Weathering & Erosion
Weathering and Erosion Weathering - processes at or near Earth s surface that cause rocks and minerals to break down Erosion - process of removing Earth materials from their original sites through weathering and transport to another place.
Mechanical (Physical) - processes that break a rock or mineral into smaller pieces without altering its composition Types of Weathering Chemical - processes that change the chemical composition of rocks and minerals
Mechanical Weathering 1. Abrasion/Scouring wind & water
Mechanical Weathering 2. Frost wedging water expands when it freezes
Frost Wedging
Mechanical Weathering 3. Exfoliation/Unloading As deeply buried rock is uplifted, pressure is reduced and the rock breaks off into leaves or sheets along joints
Enchanted Rock
Mechanical Weathering 4. Thermal Expansion/Contraction repeated, daily heating (expansion) and cooling (contraction) of rock causes stress along natural boundaries in the rock
Mechanical Weathering 5. Plant/Animal Activity plant roots, animal burrows
Mechanical Weathering 6. Crystal Growth water evaporates leaving minerals (salt) behind; crystals grow and fracture the rock.
Chemical Weathering Rock reacts with water, gases and solutions which add or remove elements from minerals
Chemical Weathering 1. Oxygen Iron combines with oxygen in the presence of water in a processes called oxidation. The product is rust.
2. Carbon Dioxide Chemical Weathering CO 2 dissolves in rain water and creates carbonic acid which easily weathers limestone and marble
1908 to 1969 Acid rain has eaten away this limestone statue
Chemical Weathering 3. Biological Activity Lichen that grow on rocks produce weak acids that chemically weather rock
Weathering and Erosion Weathering - processes at or near Earth s surface that cause rocks and minerals to break down Erosion - process of removing Earth materials from their original sites through weathering and transport to another place.
Erosion is caused by 1. Water Rain, Streams, Oceans, Ice
2. Wind Erosion is Caused by
3. Gravity Erosion is Caused by
Gravity Mass Wasting - Movement of large amounts of material downhill Creep, landslide, mudflow, slump, mudslide, avalanche Caused by Rain Over-steepening Deforestation / Devegetation Earthquakes
PARTS OF A STREAM Channel - path of river Bed - bottom Head - beginning/source Mouth - ending Gradient - steepness of slope Base level - lowest point to which a stream erodes (ocean, another river, lake) ultimately the oceans
Longitudinal profile of a stream Channel Base level
Mississippi River Drainage Basin
A drainage basin is the land area that contributes water to a stream. A divide is an imaginary line that separates the drainage basins of one stream from another.
Drainage Patterns Dendritic Radial Rectangular Trellis
What streams Fig. 10.37a do Erode Transport Material Deposit Dissolved load Suspended load Bed load
Factors that affect erosion and deposition Velocity Gradient (slope) Channel shape Channel size Channel roughness Discharge amount of water flow per unit (second/hour/day) time, as discharge increases, load increases
Velocity
Upstream-Downstream Flow Changes Velocity is determined by; Gradient Channel roughness Channel size Channel shape Increase Velocity Discharge Channel size Decrease Gradient Channel roughness Fig. 10.37d
Channel shape and roughness Semicircular Flow fast Wide channel Flow slow Rough bottom Flow slow
YOUNG River Valleys Rapids/Waterfalls Narrow V-shaped valley Steep gradient (SLOPE) Erosion is mostly downcutting Little or no floodplain
A narrow V-shaped valley
The Yellowstone River in Wyoming
MATURE River Valleys Stream is near base level Stream energy is directed from side to side Some meandering (curves) Less downward erosion Flood plain is evident Slower flow than young river
Continued erosion and deposition widens the valley
Wye River in England
OLD River Valleys Close to base level Slowest velocity & widest channel Low gradient (slope) Meanders well-developed Wide flood plain
The resulting wide stream valley is characterized by meandering on a welldeveloped floodplain
Satellite view of the Missouri River flowing into the Mississippi River near St. Louis
Same satellite view during flooding in 1993
Erosion and Deposition Along a Meandering Stream
Stream Deposition*(add to your notes) Floodplain deposits Natural levees: form parallel to the stream by successive floods over many years Alluvial fans Develop where a high-gradient stream leaves a narrow valley Slopes outward in a broad arc Deltas Forms when a stream enters an ocean or lake
Flood plans deposits
*also happens at Submarine Canyons Alluvial Fans
River delta--- bird s foot delta
Wind Erosion
How the wind transports and erodes material When the particles carried by the wind are dragged across a surface, they act like sandpaper, etching, polishing and pitting surfaces. Wind can erode by abrasion or deflation
Deflation ---the process of removing surface materials by wind erosion High elevations are left due to vegetative stabilization
Natural arch Typically formed by a combination of wind and water Narrow ridges are formed and the softer substrata is weathered first, forming a bridge/arch overhead
Ventifact Stones that have been sculpted by the wind Wind carries fine particles that work like a sand blaster (i.e. sand, silt, clay, and ice particles) The windward face of the rock is flattened and smoothed
Dreikanter A type of ventifact that has three ridges Multiple faces have been formed by either a changing prevailing wind or movement of the actual rock being weathered
Desert pavement Formed in arid environments when wind carries finer, more lightweight particles such as sand away Large particles are left behind and protect from further erosion
Sand Dunes Piles of sand Deposited by wind Leeward side (slipface) has a steeper slope Windward side is more gradual
Barchan Dune Crescent shaped dunes Concave side is the leeward side Formed from a unidirectional wind
Transverse dunes A dune that elongates perpendicular to the prevailing wind (rippled appearance) Caused by a combination of wind directions
Parabolic dunes Crescent shaped dunes, U-shaped Concave side is the windward side Found in coastal deserts Arms of the crescent lag behind because they are held in place by vegetation
Linear dunes Form in areas with limited sand supplies and winds that converge from different directions
Star dunes Form in areas of abundant sands and variable wind directions
Huge dune fields Ergs
Ripple marks Miniature dunes within a dune (not more than 2 inches tall) Formed by wind May form from cross winds and appear to be traveling across a larger dune
Pedestal rock Base of a rock is weathered and eroded more quickly due to sand blasting
Yardang Streamlined, bedrock hills extensively grooved, fluted, and pitted by wind erosion. alternating ridges parallel to the wind direction
Glacier Notes
Cryosphere All of the frozen areas on Earth's surface where water exists in its solid form sea ice ice shelves icebergs ice sheets glaciers lake ice river ice snow permafrost
Glaciers Mass of ice formed by the recrystallization of snow under its own weight How it s formed: Compacted snow becomes firn More snow has to be added than melted accumulate in layers
Types of Glaciers 1. Ice sheets (continental glaciers) -- cover large areas of land 2. Valley (alpine) glaciers -- form at mountain tops and flow down valleys
How Glaciers Move
Glacial Zones Zone of Accumulation Snowfall exceeds ablation Ablation reduction in glacial ice by sublimation, melting, or calving Zone of Melting (Ablation) (Wastage) Ablation exceeds snowfall
U-Shaped Valley / Fjord / Trough Are formed from alpine glaciers!
Glacial Deposits Glacial deposit is called till. Glaciers pick up everything in their path, even the largest boulders. Large amounts of sediment can be carried long distances by glaciers.
Where are all the Glaciers now?