Download Chapter 14 – Weathering and Erosion

Chapter 14 – Weathering and
Erosion
Section 1 – Weathering Processes
Section 2 – Rates of Weathering
Section 3 – Soil
Section 4 - Erosion
Section 1: Weathering Processes
• Weathering – the physical breakdown or
chemical decomposition of rock materials
exposed at the earth’s surface.
Mechanical Weathering
• Strictly a physical process. It does not change the
composition of the rock .
• Agents of mechanical weathering are ice, plants and
animals, gravity, running water, and wind.
• Granite is often formed deep in the earth. As
overlying rocks erode, the pressure on the granite
lessens as the rock is exposed. Due to less pressure,
the granite expands. Small cracks, called joints,
develop. When the joints are parallel to the surface
of the rock, curved sheets peel away in a process
called exfoliation.
• Ice wedging – a type of mechanical weathering that
happens in cold climates. Water seeps into the
cracks in the rock and then freezes. When the water
freezes, the volume increases by 10% and creates
pressure on the surrounding rock. Every time the ice
melts and refreezes, cracks in the rock widen.
• Abrasions – caused by gravity, running water and
wind. It is the collision of rocks against rock that
breaks the rocks over time.
• Organic activity – As plants grow, the roots grow and
expand to create pressure that wedges rock apart.
Burrowing animals also cause weathering.
Ice Wedging
Chemical Weathering
• The process by which rock is broken down
because of chemical interactions with the
environment. The chemical reactions occur
with the minerals of the rock. The reactions
occur between rock, water, carbon dioxide,
oxygen, and acids.
• Chemical weathering changes both the
chemical composition and physical
appearance of the rock.
Types of Chemical Weathering
• Oxidation – the process by which elements
combine with oxygen. It commonly occurs in
rocks and soils that contain iron. The oxygen
dissolved in the water forms red iron oxide.
• Hydrolysis – water causes a change in
composition of minerals. Often this change
form clay. The water can carry the dissolved
minerals to lower parts of the rock in a
process called leaching.
Flashflood – notice the red rock
walls
• Carbonation – When carbon dioxide in the air
dissolves in water a weak acid is formed called
carbonic acid. When certain minerals come in
contact with carbonic acid, they are changed into a
new mineral called a carbonate. Carbonic acid reacts
with calcite, a major component of limestone, and
forms calcium bicarbonate. Calcium bicarbonate
dissolves easily in water, and so limestone weathers
away.
• Organic Acids – these are produced naturally by
certain living things. Lichens and mosses grow on
rocks and produce weak acids that can weather the
surface of the rock. The acids seep down into the
rock.
• Acid Precipitation - (Acid Rain) – natural
rainwater is slightly acidic, but when it
combines with nitrogen oxides and sulfur
dioxides found in the air (due to burning fossil
fuels), nitrous acid or sulfuric acid is formed in
the rainwater. When this acid rain falls to the
ground, it weathers some rocks faster than
normal precipitation.
• Many historical monuments and sculptures
have been damaged by acid rain.
Section 2: Rates of Weathering
• The processes of mechanical and chemical
weathering are usually very slow.
• Carbonization of limestone dissolves only .2
cm every 100 years.
• The rate at which rock weathers depends on a
number of factors – including rock
composition, climate, and topography.
• Differential weathering – the process by which
softer, less weather-resistant rock wears away
and leaves harder, more resistant rock behind.
• Rock composition – Limestone and other
sedimentary rocks that contain calcite are weathered
most rapidly due to carbonation.
• Amount of Exposure – The more exposure to
weathering agents a rock receives, the faster the rock
will weather. Fractures and joints in the rock also
help it weather more quickly. One reason why, is
that there is a greater surface area available and
exposed to the elements.
• Climate – Climates with alternating periods of hot
and cold weather, have faster rates of weathering.
The slowest rates of weathering are in hot, dry
climates.
