Weathering, erosion, deposition (teacher background)

How are Landforms Made?

• The forces that create the different
landforms are, broadly speaking:

• Constructive forces

• Destructive forces

Constructive forces
• Are those that build up the land.

• Landforms such as mountains ranges,
volcanoesand plateaus are built by the
movement of the Earth’s plates

• Landforms such as deltas, plains and sand
dunes are created when rocks and soil
resulting from weathering and erosion are
carried away and deposited in new areas.

Destructive forces

• Those that wear down the land, like weathering
and erosion.

(Don’t let the name “destructive forces” mislead you.
Destructive forces create landforms like canyons,
valleys, etc. but to do so they first had to destroy
some other landforms, mountains, plateaus, etc.)

The two players in the destructive process are:

Weathering
and

Erosion

All rocks do not weather at the same rate.

Further we will find that parts of some
rocks weather faster than other parts of
the same rock. Why?
Whatever the reason, one can find some
very odd looking weathered rock:

Why?

Grand canyon

Image courtesy of National Geographic

Why?

Is it because the
rock on top is
harder than the
rock below?

http://www.flickr.com Wolfgang Staudt

Yes, the rock on the bottom is softer than the
rock on top of this formation. The top rock
weathers more slowly.

What causes weathering; that is what
causes rocks to break into smaller and
smaller pieces?

• Nonliving things and living things can
break bigger rocks into smaller pieces.

We will look first at non living things that
break up rock.

Nonliving things that break rocks into pieces.
1. Water running over the rock
2. Water freezing in cracks in the rocks
3. The temperature of rocks changing
from hot to cold
4. The abrasion of rock by the blowing
wind carrying sand
5. Water with acid in it

Rapidly moving water particularly high up in
the mountains or a canyon can break off pieces
of rock.
http://www.ngu.no/en-gb/hm/

Seen here, a small,
fast running
stream of water is
dramatically
cutting through
the rock in the side
of this mountain.

Rocks carried by fast moving water hit other rocks
breaking them into pieces. Moving sand acts like
sandpaper on the larger rocks in the river bed
rubbing off pieces of rock.
These
smallerpieces
are then
carried
downstream
by thefast
movingriver.
http://www.flickr.com kia4067

Here you can see pieces of rock created by
fast moving water.

http://www.flickr.com Randy OHC

Rocks that have
been tumbled for
a long time in
rivers and streams
become smooth
and rounded.

http://www.flickr.com
Dawn

Here we can see
cracks in large
mountain rocks.

Copyright 2004 by Andrew Alden,
geology.about.com, reproduced under
educational fair use http://www.ct.gov/

Rain fills these cracks with water.
When the water freezes, it expands within
the crack pushing the rock apart.

Image courtesy of the geology department umd

As the ice melts, the water flows
deeper into the crack.
Then the water freezes again, further
cracking the rock.

This repeated of
freezing and
thawing forces the
crack open more
and
more, eventually
shattering the large
rock into smaller
pieces.

3. Changing temperature also causes
weathering.
As rocks heat up, they expand. As they
cool, they contract.
This process is repeated over and over again
in nature.
Eventually this process causes them to
break apart.

Here we see
large rock in a
desert
environment
that has
probably
been exposed
to the freeze-
melt cycle.

Hoggheff aka Hank Ashby aka Mr. Freshtags'

4. Abrasion by windblown sand also
weathers rocks particularly in deserts. It
is similar to rubbing sandpaper over a
piece of wood or sandblasting concrete.

http://www.flickr.com beige alert

Frequent sandstorms in the desert
weather exposed rocks.

http://www.flickr.com nukeit1

The scouring of the rock by the wind
carrying sand wears off fragments of rock.

http://www.flickr.comlumierefl

Carbonic acid is very common in nature. It is
produced when carbon dioxide combines
with water.

When this weak carbonic acid trickles
into cracks in limestone, it dissolves the
rock and eats “holes” in it.

The mildly acidic rain
water flows into
cracks in the ground.

Sometimes it eats
huge holes in the
rock--caves.

http://www.esi.utexas.edu

The same acid that made this rock “holy”
when it was buried in the ground, also
works to make caves

stalactites

stalagmites

This is a picture of a cave with stalactites
and stalagmites.

When the acid water dissolved the rock
evaporates, crystals of calcite are left behind.

