Transcript Erosion Notes NO PICS File

11) Solid Earth. The student knows that the
geosphere continuously changes over a range of
time scales involving dynamic and complex
interactions among Earth’s subsystems. The
student is expected to:
a. Compare the roles of erosion and deposition through the actions of
water, wind, ice, gravity, and igneous activity by lava in constantly
reshaping Earth’s surface;
•Student understands the difference between mechanical and chemical
weathering
•Students know events of the Hydrologic cycle (precipitation,
infiltration, run-off)
•Students will be able to describe the features produced by glacial
erosion and deposition
•Students will be able to list the types of wind deposits and
describe the features of wind deposition
•Students will know that gravity is the controlling force of
mass wasting
If you have ever been to the Grand Canyon, then you know the splendor of the
natural Earth altering process we call weathering. For millions of years, the
mighty Colorado River has etched its trail throughout the American west,
carrying with it the remains of the land it is carving out.
Mechanical weathering is
The materials that make up
sedimentary rocks largely come from simply the process by which
larger rocks are broken down
the bedrock, which forms the
into smaller rocks in a noncontinents.
chemical fashion. The small
fragments that are chipped
Agents such as water and ice act upon away from the parent rock are
this bedrock, which tend to break the not chemically altered.
rock into smaller and smaller pieces.
Some of the mechanisms of
mechanical weathering include
The breaking down of rock into smallerfrost, temperature, plants and
animals (organic activity), wind
pieces, or the changing of the
substances making up rocks into other (abrasion), water, and gravity
(landslides).
substances is called weathering.
The type of weathering by
Rocks can be altered in two ways:
plants as seen in the photo to
either mechanically or chemically. the left is known as “root-pry”.
When chemical weathering
occurs, changes in the mineral
composition of rocks become
apparent.
As these changes take place,
minerals can be added to or
removed from rocks.
The minerals can also be broken
down into completely different
substances in a process called
decomposition.
Many substances react
chemically with rock to break
them down.
Most all chemical
caused by water.
weathering
is
Do not confuse this type of erosion
with the mechanical type spoken of in
the previous section when I outlined
abrasion.
Water is strictly the
medium which carries the abrasive
agent which was tiny jagged particles
of sand.
When water is an agent of chemical
weathering, it is quite different.
When water comes into contact with
certain kinds of rocks and minerals, it
combines chemically with that mineral
in a process known as hydration.
Rocks that dissolve easily in water are
said to be soluble. Gypsum forms this
way.
Water can also combine with certain atmospheric
gases (like CO2) and become acids. These acids,
once in contact with the minerals, will speed up the
process of decomposition.
Water can combine with a mineral to form a
completely different mineral. When feldspar is
dissolved in water, it forms a type of clay called
Kaolinite.
Feldspar is a part of granite, and when it is affected
by water in this way, the structure of the granite is
weakened, and more susceptible to the action of
mechanical weathering. Thus chemical and
Pink Granite
mechanical weathering are linked.
Granite Decomposition through hydration:
1. The feldspars will undergo hydrolysis to form kaolinite and Na and K ions
2. The sodium and potassium ions will be removed through leaching and will be carried in solution in running
water to the surrounding environment.
3. The biotite and/or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides.
4. The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to
weathering.
Abrasion is an erosive process that
occurs when wind (called a haboob), or
water picks up and carries small sand
sized particles, which scrape along the
rocks on the Earth’s surface. Wind is a
mechanical process, and water is
chemical, and mechanical.
These tiny particles have sharp edges,
and over a long period of time, the
action of them rubbing against the
surface of rock is quite noticeable.
Abrasives can create interesting shapes in the rocks on which they act. This
interesting rock in the shape of a mushroom is called a “hoodoo”, and the arch is a
natural bridge. These have been battered for eons by tiny particles of sand and
stone. See here what water carrying abrasives can do.
The type of wind erosion that removes loose materials such as
clay, silt, dust and sand from the land is known as deflation.
The finer particles, like dust, are carried many meters up into the
atmosphere, whereas the larger, dense materials are only lifted a
few centimeters.
The amount of damage this type of erosion accomplishes is
dependent on four things:
• the size of the particles
• the strength of the wind
• the length of time the wind is blowing.
• the resistance of the rock that exposed
Sand dunes are structures found in
desert ecosystems, and beaches.
They are simply mounds of sand
deposited by wind.
When wind in a desert erodes sand
down to a depth where there is water
present, then an oasis is formed. The
exposed water will form a green, fertile
area, where plants and animals will be
sated.
Sometimes sand and silt particles are not
deposited in dunes. Instead, with a strong
enough wind, they are carried many kilometers
away from where they were picked up.
When they are redeposited in another area, a
thick layer of loess is formed. Loess deposits
can be found hundreds of meters thick in China
that have been blown in from the Gobi desert.
Loess makes for very fertile topsoil.
