Weathering and Mass Movement

Weathering is the breaking down of rock and soil that are exposed to the weather. The broken-down material does not move from the place where the weathering occurred.

There are 2 types of weathering:

• Mechanical weathering

• Chemical weathering

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Erosion

Erosion is the breaking-down of rocks and soil and transportation of the erosion material.

Erosion is caused by:

• Moving water: rivers and seas
• Moving ice: glaciers
• Moving air: wind

Mechanical weathering

Mechanical weathering breaks up rocks into smaller pieces by putting pressure on the rock.

Example of mechanical weathering: Freeze-thaw action

Freeze-thaw action

Freeze-thaw action occurs high up in mountainous areas where there is a lot of precipitation (rain, sleet and snow) and the temperatures regularly rise above and fall below freezing point.

Temperatures that repeatedly rise and fall: fluctuating temperatures.

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1. During the day, water seeps into cracks in the rock
2. At night, temperatures fall below freezing point. The water in the cracks freezes and expands by 9%, putting pressure on the rock.
3. The process continues over time, widening the cracks and joints in the rock , causing it to weaken, and eventually pieces break off. The broken-down rock is known as scree. Scree often builds up at the base of a slope.

Chemical weathering

Chemical weathering is when rocks are dissolved because of a chemical reaction.

Example of chemical weathering: Carbonation

Carbonation

• The atmosphere contains a gas called carbon dioxide (CO2)

• Rainwater (H2O) mixes with the CO2 to form a weak carbonic acid (H2CO3)

• The weak carbonic acid has a huge effect on limestone rock beacause limestone contains 80% calcium carbonate

• The weak carbonic acid reacts with the calcium carbonate in the limestone and dissolves it

• Limestone is a permeable rock

• Permeable rock: rainwater can pass through the rock

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Carbonation and karst landscapes

Karst landscapes: areas where limestone rock is exposed to carbonation

Limestone pavement

• The surface of a karst landscape: limestone pavement
• Rainwater falls onto the exposed limestone and passes through vertical joints or cracks in the limestone pavement
• Carbonation makes the joints wider by dissolving the calcium carbonate in the limestone until they then look like grooves or gaps in the limestone. These grooves of gaps are called grikes
• The blocks left in between the grikes are called clints
• Joints: long vertical cracks in limestone rock

Swallow hole

How a swallow whole is formed:

• As a river flows over the limestone, the water widens the joints in the rock through carbonation
• Eventually the river disappears from the surface and begins to flow underground along the bedding planes
• Bedding planes: the horizontal paces between the layers or strata in limestone
• Carbonation and the physical force of the moving water, called hydraulic action, enlarge the joints and bedding planes
• The place where the river disappears underground is called a swallow hole
• The river continues to flow underground, help to form underground karst features such as passages and caves

Caves and caverns

As a river flow underground through swallow holes they enlarge the passage through which they flow

• This occurs by the processes of carbonation and hydraulic action, the physical force of the moving water
• These processes can wide the spaces underground to create caves and caverns, such as the Aillwee Caves in the burren, Co. Clare

Stalactites

As the water seeps through the rock, it carries dissolved limestone with it. Some of the water eventually reaches the roof of a cave or a cavern below the ground.

• The drops slowly evaporate and leave behind small deposits of pure limestone called calcite
• Gradually the calcite builds up to form icicle-like shapes called stalactites
• Stalactites: hang from the roof

Stalagmites

Drops of water may also seep through the rock and fall on the ground

• The water evaporates and leave deposits of calcite on the floor of the caves directly below the stalactites
• The calcite builds up, forming stalagmites
• Stalagmites: on the floor

Pillars

After thousands of years a stalactite and a stalagmite can meet and join, resulting in the formation of a pillar or a column

Mass Movement

Weathering and erosion produce loose material called regolith. Mass movement is when regolith moves down a slope due to gravity.

Factors that effect mass movement

1. Gradient

   Gradient: Steepness of the slope. The steeper of the slope the faster the movement of the regolith

2. Water content

   Water content makes regolith heavier. Water also acts as a lubricant, which makes the movement of the regolith easier. Heavy rain can speed up mass movement.

3. Human activity

   People sometimes dig into slopes in upland area for quarrying, mining and construction => wind turbines and roads. Digging into the slope makes the slope steeper => steep gradient

4. Vegetation

   Grass and shrubs protect the soil from weathering and erosion. The roots of the plants bind the soil and keeps it stable to prevent mass movement

5. Animals

   Burrowing animals can loosen soil, making it unstable. Overgrazing removes vegetation cover exposing the soil.

Types of Mass movement

Mass movement is classified by the speed at which it happens. There are slow => soil creep, and fast => bog bursts, mudflows, landslides and avalanches

Soil creep

Soil creep is the movement of soil down a slope under the influence of gravity. It is the slowest form of mass movement. It becomes evident only when you examine the landscape closely:

• Trees grow at an angle, with the base of the tree turned downslope.
• As the soil moves downslope, a series of steps called terracettes can form, giving the surface a wrinkled appearance.
• Fences bend and telegraph poles tilt downhill.
• Walls crack and break when soil piles up behind them.

Bog bursts

A bog burst, or bog slide, occur when a mass of bog or peat moves down a slope after a period of heavy rainfall. The peat becomes so saturated that water can no longer soak downwards that makes it unstable, so the peat moves downslope and can block roads, knock down trees, damage road and buildings, and polute lakes and rivers

Mudflows

Mudflows occur when soil and regolith become saturated with water after periods of heavy rainfall and move downslope like a river of mud. They are one of the fastest forms of mass movement and can reach speeds of over 100 kph

Mudflows can also occur in the aftermath of a volcano in snowy regions, a volcanic eruption will cause snow and ice to melt quickly. This meltwater mixes with ash, soil and rock fragments to create a particularly dangerous kind of mudflow known as a lahar

Landslides

A landslide is the rapid movement of regolith down a steep slope that has become unstable. Causes of landslides include coastal erosion, deforestation, heavy rainfall, earthquakes and undercutting by road building or quarrying

A landslide caused by heavy rainfall in south-west China in June 2017 buried 62 homes and more than a hundred people. Thousands of rescue workers with sniffer dogs and life-detection equipment searched in the rubble for the missing people. They also worked on unblocking the local river and roads that had been filled with material from the landslide

The lack of vegetation on the hillside made the disaster much worse than it might have been.

Avalanches

An avalanche is the rapid movement of snow and ice downslope when the weight of the snow is too much for the slope to hold.

An avalanche in a ski resort in the Italian Alps killed three skiers in March 2017. The skiers were skiing off-piste through fresh and unstable snow. Heavy snowfalls and strong winds probably caused the avalanche.

Controlling Mass Movement

Humans can try to control natural disasters caused by mass movement. These hazards can be reduced or prevented in the following ways.

• Vegetation such as trees and vines can be planted on slopes to stabilise them.
• Steps can be built into a mountainside to trap moving material.
• Restricting overgrazing keeps vegetation cover and stops soil creep.
• Controlled explosions in mountainous areas can be used to trigger small avalanches, making large, life-threatening avalanches less likely.

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