Transcript Document
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 !
Which comes first
Erosion or weathering?
Weathering must happen before erosion can take place!
Mechanical Weathering
•The physical breaking or fragmentation of rock 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
What things can cause mechanical weathering?
1. Ice 2. Abrasion 3. Crystal Growth 4. Biological Activity: Plants and Animals 5. Temperature changes 6. Pressure changes
1. Ice or Frost wedging
Repeated freezing and thawing of H 2 O in rocks -Can form potholes in cold climate areas -Can be responsible for splitting trees as well
2. Abrasion
• Abrasion is the weathering of rock due to impact and wearing down.
• This can happen because of water, wind, waves or ice.
• Abrasion happens when rocks get hit over and over and over and over
3. Crystal Growth
Weathering starts when water brings dissolved minerals to the rock surface. When the water dries, the minerals form crystals that force small particles to break off.
4. Biological Activity
Plant roots in search of minerals and water grow into fractures, and as the roots grow, they wedge the rocks apart.
Burrowing of Animals
5. Temperature Changes
When heated up by solar radiation each different mineral will expand and contract a different amount at a different rate. With time, the stresses produced are sufficient to weaken the bonds, and thus breaking of fragments .
6. 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
Jointing in upper Navajo Sandstone, Grand Staircase-Escalante National Monument
Chemical Weathering
• The process that breaks down rock through chemical changes.
• The agents of chemical weathering – Water – Oxygen – Carbon dioxide – Living organisms – Acid rain
Water
• Water weathers rock by dissolving it • Water is by far the most important agent of chemical weathering.
• Pure Water is nonreactive, yet a small amount of dissolved material is generally all that is needed to activate a chemical reaction.
Oxygen • Oxygen dissolved in water will oxidize some materials.
• Iron combines with oxygen in the presence of water in a processes called oxidation • The product of oxidation i s rust
Carbon Dioxide
• CO 2 dissolves in rain water and creates carbonic acid • Carbonic acid easily weathers limestone and marble
Pollution…Acid Rain
• Compounds from burning coal, oil and gas react chemically with water forming acids.
• Acid rain causes very rapid chemical weathering
Living Organisms
• Lichens that grow on rocks produce weak acids that chemically weather rock.
• Also, decaying organic material will produce CO2. As water moves through the soil, it can become slightly acidic.
Spheroidal Weathering
Fluids enter rock along joints or fractures and the edges become rounded.
Rates of Weathering
Factors influencing weathering:
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Mineral makeup
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Climate
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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
Pinnacles in Australia Pinnacles in Bryce Canyon NP
Arches National Park, Utah:
Hoodoos have a cap of harder less eroded rock: Utah Canada
Chemical Weathering of Granite
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
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
Soil composition
-Soil forms in layers
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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) -Leaf litter, high decomposition O zone 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
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The Role of Water Oversteepene d Slopes Vegetation Earthquakes
Flows, Slides, and Falls (All types of Mass Wasting)
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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
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.