Transcript Document

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Weathering & Erosion
Weathering
 When Rocks undergo changes at or near the Earth’s
surface it is called
Weathering
 What type of rock weathers the fastest?
ans. Sedimentary
 There are 2 Types of Weathering:
1. Mechanical or Physical
2. Chemical or Decomposition
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Rates of Weathering
 Three Factors affect the rate of
weathering:
 1. Rock Characteristics
 2. Climate
 3. Surface Area
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Climate
 Climatic features, especially
TEMPERATURE & MOISTURE, have
a strong effect on the rate of
weathering.
 Why?
Controls Freeze-thaw cycles – which in
turn controls amt. of Frost Wedging.
Controls amt. of Vegetation – which in
turn affects amt. of humic acid released by
plants.
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Rock Characteristics
 Cracks – allow rocks to weather because
they influence the ability of water to
penetrate rock.
 Mineral composition – can have a big effect
on weathering. Ex.
– Granite – weathers slow and resistant to
chemical weathering
– Marble – undergoes rapid chemical weathering
Ex. Gravestones
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What is the BEST climate?
High Temperatures
and
Abundant Moisture
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Differential Weathering
 Different parts of rock masses will
weather at different rates.
 Why
Because of
different
mineral
compositions
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Different layers of the rock are weathered at different
rates depending on the minerals within the rock. The
layers sticking up most are the most resistant
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Rates of weathering will be
influenced by...
 Surface area
exposed weathering
occurs on the
surface. More
surface
exposed, the
faster the
weathering
will occur.
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Mechanical Weathering
 Mechanical – A type of Physical Weathering
in which no new substance is formed. Rock is
only broken down physically into smaller
pieces of rocks.
– Occurs mostly in temperate zones where you have
alternating freezing and thawing.
– Occurs when plants take root in cracks and break
rock up.
– Occurs when animals trample rocks and break them
down
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Types of Mechanical
Weathering (Disintegration)
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
1.
Joints
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Unloading or Joints– Mass of rock
is pushed upward at the surface
reducing the pressure on the rock
layer.
 Result = slabs of rock break free and
gravity pulls them down slope.
(Talus)
 When granite expands into long
curved cracks parallel to the surface.
When joints develop on the surface of
the rock, the rock breaks into curved
sheets that peel away from the
underlying rock in a process called
Exfoliation.
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example: Stone Mountain,
Georgia.
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•T h e r m a l E x p a n s I o n •repeated daily heating and cooling of
rock;
•heat causes e x p a n s i o n;
cooling causes contraction.
•different minerals expand and contract
at different rates causing stresses along
mineral boundaries.
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2. Ice Wedging
 A type of mechanical weathering where water
seeps into cracks or joints and freezes.
 When water freezes it expands by
10%. This
process widens cracks in the rock.
 Over time the fracture will expand as the
freeze/thaw cycle continues.
Can result in the formation of a talus slope
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Rock Fall - the freefall of rock from a cliff. Rock falls
are often the result of physical weathering such as ice
wedging.
The rock typically accumulates at the base of
the cliff in the form of talus (loose rock).
Rock falls are often triggered by
earthquakes.
3. Organic Activity
Root wedging occurs when a plant,
especially trees, sink root systems into
existing joints and fractures.
As the root grows it forces the fracture to
expand. Sometimes this cracks rocks into
smaller pieces.
 Animals that burrow in the ground expose
rocks and cause them to be weathered.
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4. Abrasion
 Moving material can impact or abrade on a
surface.
The surface can fracture, flake or loose
small particles.
Rounded rocks have been
exposed to more
mechanical weathering;
they have traveled further,
they have been around for
longer, or existed in areas
with more rubbing and
abrasion
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Types of Chemical
Weathering (Decomposition)
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Decomposition
 Decomposition – A type of Chemical
Weathering in which new substances are
formed and the rocks are altered in
composition.
– Occurs mostly in warm climates
– Typically replacing strong minerals with weaker
minerals
– 3 common chemical reactions associated with
chemical weathering are dissolution, hydrolysis,
and oxidation.
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1. Dissolution
 Dissolution occurs when rocks and/or
minerals are dissolved by water. The
dissolved material is transported away
leaving a space in the rock. One
consequence of this process is the formation
of caves in limestone areas.
 rain + carbon dioxide (from air)  carbonic
acid (reacts with rocks)
 H2O + CO2  H2CO3
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The details of marble statues and tombstones
will be gradually worn away by dissolution.
Image courtesy of Dr. Annabelle Foos.
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Dissolution
Acidic waters (from pollution
or natural) dissolve limestone
allowing for additional water
to gain entrance. Can cause
sinkholes and karst features
as well as dissolution of
statutes and grave stones.
