Weathering

PHYSICAL WEATHERING

What is physical (mechanical weathering)?Phy

Physical weathering- Breaking down of rocks into smaller particles (sediments) without changing the composition of the rock

Frost Action/Ice Wedging:

• Repeated freezing and thawing of water in the cracks of rocks

  • Ex. Potholes on the streets, broken rocks in the mountains

Abrasion:

• Bumping and rubbing of rocks as they collide

• Rocks become smaller, smoother, and rounder

• Sediments carried by a river or a stream

• Ex. Rounded pebbles found near water and in deserts (wind)

CHEMICAL WEATHERING

What is chemical weathering?

• A change in the rock that forms a new compound

• Mostly caused by water reacting with the mineral grains in rocks to form new minerals

• These reactions occur particularly when the water is slightly acidic from acid rain

• Takes long and signs aren’t obvious

  • Look for bubbles released, substances dissolved, change in color

• Composition of rock determines the effects of chemical weathering

  • Ex. Calcite decomposes quickly in acidic water

    • Limestone and marble are made from calcite

• Agents include water, oxygen, carbon dioxide, and acid precipitation

Oxidation:

• When iron-bearing rocks are exposed to air and water, the iron atoms combine with oxygen to form iron oxide (rust)

• Ex. Rusty nails, planet Mars

• Common mineral that contains oxidized form of iron is hematite

Carbon Dioxide (CO2):

• Contributes to chemical weathering

• Combines with water and forms the acid carbonic acid and falls as precipitation

• Can dissolve rocks such as limestone

• Decaying matter and respiration can produce high levels of carbon dioxide

• Acidic water from precipitation combines with this and forms stronger agent

• Carbonic acid reacts with calcite to dissolve rocks

Carbonation:

• Acid rain dissolves rocks containing the mineral calcite

• Eg. caves and sinkholes in limestone

Hydrolysis:

• Takes place when water reacts with some of the minerals in a rock (ex. Feldspar in granite) to produce clay and salts

• Ex. statues made of granite, mountain

Chemical reaction of ions:

• The formation of secondary minerals in soils generally results from the recombination and addition of ions and molecules from the soil solution to the solid phase

• Process is slow but happens

• Ex. The weathering of feldspar to kaolinite

• Next soil solution Al3+ will react with soluble silica to form kaolinite:

RATE OF WEATHERING

Factors that affect the rate of weathering:

1. Climate

• The weathering of rocks is influenced by temperature and rainfall intensity

• Strong chemical weathering occurs in warm and very humid climates (ex. rainforest)

• Strong physical weathering (frost action) occurs in cold and humid climates (ex. Mountains and higher latitudes)

• Very slight weathering occurs in arid (dry) climate with low amounts of precipitation

• The amount of chemical weathering will increase if air temperature increases and precipitation increases

How does climate affect landscape development?

2. Exposed Surface Area

• The more surface area exposed while a particle is weathering, the faster it will weather (break)

3. Mineral Composition

• Rocks weather at different rates depending upon mineral composition

• Rocks composed of harder, more resistant minerals will weather away more slowly

• Rocks composed of softer, less resistant minerals will weather away more rapidly

SOILS

How does soil form?

• Soil: the mixture of weathered rocks and biological activity over long periods of time

• Soil profile:

  • The top layer (horizon) contains the highest amount of organic matter and weathered material

  • The bottom layer of soil contains the most amount of weathered material

EROSION AND DEPOSITION

Vocabulary:

Agents of Erosion:

What are sediments?

• Described in 3 ways: size, rounding (well rounded/angular), and sorting (well sorted/poorly sorted) based on size

How are sediments deposited?

Erosion-Deposition by waves:

How do ocean waves erode and deposit sediments?

• Shoreline erosion is an ongoing process caused by the force of ocean waves

• Waves are formed by the wind blowing over the water

• As waves crash onto the shore, they carry sand and pebbles that act like natural sandpaper

  • Gradually wearing down the coastline overtime = abrasion

• Waves also transport eroded materials (ex. sand) along the shore

• Coastal erosion:

  • Affected by urbanization (building moves sediments around)

Erosion-Deposition by wind:

How does wind erode and deposit sediments?

• The wind picks up small particles such as sand, silt, and slay, and carries them to different locations

Erosional features formed by wind:

• Arches National Park

• Monument Valley

Erosion-Deposition by gravity (mass movement):

How does gravity affect erosion and deposition?

• Mass movement: downhill movement of rock or sediment due to gravity

• Sediments eroded by gravity are ANGULAR and UNSORTED

Here’s the revised list with at least four points for each section:

Physical Weathering:

1. Mechanical breakdown of rocks

2. No change in chemical composition

3. Caused by temperature changes, water, ice, and wind

4. Often results in smaller, angular rock fragments

Chemical Weathering:

1. Chemical alteration of minerals

2. Changes the chemical composition

3. Caused by reactions with water, oxygen, acids, and carbon dioxide

4. Often leads to the formation of new minerals and soluble substances

Similarities (Overlap):

1. Both contribute to the breakdown of rocks

2. Both influence the Earth's surface

3. Both are natural processes

4. Both can work together to shape landscapes over time