Wednesday, October 10 th Page 13a (left side) / Place Lab on table Objective: We will describe the different types of weathering and erosion and identify evidence of each type. Warm-up: 1. What is weathering? 2. In our Rock Cycle Lab, what was our weathering agent?
Predict Tell a story about how the following rock feature was formed.
Weathering
Weathering: the disintegration, or breakdown of rock material
Mechanical Weathering: no change in chemical composition--just disintegration into smaller pieces
Chemical Weathering: breakdown as a result of chemical reactions CaCO 3 +CO 2 +H 2 O ---> Ca 2+ + 2HCO 3-
Mechanical Weathering Physical breakup pressure release water: freeze - thaw cycles crystallization of salt in cracks thermal expansion and contraction All this increases the total surface area exposed to weathering processes.
Types of Mechanical Weathering Exfoliation Frost wedging Abrasion Biological Activity
Mechanical Weathering Exfoliation: Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.
Weathering Mechanical Weathering
Sheet Joints (Exfoliation)
Stone Mountain, GA Half Dome, Yosemite, CA
Exfoliated Domes, Yosemite
Stone Mountain, Georgia, showing the product of exfoliation due to unloading Stone Mountain, GA
Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints
Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart.
Thermal expansion due to Weathering extreme range of temperatures can shatter rocks in desert environments. Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks.
Abrasion: scraping of rock surface by friction between rocks during transport. Can also have wind abrasion.
Role of Physical Weathering 1) Reduces rock material to smaller fragments that are easier to transport 2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering
Surface Area and Weathering
Rates of weathering Joints in a rock are a pathway for water they can enhance mechanical weathering
Erosion The movement of sediments through processes such as wind or water flow.
Physical Weathering/Erosion Lab In lab groups, send one person to front table to gather materials Chalk piece Mason jar with water
Physical Weathering/Erosion Lab Place piece of chalk in the jar Let sit for 3 minutes Write 3 observations in complete sentences
Physical Weathering/Erosion Lab Put lid on tightly. Shake vigorously for 5 minutes Make three observations about how the chalk piece has changed.
Physical Weathering/Erosion Lab Dump out water in sink Fill up jar with warm water Place another chalk piece in the warm water 3 minutes Record observations (3)
Physical Weathering/Erosion Lab Put lid on tightly Shake vigorously for 5 minutes Make observations (3)
Physical Weathering/Erosion Lab Conclusions: answer the following questions What changes in the chalk did you notice? What was affect did temperature play on the erosion of the chalk? Where might you encounter this type of erosion in nature?
Chemical Weathering Definition: transformation/decomposition of one mineral into another Mineral breakdown carbonate dissolves primary minerals --> secondary (mostly clays) minerals Net loss of elements retained in the soil.
Chemical Weathering Water is the main operator: Dissolution Many ionic and organic compounds dissolve in water Silica, K, Na, Mg, Ca, Cl, CO 3, SO 4 Acid Reactions Water + carbon dioxide <---> carbonic acid Water + sulfur <---> sulfuric acid H + effective at breaking down minerals
Dissolution H 2 O + CO 2 + CaCO 3 --> Ca +2 + 2HCO 3 - water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion Biological activity in soils generates substantial CO 2 Bicarbonate is the dominant ion in surface runoff.
Chemical Weathering Oxidation Oxygen dissolved in water promotes oxidation of sulfides, ferrous oxides, native metals Organic Activity Plant material makes H + ions available
Chemical Weathering Hydration: attachment of water molecules to crystalline structure of a rock, causing expansion and weakness Hydrolysis: combination of hydrogen and oxygen in water with rock to form new substances
Chemical Weathering Solution: process by which rock is dissolved in water Is strongly influenced by ph and temperature When water becomes saturated, chemicals may precipitate out forming evaporite deposits. Calcium carbonate (calcite, limestone), sodium chloride (salt), and calcium sulfate (gypsum) are particularly vulnerable to solution weathering.
Resistance to Weathering First to Crystallize Fast Weathering Bowen s Reaction Series Goldrich Stability Series Last to Crystallize Slow Weathering
Olivine/pyroxene to clay + H 2 CO 3 (acid)
Feldspars to clay + H 2 CO 3 (acid)
Quartz to quartz (!) + anything
Calcite to. + anything nothing
This photo of Lime Sink was taken on 20 July 1932, over a week after the drawdown, which occurred over the night of 9-10 July. Karst landforms develop in areas underlain with limestone
Biological Weathering Can be both chemical and mechanical in nature. roots split rocks apart roots produce acids that dissolve rocks. tree throw burrowing animals
Weathering Rates of weathering Climate Temperature and moisture characteristics Chemical weathering Most effective in areas of warm, moist climates decaying vegetation creates acids that enhance weathering Least effective in polar regions (water is locked up as ice) and arid regions (little water) Mechanical weathering Enhanced where there are frequent freeze-thaw cycles
Mechanical and Chemical Weathering Fracturing, disintegration caused by mechanical weathering exposes more surface area. Greater surface area, means more places for chemical action to occur.
Alaska Seattle Amazon Altiplano
Quick Questions (QQ) 1. What is the difference between chemical and physical weathering?
Quick Questions (QQ) 2. Which of the following is an example of Chemical Weathering? a. Wind erosion b. Rust c. Exfoliation d. Abrasion
Quick Questions (QQ) 3. What role does surface area play into weathering and erosion?
Quick Questions (QQ) 4. Which of the following is not an agent of chemical weathering? a. Water b. Carbon dioxide c. Oxygen d. Wind
Quick Questions (QQ) 5. What was likely the weathering agent that caused the following rock feature?
Pre-Lab Questions: Page 13 (b or c) 1. List one role of physical weathering in the rock cycle. 2. If we think of a cake as a rock, what would physical weathering be? 3. What would chemical weathering be?
Ancient Text But the mountain falls and crumbles away, and the rock is removed from its place; the waters wear away the stones; the torrents wash away the soil of the earth; so you destroy the hope of man. -Job 14:18-19 Written nearly 4000 years ago What is this passage talking about?
1. The four limestone samples illustrated below have the same composition, mass, and volume Under the same climatic conditions, which sample will weather SLOWEST? a. Sample A b. Sample B c. Sample C d. Sample D
2. As particles of sediment in a stream break into several smaller pieces, the rate of weathering of the sediment will a. decrease due to a decrease in surface area. b. increase due to a decrease in surface area. c. decrease due to an increase in surface area. d. increase due to an increase in surface area.
3. Base your answer to the question on the graph below, which shows the effect that the average yearly precipitation and temperature have on the type of weathering that will occur in a particular region. Which type of weathering is most common where the average yearly temperature is 5 C and the yearly precipitation is 45 cm? a. Moderate chemical weathering with frost action b. Moderate chemical weathering c. Very slight weathering d. Slight frost action
4. Which event is an example of chemical weathering? a. rocks falling off the face of a steep cliff b. feldspar in granite being crushed into clay sized particles c. water freezing in cracks in a roadside outcrop d. acid rain reacting with limestone bedrock