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. Landslides
b. Mudflows
c. Slump
d. Creep
Table of Mass land Movements
Type / Description / Speed / SlopeLandslides / Large or small rocks and soil / Rapid / Steep
Mudflows / Mixture of high water, rock and soil / Rapid / Steep to Gentle
Slump / Rock and soil at a bottom (base) which is soaked-rich in clay / Rapid / Extremely Steep – almost vertical
Creep / Rock particles and water, roots loosen by water, burrowing animals / Extremely Slow – so slow it’s barely noticed (signs are tilted fences) / Varies from Steep to Gentle
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