IOHS NOTES: Rocks, Weathering, Erosion and Deposition
I. Rocks and Weathering
•What is the difference between weathering and erosion?
Weathering: The process that breaks down rock and other
substances of Earth’s surface.
Erosion: Forces that carry away pieces of sediment left by
weathering of rock; the transportation of sediment by
wind, water, ice or gravity.
A. Types of Weathering
1. Mechanical Weathering
a. Definition: Where rock is physically broken into
smaller pieces of the same composition. Works
slowly over very long periods of time and can
eventually wear away whole mountains.
b. Causes and Descriptions:
1) Freezing and Thawing – water freezes in cracks
in rocks; it expands and makes cracks larger.
• Ice wedging – process where wedges of ice in
rocks widens and deepens cracks. Ice melts;
water seeps deeper into cracks; ice wedges
itself deeper - a repeated process - cracks
slowly expand until pieces of rock break off.
2) Release of Pressure – the surface of a mass of
rock erodes, pressure on the rock is reduced –
this causes outside of rock to crack and flake
off like the layers of an onion
3) Animal Action - burrowing animals such as
moles, gophers, praire dogs, and some insects
loosen and break apart rocks in the soil
4) Plant Growth – roots of trees and plants enter
cracks of rocks and as they grow, cracks are
forced farther apart. Overtime any plant’s roots
can pry apart rocks.
5) Abrasion – the grinding away of rock by rock
particles carried by water, ice, wind, or gravity
like sandpaper on wood
2. Chemical Weathering
a. Definition: The process that breaks down rock
through chemical changes.
• Rocks are made of one or more minerals that can
produce new mineral through chemical weathering
• Example: Granite (includes the minerals feldspar,
quartz, mica) plus chemical weathering changes
feldspar into clay minerals.
• Creates holes or soft spots in rock
• Chemical and Mechanical weathering work together
b. Causes and Descriptions:
1) Water – most important cause of chemical
weathering. Dissolves rock to form a solution
2) Oxygen – Iron + Oxygen gas + water = Rust.
Rust makes rock soft and crumbly; red/brown
3) Carbon dioxide – as a gas in air it dissolves in
rainwater and the water that sinks through air
pockets in soil and forms carbonic acid. This
easily weathers rocks like marble and limestone.
4) Living organisms – plant roots push into cracks
in rock; roots produce weak acids that dissolve
rocks around the roots such as “Lichens”
5) Acid Rain – the burning of large amounts of
coal, oil and gas produce pollution of air with
sulfur, carbon and nitrogen compounds plus
water vapor in clouds = acids in rain drops.
Causes very rapid chemical weathering.
II. Erosion and Deposition
A. Process of erosion: Where natural forces move weathered rock
and soil from one place to another
1. Sediment: material moved by erosion such as pieces of rock
or soil or remains of plants and animals
2. Deposition: where agents (causes) of erosion deposit or lay
down sediment. Changes the shape of land
B. Geologic Cycle: weathering, erosion and deposition act together
in a cycle that wears down and builds up Earth’s surface. – Has
continued for billions of years.
C. Causes of erosion (agents)
1. Gravity
2. Running water
3. Glaciers
4. Waves
5. Wind
D. Gravity
1. Definition: gravity is the force that moves rock and other
materials downhill
2. Define Mass Movements: caused by gravity it is any one of
several processes that move sediment downhill. Frequently
triggered by storms and earthquakes but not always.
3. Types of Mass Movements
a.
b.
c.
d.
Type
Landslides
Mudflows
Slump
Creep
Landslides
Mudflows
Slump
Creep
Table of Mass land Movements
Description
Speed
Slope
Large or
Rapid
Steep
small rocks
and soil
Mixture of
Rapid
Steep to
high water,
Gentle
rock and soil
Rock and soil
Rapid
Extremely
at a bottom
Steep –
(base) which
almost
is soakedvertical
rich in clay
Rock
Extremely
Varies from
particles and
Slow – so
Steep to
water, roots
slow it’s
Gentle
loosen by
barely
water,
noticed
burrowing
(signs are
animals
tilted fences)
E. Water Erosion
• What process is mainly responsible for shaping the surface of
the land? Water running downhill
1. Runoff
a. Definition: water that moves over Earth’s surface
b. What are the 5 main factors that determine the amount
of runoff?
