What is a Landslide and How Are They Caused?

Written by Emmanuel Kingsley
Updated: May 26, 2022
Image Credit iStock.com/Josephine Jullian
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Suppose you live in Alaska, Hawaii, the Appalachian Mountains, or the Rocky Mountains, you may have experienced landslides or seen the aftermath. Regardless of where they happen, all landslides have one thing in common: the terrain becomes changed or damaged. In most cases, homes, properties, and even towns are lost. In severe cases, people may die or get badly injured.

In this article, we will examine this unfriendly phenomenon called a landslide – what a landslide is, the likely causes, and ways to prevent it from disrupting life as you know it.

What is a Landslide?

Landslide is the movement of large deposits of soil, rocks, or debris down a slope.

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When large deposits of soil, rocks, or debris travel down a slope, the movement is called a landslide. Although the downward fall is mainly influenced by gravity, the terrain can also determine how easily the materials move. Landslides often occur on slopes. They can be caused by heavy rainfall, winds, erosion, rapidly melting snow, earthquakes, volcanoes, and human activity. 

Types of Landslides 

Landslides can be categorized according to the nature of their movement and the kind of material involved. 

Landslides involving rocks are called rock falls, while those involving debris are called debris flows. Then slides caused by volcanic eruptions, which often contain ash, are called lahars. Rock falls and debris flows are the most common. 

When a landslide involves heavy blocks or rocks, the movement is called a fall or a topple. This describes how rocks, boulders, or stones would fall one after the other when separated from a slope. Slides that happen when an earthquake shakes down the upper layer of a stable slope are called translational slides. The third type of slides under this category is a lateral spread. This is when the mass of material spreads sideways. It takes a strong force, like an earthquake, to move the substances very quickly in such a manner. 

How Do Landslides Happen?

When there is a change in the natural stability of a slope, landslides happen.

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A landslide happens when there is a change in the natural stability of a slope. Forces like heavy rainfalls, winds, or massive energy from volcanic eruptions can cause such disability. When a hill is compromised, the influence of gravity becomes more potent than the friction holding the rocks, soil, or debris together. For instance, big rocks will remain the same until a heavy volcanic eruption happens and blasts the rock to pieces or if constant rainfall erodes too much of the soil that helps it stay in place. The aftermath of such events is that the force of gravity will pull down the rocks till they reach the lowest point. The same thing can happen to debris from construction sites or demolitions. 

Where Do Landslides Happen the Most?

The Appalachian Mountains, the Rocky Mountains, and Pacific Coastal Ranges have the most landslides.

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Landslides happen in all states across the US territory. However, the Appalachian Mountains, the Rocky Mountains, and Pacific Coastal Ranges have the most landslides. The analysis of the United States Geological Survey states that Hawaii, Alaska, Oregon, Washington, and California are more prone to landslides.

Alaska, Hawaii, and the Appalachian mountains and Rocky mountains are prone to heavy storms and strong winds. As a result, these areas experience average to severe landslides. Washington and California are regarded as landslide-prone due to rapid snowmelt. In fact, one of the deadliest landslides in the world’s history was triggered by a mountain explosion in Washington in 1980. 

Causes of Landslides: Natural Causes

Landslides are typically natural disasters that have always existed on the planet. Some of the most significant landslides are estimated to have happened several million years ago. They can occur after any of the following: rapid snowmelt, storms, volcano, rainfall, droughts, earthquakes, and repeat slides.

Earthquakes and rainfall are the most common natural causes of landslides. When there are earthquakes in places with steep slopes, it upsets the balance of soil, vegetation, or other materials in the area and could cause a landslide. 

Causes of Landslides: Human Causes

Many human activities can trigger landslides. Construction work, poor agricultural practices, and disturbance of old landslides. Often, constructions are undertaken without consideration for the terrain or slope grading. Where poorly planned drainage alterations or house constructions without proper slope grading is done, the slopes weaken or destabilize. There could be changes in water circulations and the creation of slopes which will, in turn, trigger landslides. 

Other practices like the indiscreet cutting of hills and trees can aggravate landslide events. Trees, in particular, serve as an anchor for soil. They prevent rain and floods from sweeping off too much soil. Taking out trees (deforestation) affects the stability and makes the land more prone to landslides. Disturbing old landslide sights could also trigger new ones. There have been several cases of repeat landslides resulting from debris flow. 

Significant Landslide Occurrences in History

Statistics from the United States Geological Survey show that an average of 25 to 50 people die from landslides in the US every year. These events occur rather frequently worldwide, although in various degrees and with far-reaching consequences. The aftermath of a landslide ultimately depends on the size and its location, as densely populated areas often result in more casualties.

The largest recorded landslide in the United States happened after a volcanic eruption in Mount St. Helen, Washington in 1980. The force of the explosion caused blasted rocks and ash to spread. The volume of the material was 2.8 cubic km (0.67 cubic miles) and it traveled down for 22.5 km (14 miles). About 57 people were killed during this event, and several animals in the region died.

The 1983 Utah Slide takes the position of the most expensive landslide in US history. It is also the worst natural disaster to have ever happened in Utah. It happened in Thistle following deep winter snow and rapid snowmelt. The slide created a 160 feet deep lake, which resulted in a flood that wiped out the entire town. At the time, damages were estimated to cost between 200 and 400 million dollars. Due to this tragic event, Thistle is a ghost town to date. 

The largest-ever known landslide occurred almost 50 million years ago in Wyoming. Still, its effects are very much evident to date. It is known as the Heart Mountain landslide. A 1600-feet thick limestone covering over 400 square miles detached and began a descent. As it moved, it broke into several small pieces, which are now scattered across about 1300 square miles. This is the largest ever landslide recognized by geologists. However, much bigger landslides may have occurred in earlier geologic history.

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