Water Shapes the Land
J. Quigley
4/2012
Water Shapes the Land
Water Shapes the Land
• The Mississippi River is
like a thousand dump
trucks… every year it
carries more than 300
million tons of sediment
to the ocean!
• This sediment is being
washed away from the
surface of the land and
is called erosion.
Water Shapes the Land
• The primary force of
erosion is gravity.
• Gravity pulls sediment
and water downhill
• The end result of erosion
is the deposition of
sediment
• Deposition is the process
in which sediment is laid
down in new locations
Water Shapes the Land
• Most sediment is moved and deposited by flowing water.
• Flowing water is the major agent of erosion responsible
for shaping Earth’s surface
Running Water Erodes the Land
• Water from rain or melted snow may soak into
the ground, evaporate or flow over the
surface as runoff.
• Erosion begins when runoff carries small
particles of soil downhill.
• Runoff turns into streams, which turn into
rivers… each larger body of water carries more
sediment and does more erosion and
deposition
Running Water Erodes the Land
• Streams and rivers erode larger particles of sediment by
saltation.
• Saltation is the process of particles bouncing along a
stream bottom.
• During floods, a fast moving river can move boulders
the size of a small car!!
• This breaks down the rocks into sediment.
Running Water Erodes the Land
• A stream’s ability to
erode depends mainly
on its speed.
• Faster streams carry
more sediment than
slow streams of equal
size
• Fast streams can also
carry larger sized
sediment than a slower
stream
Features Formed by Water Erosion
• Water erosion forms:
–V-shaped valleys
–Waterfalls
–Flood plains
–Meanders
–Oxbow lakes
Features Formed by Water Erosion
• V-shaped valley- near a
stream’s source, the
stream flows fast as it
plunges down steep
slopes. The stream
erodes the rock on both
sides making the valley
even steeper
• They often contain
rapids and waterfalls
Features Formed by Water Erosion
• Waterfall- A water fall
develops where a stream
crosses rock layers that
differ in hardness. The
harder layers resist
erosion, forming the top
of the waterfall. The
softer rock layers are
worn away, leaving a cliff
over which the waterfall
tumbles.
Features Formed by Water Erosion
• FLOOD PLAIN -Where a river or stream crosses
gently sloping land, a flood plain forms.
• As a river flows across a flood plain, it deposits
sediment
• The sediment builds up into long, low ridges
called natural levees
• These levees prevent the river from spilling
over the banks and make a channel for the
river
Flood plain
Features Formed by Water Erosion
MEANDERS• Where a river curves slightly, the water on the
outside of the curve moves more rapidly than
water on the inside.
• Fast moving water causes more erosion, there
fore the river tends to remove more soil from the
outside of the curve
• Sediment is deposited on the inside of the
curve… over time, this process forms a loop like
bend in the river called a meander.
Meander
Features Formed by Water Erosion
• Oxbow lake- Sometimes
during a flood, a river
erodes through a
narrow meander and
forms a new path.
Sediments build up and
cut off the old meander
creating a separate,
curved lake.
Features Formed by Water Deposition
• As a stream or river slows down, it begins to
deposit sediment.
• Features deposited by flowing water include
alluvial fans and deltas.
Features Formed by Water Deposition
• As a stream flows out of
a mountain and onto
the plains, it slows
down and sediment
settles out.
• The result is a fanshaped deposit of
sediment on land called
an alluvial fan
Features Formed by Water Deposition
• When a stream flows into
a lake or ocean, the water
slows down.
• The sediment that the
stream was carrying is
deposited in the form of a
delta
• A delta is a mass of
sediment deposited
where a river enters a
large body of water
Groundwater Erosion
• The process of chemical weathering causes
much groundwater erosion, including the
formation of caves, stalactites, stalagmites
and sinkholes.
Groundwater Erosion
• As CO2 in the air
combines with
rainwater it
makes acid rain.
The acid rain
erodes
limestone
making caves
and caverns
Groundwater Erosion
• Sometimes water drips
into the cavern or cave
from the rock layers
above carrying
minerals.
• Where the water drips
from the cavern ceiling,
a stalactite is formed.