When the water from many, many drips at the
top of a cave evaporates, a stalactite forms. (the
one on the ceiling stuck tight ...stalactite)

Drips that fall on the cave floor cause stalagmites
to grow. (The stalagmitesmight have stuck to the
ceiling but they didn’t.)

More pictures of
stalactites and
stalagmites, do
you remember
which is which?

Sometime other minerals in rocks react with
the weak acid in water to form other weaker
substances.
These weaker substances are then more easily
worn away by weathering.

Feldspar changes
to clay.

Living things
that cause
weathering

These are some of the living things
that break rocks into smaller pieces:

1. Plant roots—particularly tree roots

2. Lichen growing on rocks

3. Burrowing animals

The roots of plants, particularly tree roots,
are amazingly strong. When they start
growing as tiny root hairs they can fit into
the smallest of cracks.

As these tree roots continue to
grow, cause the cracks to get bigger and
bigger breaking the rock apart.

Here the
roots of the
tree are
growing in
the cracks in
the rocks
making the
cracks larger.

Chazz Layne

http://media.photobucket.com/ city bumpkins

Lichens appear in the form
of small patchy crusty
colors of green, brown,
and orange patches. They
often grow on rocks and
break them apart.

http://www.flickr.combrian http://www.flickr.comSeaDavid

3. Burrowing animals
When animals burrow in rocks or between
the rocks, they carry seeds which germinate
in the cracks in the rocks.

So far we have examined weathering Now
we will move to the concepts of
erosion and deposition.

They will be considered together because it
is hard to separate them; they occur at the
same time.

Erosion—the movement of rocks and sediment
from one place to another. The main agents are:
1. Water 3. Ice
2. Wind 4. Gravity

Deposition—the dropping of sediment and/or
rocks in another place follows weathering and
erosion. Deposition occurs when:
1. Water carrying the sediment slows down.
2. The wind carries the sediment dies down.
3. The glacier carrying the sediment melts.

Here we see
the processes
acting of
weathering, Gravity

erosion and
deposition Deposition

working
together.

Water is the main agent of erosion.

Running water carries weathered pieces of
rock from one location to another.

It can carry big pieces of rock as well as
smaller rock pieces and soil.

It can also weather rock at the same time as
it is eroding it (carrying it away).

Rock is worn away (weathered) at the same
time that is carried away (eroded) by fast
moving water.

Rocks hit
one
another
causing
them to
break.

Fast moving water can
move (erode) very large
rocks transporting
them downhill along
with smaller rocks.

http://www.flickr.comHamed Saber
http://www.flickr.com Diego

A slow moving river carries mainly pebbles,
sand, silt and clay (sediment. The slower
water cannot carry bigger rocks.

http://www.flickr.com traveling lunas
http://www.flickr.comwatchsmart

As soon as
the rushing
water slows
down, the
larger pieces
of weathered
material it
had been
carrying is
deposited.
Photo courtesy daneen_vol of Flickr under
Creative Commons license

As can be seen by this diagram, as the water slows
down, first the larger pieces fall out. And then as it
flows slower and slower, smaller and smaller pieces
are deposited.

Soil and pebbles
may be carried for a
great distance as
sediments in the
river. As we said
before, a river
carrying a lot of
sediment looks
muddy. Shown
here, sediments are
being deposited at
the mouth of a river
Wikipedia Commons
in Lake Genoa.

http://www.arthursclipart.org/

When sediments (gravel and soil) are
deposited at the mouth of a river, a delta may
be formed. It becomes a fertile area for to
grow crops such as rice.

Sediment deposited at the mouth of a
river also may build a sandy beach.

Sediment deposited at the mouth of a
river also may build a sandy beach.

• Some of the sediment is deposited
immediately at the mouth of the river.
• Most of the sediment becomes suspended
in sea water and is carried along the coast by
the longshore current, a stream of water
flowing parallel to the beach. This current is
created by waves breaking at an angle to
shore.

Wind, the second agent of erosion.

Wind picks up
small pieces of
rock or soil and
transports them
from their
source to
another location
where they are
deposited. http://www.flickr.com nukeit1

When the wind blows in the desert, sand is
continuously deposited in a different places.
When the
wind
stops
blowing,
new sand
dunes
may have
been
formed.

In this section we see will see mesas,
buttes, arches, canyons and and other
strange rock formations that were created
through both weathering and erosion.

The wind’s remarkable ability to sculpt
such odd and beautiful landforms is
explained by the hardness of the rock
involved. Some rocks are softer and
weather faster than others.