• Topography or elevation – temperatures are
generally cold at high elevations and ice wedging is
more common at high elevations. On
mountainsides, weathered rock fragments are pulled
downhill by gravity and washed out by heavy rain.
Due to all this, new surfaces of the mountain are
continually exposed to weathering.
• Human activities – mining, construction, recreational
activities such as hiking or riding all-terrain vehicles.
• Plant and Animal Activities – rocks are disturbed
and broken by plants and animals. The biological
wastes (guano) can cause chemical weathering.
Section 3: Soil
• Regolith – the layer of weathered rock
fragments that covers much of Earth’s surface.
• Bedrock – the solid, unweathered rock that
lies beneath the regolith.
• Soil – complex mixture of minerals, water,
gases, and the remains of dead animals – very
fine particles.
• Characteristics of soil y
–
– Depends mainly on the parent rock from which it
came. If it stays near the parent rock, it is called
residual soil. If the soil has been carried away by
water, wind, or glaciers, it is called transported
soil.
– Parent rocks rich in feldspar and aluminum
produces soils that contain large amounts of clay.
– Black soils are rich in organic material
– Red soil often form from iron-rich parent rocks
– Soil texture – Three main types
• Clay diameter from .004 mm
• Silt
diameter from .004-.06 mm
• Sand diameter from .06-2 mm
• Soil Profile is a cross-section of the soil and its
bedrock. The different layers of the soil are
called horizons.
• Residual soils have 3 horizons
– Horizon A – topsoil. It is a mixture of organic
materials and small rock particles. Almost all
organisms live in Horizon A. When the organisms
die, they decompose and become humus.
– Horizon B – subsoil. Contains minerals leached
from the topsoil, clay, and sometimes humus.
– Horizon C – contain s partially-weathered bedrock
Section 4: Erosion
• When rock weathers, the rock particles do not
always stay near the parent rock. The process
where particles move is called erosion.
• Farming and ranching increases soil erosion.
Ground cover is destroyed and increases
erosion rate.
• Soil erosion is considered one of the major
environmental problems of the day by some
scientists.
Types of erosion
• Gullying and sheet erosion – Gullying is where
water runs over a surface with a small furrow.
As the water increases speed, the furrow
increases into size to a gully. Sheet erosion is
where the entire layer of topsoil is washed
away.
• Rockfalls and landslides – the fastest kinds of
mass movement. Rockfalls are the fall of rock
from a steep cliff. Landslides are when masses
of loose rock begin to slide down a steep
slope.
• Mudflows and slumps – A mudflow is the rapid
movement of a large amount of mud usually in a
dry, mountainous region after there has been a
sudden, heavy rainfall. A slump is a large block of
soil and rock that becomes unstable and moves
downhill in one piece. This often occurs after
being saturated by water.
• Solifluction – the movement of water-saturated
soil over hard or frozen layers – usually topsoil.
• Creep – an extremely slow downhill movement of
weathered rock
• Talus – rock fragments that accumulate at the
base of a slope
Soil Conservation Methods
• Contour plowing – soil is plowed in curved
bands that follow the contour or shape of the
land
• Strip-Cropping – crops are planted in
alternating bands, usually with one band
being a cover crop like alfalfa or clover.
• Terracing – constructing step-like ridges that
follow the contours of a sloped field
• Crop Rotation – farmers plant one type of crop
one year and a cover crop the next year
• The three major landforms that are shaped by
weathering and erosion are mountains, plains, and
plateaus.
• Erosion of Mountains – During early stages of a
mountain, the mountain undergoes uplift. Uplifted
mountains have sharp peaks and deep, narrow valleys.
Weathering and erosion wear down the rugged peaks to
rounded peaks and gentle slopes.
• Erosion of plains – A plain is a relatively flat landform
near sea level.
• Erosion of plateaus – A plateau is a flat, broad landform
that has a high elevation. It has more erosion than a
plain. In a dry climate, resistant rock forms flat-topped
mesas. As they continue to erode, the become buttes.
Both have steep sides and flat tops.
Mesa in Arizona
Buttes