The Gros Ventre slide is an especially dramatic example of an important process that usually is more boring: mass movement. This is the name given to the downhill motion of rock, soil, debris, or other material when the flow is not primarily in wind or in a glacier, or in water (if the material is washed along by a river, we call it a river)
Water is usually involved in mass movement, however, and most mass movements occur when soil or rock is especially wet. Water helps cause mass movement for four reasons: 1) water makes the soil heavier; 2) water lubricates the motion of rocks past each other; 3) water partially floats rocks (a rock pushes down harder in the air than in water) so that the rocks in the water are not as tightly interlocked and can move more easily past each other; and 4) filling the spaces in soil with water removes the effect of water tension.
Number four, above, may deserve a bit more explanation. Think about going to the beach and building sandcastles. Dry sand makes a little pile with sides rising at maybe 30 degrees (steep, but not too steep; see the diagram below). Totally saturated (wet) sand flows easily, forming a pile with a much more gradual slope. But people making sandcastles want damp sand, which can hold up a vertical face. You can even make and throw damp sand balls (be careful where you throw them).
Now watch a demonstration of the process followed by a video explanation.
Video: Mass Movement and Sandcastles Demonstration (:55 seconds)
So, let's see what happens with piles of sand. This is a protractor and I'm adding a little playground sand here. This is dry sand and you'll see it makes a moderate slope. It's going to be about 30 degrees there, as you see on the protractor. Now I'll add a little bit of water, but we're just going to make it damp, not wet and you're not going to be impressed by my skills at making sand castles. But you know when you've seen a sand castle that you can now get a very steep slope. But if we take that sand castle and a wave comes in, the tide comes in, we pour a lot of water on it, you see what happens. It flattens way out. And so we have very different behaviors depending on whether it's dry or damp or wet.
Video: Mass Movement and Sandcastles Explanation (2:09 minutes)
So what's going on with this dry damp wet sand, really what's going on down in it. Well let's take a zoom in on the dry sand. These are individual grains of sand made bigger and they make a nice pile that is sort of like this one. You could have a pile like that underwater and totally wet, but the water supports some of the weight of the grains of sand. The water makes it easier for the sand grains to move over each other and so even a little shake or a little current tends to knock down the pile and give you something very broad and not at all steep on the edges such as what we show here. In between the damp case, water molecules tend to stick to other water molecules and they tend to stick to the sand. So when the water is sticking to the sand and the other water, it sort of forms like you see in this diagram. And if we zoom in, here's two grains with the water in between sticking to the sand grains and to the other water molecules. If you were trying to pull the sand grains apart, it takes a force because you either have to pull a sand grain or both sand grains out of the water and break that attraction or you have to pull the water apart and break that attraction. And so the strength of the water sticking to the sand can hold up a sand castle or it can hold up most of the hillside slopes on the earth, most of the time. So here are three dr,y which might give you a sand dune, and damp, which can give you really spectacular sand castles and holds up most of the slopes on Earth, most of the time. But when you make them really wet they may make landslides and run down the hill and that's not good if you're in the way.
The details of the surface physics involved are a bit complicated, but basically, a drop of water will sit at the junction of two sand grains. If you pull the sand grains apart, both grains will end up wet, so you had to “break” the water from one continuous film into two. There is a similarity to a dripping faucet. A water drop doesn’t fall off immediately but first becomes large and heavy. Water molecules stick to each other, and to the faucet, so strongly that they can hold up a large drop of water before it falls. (In situations such as this, the attraction of water molecules for each other is usually called surface tension.) Damp sand thus is strong—a landslide would require some sand grains to move rapidly past other sand grains, breaking the water bonds between the grains. In fully wet sand, however, the grains move more freely in the water without ever breaking it, so motion is easy. Hence, wind can blow dry sand into dunes, damp sand tends to stay where it is, but wet sand flows easily.
Classifications of Mass Movements
There are elaborate classifications of mass movements, depending on how fast, how wet, how coarse, how steep, and how "other" they are. Most of the names make sense: falls are rocks that fell off cliffs, topples are rocks that toppled over from cliffs, landslides, debris flows, and debris avalanches are fast-moving events, and slumps are something like a person slumping down in a chair (failures of blocks of soil along concave-up curved surfaces).
One fascinating and scary type of mass movement occurs in “quick” clays. You can read about these in the Enrichment. Quite literally, in certain places at certain special times, the foundations of a town built on sediments made of certain types of clay may liquefy and flow down the river, killing people. (Most people don’t need to worry about these, though!)
Enrichment
The quick clays that cause large, dangerous landslides generally start off as clay layers deposited rapidly in a shallow ocean, that then is raised above sea level. This often occurs near a melting ice sheet at the end of an ice age. The melting ice dumps a lot of sediment including a lot of clay, and then, as the weight of the ice is removed, the land rebounds above sea level. Clay particles tend to be platy and may look a little like playing cards. When these particles are deposited rapidly in the ocean, the particles may make a house-of-cards structure, with lots of big spaces. The saltwater supplies large ions that sit in the spaces and help hold the “cards” in position, something like little bits of glue helping hold up a house of cards.
After the clay is raised above sea level, rain supplies fresh water that slowly washes out the salt, like removing the glue that was holding up the house of cards. Eventually, a small disturbance may start a collapse, and this tends to make the clay “run away”, failing catastrophically from a solid to a liquid almost instantaneously, and generating a flow.
Flows from such clays are known especially from parts of Canada and Scandinavia. A quick clay failure at Saint Jean Vianney, Quebec in May 1971 destroyed 40 houses and killed 31 people in Canada, and a similar one at Nicolet, Quebec in 1955 killed 3 people. The Norwegian Geotechnical Institute released an amazing report and video about the Quick Clay Slide at Rissa in 1978; this is generally available online, if you search for it, and is truly fascinating. A man with a new (in 1978) camera filmed part of it but then had to run for his life as the slide expanded toward him. (When this was being written, you could find the video on YouTube and elsewhere.)
Sometimes, a quick clay slide will be small and will generate a flow that crosses a road. Bulldozing the clay out of the way does little good; more just flows across. But throwing a bag of salt into the flow near the road and driving a tracked vehicle through to mix the salt and clay may cause the flow to solidify so that it can be bulldozed away.