Weathering and mass movement

By Simone Pash

Mechanical weathering

It is the physical break down of rocks by wind, water, cold and heat or known as environmental factors.

Nature is always active so mechanical weathering is constantly happening very slowly.

We can feel temperature fluctuate but we can't see the rocks expanding and contracting which cause the rocks to weaken by making fractures and breaking down.

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Thermal expansion


Is when temperature fluctuations effect rocks which cause the minerals in rocks to expand and contract from the temperature at different rates. This then causes stress on rocks to form small cracks and gradually break down.

From thermal expansion, Grus are created by the accumulation of coarse-grained and loose fragments left behind from weathering. This is the direct result of physical breakdown and weakening of the rocks.

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Frost shattering


Is a type of mechanical weathering that breakdown the rocks by the expansion of ice and the formation of tiny ice crystals in the rocks crevices. When the temperature goes warm, water then trickles into the cracks and pores of rocks and when the tempertures cool again the water freezes and expands. When the frost is created the rocks widen and shatter portions of the rock.

In coastal areas the water could contain salt that if the water evaporates it can form salt crystals in the cracks that expand and when exposed to heat can become wider.

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Plant Activity


It is the crevices in rocks that provide enough room for living organisms to enter and grow causing another way to mechanically weather. The roots of plants and trees grow into cracks within the rocks that makes the cracks widen and pushes the pieces of rocks apart.

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Abrasion


It is the breakdown of rocks by friction. Stationary rocks are scraped away by loose rock and mineral fragments which happen by air or water directly hitting the stationary rocks in rivers.

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Chemical weathering

It is when rocks are Broken down by chemical reactions like water, oxygen, carbon dioxide and acids that can also alter the minerals in rocks. Differerent types of chemical weathering are hydrolysis, oxidation, carbonation, acid rain and the acids produced by lichens. After the chemical weathering has taken place there is a different substance than the one we started with.
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Hydrolysis


Is the chemical breakdown of a substance combined with water, for example the minerals in the rocks with rain water. The most common hydrolysis is feldspar in granite changing to clay from rain water causing it to weaken.

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Oxidation


Is the reaction of a substance with oxygen, that can cause rust when contain iron. The iron reacts with oxygen forming iron oxide that weakens and crumbles the rocks easier making them look red.

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Carbonation


It is the mixing of water with carbon dioxide in rocks to make carbonic acid, which is the formation of caves. The dissolved carbon dioxide in rainwater or in moist air, forms carbonic acid which reacts with the minerals. The mineral calcite , common in limestone is vulnerable to carbonation causing it to get washed away and hollow to form a cave.

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Lichens and acid rain


Lichens are a combination of fungi and algae that's grown on rocks which produces acids that break down the minerals in rocks. When the lichens are mixed with water it produces a weak acid that's common in rivers and streams. The lichens are similar to acid rain which is air made acidic by certain pollutants. When Sulfur dioxide, carbon dioxide and nitrogen dioxide are produced by burning fossil fuels are realeased into the air mixing with the moisture, they from acids that fall with the rain.

The factors that effect the rate of weathering

Exposure to the atmosphere


The rocks that are covered by the ground are relatively protected from environmental elements that mechanically weather rocks like wind, water and day-night temperature fluctuations. Some rocks from exposed atmosphere can cause cracks to widen due to the expansion of ice freezing and plant roots growing into cracks. The cracks provide a wider surface are where chemical weathering can occur and accelerate the weathering rate.

Composition of Rock


The minerals are a solid substance in rocks that have a distinct chemical composition, which is most reactive with water oxygen or other elements to weather more rapidly. Iron is a mineral in rocks that reacts with oxygen to form iron oxide known as rust. It weathers more quickly and beaks apart easily. The minerals are softer weather more rapidly like calcite in limestone and vulnerable to chemical weathering.

Climate


It is mostly based on temperature and rainfall in the region. Water is important as wetter climate is prone to faster weathering. Chemists use heat to speed up chemical reactions. Warmer climate accerlatechemical watering. The warm humid environment like tropics experience higher rates of weathering then cold, dry deserts

Mass movement

Is the large movement of rocks, soil and debris going downward by force of gravity. Another way to say it that the earths outer crust is being wasted away on a massive scale that's falling to lower elevations. It is a type of erosion that makes big changes to the side of a mountain which can happen suddenly or slowly over time. It can be thought of as a landslide but it has many triggers and types of erosion on a large scale.
Causes of mass movement


The cause is gravity that's constantly pulling rocks and debris down slopes on mountains. But mountains at the same time are resisting by their cohesive strength and internal friction between the materials causing their shear strength to pull back against gravity. The shear strength works by maintaining the slopes stability to keep the materials in place. When mass movement occurs it's the gravitational force overcoming the resistive force. The increase of slope steepness also increases the mass movement causing the gravitational force to act greater on a steep slope than a gentle slope. Another increase to mass movement is increased water that washes away small particles which helps the mountain side intact. The next increase to mass movement is decreased vegetation that stabilises the soil particles on the surface and anchors soil under surface through the root system. The last is earthquakes that have the ability to break of sections of mountains so or hills from shaking causing them to slide down the slope.

Slumps and rockslides


Mass movement can occur by a number of factors, the first one is when a large portion of slope fails and slides or rolls downhill called a slump, it can be debris sliding down a mountain mixed with water or air. This can also be described as coherent rock sliding along a curved surface that often happens from undercutting of the mountains base from for example the water might erode at the base of the cliff causing the mountain to slump down at the base as a unit or multiple units.

The second factor is a rockslide, the sliding of rock materials down a mountain. A rockslide is similar to a slump but does not move along curved surfaces instead slides along a pre-existing surface like underlying layer of rock and at the end of the base there is a collection of fallen rock.

Debris flow and earth flow


It is mass movement involving downward movement of unconsolidated materials referred to as material flow down a slope. Debris flow is water-laden mass of loose mud, sand, soil, rock and debris down a slope. It can be slow or fast down a steep slope that reaches 100kms per hour sweeping away trees, bridges, houses or roads.

Earth flow is fine-grained material developing at the lower end of a slope. Clay, silt and fine-grained volcanic material can become saturated with water becoming susceptible to earth flow and more water the more speed.

Sinkholes

A sinkhole is any hole in the ground created by erosion and drainage of water. They can be a few metres or big enough to swallow buildings. Sinkholes are often created by natural processes but can also be from human activity. Sinkholes then threaten water supplies that drain water supplies from lakes, streams and wet lands into aquifer and under ground water supplies. There are two types, created slowly over time called cover-subsidence sinkholes or ones that appear suddenly cover-collapse sinkholes. They mainly occur in karst terrain, which is areas of land with soluble bedrock like limestone or gypsum dissolved from water.

Where they are located


Sinkholes are found in the countries Laos, Guatemala Bolivia, China, Mexico, Croatia, PNG, USA and Italy. There is about 20% of sinkholes occur in the USA. There is mainly Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. Sinkholes rant tracked but in Florida CNN reports sinkholes from 2006-2010.