If you recall in your American History, the time of the “Dust Bowl” occurred in
the 1930s and was a period in which there was a great drought in the Great
Plains stretching from Texas to South Dakota.
Lack of rain caused the crops to wither away, leaving the fields dry and
unprotected from the damaging force of the wind. The wind carried thick
clouds of dust all the way to the Atlantic Ocean at this time. When the
particles finally settled, they were considered loess deposits.
Running water is the greatest erosional force on the planet.
Whether it is in the form of gentle raindrops, raging rivers, or the
ebbing and flowing tides of the oceans, water is responsible for
more change on the Earth’s surface than any other agent of
erosion.
Let’s review the
Hydrologic Cycle
What processes in
the water cycle could
be responsible for
weathering?
A flowing body of water has the potential to
do great damage to the surrounding
landscape. As the velocity of the river or
stream increases, so does that potential.
Factors that affect velocity of water:
• Gradient
Gradient, or slope
Channel characteristics including shape, size,
and roughness
• Discharge
Discharge—The volume of water moving past a
given point in a certain amount of time
Stream sediments are known as alluvium. The greater the
gradient, and discharge, the more alluvium the stream carries.
Deposition of sediment by flowing water:
•Delta—Body of sediment which forms where a stream enters
a lake or the ocean, results from a sudden decrease in the
velocity of the flowing water.
Natural levees—Ridges of sand
and
gravel form
Lena Delta
in Siberia
parallel to the stream channel by successive floods
over many years
Sediments are usually deposited on the
riverbank where a river bends. Materials along
the outside of the curve are readily eroded, as
the velocity of the stream is greatest there,
while along the inside curve, where the velocity
slows, materials are deposited.
As the stream or river ages, this tends to
influence it’s shape into a series of twists and
turns known as “meanders”. Eventually, over
time, the river will merge with itself where the
meanders are greatest, and create an oxbow.
If you have ever seen a glacier,
you may wonder how such a
large and seemingly immovable
object could possible change the
physical features of the surface
of the planet.
Glaciers are capable of great
Glaciers are in fact, very movable.
Glaciers are large masses of moving amounts of erosion. The
ice and snow. They are basically rivers evidence of erosion is clear when
of ice which flow downstream due to observing the terminus of an
alpine glacier. The terminus is the
the force of gravity.
farthest point that a glacier
Even when glaciers are retreating, or
apparently moving backwards, the sun’s advances at a given time.
energy is responsible for the melting
them and making this happen.
As the ice in the glacier moves along, it carries
with it sand, pebbles, rocks, boulders, and
even larger pieces of debris.
These particles act as a sort of “sandpaper”
barrier beneath the ice, and it scrapes and
scratches the surface. In the picture below see
glacial striations, and polish. Sometimes,
glacial surfaces are polished to a complete
shine. Also note the erratics, or large rocks left
behind as the glacier retreated.
One of the most spectacular
sites to see in Yosemite
Valley is the gigantic igneous
intrusion known as Half
Dome. With flat side
towering nearly 5,000 feet
above the valley floor, it’s
geological history is
remarkable.
The rounded summit is the
result of exfoliation,
caused as rock withstood
thousands of nights of
freezing temperatures,
followed by thousands of
days warming into the 90s.
Weathering actually occurs
as rock cracks in sheets
parallel to the surface of the
exposed granite, and then
slides off.
While Half Dome’s ascent was deemed impossible in the mid-eighteen hundreds, its
summit was conquered in 1875. Hikers today can climb in relative comfort. What type of
erosion do these hikers have the potential to cause?
Mechanical/organic
Sediment that is carried
by a glacier is known as
LOAD. An advancing
ice sheet carries an
abundance of rock that
was plucked from the
underlying bedrock;
only a small amount is
carried on the surface
from mass wasting.
The rock/sediment load
of alpine glaciers, on
the other hand, comes
mostly from rocks that
have fallen onto the
glacier from the valley
walls.
Ice sheet glacier in Antarctica
Mountain Alpine Glacier
• Glacial flour - rock ground to the texture of a fine powder. It
usually flows out of a glacier as sediment in a glacial meltwater
stream running from the glacier.
• Till - refers to unsorted mixture of sediment, clay, gravel, and
rocks deposited by a glacier.
• Moraine - a French word that refers to any glacier-formed
accumulation - there are a variety of moraines:
 Terminal moraine - an accumulation at the outermost edge
of where a glacier or ice sheet existed.
Lateral moraine - ridges of till on the sides of a glacier.
 Medial moraine - a moraine formed when two glaciers
merge (a tributary and trunk glacier) and their lateral moraines
come together to form a single moraine.
• Glacial erratics - large boulders that were been carried by the ice
and deposited. They are much different in size than surrounding
till. They look ‘out of place’, and are usually easy to spot.
Gravity is another agent of
mechanical weathering.
When gravity pulls loosened
gravel down a mountain, it is
called a landslide, or “mass
wasting”.
