Weathering and Mass Movement

What is weathering?

Weathering - process of breaking down and changing rock at or near the Earth’s surface.

The two types:– Mechanical weathering

– Chemical weathering

Mechanical Weathering

The process of physical forces breaking down rocks into smaller and smaller pieces without changing the type of rock.

The causes of mechanical weathering are caused by gravity, water, wind and waves!

What is frost wedging?

The freezing and thawing of water in cracks.

The broken down pieces that result are known as talus.

What is unloading?

Unloading is when large masses of igneous rock are exposed through uplift, and erosion of over lying rocks.

This reduces the pressure on the rock causing the rock layers to break away which is known as exfoliation.

What is biological activity?

Biological activity are the activities of living organisms that cause the breaking up of rock.

– Roots of trees break rock apart– Burrowing Animals move rocks to the surface where weathering is

more rapid.– Decaying Organisms- produce acid compounds that cause

chemical weathering.– Humans- accelerate mechanical weathering through deforestation

and blasting in search of minerals or in the creation of new roads.

What is chemical weathering? Chemical weathering is the process when rock turns

into one or more new compounds.– Analogy:

mechanical – tearing paper chemical – burning paper

Water is the most important agent of chemical weathering.– water absorbs gases from the atmosphere and ground.– these dissolved substances chemically react with various

minerals. Rust - is iron oxide that forms when iron containing objects are exposed to

water. Acid rain – is forms when rain absorbs carbon dioxide as it falls. This acid

breaks down rocks.

What are 2 factors that affect the rate of weathering?

Rock Characteristics – Breaks rock into smaller pieces, which increasing the surface area

of exposed rocks & accelerates chemical weathering by.– Physical characteristics such as cracks can effect the ability for a

rock to be penetrated. – Mineral composition increase or decrease rate of weathering.

Climate– controls the frequency of freeze-thaw cycles and affects the rate of

mechanical weathering.

Mass Movements

The transfer of rock and soil down slope due to gravity is called mass movement.

Among the factors that commonly trigger mass movements are

– Water: saturation of surface materials with water decreases friction.

– Slopes: Steepening of slopes unstabilizes angles – Removal of vegetation: Loosens soil– Earthquakes & Volcanic eruptions: Destabilizes rock

Types of Mass Movements

Geologists classify mass movements based on the kind of material that moves, how it moves, and the speed of movement. Rockfalls Slides Slumps Flows Creeps

Rock fall – all th

e broken

sediments fall to the base

Rockslide on I-40 near Asheville

Nat Geo videoclip

Slump: block of material moving down slope along a curved surface

Creep…slowest of all mass movements!

Let’s do a lab where we model MASS MOVEMENT!

Water Erosion

The four types of water erosion are as follows:– the movement of soil by rain splash and its

transport by this surface flow. – Rill erosion by concentrated flow in small rivulets. – Gully erosion by runoff scouring large channels

(deeper than 1 foot). – Streambank erosion by rivers or streams cutting

into banks.

Stream Erosion

Streams generally erode their channels, lifting loose particles by abrasion, grinding, and by dissolving soluble material.

A stream’s bedload is solid material too large to carry in suspension.

Deposition occurs as stream flow slows down– Larger, heavier particles are deposited first

Glacier Erosion

Glaciers mainly erode in two ways: – Plucking is when the bottom layer of a glacier melts and

the water seeps into the surrounding rock. This water then refreezes and cracks and splits the rock. These rocks are then incorporated into the glacier.

– Abrasion occurs when a glacier made of ice and rock moves over a bedrock surface which is then scraped and smoothed by the glacier.

Glacial drift applies to all sediments deposited by glaciers.

– There are two types of glacial drift: till and stratified drift.

Glacier Erosion

Glacial drift applies to all sediments deposited by glaciers. – Till is deposited as the glacier melts and drops its

load of rock debris. Ice cannot sort the sediment it carries, therefore till deposits are usually unsorted mixtures made up of many particle sizes.

– Stratified drift is sediment laid down by glacial meltwater. It contains particles that are sorted according to size and weight of the debris.

Wind Erosion

In general, most erosion results from running water, although in deserts, wind erosion is more significant than elsewhere because deserts soils are dry.

Wind erodes in the desert in two ways: deflation and abrasion.

Deflation

Deflation is the lifting and removal of loose particles such as clay and silt.

Deflation can lower elevation of land by a meter or more.

Deflation also results in shallow depressions called blowouts.

Deflation creates a stony surface layer called desert pavement when it removes all the sand and silt and leaves only coarser particles.

Abrasion

Abrasion happens when wind-blown sand cuts and polishes exposed rock surfaces.

Blowing sand can grind away at boulders and smaller rocks, sometimes sandblasting them into odd shapes.

How do humans affect erosion?

Erosion is strongly influenced by human activity: Removal of natural vegetation, such as

farming, logging, & construction greatly increase erosion.

Over-steepened slopes due to construction can lead to instability of soil and mass movements.

River sediment has increased since humans have appeared