Weathering, Soil and Mass Wasting

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? Weathering must happen before erosion can take place!

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

Weathering Mechanical 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

Frost wedging -Repeated freezing and thawing of H2O in rocks -Can form potholes in cold climate areas -Can be responsible for splitting trees as well

Biological Activity Plant 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.

Mechanical Weathering Many igneous rocks, particularly granite, are formed at great depth. When they come to the surface, they expand. Creates a morphology with exfoliation slabs – slabs sheet off in onionlike layers Stone Mountain, Georgia Expansion of rocks pose a risk in deep mines – explode off walls These fractures, or joints can enhance frost wedging and other erosive processes

Sheet Joints (Exfoliation)

Half Dome, Yosemite, CA Stone Mountain, GA Summit of Half Dome, Yosemite, shows exfoliation Half Dome, Yosemite, CA Stone Mountain, GA

Exfoliated Domes, Yosemite

Stone Mountain, Georgia, showing the product of exfoliation due to unloading Summit of Half Dome, Yosemite, shows exfoliation Stone Mountain, GA

Chemical Weathering of Statues, Bath, UK 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 Weathers Potassium Feldspar + carbonic acid + water  Chemical 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.

Factors influencing weathering: Differential Weathering Rates of Weathering Factors influencing weathering: Mineral makeup Climate Differential Weathering Granite versus Marble

Differential Weathering Rocks 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

The four components of soil: What Is Soil? Soil is the layer of rock and mineral fragments along with organic matter, water and air that supports the growth of plants. The four components of soil: Decomposed rock Humus Air Water

Dirt is what gets on your clothes and into your house 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 Structure Point A: 10% silt 40% clay 50% sand Clay loam has no single particle size percentage that dominates!

Controls of Soil Formation If the parent material is bedrock, then we get residual soils On unconsolidated sediments, we get transported soils

How quickly is soil formed? 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 Horizons 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

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 Processes Earthflows 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 Movements Slump 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

The frozen bottom layer stays in place. Solifluction Solifluction occurs in regions of permafrost when the thawed upper region flows down slope. The frozen bottom layer stays in place.

Rock Avalanches The 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. What does this coin have to do with Rock Falls?

The Old Man is no more!!