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

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: $fluctuating temperatures.$

Freeze-thaw action diagram

During the day, $water$ seeps into cracks in the rock
At night, $temperatures fall$ below freezing point. The water in the cracks $freezes$ and $expands$ by $9%$ , putting $pressure$ on the rock.
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
$Permeable rock: rainwater can pass through the rock$

Carbonation and karst landscapes

Karst landscapes: areas where limestone rock is exposed to carbonation

Limestone pavement

The surface of a karst landscape: $limestone pavemen$ t
$Rainwater$ 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

$Gradient$

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

W $ater content$

$Water$ 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.

$Human activity$

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

$Vegetation$

$Grass$ 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

$Animals$

$Burrowing$ 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 $heavy rainfal$ l 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.