Environmental Geology Lecture Outline
- the upper layer of the earth that may be
dug or plowed and in which plants grow (Websters).
- unconsolidated material at the earth's surface.
What are the initial
materials soils are formed from?
- minerals: inorganic, crystalline, naturally
- types of?
most common type, (SiO4)-4 the common ionic group,
includes quartz, feldspars, micas.
- oxides: form
in presence of oxygen, by weathering. Hematite as nature's pigment.
Common in some soils because communication with the atmosphere
produces an oxidizing environment. Other soils reducing.
- salts: precipitate
out of water.
- sulfides: form
often in oxygen poor environments.
soluble so relatively mobile.
- clays: a special
mineral group important in soils.
- submicroscopic, and very variable and complex.
- usually dominated by sheet silicates that
provides them with unique traits (shrink-swell behavior, adsorption
capabilities, low strength).
- rock fragments:
some common rock types (represent source material for sediment
- igneous: granite,
sandstone, shale, limestone.
schist, gneiss, slate.
factors control soil formation? Students should be able to
expand a bit on each of the below.
- climatic factors?
- source factors?
- time factors?
- slope factors?
- biologic factors?
occur during soil formation and what approaches
could you take to study these processes? Rephrasing the first
question - how does rock turn into soil.
- Types of approaches to take in studying:
- literature search
(but what if not studied before?).
- experimental approach (but limiting time factor).
- theoretical approach
(but theory may be limited).
- field studies:
spatial variation can be a function of history.
- Field studies indicate great diversity, but
general trend of loss of soluble elements (see below). Effects
diminish from the surface downward.
- Below is a table taken from Goldich (1938)
that shows the chemical analysis of a quartz-feldspar-biotite
gneiss and variably weathered versions of it. The inference is
that this is a temporal progression, but this is not necessarily
the case (think of other factors that control the degree of weathering).
|| fresh rock
|| least weathered
|| moderate weathering
|| most weathered
| Fe O
- Below is a table from the same source showing
some of the mineralogic changes.
|| fresh rock
|| least weathered
|| moderate weathering
|| most weathered
- Physical weathering processes:
- rock jointing and fracturing: most common
type of structure.
- abrasion by wind, water or ice carrying particles.
- Chemical weathering processes:
- silicate hydrolysis.
of weathering pattern in a shallow Tertiary intrusive from the
foot of the Chisos Mountains in Big Bend, Texas. Note the rectangular
fracture pattern that outline weather blocks. The rock was originally
all uniform in color and the banding represents oscillatory iron
staining and reflects weathering that worked from the fracture
margins into the interior of the blocks. The middle of the blocks
are less weathered than the fracture margins. This is the same
geometry and process that gives rise to spheroidal boulders when
the more weathered material is eroded away.
View of desert soil near Moab, Utah. The reddish material
is wind blown fine red sand. The red coloration is due to oxidation
of the small bit of iron in the grains. Of distinct interest here
is the dark, bumpy covering. To the touch it has a leathery feel,
and under the microscope it is a complex community of unicellular
plants and organisms called a cryptobiotic crust. This community
thrives under the harsh arid desert conditions. It also serves
as an important element in soil formation, by trapping sediment,
moisture and seeds. The germination of seeds and growth of larger
desert plants often depends upon this substrate being developed.
Unfortunately the crust doesn't thrive underfoot, and new efforts
are being made to educate people about the importance of this
cryptobiotic crust to the desert soils and ecosytem.
What are the soil
constituents that result from these processes?
- mineral and rock fragments.
- precipitates: oxides, salts and sulfates.
- water and ice.
- air, in pore space.
(order) exists in soils?
- Clear that soils are zoned, from the surface
- Soil exposed along bluffs of Elkhorn River just west
of Omaha. Letters A, B, and C show approximate positions of layers
described before. The soil is developed in material mainly derived
from the wind blown loess (material with columnar fractures at
base of cliff). A is evident by the dark coloration produced
by the collection of organics. B is evident by orange and white
staining and C is the transition zone to unaltered material below.
- soils with clays also have unique fracture
How can soils be
- agricultural, scientific, engineering - i.e.
- climatic, by process. Some examples.
- oxisols: tropical
environments, intense chemical weathering, oxide accumulation,
laterites old term for one type.
arid environments so get evaporation out of soil, accumulation
of calcium carbonate a key.
- cryosols: freezing
important part of process, permafrost, patterned ground one result.
What are environmental concerns
associated with soils?
- what are natural rates of erosion? how would
- what are mechanisms of erosion?
- sheet wash.
- gullying: headward retreat, fractal pattern
- wind erosion.
- human activity the promotes soil erosion?
- logging, especially clear cutting.
- improper farming.
- off road vehicles, intense foot traffic.
- effect of plumes of sediment on aquatic ecosystems.
- Dust Bowl Era - early 1930s. Dust in the
wind was topsoil from the field. Cause?
- Soil Conservation Service, born April, 1935.
- possible solutions to soil erosion?
- maintenance of plant cover.
- minimum tillage, contour plowage, minimize
- gully sediment traps (prevent headward retreat).
- contour land.
- cycles of irrigation and some numbers to
- salinity of seawater: circa 35,000 ppm
- salinity of input irrigation waters 350 ppm.
- >2000 to 3000 ppm toxicity level for many
- critical question: if the climate is arid
enough that water evaporates out of soil where does the salt
- extent of concern?
- San Joaquin valley in California, Pakistan,
"half of the world's irrigated land .... suffers from reduction
of crop yields due to salinization."
- possible solutions:
- desalinize irrigation water (very expensive).
- direct trickle irrigation (water usage reduction).
- tile fields: can flush salts through soil,
run off waters high in Selenium in California, expensive.
- salt resistant crops (will only delay problem).
runoff associated with soil erosion includes dissolved and suspended
load material that serve as nutrients for algal growth. This is
especially true for urban areas, and for fields were agrichemicals
are used. When these standing bodies of water the nutrients can
cause algal blooms, oxygen depletion and associated problems.
The Mississippi provides enough nutrients to cause algal blooms
and what are known as anoxic events in the nearshore Gulf Coast
area. One is this is a major disturbance in the Nitrogen cycle.
- differential compaction (settling).
- good local examples on campus: library, parking
- are sediments naturally compacted or not:
soil tests are critical.
- organic rich soils an extreme of 20' to 1'
compaction (contributes to subsidence problems).
- muds can easily compact from several feet
to 1'. Sand compacts a little.
- swelling clays.
- $2-7 billion annual damages in U.S. (Krohn
& Slassna, 1980)
- Dallas area alone $10 million a year.
- common where volcanic ash a major component
in soils or where have marine clays (Denver and Houston).
- soil creep.
- drainage characteristics.
Return to Environmental
Geology course index.
Return to Harmon's
© Harmon D. Maher Jr.. This page may be
used for non-profit educational purposes. For any other use please