoes erupt. But overall, nature tears down mountains about as rapidly as they form, and we can watch and measure the tearing-down, too. The slow disappearance of names from old tombstones, the hubcap-rattling holes in late-winter city streets, and the maintenance budget for university buildings all attest to the effects of nature on human-made things. Here, Dave Witmer takes you to Bryce Canyon, one of the many, many places where you can see nature removing natural things.

Erosion at Bryce Canyon National Park

Erosion at Bryce Canyon National Park
Click Here for Transcript of Erosion at Bryce Canyon National Park Video

Hi, I'm standing here up at the top of Bryce Canyon. And right over here we have a great example of how quickly erosion takes place here. As you can see, the roots of this tree are exposed. And this tree is only about 100 years old, but it looks like it's trying to jump out of the ground. And that's because of the ground that was up here has been washed down the canyon exposing the tree's roots. Luckily for this tree, the roots go very deep into the ground to get all the moisture it can. And that's why it's still able to surviv.

Credit: Dr. Richard Alley

Geologists observe the wear-and-tear of nature on human-made and natural things, gaining clues to help understand how mountains are torn down. When climbing the sheer cliffs of Zion National Park into the mysterious crevice of Hidden Canyon, the intrepid hiker clings to a rather precarious-looking chain to avoid falling into the stream-carved potholes just beside the trail, and on down to the Virgin River, in the Canyon a hair-raising drop below. In these two clips, Dave Witmer and Dr. Anandakrishnan show how rocks are worn away, a little at a time, and what this has to do with south-Indian cuisine. You might begin thinking about what this wearing-away of rocks has to do with the Virgin River in the Canyon far below.

Chain of Events / Zion National Park

Chain of Events, Zion National Park
Click Here for Transcript of Chain of Events, Zion National Park Video

Hey Sridhar, look here.

What's going on there?

The chain that we are using to climb up the hillside here in Zion is actually causing some mechanical weathering. And you can see how everybody that grabs on to the chain as it goes up--

Ah, cool. Check it out.

It carves into here. It actually carves into the sandstone.

It's made these little scallop marks where the chains that are turned in this way go in a little bit deeper. And the chains that are out that way are back a little bit farther.

Yep. It would be just like a rock in a stream or even a glacier pushing stuff along. It's just scooping things up at a constant pressure along the side of the rock.

And so this stuff is actually fairly friable. It breaks apart reasonably easily. This chain's probably only been here 10, 15 years. And already it's cut in, what 1/25 of an inch, an 1/8 of an inch, something like that.

Credit: Dr. Richard Alley

Pothole Grinding / Zion National Park

Pothole Grinding, Zion National Park
Click Here for Transcript of Pothole Grinding, Zion National Park Video


So the water came along this way and carved out those holes coming around the bend. And then as it started to make the corner, it came down and it swirled around in these big holes. Probably, you can even see down there, there's some pebbles stuck in there and gravel size bits. And in a big flood, it would carry those size rocks and larger, bring them down.

And it's just like my grandmother grinding rice to make dosas and idlis which are these magnificent South Indian dishes, where you take rice, you put them down in a mortar and a pestle, and you just grind them around, add a little bit of water, grind them around. And you get this wonderful flour that comes out of it. That's the exact same thing happening here.

The water comes pounding down, carrying rocks, and it swirls them around because it's making the turn. And it's going fast enough that as it makes a turn it can't just go straight, it's got to curve around. It's just beautiful, it's magnificent. And we can even see some of the rocks left down in there from the last time we have a flood coming through here.

Credit: Dr. Richard Alley

Rocks and soil on hillsides really do slide down, whether rapidly or slowly, in big chunks or small ones. When a river erodes downward to make cliffs in the riverbank, or when a volcanic eruption or an earthquake makes a cliff, rocks fall or creep or slump or are washed off the steep slope, smoothing it over time. Here, Dr. Alley shows this for a tiny "canyon" in the bottom of the truly Grand Canyon.

Making a Sand Canyon / Grand Canyon National Park

Making a Sand Canyon, Grand Canyon National Park
Click Here for Transcript of Making a Sand Canyon, Grand Canyon National Park Video


If you make a canyon, and you deepen that canyon, the sides fall in. And one can see the sides avalanching in there merrily, eating back over time. See waves of adjustment going on. It's really beautiful to see.

I steepen it, and it steepens at the bottom. And then it eats its way back, and then it eats back at the head, eventually. Well, that's exactly what's going on here.

And so we can see across the river. The river is cut down. And then above it, there's that slope of rocks that have fallen off of the cliff. And as the river cuts down, that slope will be lowered and rocks will fall off of it and come avalanching down in exactly the same way as what we're seeing going on here in a very small scale. And you'd see all these little waves and nick points, and there are quite a number of very exciting things going on here.

Credit: Dr. Richard Alley

Want to see more?

Enrichment: More movies for you to enjoy—and these two won't be on a quiz. Erosion has carved fantastic forms from rocks, and weathering is important in loosening pieces to be transported away. Here, National Park Service Ranger Jan Stock and the CAUSE students explore weathering in Bryce National Park, and then Dr. Richard Alley explains changes at Delicate Arch in Arches National Park.

Bryce Windows and the Freeze/Thaw Cycle, (Transcript)

Forming Arches (Transcript)

Here are some optional animations you might also want to explore! (No, these won't be on the quiz!)

Soil Erosion
(An extensive collection of animations on this subject)

Mass Wasting/Landslide Animations
(An extensive collection of animations on this subject)