Ch. 6 - Weathering and Soils

6.1 Weathering

Define weathering and distinguish between the two main types.

Weathering

Presentation content:

• Weathering involves the physical breakdown and chemical alteration of rock at or near Earth’s surface
  • Mechanical weathering — physical forces breaking rocks into smaller pieces
  • Chemical weathering — chemical transformation of rock into new compounds
  • Both work simultaneously and reinforce each other. Mechanical weathering, by breaking rock into smaller and smaller pieces, increases surface area for chemical weathering attack

Notes from the lecture:

• Important in the sense that it is destroying land of agricultural production
• Modern example: a lot of rain, flooding the soil
  • Result on large amount of transport of the sediment
• What is weathering?
  • Physical and chemical breakdown of rock
  • Chemical:
    • Example: water gets in it and starts to dissolve it
    • Breaking into ions.

6.2 Mechanical Weathering

List and describe four examples of mechanical weathering.

Frost Wedging (Mechanical Weathering)

Presentation content:

• Two different methods
  • Water works its way into cracks in rocks and the freezing enlarges the cracks in the rocks
  • Lenses of ice in soil grow larger as they attract liquid water from surrounding areas

Notes from the lecture:

• When water freezes, it expands, and as a result it cracks concrete
• Geologic version: fractures created when you get frozen water inside a rock

Salt Crystal and Sheeting

Presentation content:

• Salt Crystal Growth
  • Sea spray or salty groundwater penetrates crevices and pore spaces in rocks
  • As the water evaporates, salt crystals form and enlarge the crevices
• Sheeting/Unloading
  • Large masses of igneous rock are exposed by erosion and concentric slabs break loose due to release of confining pressure
  • An exfoliation dome is formed after continued weathering causes slabs to separate and spall off

Notes from the lecture:

• As it is eroding it, it is more exposed and gas is able to expand

Biological ACTIVITY

Presentation content:

• Plant roots grow into fractures in a rock, causing the cracks to expand (root wedging)
• Burrowing animals break down rocks by moving fresh material to the surface, enhancing physical and chemical weathering
• Human impacts (rock blasting) is very noticeable-can produce effects much like unloading

Notes from the lecture:

• Organic assets:
  • Trees that tend to produce more carbonic acids
  • This dissolves limestones and some other minerals at surface and facilitates that chemical weathering process

6.3 Chemical Weathering

Discuss the role of water in each of three chemical weathering processes.

Water

Presentation content:

• The Most Important Agent Is Water
  • Responsible for transport of ions and molecules involved in chemical processes
• Processes of Chemical Weathering
  • Dissolution
  • Oxidation
  • Hydrolysis

Notes from the lecture:

• Saturation and acidity drives chemical weathering processes
• When it rains, the rain wills tart to break down those limestone blocks
• Clay
  • If you add water, eventually you saturate it enough to the point that it starts to break down
  • Same case with silicate dominant igneous rocks

Dissolution

Presentation content:

• Certain minerals dissolve in water
  • Halite is one of the most water-soluble minerals
• A small amount of acid in water increases the corrosive force of water, causing dissolution
  • Carbonic acid is created when carbon dioxide dissolves in raindrops
  • Calcite is easily attacked by weakly acidic solutions
  • This process is responsible for the formation of limestone caverns

Notes from the lecture:

• Limestone:
  • As water starts incorporating, it becomes more acidic, that water enters porous sediments and starts to dissolve it.
  • Waters full of calcium carbonate act as dissolving agents
  • Starts to grow "escalactitas" - "stalactite"
  • Example: Carlsbad caverns:
    • Carbonate sedimentary rocks

Oxidation

Presentation content:

• Rusting of iron-rich minerals
• Oxygen combines with iron to form iron oxide
• Process is slow in dry environments
• Water increases the speed of the reaction
• Important in decomposing ferromagnesium minerals like olivine, pyroxene, hornblende, and biotite
• Oxidation can only occur after iron has been freed from the silicate structure by hydrolysis

Notes from the lecture:

• Iron is deposited into our sedimentary rocks, as time passes it starts to oxidize and get rust.
• Redish color- orange color sedimentary rocks
  • Most likely a sort of mafic igneous rock (rich in Iron)

Hydrolysis

Presentation content:

• The reaction of any substance with water
• A hydrogen ion attacks and replaces another ion
• Silicates primarily decompose by hydrolysis
  • Clay minerals are the most abundant product of weathering
  • Clay minerals are very stable under surface conditions
• Acid greatly accelerates hydrolysis