Groundwater Erosion
• If the water drips down to the floor, a pillar of
minerals called a stalagmite is formed.
Groundwater Erosion
• If erosion weakens a layer of limestone, entire
portions of the ground can suddenly collapse
resulting in a sinkhole.
Glaciers and Wind
• Glaciers form in places
where more snow falls
than melts or
sublimates.
• As the layers of snow
pile up, the weight on
the underlying snow
increases, eventually
turning the snow into
ice or a glacier
Glaciers
• Even though glaciers
appear to be stationary,
they are actually
moving!
• The force of gravity
pulls the ice down hill,
the ice flows very
slowly.
Glaciers
There are two types of glaciers:
• Continental glaciers
• Valley glaciers
Glaciers
• Continental Glacierthick sheet of ice that
covers a huge area,
such as a continent or
large island.
• Most of earth’s fresh
water is frozen in the
continental glaciers that
cover Antarctica and
Greenland
Glaciers
• Valley Glacier- Occur
high in a high mountain
valley
• A valley glacier usually
begins near a mountain
peak and winds down
through a valley formed
originally by a stream
Glacial Erosion and Deposition
• Glaciers are very effective at eroding rock.
• Glaciers erode rock through abrasion and
plucking.
• Plucking is when glacier ice widens cracks in
bedrock beneath the glacier, and those pieces
are frozen and stuck to the glacier. As the
glacier moves, the rocks get “plucked” out of
the ground and pulled down with the glacier
Features formed by glacial erosion
• Glaciers cause many distinctive features in the
landscape, including:
– Cirques
– Horns
– U-shaped valleys
– Glacial lakes
Features formed by glacial erosion
• Cirque- a large bowl
shaped valley in the
mountain side caused
by a glacier. They look
like they were made
with a giant ice cream
scoop.
Features formed by glacial erosion
• Horns- If several cirques
form close together, a
ridge my be left
between them, if
several ridges connect
to form a pyramidshaped peak. The sharp
peak is called a horn.
– The peaks of the Swiss
Alps were made by
glaciers
Features formed by glacial erosion
• U-Shaped ValleyGlaciers widen and
deepen existing valleys.
When a glacier flows
through a V-shaped
valley it carves out the
rocks through plucking
and abrasion, making it
a U shape instead
Features formed by glacial erosion
• Glacial lake- Continental
glaciers can enlarge and
deepen natural
depressions in the
ground. These
depressions then fill
with water when the
glaciers retreat.
• The Great Lakes in the
USA are glacial lakes.
Features formed by glacial deposition
• A glacier gathers and transports a huge
amount of rock and soil as it moves.
• When a glacier melts, it deposits its load of
sediment, creating a variety of landforms.
Features formed by glacial deposition
• Glacial sediment is called
till.
• Till is an unsorted mixture
of sediment containing
fragments of many sizes.
• Giant boulders, gravel,
sand and rock dust are all
found in till
• Glaciers deposit till as
they melt
Features formed by glacial deposition
• Moraines form from till.
• A moraine is a mound
of sediment at the
downhill end of a
glacier and along its
sides.
Wind Erosion and Deposition
• Wind can also cause
erosion and deposition
• The speed of the wind
determines the size of
the materials it carries.
• Slow wind carry only
small particles, while
fast winds can move
large boulders!
Wind Erosion and Deposition
• Wind erodes the land
by deflation and
abrasion.
• Deflation occurs when
wind picks up and
carries away loose
surface materials.
Wind Erosion and Deposition
• Dry regions where winds
are strong, sand and dust
are lifted from the surface
and carried away.
• Over time, the surface of
the ground is lowered,
larger rocks are left
behind, forming a rocky
surface that covers much
of the land in dry regions
Wind Erosion and Deposition
• Abrasion is a form of mechanical weathering.
• Abrasion by wind occurs in much the same
way as abrasion in flowing water.
• Wind blows sand against other rocks slowly
sandblasting them away and removing the
weathered particles
Effects of Wind Deposition
• When wind slows down it drops the sediment
it is carrying.
• Features deposited by wind include sand
dunes and loess deposits
Wind Erosion and Deposition
• Deposits formed from
windblown sand are
called dunes.
