Soil Creep
The most important mass-movement type in terms of transferring material downhill is soil creep, the slow (typically inches, or centimeters, per year or less) downslope motion of soil. Creep may be just a very slow landslide. It may occur from freeze-thaw processes—a column of ice that grows under a small pebble on a cold night pushes that pebble out from the hillslope, and the pebble falls straight down when the ice melts, effectively moving a tiny distance down the hill (see the video above). When trees fall over and uproot soil, or when groundhogs and even worms dig up rock grains and allow them to move downhill, creep is occurring. If you look at a typical hill slope, streams on the lower slopes are present to move water and rock downhill, but the upper slopes lack streams. There, soil creep moves the material downhill.
Naturally, hillslopes typically reach a balance, in which weathering breaks down rocks about as rapidly as mass movement and streams take the broken rocks away. The balance may occur with bare rock sticking out (making cliffs, for example), or with a lot of soil covering the rock. If soil creep dominates the mass movement, the hillslope may be close to balance at all times. If landslides dominate, then the soil will build up for a while before suddenly sliding off, and you have to watch for a long time to see the balance. Over a very long time, the hill will usually get flatter, causing the mass movement to slow. However, the soil will very gradually thicken to slow the weathering as the hillslope is reduced, and near-balance will be maintained.
Humans are greatly upsetting this balance worldwide. Our activities—bulldozing, cutting trees whose roots held the soil, plowing, and more—are moving more material than nature moved before we were involved. Landslides are becoming more common, and causing more damage as we build in more dangerous areas. Soil erosion has increased from our farm fields, making it harder for us to feed ourselves. We could slow or reverse many of these damaging trends if we decided to work at it.
Video: Soil Creep/Frost Heave (1:59)
There are many processes that move soil and loose rocks slowly downhills as soil creep. One of them we're going to show you here. We've drawn for you a hill with a pebble, just a little piece of rock, less than a half an inch or so. There would be lots of pebbles on this hill. Now sometimes on a cold night, ice will grow beneath the pebble and it will push it away from the hill as is shown here. This is a picture of how that process looks in the real world. So this is ice and it has shoved these pebbles up by about one inch from a little hillside. These are pine needles and these are pieces of leaves and bark. So, you can see this is at Colyer Lake in Central Pennsylvania. Now what will happen next is that the ice will melt during a warm day and when the ice melts the pebble will tend to fall down. And the net from that will be that the pebble has moved a little bit down the hill in a day or so. Later in the semester we will see that this process also happens to bring rocks out of the soil, up to the surface, so they can then be moved downhill. Pennsylvania's hillsides had a lot of this process during the Ice Age when we were in a permafrost climate. And our hillsides now are covered with big rocks that will twist your ankle if you're hiking if you're not careful and this is part of soil creep.
Here is a simplistic diagram. See if you can describe what is happening to a friend and then take a look at some truly amazing landslides from around the globe.
A Slideshow of Landslides Around the Globe
Optional Enrichment
These will not be on the quiz but might prove interesting. First, let's start with a video of some amazing landslides.
Video: Top 5 Massive landslide Caught on Camera (10:40)
You can hear background noise, but no true conversation.
And now a retro video about one of the National Park's most iconic elements.
Video: Formation of Delicate Arch (2:40)
Here we are at Delicate Arch. With the possible exception of Old Faithful, this is the most famous feature, the symbol of the parks. This is one of the very many, more than 2,000 arches, that are in Arches National Park, and it shares with them the same origin. These are not carved by running water shooting underneath them. Nor are they carved by wind blasting a hole through it. They're carved mostly by some very odd processes.
Down below, there's a layer of salt that was deposited in a sea a long time ago that was drying up here. Salt, when it's down deep and there's something sitting on top of it, is soft, and it flows. And the salt has flowed into sort of a mound, something like a lava lamp bulging up, and that mound has warped the rocks. And if you take rocks and sort of warp them up, they crack. And so there are these cracks that have made standing vertical walls of rock.
Well, the next thing that happens, if we look at that running along towards the bottom, there's a line on both sides of it. That's a soft layer, and it weakens things. Well, when you start undercutting a cliff-- we have these cliffs, and they get undercut by water leaking out along these little cracks-- then what happens is pieces fall off. And enough pieces fall off that eventually one of the pieces breaks through, and then it's an arch.
This arch is not long for the world. We see on the left, about halfway up, how thin it's getting. And we also see at the top a whole bunch of cracks that are developing. That layer across the top is trying to sag, and as it does, there are cracks that are forming. And some of those cracks sort of look like Pennsylvania keystone sitting up there.
And so it might last thousands of years. It might go in the next big storm. But it is not terribly long for the world. We know that many of the prominent and famous features in the park have changed in the time people have been watching. It will be a loss, but an educational one, when this very famous feature of the Park Service also changes.
Soil Erosion
(An extensive collection of animations on this subject)
Mass Wasting/Landslide Animations
(An extensive collection of animations on this subject)