Weathering, Erosion, and Transportation
• Rocks exposed at Earth’s surface are constantly changedby water, air, temperature variations and other factors
• Weathering is the group of destructive processes that change physical and chemical character of rocks at or near Earth’s surface
• Erosion is physical picking up of rock particles by water, ice, or wind
• Transportation is the movement of eroded particles by water, ice, or wind
Types of Weathering
• Mechanical weathering– Physical disintegration
– Frost action, pressure-release fracturing, plant growth, burrowing animals, salt wedging, thermal cycling
• Chemical weathering– Decomposition of rock from exposure to
atmospheric gases (oxygen, water vapor and carbon dioxide)
– New chemical compounds (minerals) form
– Rate increased by increased rock surface area Spheroidal weathering
Mechanical Weathering
• Frost action– Mechanic effect of freezing (and
expanding) water on rocks
• Pressure release– Removal of overlying rock allows
expansion and fracturing
• Plant growth– Growing roots widen fractures
• Burrowing animals
• Thermal cycling– Large temperature changes fracture rocks
by repeated expansion and contraction
But mostly physical weathering is a matter of things just falling down. So in a sense, gravity, is the primary cause of physical weathering.
Chemical Weathering• Oxidation
– Chemically active oxygen from atmosphere
– Iron oxides are common result
• Soil and sedimentary rocks often stained
with iron oxides
• Acid dissolution– Hydrogen cations replace others in
minerals
– Carbonic acid from atmospheric CO2
dissolved in water
– Sulfuric, hydrofluoric acids emitted by volcanic eruptions
– Some minerals, such as calcite, may be totally dissolved
– Human activity, such as mining and burning of fossil fuels, produces acids
Soil• Soil - a layer of weathered,
unconsolidated material on top of bedrock
– Common soil constituents:
• Clay minerals
• Quartz
• Water
• Organic matter
• Soil horizons– O horizon - uppermost layer; organic material
– A horizon - dark layer rich in humus, organic acids
– E horizon - zone of leaching; fine-grained components removed by percolating water
– B horizon - zone of accumulation; clays and iron oxides leached down from above
– C horizon - partially weathered bedrock
Soils and Climate• Soil thickness and composition are
greatly affected by climate– Wet climates:
• More chemical weathering and thicker soils
• Soils in moderately wet climates tend to have significant clay-rich layers, which may be solid enough to form a hardpan
– Arid climates:
• Less chemical weathering and thinner soils
• Subsurface evaporation leads to build-up of salts
• Calcite-rich accumulation zones may form, cementing soil together into a hardpan
– Extremely wet climates (e.g., tropical rainforest)
• Highly leached and unproductive soils (laterites)
• Most nutrients come from thick O/A horizons
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Soil formation: Goldilocks Solution• The mineralogy and thickness of soil layers depends on source rock, climate
(temperature and rainfall), and age.
• Which of these soil types would you rather farm?
“Desire Path: A term in landscape architecture used to describe a path that isn’t designed but rather is worn casually away by people finding the shortest distance between two points.” http://www.yourdailyawesome.com/
Path of desire or path of least resistance?
Running Water
• Running water is the most important
geologic agent in eroding, transporting
and depositing sediment
Hydrologic Cycle
• Hydrologic cycle - the movement and interchange of water
between the sea, air, and land
– Evaporation
• Solar radiation provides energy
– Precipitation
• Rain or snow
– Transpiration
• Evaporation from plants
– Runoff
• Water flowing over land surface
– Infiltration
• Water soaking into the ground
Running Water
• Stream - a body of running water, confined to a
channel, that runs downhill under the influence of gravity
– Headwaters - upper part of stream near its source in the mountains
– Mouth - place where a stream enters sea, lake or larger stream
– Delta – where sediment is dumped into the lake or ocean
– Channel - a long, narrow depression eroded by a stream into rock or sediment
– Floodplain - flat valley floor composed of sediment deposited by the stream
Drainage Basins
• Drainage basin - the total area drained by
a stream and its tributaries
– Tributary - a small stream flowing into a larger one
• Divide - ridge or high ground that divides one
drainage basin from another
– Continental Divide separates the streams that flow into the Pacific from those that flow into the Atlantic and Gulf of Mexico
Drainage Patterns
• Drainage pattern - the arrangement, in map view, of a stream and its tributaries
– Most tributaries join the main stream at an acute angle, forming a V or Y pointing downstream
– Dendritic - drainage pattern resembling
the branches of a tree
– Radial pattern - streams diverge outward like the spokes of a wheel
• Typically form on conical mountains (volcanoes)
– Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles
– Trellis pattern - parallel streams with short tributaries meeting at right angles
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1306/es1306page01.cfm?chapter_no=visualization
Glaciers
• A glacier is a large, long-lasting mass of ice, formed on land, that moves downhill under its own weight
• Glaciers are part of Earth’s hydrosphere
• Along with sea ice, glaciers are known as the cryosphere
• About 75% of the world’s supply of fresh water is locked up in glacial ice
Numerous photos from http://www.earthscienceworld.org/images/ , some from http://cdm.lib.uiowa.edu/cdm4/index_geoscience.php?CISOROOT=/geoscience and
http://www.env.duke.edu/eos/geo41/gla.htm
Distribution of water in the hydrosphere
OceansGlaciers and other iceGround waterLakes
LakesLakes
Soil moistureAtmosphereRivers
97.22.150.61
0.0090.0080.0050.0010.0001
Reservoir________________(%)__
Formation of Glaciers• A glacier can only form where more
snow accumulates during the winter than melts away during the spring and summer
• Two types of glaciated terrains on Earth:
– Alpine (mountain glaciers) occurs in mountainous regions in the form of valley glaciers
– Continental covers large land masses in Earth’s polar regions in the form of ice sheets, e.g. Antarctica, Arctic Ice Cap, Greenland
Alpine glacier
Continental glacier
Glacial Erosion
• Glaciers erode underlying rock by plucking of rock fragments and abrasion as they are dragged along
– Basal abrasion polishes and striates the underlying rock surface and produces abundant fine rock powder known as rock flour
Erosional Landscapes• Erosional landforms produced by valley glaciers
include:
– U-shaped valleys
– Hanging valleys
• Smaller tributary glacial valleys left stranded above more quickly eroded central valleys
– Cirques
• Steep-sided, half-bowl-shaped recesses carved into mountains at the heads of glacial valleys
– Arêtes
• Sharp ridges separating glacial valleys
– Horns
• Sharp peaks remaining after cirques have cut back into a mountain on 3+ sides
Glacial Erosion• Erosional landforms produced by valley glaciers
include:
– U-shaped valleys
– Hanging valleys
• Smaller tributary glacial valleys left stranded above more quickly eroded central valleys
– Cirques
• Steep-sided, half-bowl-shaped recesses carved into mountains at the heads of glacial valleys
– Arêtes
• Sharp ridges separating glacial valleys
– Horns
• Sharp peaks remaining after cirques have cut back into a mountain on 3+ sides
Glacial Deposition
• General name for unsorted, unlayered glacial
sediment is till
– Deposits of till left behind at the sides and end of a glacier are called lateral, medial and terminal moraines, respectively
• Lateral moraines are elongate, low mounds of till along sides of valley glaciers
• Medial moraines are lateral moraines trapped between adjacent ice streams
• Terminal moraines are ridges of till piled up along the front end of a glacier
• Successive end moraines left behind by a retreating glacier are called recessional moraines