Plateau with mesas, butte and gully

/
puroticorico

Photo of a mesa—this is what is left of the plateau
that made up this entire landform before weathering
and erosion carried much of it away.

http://pics4learning.com/

Photo of buttes

What could the red line be depicting? http://pics4learning.com/

Photo of arch

Why was the middle of the arch
eroded away and not the supporting sides?. Photo courtesy of USGS

The difference in hardness of the rock composing the “cap” and
the rock below the cap explains why the under rock weathered
faster than the cap itself.

a hoodoo.

http://pics4learning.com/

Here we can see that the weathering and erosion from these
boulders is filling in the area beneath them.

Over hundreds, maybe thousands of years,
it could have happened something like this.

On their way to the sea, some rivers wind across
plateaus, carving deep valleys and taking sediment
down stream with them.
Once the valley walls become exposed, these
rocks are further weathered by the wind, rain
and changing temperatures.
Over millions of year, these valleys can become
giant, majestic chasms called canyons.

The Grand Canyon is just such a canyon, it is
continually being carved by the Colorado River.

The Grand Canyon

http://www.pdphoto.org

Palo Duro Canyon

Wikipedia commons

Agents of erosion, weathering, and deposition:
Ice--Glaciers

Glaciers—great sheets of ice-- create
landforms through both erosion and
deposition.

Glaciers form when, over many years more
snow falls each year than melts. As a
result, a deep layer of compacted snow
accumulates. This layer of snow becomes
compressed into a thick sheet of ice.

What makes glaciers unique is that they
move. Due to their mass and the force of
gravity, glaciers flow down hill a few
inches or feet per year.

There are two types of glaciers:

1. Continental glaciers--glaciers that
form over large areas of continents
close to the North and South Poles.

2. Mountain glaciers--relatively small
glaciers that form near the tops of
mountains.

Continental glacier in Antarctica

http://www.coolantarctica.com/gallery2/glaciers/Glacier-10.html

Mountain glacier in the Rocky Mountains

http://pics4learning.com

During the past Ice
Ages, very thick
continental ice
sheets overlaid much
of the continent of
North America as
shown in this diagram.
These very thick
glaciers covered all
but the highest
mountains and
resulted in significant
Grey shaded area—continental glaciers
erosion.

Many lakes in North America including the Great
Lakes, were created by glacial moving over the
rock and gouging out deep “holes” which filled
with water when the glaciers melted.

Wikipedia Commons

In the last glaciation period, which ended
approximately 10,000 years ago, 32 percent of
Earth's land area was covered with glaciers.
Glaciers now cover only about 10 percent of
the land area.
That glacial ice is found mainly over Antarctica.
Most of the other glaciers overlie Greenland; the
remaining small percentage are mountain glaciers
found in places such as Alaska, the Canadian
Arctic, Patagonia, New Zealand, the Himalayan
Mountains, the Rocky Mountains, and the Alps.

That glacial ice is found mainly over Antarctica.
Most of the other glaciers cover Greenland; the
remaining small percentage are mountain glaciers
found in places such as Alaska, the Canadian
Arctic, New Zealand, the Himalayan
Mountains, the Rocky Mountains and the Alps.

Glaciers are not landforms. The action of
glaciers, however, creates landforms.

Glaciers move, and as they do, they scour
the landscape, "carving" out landforms
and eroding material.

This happens because friction breaks the
bedrock into pieces of smaller rock and
soil. This debris becomes embedded in
the bottom of the moving glacier and is
carried downhill.

Like a big bull dozer, glaciers drag and push
rocky debris downhill to the end of their travel
for that winter (or for that ice age).
As they
move, the rocks
and soil stuck in
the bottom of
the glacier
scrape and
scratch the land
underneath
them.
In the photo above, you can see these “scratches”.

Imagine that scrapping continuing for millions
of years and you can see how glaciers turn
V shaped valleys into U shaped valleys.

http://www.flickr.com/theslowlane

As the glacier moves through a V-shaped valley,
it sculpts a “U”.

A horn is a
pyramid-shaped
mountain peak
created by
several glaciers
eroding away at
different sides of
the same
mountain.
http://www.geology.wisc.edu

Glaciers leave a predictable mark on the
landscape through their deposition.
As the weather warms, glaciers pull
back, leaving deposits of the rocky
material along the sides and at the
end of their travels.

This deposition creates landform
features called moraines.

