Mass movements range from small to huge movements, slow to fast and less dangerous to catastrophic movements. These are some of the hazardous and often catastrophic mass movements that leave a trail of destruction behind.
Most hazardous mass movements are caused by heavy rainfalls, earth movements and human activities on slopes. Tectonic areas and tropical areas are most at risk from hazardous mass wasting.
Avalanche
Avalanches are one of the deadliest and catastrophic mass movements moving at speeds of more than 300km/hr. Avalanches are capable of killing thousands and destroy anything in their wakes from humans, vehicles, trees or even change the slope shape.
Avalanches are a mixture of debris and snow racing down a mountain slope. Some less disastrous avalanches have a powdery snow appearance while huge ones involve slabs of ice or a glacier ice moving downslope.
Several factors can trigger avalanches such as earthquakes, volcanic eruptions, torrential rainfalls or even vibrations from human activities, vehicles and skiers. Tectonic movements are the most triggers of Avalanches making mountainous regions such as the Andes (S. America), Rockies (West. USA) and Alps (Europe) dangerous.
In 1975, Peru, 18 000 people were killed by an avalanche in the Andes mountains. The avalanche travelled 17km in 15 minutes destroying nine towns in the Santa Valley.
Landslides
A landslide involves a block of soil (land) sliding down a slip plane. Landslides are caused by water lubricating lines of weakness causing en mass movement of material downslope.
Settlements on or below slopes are severely impacted and buried. For example, the 2005 LaConchita landslide, California, USA which killed 10 people and destroyed several houses below the slope.
Several slums (favelas) built on unstable slopes in Brazil are destroyed and buried by landslides killing hundreds of people or more if they’re asleep. Frequent landslides occur in Brazil.
On March 1, at least 100 people were killed by a landslide in Bududa, Uganda following heavy downpours that lasted seven hours.
Rotational slides
Rotational slides or slumps are similar to landslides except that they occur over a curved plane and blocks of land (soil) rest on top of another.
Mudflows
Mudflows are a result of soil, especially cohesive clays, being soaked in water causing rapid movement downslope. Mudflows are mainly triggered by heavy downpours saturating and increasing soil volume.
They’re are capable of destroying people and burying houses in their paths. The 2019, Cyclone Idai caused widespread mudflows and killed or buried hundreds in Eastern Zimbabwe and Mozambique.
Landslides sometimes can convert to mudflows if there’s more water.
Lahars
A volcanic version of mudflows called a lahar is one of the hazardous mass movements. A lahar is a mixture of volcanic debris and water or snow. These mass movements are common in volcanic regions such as the Andes, Alps, Mt St Helens and many others.
In 1985, Colombia, 25000 people in the town of Armero and other surrounding areas were killed by a lahar which was triggered by an eruption of volcano Ruiz. The 1980 Mt St Helens (west USA) eruption also generated huge lahars.
Rockfalls
As the name suggests, rockfalls involve large boulders falling down from a scarp. Earth movements, weathering and torrential rainfalls influence and triggers rockfalls.
Weathering such as frost action along joints can cause rocks to crumble down. Heavy rainfalls can also weather down and corrodes lines of weakness triggering rockfalls.These mass movements have the ability to crash humans, settlements and cars.
Rockslides
Rockslides involve boulders sliding down a bedding plane on rocks such as limestone. They’re less hazardous than other mass movements above as they impact often isolated areas.
