Transcript Weathering, Soil, and Erosion

Weathering, Soil, and Erosion
Chapter 5
Weathering
 Def:
the breakup of rock due to exposure to
processes that occur at Earth’s surface
 2 types:
 Mechanical-
the breakdown of rock that takes place
when a rock is broken into smaller pieces of the same
material without changing its composition
 Chemical- the breakdown or decomposition of rock that
takes place when minerals are changed into different
substances
Mechanical Weathering
 Processes
 Frost wedging- when water freezes in the cracks of a
rock and wedges (moves) it apart
Mostly in porous rocks and in rocks with many
cracks
In places where there are frequent freezes and
thaws
Ice Heaving- creates potholes; ice lifts pavement up
and it collapses when it thaws
Examples of Frost Wedging and Ice Heaving
Mechanical Weathering cont…
 Abrasion-
the wearing
away of rock material by
grinding action
 Water,
wind and ice are
capable of moving rocks.
 Sand is a product of
abrasion.
Mechanical Weathering cont…
 Biological
Weatheringfrom Plants and Animals
 Growth
contributes
 Roots
wedged into tiny pores
and crevices
 Burrowing
 Digging
contributes
holes into soil
 Bringing rock fragments to
surface
Mechanical Weathering cont…
 Upward
Expansion
 Exfoliation-
the peeling of surface layers from exposed
bedrock
 Rock
is uplifted by tectonic forces (ex: granite)
 Overlying rock is worn away which reduces pressure
 Upward expansion causes the granite to break along curved
joints
 Large sheets of loosened rock break away
Upward Expansion Examples
Chemical Weathering
 Water
 Hydrolysis-
the chemical
weathering by reaction of
water with other substances
 Acids:
seeps into the ground
and reacts chemically with
many common minerals
 Calcite
Dissolves completely
 Underground caverns

Chemical
Weathering
cont…
 Acid
Rain- rainwater
that contains unusually
high amounts of acids
that can be traced back
to pollutants
 Increases the rate of
chemical weathering
Chemical Weathering cont…

Oxidation- chemical reaction
of oxygen with other substances
 Effective with minerals
containing iron
 Formation of rust, or iron
oxides
Rates of Weathering
 Weathering
is usually a slow process
 Factors that affect the rate of weathering:
 Surface
 More
Exposure
surface exposed, weathers more quickly
 Composition
of Rock
 Various
rocks are affected differently by weathering
processes
 Climate
 Warm/wet
climates: both chemical and mechanical
 Cold/dry climates: mainly mechanical
Soil
 Def:
loose, weathered rock and organic material in
which plants with roots can grow
 Formed
by weathering
 Contains air, water, organic material, mineral matter
 Parent material
 Def:
material from which a soil is formed
 Residual soil- soil whose parent material is the bedrock
below
 Transported soil- soils formed from transported materials

Deposits left by winds, rivers, and glaciers
Soil Profile
 Def:
cross section of earth exposed by the digging
 Soil horizons- three distinct zones of mature soil
 A-horizon Darker,
topsoil
contains humus (organic material), sandy
 B-horizon-
subsoil
 Red
or brown, iron oxides, clay from topsoil, calcium and
magnesium
 C-horizon
 Weathered
parent material, rock fragments
Soil Composition
3
noticeable parts: sand, silt, and clay
 Affects
 Factors
soil’s ability to hold water and air
that affect composition:
 Time
 Parent
material
 Plants and animals
 Topography
 Climate- MOST IMPORTANT!
12.3 Mass Movements and Erosion
 Mass
movements- the downward
transportation of weathered materials
 Gravity
causes materials to fall, slide, or move
at slow speeds to lower levels
 Erosion-
the removal and transport of
materials by natural agents such as wind and
running water
Mass Movements
Talus- rock fragments that have
been weathered from a cliff and
pulled down by gravity
 Landslide- movement of a mass
of bedrock or loose soil and
rock down the slope of a hill,
mountain, or cliff

Steep slopes
 Regions near volcanoes and in
earthquake-prone regions

Mass Movements

Creep- slow, imperceptible
movement of soil down a slope


Causes fence posts, poles and other
objects fixed in soil to lean
downhill
Slump- a block of land tilts and
moves downhill along a surface
that curves into the slope

Tends to occur because bottom of
slope can no longer support top of
slope
Mass Movements

Earthflows- the downslope
movement of a mass of earth
materials that have been saturated
with water


Slower and less fluid than a mudflow;
velocity affected by amount of water
present, the composition of the soil,
and the steepness of the slope
Mudflows- the downslope
movement of water that contains
large amounts of suspended clay
and silt

Rapid movement; capable of moving
rocks, boulders, trees, and houses;
lahars- volcanic mudflows
Erosion
 Topography-
uplifting and weathering/erosion are
happening at same time
 More
uplifting- rugged and sharp (Himalayas)
 More erosion- smooth and rounded (Appalachians)
 Climate-
in humid areas, water is primary agent
and causes rounded topography
 Composition of rock- some types are more
resistant than other rock types
 Volcanic
neck- the solidified lava filling the center vent
of an extinct volcano (picture pg 270)
 http://www.youtube.com/watch?v=HjVSiuj7Lxk