Rocks and
sediment
deposited on the
sides or the ends
Wikipedia Commons of a glacier
create a
landform called a
moraine.

Wikipedia Commons

When sediments
are deposited by a
river over a long
period of time into
a shallow ocean or
lake, layers of
sedimentary rock
may be formed.
Just what is
sedimentary rock?
http://www.texasbeyondhistory.net/

One of the three major rock
types, sedimentary rock is formed when
sediments such as tiny fragments of rock
including pebbles, gravel, sand, silt and
clay are naturally glued together under
great pressure.

The next few slides will help us better
understand just how sedimentary rock is
formed.

The first steps in the
formation of
sedimentary rock are
weathering, erosion and
deposition of fragments
of rock in a river.

The fast moving river carries the pieces of
rock. In the process, the rocks hit one
another and are weathered further and
further, forming tiny pieces of sediment.

Here you can see the pieces of rock
becoming smaller and smaller as the river
flows towards the sea.

As it nears the sea the river gradually slows
down, depositing bigger pieces of rock. By
the time it reaches the sea it is mainly
carrying small pebbles and soil. As it enters
the sea it slows down even more and
deposits the sediment it is carrying.

The river deposits the larger heavier
pebbles first and then gradually the smaller
and lighter sediments. In this way layers of
sediment are deposited on the floor of the
sea. The upper layers of sediment are
heavy and push the sediments beneath
closer together.

Then the process of lithification occurs. The
seawater between the grains of sediment
evaporates leaving behind the minerals that
were dissolved in the water. These minerals
glue the tiny fragments of sediment together
to form a type of rock called sedimentary rock.

Over millions of years, the sea fills up
with sediment and the water evaporates.
The result--layers of sedimentary rock.

http://www.knockan-crag.co.uk/ written permission to use

Five types of sedimentary rock are
formed in this way:
• conglomerate
• sandstone
• shale
• siltstone
• limestone

The type of rock depends on what type of
sediment is cemented together.

Particles of pebbles
or gravel cemented
together become
conglomerate.

Particles of sand
cemented together
become sandstone.

Particles of silt
cemented together
becomes siltstone.

Particles of mud
cemented together
becomes shale or
mudstone.

Particles ground up
sea shells cemented
together become
limestone.

A layer of limestone on top of shale.
Wikipedia Commons

Most rock on the Earth’s
surface is Sedimentary Rock

http://rst.gsfc.nasa.gov/

Formation of Beaches

There are two main types of beaches

1. Rocky beaches

2. Sandy beaches

Rocky Beaches
(most with cliffs)
• Are often made of volcanic rock.

• Include four kinds of landforms: sea
cliffs, sea arches, sea stacks and sea
caves.

Rocky beaches are shaped by the
weathering, erosion and desposition by the
of waves (water) and wind.

The constant beating of the cliffs and sea
stacks weather and erode the rock.

Eroded material is then deposited
becoming available to form sand bars and
barrier islands.

Because of the severe pounding of
both wind and waves, these
formations erode relatively quickly.

Over a long period of time sea stack
sometimes completely weather and erode
away.

The following series of pictures shows the
same sea stack photographed over a 100
year period.

1890

http://walrus.wr.usgs.gov/pubinfo/jump.html

1910

http://walrus.wr.usgs.gov/pubinfo/jump.html

1910

1920

Sea arches and sea
caves are created
when part of the rock
making up a cliff is
harder than other
parts. The softer
section can not stand
the erosive powers of
the waves for as long
as the harder section
and erodes faster.

The softer material in this span has already
eroded, leaving the harder rock spanning the arch.
mikebaird

The softer rock in this cliff has
already eroded, leaving a sea cave.

Sandy Beaches
• Are deposition landforms, meaning
that they are formed by deposits of
sediment.
•The sediments deposited are sand, silt and
pebbles– materials carried by a river or
stream from the inland into the ocean;
•River sediments are the source of most of
the sand on beaches.

Washed to sea by streams and rivers, two
separate processes result in the deposit of
this sand and sediment on the shore.
• Some of the sediment is deposited
immediately at the mouth of the river often
forming a delta.
• Most of the sediment becomes suspended
in sea water and is carried along the coast by
the longshore current, a stream of water
flowing parallel to the beach. This current is
created by waves breaking at an angle to
shore.

Rocks and sediment deposited at the foot
of a glacier where the glacier melts create
a landform called a moraine.

THE END

Weathering, erosion, deposition (teacher background)

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