Wind Erosion and Deposition
• Deposits formed from
windblown dust are
called loess. Loess
consists mainly of finely
ground particles.
The Restless Ocean
• Oceans surround all of the continents
• Earth’s ocean waters are divided into four
major oceans:
– The Pacific
– Atlantic
– Indian
– Arctic
• These oceans also include smaller bodies of
water such as seas and gulfs.
Exploring the Ocean
Exploring the Ocean
• Most of Earth’s water is exists as salt water in
the oceans.
• Ocean water is a mixture that includes
dissolved salts and gases
• Salinity is the proportion of dissolved salts in
water.
• On average there are about 35 grams of
dissolved salts in each kilogram of ocean
water.
Exploring the Ocean
• Salt is added to the oceans by rivers and
volcanoes.
• Rain slowly dissolves salts out of surface rocks
• All the salts get deposited in the oceans,
making them “salt water”
Changes with Depth
• Conditions in the ocean, such as the amount
of sunlight, temperature and pressure change
as you move from the surface to the ocean
floor.
• Light and temperature decrease with depth,
and pressure increases
Exploring the Ocean
• Sunlight decreases with
depth. The deep ocean
is totally dark
• Light cannot penetrate
deeper than 200 meters
Exploring the Ocean
• Water temperature
decreases with depth.
• The top 100 to 500
meters of the ocean are
well mixed and relatively
warm.
• Deeper water is much
colder and denser. Deep
water moves very slowly
and does not interact
much with the surface
layer
Exploring the Ocean
• Pressure increases
continuously with depth
in the ocean.
• At a depth of 500
meters, the pressure is
about 50 times greater
than the atmospheric
pressure at sea level.
• Few forms of life can
tolerate such great
pressures
Exploring the ocean
• At the coast line, there
are reefs and shallow
waters called the
continental shelf…
• At the edge of the
continental shelf, the
continental slope
descends steeply to the
floor of the deep ocean
Exploring the Ocean
• The ocean floor- is a
vast, flat plain dotted
with volcanic peaks.
• It also contains the mid
ocean ridges.
• There are also basins
and trenches along the
edges of some oceanic
plates
Exploring the Ocean
• Mariana Trench is the
deepest place on earth,
11 kilometers below sea
level!!!
• Overall, the average
depth of the ocean is
about 3.8 meters to the
floor!
Ocean Currents
• Currents are the
patterns of the oceans.
• Ocean Currents are
affected by winds,
earth’s rotation and the
position and shapes of
the continents.
Ocean Currents
• Surface currents- A
surface current is a
large stream of ocean
water that moves
continuously in about
the same path
• Winds blowing across
the surface of the ocean
cause the continuous
flows of surface
currents
Ocean Currents
• Unlike surface currents, deep currents are not
caused by winds
• Deep ocean currents are caused by differences
in the density of the ocean water
• Denser water results from colder temperature
or high salinity
• Because deep ocean currents are affected by
density, they are called density currents
Ocean Currents
• Density currents are
responsible for a slow
mixing of the water
between the surface
and deeper ocean.
Ocean Currents
• In certain places in the oceans, water from the
deep oceans moves upward toward the
surface. The movement of water from the
deep ocean to the surface is called upwelling.
Ocean Currents
• In upwelling, winds
blow warm surface
water aside, this allows
cold water from the
deep ocean to rise and
take the place of the
warmer water.
• This is important for
mixing oxygen and
nutrients in the ocean
Wave Erosion and Deposition
• Two physical processes,
hydraulic action and
abrasion are
responsible for wave
erosion
• Over time, wave erosion
and deposition work
together to straighten a
coast line
Wave Erosion
• In the process of
hydraulic action occurs
when waves pound on
cracks in rocks. As water
forces its way into the
cracks, rocks break off.
• The rock pieces are
then turned to
sediment through
abrasion
Wave Erosion
• Sand is continuously
eroded and deposited
along the shore by
waves and currents
• Longshore drift is the
process that moves
sand along the shore.
• As waves break, they
carry sand up the beach
at an angle. This angled
sand is longshore drift.
Deposition by currents and waves
• Sediments in the ocean
are eventually pushed
up on land and create
beaches