Download water = more weathering

Chapter 7: Weathering,
Erosion, and Soil
How does this
cartoon
relate to
weathering
and
erosion?
BIG Idea:
Weathering and erosion are
agents of change on Earth’s
surface.
• Weathering: the process by which
rocks are broken down chemically
and physically by the action of
water, air, and organisms.
I. WEATHERING PROCESSES
A. Mechanical Weathering
Strictly a physical process
 Rocks and minerals break into
smaller pieces
 No change to
chemical
composition
Agents of MECHANICAL
Weathering:
• ice
• plants and animals
• gravity
• running water
• wind
1. Ice (Frost) Wedging
• water seeps
into rock
cracks and
then freezes
What causes this to happen?
2. Pressure
• Exfoliation: outer rock
layers are stripped
away (like an onion’s
layers can be peeled)
3. Organic Activity
•Root Action: tree roots
grow within cracks in
rocks and cause the
rocks to split
4. Abrasion
• The collision of rocks with one
another, causing them to break
down
• Agents of abrasion:
 gravity
 running
water
 wind
wearing down
or smoothing
of a
material due
to
constant
collisions
Wind Abrasion
Water Abrasion
B. Chemical Weathering
• Process by which rocks and
minerals undergo changes in
their composition
– Can cause some substances to
dissolve
– Some new minerals form
– New minerals have properties
different than those of original rock
• What items have you seen
rusted?
• What could you do to such
objects to prevent or reduce
rusting?
Agents of CHEMICAL
Weathering:
• Water
• Oxygen
• Carbon dioxide
• Acid precipitation
1.Hydrolysis: reacting with
water
2.Carbonation: carbon
dioxide combines with
water to produce carbonic
acid (dissolves limestone)
3.Oxidation: reacting with
oxygen (rust)
• Carbonic acid
creates caves
Oxidation of the iron in the rock
causes a red hue
• Acid precipitation (rain):
sulfur dioxide, carbon
dioxide and nitrogen
oxides released into
atmosphere by human
activities; gases combine
with oxygen and water to
form strong acid rains
• Plant acids: from decaying
plants
II. RATES OF WEATHERING
• All rocks do not weather at the
same rate (speed)…
• Some factors that affect the rate
at which rocks weather are:
A. Rock Composition
• Igneous and metamorphic rocks
break down more SLOWLY
• Hardest minerals resist
weathering (i.e. quartz)
• Rocks containing calcite weather
QUICKLY
Which layer is MORE resistant
to weathering? LEAST
resistant?
A
C
B
D
B. Amount of Exposure
• time exposed
• surface area
Surface Area
• Which will weather faster, solid
rock or crushed rock?
• The crushed rock of course!
–the large rock is tightly packed
and has less surface area
exposed
–the crushed rock is loosely
packed and has more surface
area exposed
Increased surface area =
increased weathering
C. Climate
• Rocks in tropical regions weather
faster than rocks in dry desert areas
• More rain, humidity, wind, and
temperature range = faster
weathering
• Physical
Weathering:
works better in a
cool and dry
climate
• Chemical
Weathering:
works better in a
hot and moist
environment
(more water =
more
weathering)
Cleopatra’s Needle stood in
Egypt (hot/dry) for 3,000
years…
• It was only in New
York City for 100
years
• Rain, ice, and
pollution destroyed it
Compare the two tombstones
in the next picture…
How do they compare in
appearance?
Which appears more
weathered?
marble
slate
Which tombstone is
older?
marble
slate
This means that rocks can weather at different
rates!
marble
slate
D. Topography
• Higher elevation =
faster weathering
(colder, more ice
wedging)
• Steep slopes gravity pulls rocks
down hill
III. Weathering and Soil
Soil: loose rock
fragments and
clay from
weathered rock
mixed with
organic
material.
A. Soil Composition
A = Humus - dark
organic material
B & C = Soil – mixture
of minerals, gases,
and dead organisms
D = Regolith – layer of
weathered rock
fragments
E = Bedrock – solid
unweathered rock
B. Soil Profile
Soil Profile: cross section of the soil layers
Horizons: layers in
the soil
Two Types of Soil:
1. Residual: parent material is the
local bedrock
2. Transported: develops from
parent material that has been
moved far from original location
Residual Soils
• forms from the weathering of the
underlying bedrock
Transported Soils Due to…
IV. EROSION
• Process by which the products of
weathering are TRANSPORTED
(moved)
• Agents of erosion:
•Running WATER
•wind
•glaciers (ice)
A. Gravity and Erosion
• Talus: rock
fragments
at the
base of a
slope
Mass Movements
Rock fall: fall of rocks from a
steep cliff
Landslide: loose soil and rocks that
move down the slope of a hill
Mudflow: rapid movement of a
large mass of mud
Creep: slow downhill movement of
rocks and soil
B. Landforms and Erosion
• 3 major landforms:
–Mountains: steep, high
elevations
–Plains: flat or gently sloped,
not high above sea level
–Plateaus: high elevations, flat
surfaces