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1. Carbonation (Dissolution)
 Carbon dioxide (CO2) from the air is
dissolved in rainwater to create a weak acid,
carbonic acid (H2CO3), that preferentially
dissolves certain rocks and minerals, e.g.
limestone, marble. All rain is mildly acidic
(average pH ~5.6, compared with neutral
fluids [pH 7] and highly acidic [pH 1]
substances).
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Dissolution
Different
effects of
chemical
weathering
according to
rock type:
limestone in
foreground;
granite in left
background,
Vermont.
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Cavern Systems
 Caves form when dissolution occurs along a series
of fractures in limestone to create a larger
opening. Water passing through the rock enlarges
the cave and associated re-precipitation can form a
variety of features. The dissolved limestone is
transported through the cave and may be
precipitated to form new features such as
stalagtites that grow downward from the cave
ceiling and stalagmites that grow up from the
floor. If they meet they form a compound cave
formation such as a column.
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Sinkholes
 Not all the products of dissolution are below
ground. Sinkholes form at the surface from
the collapse of the roof of an underlying
cavern or by dissolution of rock along a
series of fracture surfaces.
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Image below shows a sinkhole, (Watlings Blue Hole) in the Bahamas.
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2. Spheroidal Weathering
When water enters along cracks or
joints in a rock, it weathers the
corners and edges very fast because
they have a greater surface area.
This is called Spheroidal weathering.
 This happens when minerals in
rocks turn to clay.
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Spheroidal Weathering
Caption: Spheroidal weathering
(a) The rectangular blocks outlined by joints are
attacked by chemical weathering processes,
(b) but the corners and edges are weathered
most rapidly.
(c) When a block has been weathered so that it is
spherical, its entire surface is weathered
evenly, and no further change in shape occurs.
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Pictured here is a phenomenon
referred to as spheroidal
weathering.
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Cracks in the
diabase break up
the rock into
adjoining blocks.
Water seeping
along the cracks
weathers
(decomposes)
each isolated
block from the
outside in
toward the
center. The
weathered rock
peels off the
unweathered
core like onion
skin.
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3. Hydrolysis (Granite Weathering)
Hydrolysis occurs when minerals react
with water to form other products.
Feldspar, the most common mineral in
rocks on the earth's surface, reacts with
water to form a secondary mineral such as
kaolinite (a type of clay) and additional
ions that are dissolved in water.
The weaker clay is readily worn
away by physical weathering.
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Feldspar alters to clay (kaolinite) plus dissolved materials
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Leaching

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Leaching is the process where
minerals that are soluble and
dissolved are carried down
through the soil by the
movement of water.
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4. Acid Precipitation
 The term acid precipitation is used to
specifically describe wet forms of acid
pollution that can be found in rain, sleet,
snow, fog, and cloud vapor.
 Precipitation is considered to be acidic
when its pH falls below 5.6
 (which is 25 times more acidic than pure
distilled water).
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Nitrogen oxides (NOx) and sulfur dioxide
Form ACID PRECIPITATION
 Most wet acid deposition forms when
nitrogen oxides (NOx) and sulfur dioxide
(SO2) are converted to nitric acid (HNO3)
and sulfuric acid (H2SO4) through oxidation
and dissolution.
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How do we measure pH?
Acid rain is measured using a scale
called "pH." The lower a substance's
pH, the more acidic it is.
Pure water has a pH of 7.0. Normal
rain is slightly acidic because carbon
dioxide dissolves into it, so it has a
pH of about 5.5. As of the year 2000,
the most acidic rain falling in the US
has a pH of about 4.3.
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Acid deposition has a variety
of effects including,
Damage to forests and soils,
fish and other living things,
materials, and human health.
 Acid rain also reduces how far
and how clearly we can see
through the air, an effect called
visibility reduction.
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Chemical Weathering – e.g
Dissolution by acid rain
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Picture of acid-generating, iron
sulfate salts on the New Albany
Shale
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5. Plant Acids
 Lichens and Mosses grow
on rocks and produce weak
acids that dissolve the
surface of rocks.
Example: Humic Acid
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Plant Acids – Eating away
at rock chemically.
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6. Oxidation
Oxygen, the second most
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common element in the air we
breathe, reacts with iron in
minerals to form iron oxide
minerals, e.g. hematite (rust).
As many minerals contain
iron, it is not unusual to see
red-colored rocks like the
example from Arches National
Park.
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Oxidation
occurring on the
door and top of
this building
National Arches State Park
Oxidation is occurring
Weathering rind in
granite produced
primarily by
oxidation.
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Identify what type of
Weathering is occurring in
the following slides.
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Identify the Following Pic.
A.
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Identify the Following Pic.
B.
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Identify the Following Pic.
C.
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Identify the
Following Pic.
D.
This Arrow is pointing at
__________ type of
weathering
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Identify the Following Pic.
E.
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Identify the Following Pic.
F.
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Identify the following Pic
G.
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