Download 05 Chapter 5_Weathering and Soils

Engineering Geology
CVL 3315
Chapter 5
Volcanoes and Volcanic
Hazards
Dr. Sari Abusharar
University of Palestine
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
2nd Semester 2014-2015
1
Outline of Presentation
Earth’s External Processes
Weathering
Mechanical Weathering
Chemical Weathering
Rates of Weathering
Soil
Controls of Soil Formation
2
Earth’s external processes
Weathering, mass wasting and erosion are
called external processes because they occur
at or near the surface and are powered by
energy from the sun
They are a basic part of the rock cycle
transforming solid rock into sediment
Earth’s external processes
Weathering ‫ –التجوية‬the physical
breakdown (disintegration) and chemical
alteration (decomposition) of rocks
Mass wasting ‫ –انھيال كتلي‬the transfer of rock
and soil downslope under the influence of
gravity
Erosion ‫ –التعرية‬the physical removal of
material by mobile agents such as water,
wind, ice, or gravity
Weathering
Weathering occurs constantly and is very
important for life on Earth because it
decompose the rock to form soils with
minerals and elements that eventually we
eat.
Weathering occurs when rock is
mechanically fragmented or
disintegrated and/or chemically altered
or decomposed.
Types of Weathering
Mechanical
weathering
Chemical
weathering
Weathering
Mechanical weathering – breaking of
rocks into smaller pieces each retaining
the characteristics of the original
material
Chemical weathering can
occur only to those
portions of a rock that
are exposed to the
elements.
Mechanical
weathering breaks rock
into smaller and smaller
pieces,
thereby
increasing the surface
area
available
for
chemical attack.
Types of Mechanical Weathering
Biological activity
Thermal expansion
Unloading
Frost wedging
Weathering
Frost wedging ‫ –توتد صقيعي‬alternate freezing
and thawing of water in fractures and
cracks promotes the disintegration of rocks
Frost wedging
As water freezes, it
expands, exerting a
force great enough to
break rock. When
frost wedging occurs
in a setting such as
this, the broken rock
fragments fall to the
base of the cliff and
create a cone-shaped
accumulation known
as a talus slope ( ‫ميل من‬
‫ركام من حطام صخري يتجمع‬
‫)اسفل سفح مرتفع‬.
Weathering
Unloading ‫ –التفريغ‬exfoliation (‫ )تقشر‬of igneous
and metamorphic rocks at the Earth’s
surface due to a reduction in confining
pressure
Weathering
Thermal expansion (‫ – )التمدد الحراري‬alternate
expansion and contraction due to heating
and cooling
• The daily cycle of temperature will weaken the
rocks, particularly in places like hot deserts
where daily variations may exceed 30°C.
• Heating a rock causes expansion, and cooling
the rocks causes contraction.
• Repeated expansion and contraction of the
minerals with different expansion rates will
exert stress on the rock’s outer shell.
Thermal expansion
Weathering
Biological activity –
Weathering is also
accomplished by the
activities of organisms,
including plants,
burrowing animals, and
humans.
Root wedging widens fractures in
rocks and aids the process of
mechanical weathering.
Weathering
Chemical Weathering
• Breaks down rock components and
internal structures of minerals
• Most important agent involved in
chemical weathering is water:
- responsible for transport of ions and
molecules involved in chemical processes
Major processes of chemical
weathering
Hydrolysis
‫التموه‬
Oxidation
‫األكسدة‬
Dissolution
‫الذوبان‬
Weathering
Dissolution
– Aided by small amounts of acid in the water
– Soluble ions are retained in the underground
water supply
Just as sugar dissolve in water
Dissolution
One of the most
water-soluble
minerals is
halite which is
composed of
sodium (Na+)
and chloride
(Cl-) ions.
Weathering
Dissolution
• Although most minerals are insoluble in
water, the presence of even a small amount
of acid dramatically increases the corrosive
force of water.
• An acid solution containing the reactive
hydrogen ion, H+.
Weathering
Dissolution
• An acid solution easily attacks calcite
CaCO3, the mineral that commonly forms
the limestone and marble.
• In nature, over periods of thousands of
years, large quantities of limestone are
dissolved and carried away by underground
water forming caves.
