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Name: ___________________________________________ Regents Earth Science

WEATHERING AND EROSION

Weathering

Weathering – Process by which rocks are broken down into smaller pieces

Weathering is a dynamic process – Rocks form under conditions of high temperature and

pressure. When they become exposed to the surface they must adjust to a new environment.

I - MECHANICAL (PHYSICAL) WEATHERING

A. Decrease in pressure at the surface- Causes jointing (Joints are vertical cracks in rock)

B. Frost Action (Frost Wedging)- Water gets into cracks, freezes, expands, widens the

crack and eventually causes the rock to break. (This

causes Potholes to form in roadways in colder climates)

C. Expansion and Contraction of the outer surfaces of rocks caused by heating and cooling

D. Plant roots and Animal burrows

E. Gravity – As rocks weather, the broken up pieces are pulled downhill by gravity. Piles of

this broken rock are called TALUS. Piles of talus at the base of a slope often

form a “fan shaped” feature called an Aluvial Fan. (similar to a Delta. But

occurs on dry land).

II - CHEMICAL WEATHERING

* Breakdown of rocks by chemical action. Results in a change in mineral composition

A. Solution (Dissolving)

1. Can occur directly by water – Example: water dissolving salt

2. Water with an Acid: (a) HO (rain) + CO (carbon dioxide) = H2 CO3 (carbonic acid)

* Dissolves Limestone, calcite, and marble rock- Forms underground

cave systems

(b) Plant roots

* Roots are usually damp and secrete carbon dioxide. This forms a

mild acid. Example: Lichens and Mosses on rocks

3. Solution occurs most often most often in warmer, more humid climates

Weathering

B. Hydration- Chemically combining with water

* Example- In granite rocks – Feldspar +Water= Clay

- Mica +Water= Clay

* Usually changes to a softer, weaker mineral

* As the mineral grains in a rock chemically combine with water they expand and break apart

C. Oxidation – Chemically combining with oxygen

* Iron + Oxygen = Iron Oxide (rust)

Fe + O2 = Fe2 O3 (hematite)

* Often causes red, orange, yellow, or brown coloring in rocks

III - EXFOLIATION

* Peeling or flaking off of the outer layers of rock (like an onion) due to weathering

IV – PRODUCTS OF WEATHERING

A. Solid Sediments – clay, silt, sand, cobbles, boulders

B. Colloids – Extremely tiny solid sediments such as silt and clay

C. Dissolved Ions – (Ca++, Na+, Fe++, Cu++) Causes hard water and salty water

V – FACTORS EFFECTING WEATHERING RATE

A. Exposure to air, water, and living things – As exposure increases so does the

weathering rate

B. Particle Size – As the particle size decreases the weathering rate increases, since smaller

particles have a greater total surface area.

C. Dissolved Composition – Different minerals have different resistances to weathering.

Mica, feldspar, and calcite are much less resistant than quartz.

So they will weather more rapidly.

D. Pollution – As the pollution rate increases so does the weathering rate

E. Climate – Physical weathering is more pronounced in moist climates where there is a range

in temperatures causing alternate freezing and thawing.

- Chemical weathering is greatest in hot, humid climates.

Weathering SOIL FORMATION

Soils are produced from the weathering of rocks

I - TYPES

A. Residual – A soil formed by weathering of the bedrock directly beneath it

B. Transported – A soil formed rock that was moved to its present location before the soil was

Formed.

II – SOIL FORMATION PROCESS This process begins at the surface and progresses downward with time

1st – Bedrock at the surface breaks does into weathered rock.

2nd- - Topsoil forms – Made of tiny pieces of weathered rock and humus (decayed plant and animal

matter – causes brown color in soil).

3rd – Subsoil forms – Minerals and colloids are washed down out of the topsoil by infiltrating rain

(called leaching). They clog up the pore spaces below forming the subsoil.

** These different horizontal layers of soil are called soil horizons

** Living things aid in soil formation

Weathering

EROSION

** Process by which sediments are picked up and transported away

Gravity is the primary FORCE behind all forms of erosion. Causes downslope movement of rock

and sediment - Results in Talus piles and Aluvial Fans at the base of cliffs.

Water, Wind, and Ice (Glaciers) are all AGENTS of erosion

I – GRAVITY TYPE EROSION

1. Landslides – Rapid downhill movement of rock and or sediment

2. Creep – Very slow downhill movement of soil

3. Avalanches – Rapid downhill movement of water, rock, and sediment

4. Mudflows – Downhill movement of water, rock, and sediment

II – STREAMS AND RUNNING WATER

** Earth’s main agent of natural erosion

1. Obtain sediments by:

a. Direct lifting of particles from bottom (stream bed)

b. Friction between carried sediments and stream bed (abrasion) – Causes rounding off of particles.

