Howdy everyone and welcome to module 12 where we will finally explore landslides which involves pretty much all of Earth's spheres. So it's a great topic to end on. In this first video, we're just going to introduce the topic and talk about all the different types of mass movements. Slopes are a very common element in our physical landscape. So, it's important that we spend at least a module discussing these landscapes and some of the destructive processes occurring here. Slopes can vary greatly. Some can be steep, moderate, or gentle. Some slopes are long and gradual, like the slopes we saw along a shield volcano, but others can be more short and abrupt, like a cliff. Slopes can be rocky and contain rubble, while others can be covered by soil and vegetation. Now, mass movements, which we'll spend a significant amount of time on in this module, refers to the downslope movement of rock, regalith, and soil under the direct influence of gravity. And these things can occur on varying scales at varying speeds. A landslide is a sudden event where large quantities of rock and soil move down steep slopes. They are considered hazards when people and communities are in their way. The term landslide really has no specific definition in geology, but it is the term we are most familiar with when talking about downslope movement of rock and soil. Mass movements are a geological process that often follows weathering. Mass movements are related to weathering because it provides material that can be moved downhill by these various mass movements and eventually transported out to sea. The combined effects of mass movements and running water, which was a topic of last module, produces stream valleys, and they are the most common and conspicuous of Earth's landforms. Here's an example of mass movements and running water working together to form the beautiful Grand Canyon. The walls of the canyon extend far from the channel of the Colorado River. This results primarily from the transfer of weathered debris downs slope to the river and its many tributaries by mass movement processes. Slopes can change through time though. Most mass movements occur in areas of rugged and geologically young mountains. As mountain building subsides, mass movements and erosional processes like running water will begin to lower the land, causing the steep and rugged mountains to eventually transform into gentler, more subdued terrain. So, why should we study landslides and mass movements? Well, they are important in shaping the landscape around us, but they also pose a significant hazard to us, which I already kind of brought up. In fact, hundreds are killed every year by landslides, especially mudslides, which we'll talk about in the next video. They can happen almost anywhere on steep or gentle slopes, and even above and underwater. Property and roads are damaged and destroyed by landslides every year, even here in Texas. Individual landslide events have killed thousands of people. The deadliest landslide in world history occurred in 1920 in China, killing over a 100,000 people. This event consisted of 675 massive slides that were triggered by an earthquake. Here's a photo of an unstable slope that is an extreme threat to fail near Yakima, Washington. Can you see where it's starting to fail? In 2020, this landslide in Norway killed 10 people and damaged many homes. Several of these homes were actually swept out at sea. Landslides are a major risk in our country as well. Here's a photo from a 2018 landslide in Ventura County, California. On January 17th, 2018, 20 people were killed by mudslides in California. These photos are from the Santa Barbara County Fire and Search and Rescue. Above shows a rescue dog searching for survivors, and at the right shows first responders rescuing a 14-year-old girl from this destroyed home. Here's a photo from the Santa Barbara County Fire Department that shows the awesome destructive power of landslides. This was once a car. You probably wouldn't be able to tell what it was without the wheel. Here's a photo of a mudslide outside Guatemala City that killed at least 280 people in 2015. Here's another one from Colombia that same year. Here is a landslide triggered by an earthquake in 2016 in Japan, leaving a prominent scar in landscape. Here's another landslide caused by that same earthquake showing a highway destroyed and another landslide triggered by the earthquake bearing a major road on June 28th. 8, 2016, a massive landslide occurred at Glacier Bay National Park in Alaska, where part of an entire mountainside collapsed. The amount of debris is estimated to be about 120 million metric tons, which would be like 60 million midsize SUVs sliding down a mountain. Here's a photo showing a house damaged by a landslide in Design, Washington in 2015. Here's a landslide that threatened apartment buildings of 16 families in Granberry, Texas in 2016. So to recap, a mass movement or mass wasting event is the downslope movement of rock, regalith, or soil. It also contains sediment, debris, snow and ice, all due to gravity. A landslide is the rapid downslope mass movement of rock or soil as a more or less coherent mass. Mass movements are a natural process that's often increased by human activity. All types of movements though are ultimately controlled by gravity, which is important because it might be a question on your activity and quiz. Landslides and other types of mass movements are natural hazards. They can either occur rapidly and catastrophically or they can occur slowly over long periods of time, but they almost always cause economic damage even if they don't cause injuries or deaths. Next, we'll take a look at a catastrophic example. In the last module, we talked a little bit about dams and some consequences if they fail. Here we will discuss a dam failure that was the direct result of a landslide. The Veant Dam in the Italian Alps is in a concrete arch dam that is 260 m high which is like an 85story building. The rock layers around this dam are tilted in towards the reservoir. Heavy rains and a rising water table began to make the sides of the canyon very unstable, which resulted in a landslide on October 9th of 1963 when 700,000 cubic meters of rock and sediment, which weighed 600 million tons, slit into the lake in all about 10 minutes. Here's a cross-section parallel to the face of the dam before the landslide. Here at the center is our reservoir sitting behind the dam. You can see that the rock layers on either side are dipping in towards the reservoir. This red line right here is where the slope actually failed along this boundary between the shell above and the limestone below. The dotted line shows the surface of the slide debris after the event which completely filled up the reservoir. But where did all that water go? Here's another cross-section, but this time showing the level of the water table. The dark blue is the level of the water table before those heavy rains, which is usually intersecting the surface at the top of where we have the reservoir behind the dam. However, these high um rainfall event caused the the water table to rise as water was quickly infiltrated into the soil. And this light blue is the water table after the saturation event. You can see how much it had rapidly rose and you can see the table elevated quite drastically which ultimately led to the failure of the slopes. The photo shows the slide scar in the landscape. It was the south wall of the valley that failed completely, causing a mass that was 1.1 miles long and 1 miles wide to slide to the bottom of the reservoir with a volume of over 240 million cubic meters. This mass hit the bottom, and when it did, it actually caused an earthquake that was felt all over Europe. This slide was a fast one, moving at 68 mph. And basically, a blast of air and a wall of water displaced by the slide that was 780 ft high washed up and over the dam and rushed down the valley. A wall of water of up to 230 ft high smashed into the town of the Leroni, killing more than 2,000 people. Surprisingly, the dam survived and was left still standing even today. Here's a map showing the dam and the landslide mass. Here's where the dam stands and still stands today. And this is the entire landslide mass that completely filled that reservoir behind it. Now, this area in this teal blue color along the river that is outlined by this dotted line that shows the resulting flood which completely inundated Voyant and Longeroni as well as impacting several other towns along the river. You can actually still see the dam standing today. And here's a current photo of the Veant Dam showing the slide scars and debris that filled the reservoir. So landslides and mass wasting are extremely important to understand. Here is a map of landslide hazards in our own country with red being very high risk which correlate with some of our major mountain belts and rivers. This zone in Texas correlates to the geology where we have a lot of exposed beds of shale which is a weaker rock and limestone which is easily dissolved. Geology as well as the topography plays a critical role in these hazards. All right, in the next video we will be getting into the different types of mass movements and some videos that show them in action. I'll see you there.