1) The amount of rain an area receives
2) The amount of vegetation which holds soil
3) Type of soil (depending on its absorption of
water)
4) Shape of the land (steep or flat)
5) How people use the land (paved or non-paved,
etc.)
c. Describe each of the ways that water flows:
1) Sheet – when runoff flows in a thin layer over
land
2) Rills – tiny grooves in the soil formed by sheet
erosion runoff
3) Gullies – a large groove, or channel in the soil
formed by rills flowing into one
another that carries runoff after a
rainstorm
4) Streams and tributaries (smaller river or stream
that flows into a stream) – formed by gullies
joined together to form a larger channel that is
continually flowing down a slope. Rarely dries
up. Small streams same as creeks or brooks.
5) Rivers – larger stream formed by streams
flowing together
2. River Erosion
a. How does a river cause weathering?
• Describe: Sediment carried by the river grinds
and chips away at the rock of the riverbed
• Describe “energy and rivers”: Energy is the
ability to do work or cause change. It’s
transferred from one object to another (energy
work). A river’s water has energy. At work it
moves sediment and grinds and chips away at
rock of the river’s bed, deepening and widening the
river’s channel.
b. What happens to the resulting weathered rock?
• The rock gets eroded, or moved, by the river
downstream toward the sea
c. How does the shape of a river channel change as
weathering and erosion continue?
• It gets deeper and wider
d. What landforms form as a result of erosion by rivers?
(name and describe each)
1) Deep V-shaped valleys – steep slopes along a
river
2) Waterfalls (or Rapids – fast flowing rivers) –
occurs where a river meets an area of rock that
is very hard and erodes slowly. Flows over this
rock and then flows over softer rock
downstream. The softer rock erodes faster than
the hard rock. A waterfall eventually develops
where softer rock was removed.
3) Flood Plains – forms where the river’s power of
erosion widens its valley rather than deepening
it. A wide river valley that occurs over more
gently, sloping land.
4) Meanders – where a river flows through easily
eroded rock or sediment; a loop-shaped bend
in the course of a river. As the river winds from
side to side, it tends to erode the outer bank and
deposit sediment on the inner bank of a bend.
Meanders become more and more curved.
5) Oxbow Lakes - a meander that has been cut off
from the main river. May form when a river
floods and high waters find a new route
downstream. As it recedes, sediment dams up
the river leaving a lake
6) Bluffs – cliffs formed by erosion along the edge
of a flood plain
3. River Deposition
a. What is river deposition?
• When a moving water slows down, it drops, or
deposits, some of the sediment – fine particles
fall to the river’s bed; large stones quit rolling
and sliding
b. What features form because of deposition by rivers?
(name and describe each)
1) Alluvial Fans – a wide, sloping deposit of
sediment formed where a stream leaves a
mountain range. Shaped like a fan.
2) Deltas – variety of shapes where a river ends its
journey it flows into a still body of water (ocean
or lake). Sediment drops to the bottom building
up a landform and water stops flowing.
3) Soil on Flood Plains – makes a river valley
fertile. Floods over from heavy rains or melting
snow and river rises above the banks into flood
plains—it retreats and deposition takes place.
Grows dense forest or crops.
4) Beaches – sand carried downstream by a river
spreads along the coast to form beaches.
4. Describe the course of a river (n/a)
5. Erosion and sediment load
a. What is sediment load?
• The amount of sediment a river carries.
b. What are the factors that affect the ability of a river to
cause erosion and deposition?
1) Slope – steep means higher speed
2) Volume of flow – the (discharge) volume of
water that moves past a point of given time
3) Shape of streambed – straight or curved
4) Speed of river – increase or decrease influences
the sediment load and power to erode
5) Size of sediment particles – size determines the
speed of sediment flow
F. Waves and Wind
1. How do waves form?
• By wind blowing over the water’s surface, wind
transfers energy to water causing water waves. It is
energy that moves forward, not waves except on the
beach.
2. Erosion by waves
a. What is the major force of erosion along coasts?