The longer particles are carried in a stream the rounder they become. c. Dissolving of solid materials (such as limestone)

2. Streams act as conveyor belts in valleys (Carry away weathered material form adjoining land)

3. Streams carry sediments:

a. Dissolved minerals in solution (as Ions)

b. Small solid sediments (silt and clay – colloids) are carried in suspension

c. Larger sediments are moved by rolling or bouncing along stream bed (called saltation)

** Sediments move slower then the stream itself (colloids, which are carried in suspension, are

carried at the same rate as the stream)

Weathering

4. Factor controlling stream velocity:

a. Slope or gradient – As the slope increases, so does the velocity. (there is a direct relationship

between slope and velocity)

b. Volume or discharge (amount of water flowing in the stream) – As the volume of the water in

the stream increases so does the velocity. (there is a direct relationship between volume and

velocity)

5. Factors determining the amount of stream load (amount of sediment carried by a stream):

a. The faster (greater) the velocity, the larger the size of the particle that can be carried

** As velocity increases – maximum particle size carried increases b. The greater the velocity, the greater the amount sediment that can be carried

** As velocity increases – amount of sediment carried increases c. the greater the discharge, the greater the amount of sediment that can be carried

** As discharge of volume increases – stream load increases

6. Stream velocity vs. position in the stream:

A. Straight stream

- Maximum Velocity:

B. Curves Stream

- Maximum Velocity:

** Maximum velocity will always be at the point of lowest friction (sources of

friction are the stream bed and the air)

Refer to your Earth Science reference chart – page 6

Weathering

III – LIFE OF A STREAM

A. Youthful

1. Rapid flow

2. Steep sloping bed

3. Rapids, Waterfalls, Lakes and Potholes

4. Most erosion is downward, therefore characteristic steep walled valley or canyon.

Valley has characteristic V-shape

5. Fairly straight course

B. Middle age (maturity)

1. Decreased velocity

2. Decreased slope, since source area has been worn down closer to base level

3. Rapids and waterfalls worn away

4. Lakes filled in or worn away

5. Downcutting slows due to decreased velocity (side cutting begind)

6. Valley walls begin to become less steep and wider – Erosion is becoming lateral or sideways

7. In late maturity Meanders begin

C. Old age

1. Slow speed

2. Almost flat slope since bed is almost as sea level

3. Very little erosion taking place

4. Wide meanders and oxbow lakes

5. Wide flood plain

6. Very wide, broad, flat valley (flood plain) filled with fine sediment – alluvium

D. Rejuvenation – A stream may be brought back to it’s youthful stage of development at any time by

uplifting of the land (Example: Grand Canyon)

*** Streams have V-shape valleys in cross section

- This widens and becomes less steep as wind, weathering and gravity causes parts of valley sides

to wear back. The materials fall into the stream and are carried away.

Weathering IV- WIND EROSION

A. Occurs mostly in arid (dry ) regions due to lack of vegetation

B. Carries only small size particles due to low speed

V- WAVE EROSION

A. Wave action rounds sediments as a reault of abrasion

B. Waves approaching a shoreline move sand parallel to the shore within the zone of breaking waves

(longshore

current or transport)

D. Erosion and deposition cause changes in shoreline features

VI – SHAPE OF PARTILES VS. FORM OF EROSION

A. Water – Produces rounded particles

B. Wind – Produces rounded particles or particles flattened on one side

C. Glaciers – Produces partly rounded and scratched particles

D. Gravity and physical weathering – Produces angular particles

VII – HUMAN ACTIVITIES AND EROSION

A. Cutting lumber and forest fires

B. Poor farming methods

C. Road and community construction

D. Mining

Weathering

Weathering

Life History of a Stream

Weathering

Seasonal cycle of a beach caused by differing wave tyoes. (A) Summer beach.

(B) Winter beach. Waves may break once on the winter sandbar, then re-form and break

again on the beach face.

Long shore Drift The movement of sediment parallel to the shore as waves strike the shoreline at an

angle longshore drift (see Figure C-13). As waves wash onto the beach, they

carry sediment in suspension. When the waves wash in at an angel, they carry

sand at the same angle. When the waves reach as far as they can go, they return

to the ocean, but

Weathering

Jetties – Sand piles up against one jetty while the other side, deprived of sand is eroded

Groins – The walls trap the sand to prevent beach erosion

Beach drifting of sand caused by angled approach of swash

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