Geologic vs Accelerated Erosion NREM 461 Dr. Greg Bruland 1
I. Review of Rock Types: igneous, sedimentary, metamorphic A. Igneous: form by solidifcation when magma cools 1. Rate of cooling determines grain size (fast-fine, slow-coarse) Fine-grained Coarse-grained Rocks w/ quartz Rocks w/o quartz 2
B. Sedimentary: form where transported material is deposited 1. Classified by transporting agent a. Alluvium: material deposited b. Colluvium: material moved downslope by c. Galcial till: material deposited by d. Loess: silty material deposited by 3
C. Metamorphic: materials altered by heat & pressure, recrystallized into new larger crystals, harder than original 1. Types include: a. Marbe: metamophosed b. Slate: metamorphosed c. Quartzite: metamorphosed d. Schist: e. Gneiss: 4
D. Stage: landscapes pass thru stages of youth, maturity & old age 1. Youth: water erosion cuts steep narrow valleys in an upland 2. Maturity: landscape has many valleys & is hilly 3. Old age: hills nearly worn away & erosion has slowed 5
E. Mass wasting, Downwearing, Backwearing 1. Mass wasting: all processes by which gravity moves soil downslope a. Rapid, dramatic: b. Slow: Rapid Slow 6
2. Downwearing: hills in landscape gradually worn down by erosion & steep valley slopes become more gentle w/ time 7
3. Backwearing (parallel retreat of slopes): valleys develop 2 types of slopes, steep backslope, & flatter footslope between backslope & stream 8
F. Landscape development, glaciation, mass wasting, erosion combine to produce a complex landscape 9
II. Geologic vs. Accelerated Erosion A. erosion, of Latin origin, derived from the verb eroder- to eat away, roder-to gnaw B. Geologic Erosion (GE): 1. Natural leveling gprocess, wears down hills & mountains, fills in valleys, lakes, bays 10
Grand Canyon National Park, Arizona 11
Canyonlands National Park, Utah 12
Badlands National Park, South Dakota 13
Waimea Canyon, Kauai 14
Li River, China 15
Guilin, China 16
2. Normal rate of erosion under natural vegetation rate of soil formation 3. Geologic erosion by water greatest in semiarid regions a. 17
Nonlinear erosion dynamics 18
4. Geologic erosion rates vary according to climate & substrate, difficult to quantify, diff. methods a. GE rate from undisturbed humid-region forests & grasslands = 0.25 tons/ac-yr b. GE rates on Oregon coast = c. GE rate for gently sloping soils = 19
C. Accelerated Erosion: occurs when humans disturb natural vegetation & soil by? 20
Land-uses Leading to Accelerated Erosion Photo courtesy of USDA NRCS. Photo courtesy of USDA NRCS. Photo courtesy of USDA NRCS. 21 Photo courtesy of Jim Mansfield.
1. Accelerated erosion 10-1000x as destructive as geologic erosion (Brady & Weil 2002) & results in: a. b. c. 2. Rates of accelerated erosion by wind & water in parts of Africa, Asia, S. America a. 22
3. In U.S., avg. erosion rate on cropland = 30 tons/ac- yr (12 Mg/ha-yr) a. 17 tons/ac-yr (7 Mg/ha-yr) from water erosion b. 13 tons/ac-yr (5 Mg/ha-yr) from wind erosion (B&W 2002) 23
4. Palouse in W. WA lost 40% of topsoil in past 100 yrs a. 800 yrs to form topsoil lost in 15 years b. (Gardiner & Miller 2008) 24
Erosion rates in different countries under different land uses in t/ha-yr (Morgan 2005) Country Geologic Cultivated Bare Soil Ethiopia 1-5 8-42 5-70 Nigeria 0.5-1 0.1-35 3-150 China 0.1-2 150-200 280-360 India 0.5-5 0.3-40 10-185 Australia 0-64 0.1-150 44-87 UK 0.1-5 0.1-20 10-200 USA 0.03-3 5-170 4-9 25
5. Know how to convert units from English to Metric Troeh pg. 623-626. Submultiples Multiples Value Symbol Name Value Symbol Name How many acres in a hectare? Know how to convert from tons/ac-yr to mt/ha-yr & Mg/ha-yr 10 1 g dg decigram 10 1 g dag decagram 10 2 g cg centigram 10 2 g hg hectogram 10 3 g mg milligram 10 3 g kg kilogram 10 6 g µg microgram (mcg) 10 6 g Mg megagram 10 9 g ng nanogram 10 9 g Gg gigagram 10 12 g pg picogram 10 12 g Tg teragram Common prefixes are in bold face. 26
D. Soil Loss Tolerance Values (T) 1. T = maximum level of erosion that will permit a high level of crop production to be sustained economically & indefinitely (Troeh pg 129) a. Mean annual soil loss of 5 tons/ac-yr is generally accepted as T, but values as low as 1 ton/ac-yr are recommended for areas where soils are thin or erodible 27