• Waves – they shape the coast through erosion by
breaking down rock and transporting sand and
other sediment
b. How do waves erode the land?
1) Impact – large waves can hit rocks along the
shore with great force. Energy in waves can
break apart rocks, overtime, making small
cracks larger and eventually causing pieces of
rock to break off
2) Abrasion – the wearing away of rock by a
grinding action. As a wave approaches shallow
water, it picks up sediment including sand and
gravel. It is carried forward by waves; hits land;
and sediment wears away rock like sandpaper
wearing away wood.
3) Describe Headlands and its Erosion Process
• Headland – a part of the shore that sticks
out into the ocean; made of harder rock
that resists erosion by waves. Eventually
waves erode headlands and even out the
shoreline (waves change direction,
concentrating on headlands)
c. What landforms are created by wave erosion?
* Ocean waves erode the base of the land along a
steep coast. Where the rock is softer, the waves
erode the land faster.
1) Sea Cave – a hollow area eroded by waves
2) Sea Arch – forms when waves erode a layer of
softer rock that underlies a layer of harder rock.
Usually part of a headland where sea caves
have eroded through
3) Sea Stack – if a sea arch collapses it forms a
pillar or rock rising above the water
4) Wave-Cut Cliff – where waves eventually erode
the base of a cliff so much that the rock above
collapses
3. Deposits by Waves
a. How does deposition occur?
• It occurs when waves slow down, causing the
water to drop its sediment. Similar to a river
delta deposition when the river slows down and
drops sediment load
b. What coastal features form by wave deposition?
1) Beaches
2) Spits
3) Sandbars
4) Barrier Beaches
c. What is Longshore Drift?
• The process where as waves repeatedly hit a
beach, some of the beach sediment moves down
the beach with the wave’s current
d. Describe each of the above coastal features below:
1) Beaches – an area of wave-washed sediment
along a coast; a “dynamic system’ where it
changes constantly in response to changes in
supply of sand, size and direction of waves and
coastal features such as headlands or people
made structures.
• Sediment – usually sand from rivers
2) Spits – formed as longshore drift deposits sand
along the shore. A beach that projects like a
finger out into the ocean; results from a
headland or other obstacle that interrupts
longshore drift or the coast turns abruptly
3) Sandbars – formed by wave action (incoming
waves; long ridges of sand parallel to the
shoreline
4) Barrier Beach – similar to a sandbar but forms
when storm waves pile up large amounts of sand
above sea level forming a long, narrow island
parallel to the coast
4. Erosion by Wind
a. Where would you be most likely to see evidence of
wind erosion?
• Wind erosion often is evident in areas where
there are few plants to hold soil in place.
• A sand dune is a deposit of wind-blown sand
where wind sweeps sand across a desert, piling
up huge, ever-changing dunes. There are few
plants so soil is not held in place
b. What are the two main ways that wind causes
erosion?
1) Deflation – the main way that wind causes
erosion; the process by which wind removes
surface materials
• Fine particles – carried through the air
• Medium-sized particles – skip or bounce
along the surface
• Large particles – slide or roll along the
surface
• Examples: Great Plains in the 1930s lost
about 1 meter of topsoil in just a few years
from winds; Mojave Desert there are areas
called desert pavement where wind has
blown away smaller sediment leaving rocky
materials too heavy to move; Mojave Desert
also has areas called blowouts where an
already slight depression produces a bowl
shaped hollow from winds
2) Abrasion – by wind-carried sand which can
polish rock and shape individual stones; causes
little erosion
• Most desert landforms are the result of
weathering and water erosion, not wind
5. Deposition by Wind
a. How does deposition occur by wind?
• All sediment picked up by wind eventually falls
to the ground; happens when wind slows down
or some obstacle, such as a boulder or a clump of
grass, traps the wind-blown sand sediment.
b. What landforms are created by wind deposition?
1) Sand Dunes – Come in many shapes and sizes:
parallel ridges, u-shaped, small or very large;
move over time and determined by wind
direction
• Examples: California coast; Bays of
Monterey, Humboldt, and San Diego; Death
Valley; and Mojave Desert
2) Loess Deposits – fine particles of clay and silt
deposited by wind in layers far from source;
forms fertile soil which produce valuable
farmlands such as the Midwestern U.S. or
central China
G. Glaciers
1. How Glaciers Form and Move
a. What is a glacier?