Notes from the lecture:

• Targets silicate minerals
  • majority of our rocks
• Also dictated by acidity, it is very important, it drives the reaction

Spheroidal Weathering

Presentation content:

• Weathering attacks edges from two sides and corners from three sides
• Sharp edges gradually wear down and become rounded
• Granite, for example:
  • Crystalline rock with joints
  • Water penetrates joints
  • H+ replaces K+ in the feldspars, disrupts crystalline structure

Notes from the lecture:

• Because of fractures, water gets in
• Those tend to break down very quickly because of a greater surface area
  • More angles = more sharp edges = greater surface area for break down
• Example: removing potassium and replacing it with hydrogen
  • Breaks down the mineral

6.4 Rates of Weathering

Summarize the factors that influence the type and rate of rock weathering.

The rate of weathering is influenced by:

Presentation content:

• Rock Characteristics
  • Dependent of mineralogy
  • Silicate minerals weather in the same order as crystallization (Bowen’s reaction series)
  • Carbonates and halides weather more quickly than silicates
• Climate
  • Temperature and precipitation are crucial
    • Frequency of freeze-thaw
    • Moisture available for dissolution
    • Conditions favoring vegetation growth

Notes from the lecture:

• If a very humid climate that affects greatly
  • You will have a lot lichens, and other organism that pushes more chemical weather

Differential weather

Presentation content:

• Variations in local climate and the composition of the rock formation will produce uneven weathering of the rock called differential weathering

Notes from the lecture:

• Example:
  • Neck of the volcano was preserved while all material surrounding it got eroded away

6.5 Soil

Define soil and explain why it is referred to as an interface.

Soil is “the bridge between life and the inanimate world”

Presentation content:

• The bridge between the various Earth systems
• Earth’s land surface is covered by a layer of rock and mineral fragments produced by weathering, called regolith
• Soil is a combination of mineral and organic matter, water, and air and is the portion of the regolith that supports the growth of plants

Notes from the lecture:

• With soil you have an intersection of all spheres of Earth
  • There is water, organic matter, gases.
• A mineral rich soil
  • Helps facilitate the growth of plants - they need minerals as we do

Soil Texture and Structure

Presentation content:

• Most soils are far from uniform
• Soil texture refers to the proportions of different particle sizes
  • This property strongly influences the soil's ability to transmit and retain water and air
• Four basic soil structures
  • Platy, prismatic, blocky, and spheroidal
  • Influences how easily the soil can be cultivated, how susceptible it is to erosion, porosity and permeability
• Parent material, climate, plants and animals, time, and topography interact to control soil formation

Notes from the lecture:

• Clay is not good, it traps water, it results in flooding, it smells, etc.
• Sand tends to be more pourus, water is filtered through.
  • If you pour water at the beach, that water is going to be infiltrated and dries fast
  • Clay will trap that water instead.
• Valleys:
  • Most productive soil
  • Very fertile, but it also comes at a risk

6.6 Controls of Soil Formation

List and briefly discuss five controls of soil formation.

Parent material

Presentation content:

• Parent material
  • The source of weathered material that forms soil
    • Residual soils—soils form from the underlying bedrock
    • Transported soils—soils that form in place from unconsolidated sediment
• Climate
  • The most influential control of soil formation
  • Key factors are temperature and precipitation
    • Determines degree and rate of mechanical and chemical weathering

Notes from the lecture:

• Climate: Helps drive weather and erosion
  • A lot of seasonality, will contribute to break down of material and transportation of sediments and soil
• See bedrock exposed digging a couple feet in College Station

Plants, animals and time

Presentation content:

• Plants and Animals
  • Influence the soil chemistry
  • Remains are converted into humus which is an important part of the organic component of soils
• Time
  • Weathering over a short period of time forms thin soils that closely resemble the parent material
  • Soils that have weathered for a long period of time are generally thick and do not resemble the parent material

Notes from the lecture:

• Plants and animals can influence the soil chemistry and physically alter it as well
• You can boil some wood of some trees and it will give an acidic solution.
• Weathering can also occur very slowly
  • This is because the climate in that location does not drive a lot of erosion or weathering
  • Not all of the weathering agents presented
  • When you do not have a lot of water, erosion is much more slower

Topography

Presentation content:

• Can vary greatly over short distances—leads to development of local soils
• Steep slopes have poorly developed soils
  • Moisture content of these areas is often insufficient for plant growth due to rapid runoff
• Flat and undulating surfaces are optimal for soil formation
  • Good drainage and minimal erosion
• Slope orientation is also important in soil formation
  • Southern-facing slopes in the Northern Hemisphere receive the most sunlight are optimal for soil formation

6.7 Describing and Classifying Soils

Sketch, label, and describe an idealized soil profile. Explain the need for classifying soils.