Missouri’s Meramec Caverns
Dissolution
Monuments (‫ )نصب تذكارية‬and buildings made of limestone or
marble are also subjected to the corrosive work of acids,
particularly in areas of Polluted air.
acidity & alkalinity
pH scale measures the degree of acidity or alkalinity
7- neutral
Below 7 is acid
Above 7 is alkaline
Weathering
Oxidation
– Any chemical reaction in which a compound loses
electrons
Iron reacts with oxygen to form
iron oxide as seen on these rusted
barrels.
Weathering
Oxidation
–
– If the water goes into a mine, the water coming
out of the mine after this reaction is acid mine
drainage that will kill aquatic organism if reaches
the fluvial system.
Weathering
Hydrolysis
– The reaction of any substance with water
– A hydrogen ion attacks and replaces other
positive ions
– The silicates, the most common mineral
group is decomposed primarily by this
process.
H2O = H+, OH-, These are very reactive
hydrogen and hydroxil ions. It is the H+ ion
the one that attacks and replaces other
positive ions in the crystal lattice ( ‫الشبكية‬
‫)البلورية‬.
Weathering
Hydrolysis
How Granite Weathers
Granite is the most abundant continental rock which consists
mainly of quartz and potassium feldspar
Weathering
Hydrolysis
• Clay is the most abundant product of the chemical
breakdown of potassium feldspar. They are very
stable and as a result they make up a high percent
of soils.
• Some silica is remove from the feldspar structure
and carried away by groundwater.
• Another component of granite, quartz, is very
resistant to chemical weathering and remains.
• Eventually it is transported by water into rivers
and the sea.
Products of Weathering
Weathering of Silicate Minerals
Weathering
Spheroidal Weathering ‫التجوية الكروية‬
• Chemical weathering causes physical changes
also.
• When water enters joints and fractures, the
chemical weathering that occurs along the
fractures results into spherical shapes.
• This is because the corners are attacked more
than the rest of the rock.
Spheroidal Weathering
Spheroidal Weathering
A. Spheroidal weathering is
evident in this exposure of
granite in California’s
Joshua Tree National Park.
B. Sometimes successive
shells are loosened as the
weathering process
continues to penetrate
ever deeper into the rock
Rates of Weathering
Advanced mechanical weathering aids chemical
weathering by increasing the surface area
Factors affecting weathering
• Surface area
• Rock characteristics
• Climate
Rates of Weathering
Advanced mechanical weathering aids chemical
weathering by increasing the surface area
Factors affecting weathering
• Surface area
Increasing the surface area will increase the rate
of chemical weathering.
Surface area
Rates of Weathering
Advanced mechanical weathering aids chemical
weathering by increasing the surface area
Factors affecting weathering
• Rock characteristics
– Rocks containing calcite (marble and
limestone) readily dissolve in weakly acidic
solutions
– Silicate minerals weather in the same order as
their order of crystallization
Rock characteristics
An examination
of headstones
reveals the rate
of chemical
weathering on
diverse rock
types. The
granite
headstone (left)
was erected four
years before the
marble
headstone
(right).
granite headstone
marble headstone
Rates of Weathering
Others factors affecting weathering
• Climate
– Temperature and moisture are the most
crucial factors
– Chemical weathering is most effective in
areas of warm, moist climates
Climate
Acid rain
accelerates the
chemical
weathering of
stone monuments
and structures
Weathering
Differential weathering ‫التجوية المتباينة‬
• Masses of rock do not weather uniformly
due to regional and local factors
• Results in many unusual and spectacular
(‫ )مثير‬rock formations and landforms
Differential weathering
Climate
Differential
weathering is
illustrated by
these sculpted
rock pinnacles
‫قمم جبلية منحوتة‬
Soil
Soil = mineral + organic mater + water +
air
• Regolith ‫فتات صخري‬: the portion of the rock
and mineral fragments produced by
weathering that supports the growth of
plants
• Humus ‫دوبال‬: the decayed animal and plant
remains, is a small but essential
component.