• A large mass of ice that moves slowly over land.
b. What are the two kinds of glaciers?
1) Continental Glacier – a glacier that covers much
of a continent or large island; about 10% of
Earth’s surface such as Antarctica (about 3 km
thick) and most of Greenland; spread out like a
pancake
2) Valley Glacier – a long, narrow glacier that
forms when snow and ice build up high in a
mountain valley; valley sides keep the glacier
from spreading; usually moves down valleys
already cut by rivers, such high mountains;
smaller than Continental Glacier but can be 10s
of km long
• California Sierra Nevada and Mt Shasta have
high mountain peaks where near freezing and
snow builds up forming ice
c. What is an ice age?
• Where Continental glaciers have covered larger
parts of Earth’s surface; about 2.5 million years
ago glaciers advanced and retreated finally
retreating about 10,000 years ago
d. How do glaciers form?
• Glaciers form only in an area where more snow f
alls than melts. Once the depth of snow and ice
reaches more than 30 to 40 meters, gravity
begins to pull the glacier downhill
e. Why do glaciers flow?
• Gravity pulls on the ice and makes the ice pile
thinner; the ice eventually melts as it flows into
warmer areas; valley glaciers can move one
centimeter to a few meters per day
• Valley surge – where a valley glacier slides
down more quickly (about +/- 6 km/yr)
2. How Glaciers Shape and Land
a. What two processes by glaciers help erode land
surface?
1) Plucking – when a glacier flows over the land it
picks up rocks in the process; the glacier’s
weight of ice can break rocks apart beneath the
glacier; rock fragments freeze to the bottom of
the glacier and are carried away
2) Abrasion – the rocks that are carried on the
bottom of the glacier are dragged along a
glacier’s flow gouging and scratching the
bottom bedrock
• Example: Yosemite Valley in Sierra
Nevada
b. What landforms result from erosion by glaciers?
1) Horn – when glaciers carve away the sides of a
mountain forming a sharpened peak; sharp
pyramid-shaped peaks when 3 or more cirques
erode a mountain
2) Cirque – a bowl-shaped hollow (depression)
where ice cuts back into the mountain walls
3) Arete – a sharp ridge separating two or cirques;
jagged ridges
4) U-shaped valley – formed when a glacier flows
into and erodes a valley, changing the valley
from its original V-shaped to a U-shaped valley;
also called glacial troughs
5) Hanging valleys – smaller glacial valleys that
join the deeper main valley; valleys form
because smaller glaciers cannot carve a valley as
deep as the main glacier. Many form waterfalls
after the ice is gone (melted)
6) Fiord – forms when the level of the sea rises,
filling a valley once cut by a glacier; long lake
7) Glacial lakes – glaciers cut valleys into basins
leaving large lakes in long basins
8) Kettle Lake – forms when a depression left in
till by melting ice fills with water
9) Small lakes – form in bowl-shaped hollows
eroded by a glacier at the base of a high peak
such as the Sierra Nevada
c. Glacial Deposition
• How does glacial deposition occur?
A glacier gathers a huge amount of rock and
soil as it erodes the land in its path. When the
glacier melts it deposits sediment it eroded
creating various landforms. Called glacial drift
• What is till?
The mixture of sediment that a glacier deposits
directly on the surface of the land; made up of
particles of various sizes – clay, silt, sand, gravel
and boulders
• What landforms result from glacial deposition?
1) Moraine – a ridge formed by till deposited at
the edges of a glacier; Terminal Moraine is when a
glacier deposits mounds or ridges of till at the
farthest point reached by a glacier; one extends
across part of Yosemite Valley, CA from the last
ice age
2) Drumlin – a long mound of till that is
smoothed in the direction of the glacier’s flow
3) Kettle – small depression forms when a
chunk of ice is left in glacial till and the ice melts
leaving a kettle (retreating glacier); the continental
glacier of the last ice age left many kettles; when
they fill with water they are called Kettle Lakes.