A landslide is a large
movement of loose rocks and
soil.
As the rocks fall, they collide
with each other and break
into smaller pieces.
Mass wasting can occur
in a rapid amount of
time, or extremely
slowly.
In either case, when the
sediments come to rest
at the bottom of the
slope, it forms what is
known as a talus slope.
The result of build-up
(deposition) of years of
talus slope is a gently
“U”-shaped valley, such
as in this picture.
Rapid Mass Wasting
• Mudflow
Many times after a particularly hard rain, the rain mixes with the soil to form mud. The
mud will begin to move downhill, and as this happens, more and more soil is picked up.
The mud gets thicker and thicker. It picks up speed, and moves down the slope with
such a force that it will carry away anything in its path, including cars and houses!
• Pyroclastic Flow/Lahar
A special kind of mudflow that occurs after a volcano erupts can be nearly as destructive
as the exploding volcano itself.
•Slump
Sometimes, a large block of rock or soil will slide quickly down the slope of a mountain,
not tumble. Slump occurs when rock or soil that is resistant to weathering lies on top of
a layer of weaker rock or soil. The weak layer begins to slip down the slope and the
entire block of earth breaks off and slides down the hill.
Slow Mass Wasting
• Earthflows
Earthflows occur after large amounts of rain have fallen, and an
entire slope moves en masse slowly downward due to gravity. It
will bring plants and even small trees with it. They are
DIFFERENT from slump, because earthflows happen slowly.
• Soil Creep
This is the slowest kind of mass wasting. Many different weathering forces
come into play, including thermal expansion, animal activity, and water. If you
look at the picture above, you will notice the soil “creeping” in what looks like
steps, down a gentle slope. This can be easily seen in agricultural areas, where
hills are covered with grasses. Ever so slowly, the earth is pulled downhill due
to gravity.
Volcanoes are found both on land and under the oceans, where
they are called seamounts. Geologists label volcanoes by their
periods of activity.
•If a volcano is erupting, it is called
active.
• If a volcano is not presently
erupting but might at some future
date, it is called dormant.
• If a volcano has stopped erupting
forever, it is called extinct.
Generally, volcanoes are labeled
extinct when no eruption has been
noted in recorded history.
Kilauea in Hawaii
Mount Fuji in
Japan
Mount Shasta,
California
It's common to think that the greatest danger coming from
volcanoes is lava, liquid rock at temperatures of about 750°C to
1250°C, and splatter thrown out of erupting craters.
Pyroclastic flows are clouds of hot
volcanic gases, and ash that can sweep
down the volcano's sides and other
steep hills at speeds over 100 miles per
hour. Small volcanic particles are mixed
with high temperature gasses, ash and
larger pieces of rock, forming a red hot
cloud that is very dense.
Imagine a cloud of sand and air straight
from a blast furnace, and you are
getting close to what a pyroclastic flow
looks and feels like.
A pyroclastic has an added killer
element; poisonous gasses at
temperatures hot enough to burn your
lungs away.
In fact, flowing lava kills relatively few
people, and unless you somehow
become surrounded by flowing lava it is
usually possible to run, or even walk
away from most lava flows!
The real killers travel at over 100 miles
per hour, move across land and sea,
flow uphill as well as down, rip trees up
by their roots, flatten buildings and kill
people and animals instantly. These
deadly processes are called pyroclastic
flows and lahars.
As you can imagine, both pyroclastic flows
and lahars have great potential to be highly
erosive forces.
Volcanoes may be
•steep sided cinder cones
 Cinders and ash fall out of the air and accumulate in steep-sided piles, but these
are easily washed away by agents of erosion so they are rarely very large. A great
example of a cinder cone is Paricutín in Mexico. It was born in February 20, 1943
in a corn field and grew to 300 feet in 5 days.
• gently sloping shield volcanoes
constructed of lava flows
Shield volcanoes are huge in size. They are built by many layers of lava flows.
Lava spills out of a central vent or caldera. A broad shaped, gently sloping cone is
formed. This is caused by the very fluid, basaltic lava which can't be piled up into
steep mounds.
• composite, or stratovolcanoes.
 These volcanoes are formed by alternating layers of lava and rock
fragments. This is the reason they are called composite. They often form
impressive, snow-capped peaks which are often higher than 2500m in
height. Between eruptions they are often so quiet they seem extinct. To witness
the start of a great eruption requires luck or very careful surveillance. Composite
volcanoes erupt explosively. This is usually caused by viscous magma.
• Like any fluid, molten lava flows downhill,
moving down valleys and off ridges.
• Frequently it will cool and solidify in the
valleys, forming a rock which is very resistant
to weathering and erosion.
• As time goes by, the surrounding softer rocks
may be eroded away, leaving the lava flows at
a higher elevation, protecting the rock beneath
them from erosion.
• In this way "inverted topography" is
developed, where those areas which were
lowest become the most elevated.