Composition of a “Good” Soil
Factors controlling soil formation
Parent
material
Slope
Time
Factors
Plants
and
animals
Climate
Soil
Factors controlling soil formation
• Parent material
The source of the weathered mineral
matter from which soils develop is called
the parent material
– Residual soil – parent material is the
underlying bedrock (‫)صخر األساس‬
– Transported soil – forms in place on
parent material that has been
carried from elsewhere and
deposited
Soil
Factors controlling soil formation
• Time
– Important in all geologic processes
– Amount of time for soil formation varies for
different soils depending on geologic and
climatic conditions
• Climate
– Most influential control of soil formation
– Key factors are temperature and
precipitation
Soil
Factors controlling soil formation
• Plants and animals
– Organisms influence the soil’s physical and
chemical properties
– Also furnish organic matter to the soil
• Slope
– Steep slopes often have poorly developed soils
– Optimum terrain is a flat-to-undulating
upland surface
Variations in soil development
due to topography
Soil
The soil profile
• Soil-forming processes operate from the
surface downward.
• Vertical differences are called horizons
‫أفق\مستوى‬, which are zones or layers of soil.
• O horizon
• A horizon
• E horizon
• B horizon
• C horizon
Well-developed soils
Well-developed soils show
distinct layers called
horizons.
Soil
The soil profile
• O horizon – organic
matter
• A horizon – organic and
mineral matter
– High biological activity
– Together the O and A
horizons make up the
topsoil
Soil
The soil profile
• E horizon – little organic
matter
– Zone of eluviation and
leaching
Eluviation ‫اإلزالة‬- fine particles are
carried away
Leaching ‫الغسيل‬- depletion of soluble
material from the upper soil
Soil
As water percolates downward
through E zone, finer particles
are carried away. This washing
out of the fine soil components
is termed eluviation.
Water percolating downward
also
dissolves
soluble
inorganic soil components and
carries them to deeper zones.
This depletion (‫ )نفاذ‬of soluble
materials from the upper soil is
termed leaching.
Soil
The soil profile
• B horizon – zone of
accumulation
• C horizon – partially
altered parent material
The O, A, E, and B
horizons together are
called the solum, or “true
soil”
well-developed
soil
A soil profile showing
different horizons
A soil profile is a vertical cross
section from the surface through all
of the soil’s horizons and into the
parent material. A. The profile
shows a well-developed soil B. The
boundaries between horizons in this
soil are indistinct, giving it a
relatively uniform appearance.
Soil
Classifying soils
• Variations in soil formation over time and
distances has led to a great variety of
recognized soil types.
• Groups have been established using common
characteristics.
• In the United States, a system was devised,
and called the Soil Taxonomy ‫ علم تصنيف التربة‬.
Soil
Classifying soils
• Soil Taxonomy
- Emphasis is placed on physical and chemical
properties.
- Six hierarchical (‫ )ترتيب ھرمي‬categories exist.
- Descriptive names are derived from Latin and
Greek.
- 12basic soil orders are recognized.
Basic Soil Orders
Basic Soil Orders
Global soil regions
Worldwide distribution of the Soil
Taxonomy’s 12 soil orders.
Soil
Soil erosion
• Part of the natural recycling of all Earth
materials that we call the rock cycle
• Natural rates of soil erosion depend on:
– Soil characteristics
– Climate
– Slope
– Type of vegetation
How Soil Is Eroded
Each drop acts
like a tiny bomb.
When it is raining, millions of water drops are falling at velocities
approaching 10 meters per second (35 kilometers per hour). When water
drops strike an exposed surface, soil particles may splash (‫ )تتناثر‬as high as 1
meter into the air and land more than a meter away from the point of
raindrop impact. Then, water flowing across the surface carries away the
dislodged soil particles. Because the soil is moved by thin sheets of water,
this process is termed sheet erosion.
How Soil Is Eroded
A. Soil erosion from this field
is obvious
B. Gully erosion ( ‫تعرية‬
‫ )اخدودية‬is severe in this
poorly protected soil in
southern Colombia.
Soil Erosion Where
Vegetation Is Lacking
When the land is dry
and largely unprotected
by
anchoring
vegetation, soil erosion
by
wind
can
be
significant. The man is
pointing to where the
ground surface was
when the grasses began
to grow. Later wind
erosion of the soil
lowered
the
land
surface to the level of
his feet.
A dust storm
A dust storm
blackens the
Colorado sky in this
historic image from
the Dust Bowl of the
1930s.
Soil
Soil erosion
• In many regions the rate of soil erosion is
significantly greater than the rate of soil
formation
• Sedimentation and chemical pollution
– Related to excessive soil erosion
– Occasionally soil particles are contaminated
with pesticides (‫)مبيدات حشرية‬
End of Chapter 5