Earth’s External Processes Weathering – disintegration of rock at or near the earth’s surface Mass wasting – transfer of material down slope in response.

Earth’s External Processes

• Weathering – disintegration of rock at or near the earth’s surface

• Mass wasting – transfer of material down slope in response to gravity

• Erosion – transportation of material by a mobile agent such as water, wind, or ice

These are non–stop, never-ending processes!

Weathering must happen before erosion can take place!

Which comes first

Erosion or weathering?

Mechanical Weathering

•Breakdown of bigger pieces into smaller pieces

•This is also referred to as physical weathering

•Only size changes occur in this process•No chemical composition change occurs

in mechanical weathering

WeatheringMechanical weathering – the physical breaking or

fragmentation of rock into smaller pieces

What things can affect mechanical weathering?

-Temperature

-Most of the temperature influence comes from the freezing of H2O

-Ice expands by 9% over the volume of liquid H2O

-Freezing H2O cracks rocks

-Repeated freezing and thawing of H2O in rocks

-Can form potholes in cold climate areas-Can be responsible for splitting trees as well

Frost wedging

Biological ActivityPlant roots in

search of minerals and

water grow into fractures, and as the roots grow, they wedge the

rocks apart. This is physical

weathering.

Pressure

-Reduced pressure is what we are talking about

-Bedrock moves upward as overlying rock is removed

-Bedrock expands upward-Cracks form as pressure is reduced

-Cracks are called joints

-Joints/cracks form parallel to the ground

Mechanical Weathering

Exfoliation:Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.

WeatheringMechanical Weathering

Sheet Joints(Exfoliation)

Stone Mountain, GA

Half Dome, Yosemite, CA

Exfoliated Domes, Yosemite

Stone Mountain, GA

Stone Mountain, Georgia, showing the product of exfoliation due to unloading

Chemical Weathering• Oxygen dissolved in

water will oxidize some materials like

nails and rocks!

(Rust)• Carbon dioxide dissolved in water forms a weak acid

called carbonic acid, which helps dissolve

rock material.

Chemical Weathering of Statues, Bath, UK

The Result of Chemical Weathering

-New Minerals Are Formed and Dissolved Particles Are Released

-The Amount of Chemical Weathering Is Determined by the Composition of the

Parent Rock-Temperature increases leads to an

increase chemical reactions

How Granite WeathersChemical weathering – the alteration of the internal structure of minerals by chemical reactions

Potassium Feldspar + carbonic acid + water

Clay mineral + Potassium bicarbonate + Silica

Spheroidal Weathering

Fluids enter rock along joints or

fractures and the edges become

rounded.

Rates of Weathering

Granite versus Marble

Factors influencing weathering:

-Mineral makeup

-Climate

-Differential Weathering

Differential WeatheringRocks weather at different rates because of different

mineral makeup, degree of jointing, and exposure to the elements.

More resistant rock protrudes as ridges and pinnacles.

Bryce National Park, Utah

What Is Soil?

The four components of

soil:

Decomposed rock

Humus

Air

Water

Soil is the layer of rock and mineral fragments along with organic matter, water and air that supports the growth of

plants.

What is Dirt?

Dirt is what gets on your clothes and into your house

Controls on Soil Formation• Parent material – Bedrock

vs. Unconsolidated• Time – longer time, more

soil• Climate – temperature and

precipitation• More plants and animals =

more organic matter• Poorly developed soils on

steep slopes

Soil Texture and StructurePoint A:

10% silt

40% clay

50% sand

Clay loam has no single

particle size percentage

that dominates!

Controls of Soil FormationIf the parent material is

bedrock, then we get residual soils

On unconsolidated sediments, we get transported soils

Soil composition

-Soil forms in layers-Larger materials are found on the bottom

How quickly is soil formed?

-It may take several hundred years for 1cm to form

Soil profile 

A vertical column showing the

sequence of layers of particles in the

soil

Soil Horizons-A distinct layer or zone within a profile-There are four major zones (O, A, B, C)

O zone -Leaf litter, high decomposition

A Zone

-High organic matter

-Generally the darkest in color

-High humus content

B Zone

-Less developed

-Lighter in color

-Zone of accumulation (highly soluble mineral build-up)

-Red or brown in color

-Clay accumulated (Hard pan formation)

C Zone

-Contains weather parent material

Soil HorizonsO zone

-Leaf litter, high decomposition

A Zone-High organic matter-Generally the darkest in color-High humus content

B Zone-Less developed-Lighter in color-Zone of accumulation (highly

soluble mineral build-up) -Red or brown in color-Clay accumulated (Hard pan

formation)C Zone

-Contains weather parent material

Rill and Gully Erosion

Eroded channels ranging in size from rivulets to small gullies caused mainly by runoff waters rather than raindrop dislocation.

Gully erosion in Tanzania. What is the main factor for it’s cause?

Who cares about soil erosion?

Mass Wasting: The Work of Gravity

Mass wasting refers to the down slope

movement of rock and soil

under the influence of

gravity.Hurricane Mitch in 1998 caused severe mud

flows in Central America

Triggers of Mass Wasting

• The Role of Water

• Oversteepened Slopes

• Vegetation

• Earthquakes

Flows, Slides, and Falls (All types of Mass Wasting)

• Type of material, the kind of motion, and the velocity of movement dictate kind of mass movement

• Falls involve the free-fall of debris or rock• Slides occur when the material remains coherent

and moves along a well-defined surface• Flow occurs when material moves down slope as

a viscous fluid (liquidy)

Mass Wasting Processes

A debris flow involves a flow of soil

and regolith containing a large amount

of water.

Mass Wasting ProcessesEarthflows

form on hillsides in

humid areas during times of heavy rainfall.

Slow Movements

Creep involves the gradual

downhill movement of

soil and regolith.

Freeze and thaw cycles

contribute to creep.

Slow MovementsSlump

Landslide movement along a curved understructure

Occur on thick soils with moderate slopes

Common after rainfalls

Mixing of debris

Crescent shaped scar on the landscape

Triggered by rain or EQ

Solifluction

Solifluction occurs in regions of

permafrost when the thawed upper region flows down

slope.

The frozen bottom layer stays in

place.

Rock AvalanchesThe high

speed of rock avalanches

suggests that trapped air has helped them attain velocities of

125 miles per hour.

What does this coin have to do with

Rock Falls?

"The Old Man of the Mountain" is a rock formation that can be found on Mt. Cannon in the Franconia Notch

gateway to Northern New Hampshire. From the right view, this unique rock formation, comprised of

five layers of Conway red granite, depicts the distinct profile of an elderly man gazing eastward.

Geographers believe that the layers of granite were positioned by the

melting and slipping away action of an ice sheet that covered the

Franconia Mountains at the end of the glacial period - some 2,000 to

10,000 years ago. Today, the formation, measuring over 40 feet

high with a lateral distance of 25 feet, is held in place by cables and turnbuckles to prevent further

slipping and possible destruction.

The Old Man is no more!!