Have you ever noticed that the coast is a very dynamic area? Do you realize that coastal landforms are constantly changing? You may have re-visited a stretch of coast after a long time and observed debris at the foot of the cliff, or you may have seen the cave in a headland grow larger. These changes are the result of various processes. One such set of processes that continuously works to alter the coast and its landforms is mass movement. To learn more about the other processes that affect coastal landforms, read our explanations on Waves and Weathering.
Mass Movement Definition (Geography)
Before diving into the types of mass movement, you must understand what mass movement is. In geography, a mass movement is the movement of material downslope due to gravity. Rainwater usually acts as a lubricant in a mass movement. The extent of the weathering on the slope impacts the scale of the mass movement.
Mass movement may be fast or slow, depending on the following:
- The angle of the slope
- The type of material
- The structure of the rocks.
Mass movement is also sometimes called mass wasting.
Coastal Mass Movement
Mass movement is a critical process along the coast that influences the formation of coastal landforms. Coastal mass movement influences the rate of coastal retreat and contributes to coastal erosion. When a coastal cliff or slope is continuously eroded and weathered at its base, it becomes weak. In its weakened state, the base cannot support all the material above it, thereby making the cliff or slope susceptible to the force of gravity and mass movement. This is aided by rainwater, which lubricates the cliff/slope material. When mass movement causes the material to move downhill, the result is that the cliff or slope moves backwards.
Effects of mass movement
The material removed by mass movement is transported by the ocean and deposited along other parts of the coastline. Therefore, mass movement plays a major role in the development of both coastal erosion landforms and coastal depositional landforms.
In addition to altering the coast, coastal mass movement can have negative physical, social and economic impacts. Some of these are:
- Damage to or loss of land
- Damage to or loss of property
- Damage to or loss of infrastructure
- Injury to people
- Unstable ground surfaces
Types of Mass Movement
The types of mass movement that you need to know are:
- Slumping
- Sliding – Landslides
- Rockfalls
Slumping Mass Movement
In slumping, the soil or rock debris moves downhill along a concave or curved plane in a rotational manner. The upper part of the slump moves vertically downward, but the lower part moves outward, forming what is known as a toe at the bottom of the slump area.
Slumping occurs where permeable rocks overlay impermeable rocks, such as clay. Rainfall penetrates the permeable rocks, causing them to become saturated and heavy. The rain also causes the lubrication of the top of the impermeable layer. The extra weight of the permeable layer causes a curved plane to be formed. The rock debris and soil then slump along this plane.
Mass Movement Sliding
A slide occurs when a portion of the soil or rock along a steep slope suddenly gives away and moves downhill, usually aided by heavy rainfall. Sliding mass movement occurs on slopes weakened by weathering such as landslides, rockslides, and mudslides. We will look specifically at landslides.
Let's distinguish between rockslides and mudflows:
Rock slides refer to sliding mass movements where large amounts of rocks rolling down a cliff.
Mudslides are when loose, water-saturated soil slides downhill, usually during heavy rainfall.
Landslides
As stated above, a landslide is one type of sliding mass movement. A landslide is a sudden, rapid, downslope movement of large blocks of rock debris and soil along a definite or slide plane.
A definite or slide plane is a line of weakness in the rock, such as a fault or a bedding plane.
Landslides occur when the rock and soil are saturated with water from either rainfall or repeated wetting by waves on slopes at the coast. The rock becomes heavier due to the addition of water and begins to fall along the plane. Landslides occur on slopes with a steep gradient and are common in areas:
- Of high relief
- Where rocks are being undercut, e.g., at the coast
- Where water gets into joints in the rocks
Fun Fact: The largest landslide ever recorded occurred in 1980 after the eruption of the Mt. St. Helens volcano in the United States. The entire north side of the volcano, estimated at a volume of 2.8km3 in material, slid downhill.
Mount St. Helens today, where you can still see evidence of the massive landslide-Roy Luck, CC-BY-2.0, Wikimedia Commons
Rockfalls
A rockfall is the fastest moving type of mass movement where rocks break away from the cliff face and fall, bounce or roll downslope. Rock falls are typical in areas that experience freeze-thaw mechanical weathering, which causes chunks of rock to lose contact with the cliff face and fall. The fallen rocks form a scree slope at the base of the cliff.
Examples of Mass Movement
Now that we know the different types of mass movement, it's time for us to examine some examples of where a mass movement has occurred.
Slumping
Hornsea is a coastal town located in East Riding of Yorkshire. The area to the south of the sea defences is prone to slumping because the beach area is not expansive enough to protect the cliffs from the sea. Major slumping occurred here in 2008, where 8.75 m of land slumped. In 2019, about 50 m of the cliff slumped. In this latest slumping event, the scar left by the slump was initially 95 cm deep but grew to 300 cm within a week. Slumping in this area has resulted in coastal cliff retreat. As a result of the constant slumping, some people have had to relocate their holiday homes which are located along the cliff, as there is the fear of them being lost due to the slumping mass movement. The image below shows the extent of the slumping.
Evidence of slumping near houses along the Hornsea coastline - Ian S, CC BY-SA-2.0 Wikimedia Commons
Landslides
Sidestrand is a village which is located along the Norfolk coast. The coastline cliffs in the village are prone to landslides, which may also be accompanied by mudflows. Heavy rains in June 2019 resulted in a landslide which caused a large portion of the cliff to collapse onto the narrow beach and ocean below. After the landslide, The Sidestrand Hall School, which is located on the land above the cliff, had to have a survey completed to determine whether its perimeter fence would need to be moved.
Example of a landslide along the coast at Sidestrand, Norfolk- Evelyn Simak, CC-BY- SA-2.0, Wikimedia Commons
Rockfalls
In April 2021, the UK experienced its biggest rockfall in 60 years, when 300 m of the cliff along the Jurassic Coast in Dorset collapsed onto the beach below. It was estimated that approximately 4,000 tonnes of material fell as debris. At the time of the rockfall, persons were warned to stay away from the area as further cliff failure was expected. The impact of rockfalls like this is that they help to change and shape the coastline.
Mass Movement - Key takeaways
- Mass movement is the downhill movement of material under the force of gravity.
- Coastal mass movement contributes to developing both coastal depositional and coastal erosion landforms.
- Slumping, landslides and rockfalls are three types of mass movement.
- Rockfalls are the fastest type of mass movement.
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