The Soil Profile

Presentation content:

• Soil-forming processes operate from the surface downward
  • Soil is divided into horizontal layers called horizons
  • A vertical section through all the soil horizons is called a soil profile
    • A mature soil has well-developed horizons
    • An immature soil may lack soil horizons

Notes from the lecture:

• Potassium breaks down to form clay
  • It does form as a natural product
• C horizon
  • Partially altered parent material

Variations in soil formation

Presentation content:

• Variations in soil formation over time and distances has led to a great variety of recognized soil types
• Groups have been established using common characteristics
• In the United States, a system was devised and called the Soil Taxonomy
  • Includes 6 hierarchical categories
  • System recognizes 12 soil orders and more than 19,000 soil series

Notes from the lecture:

• Some soils reflect climate conditions

6.8 The Impact of Human Activities on Soil

Explain the detrimental impact of human activities on soil and list several ways to combat soil erosion.

Taking care of soil

Presentation content:

• The agricultural productivity of soils can be improved through fertilization and irrigation
• Soils can be damaged or destroyed by careless activities
  • Soils are crucial for providing food, fiber, and other basic materials
  • Soils are one of the most abused resources
• Examples - Clearing the Tropical Rain Forest = Loss of nutrients and promotes erosion

Notes from the lecture:

• Soil gets washed away when it rains, nutrients get loss quickly

Soil Erosion: Losing a Vital Resource

Presentation content:

• Soil erosion is a natural process in the rock cycle. However, soil formation is slow
• Erosion rates are dependent on climate, slope, and type of vegetation
  • Deforestation and farming practices can enhance soil erosion
  • Water flowing on deforested ground starts as sheet erosion on the surface, this becomes tiny channels called rills, that grow into gullies

Notes from the lecture:

• Sheet erosion and gully erosion

Controlling Soil Erosion

Presentation content:

• On every continent unnecessary soil loss is occurring
  • When steep slopes are farmed, constructing terraces can help slow runoff and decrease soil loss
  • Strips of grass or cover crops parallel to slope also slow runoff and trap sediment
  • Creating grassed drainage ways prevents the formation of gullies and traps soil

Notes from the lecture:

• A maze shape:
  • Helps prevent weather and erosion
  • The wind will not always blow away the soil

End of Chapter 6 - Concept Checks

6.1 Weathering

1. What are the two basic categories of weathering?
2. How do the products of each category of weathering differ?

6.2 Mechanical Weathering

1. When a rock is mechanically weathered, how does its surface area change? How does this influence chemical weathering?
2. How do joints promote weathering?
3. Explain how water can cause mechanical weathering.
4. How does biological activity contribute to weathering?

6.3 Chemical Weathering

1. How is carbonic acid formed in nature?
2. What occurs when carbonic acid reacts with calcite-rich rocks such as limestone?
3. What products result when carbonic acid reacts with potassium feldspar?
4. List several minerals that are especially susceptible to oxidation and list two common products of oxidation.

6.4 Rates of Weathering

1. Explain why the headstones in Figure 6.14 weathered so differently.
2. How does climate influence weathering?

6.5 Soil

1. Explain why soil is considered an interface in the Earth system.
2. How is regolith different from soil?
3. Why is texture an important soil property for agriculture?
4. Using the soil texture diagram in Figure 6.18, name the soil that consists of 60 percent sand, 30 percent silt, and 10 percent clay.

6.6 Controls on Soil Formation

1. List the five basic controls of soil formation. Which factor is most influential in soil formation?
2. How might the direction a slope is facing influence soil formation?

6.7 Describing and Classifying Soils

1. Sketch and label the main soil horizons in a well-developed soil profile.
2. Describe the following features or processes: eluviation, leaching, zone of accumulation, and hardpan.
3. Why are soils classified?
4. Examine Figure 6.23 and identify three particularly extensive soil orders that occur in the contiguous 48 United States. Describe two soil orders in Alaska.

6.8 The Impact of Human Activities on Soil

1. Why are soils in tropical rain forests not well suited for intensive farming?
2. Place these phenomena related to soil erosion in the proper sequence: sheet erosion, gullies, raindrop impact, rills, stream.
3. Explain how human activities have affected the rate of soil erosion.
4. Briefly describe three ways to control soil erosion.