GEOLOGY 101

Today: Chapter 6 “Weathering and Erosion” Instructor: Professor Matt Fouch Email: [email protected]

Office: PSF-540 965-9292 TA: Ayelet Blattstein Email: [email protected]

Office: PSH-452 Course Website: http://fouch101.asu.edu

How do we move material to lower elevations?

• Weathering – Physical – smaller pieces, but composition isn’t altered – Chemical – composition altered • Erosion – Transfer of material by water, wind, ice • Mass Wasting – Transfer of rock & soil downhill via gravity

Weathering

• Physical and chemical changes that occur in sediments and rocks when they are exposed to the atmosphere and biosphere • Not the same as erosion!

Why Care About Weathering and Erosion?

• Among other reasons, they produce soil • Important natural resource – Supports plant life which support us – Acts as a storage site for CO 2 • Causes of landslides • Septic system design • Building foundation design • Landfill design • Climate history

The Rock Cycle

What Controls Weathering?

• Rock properties – Hardness, composition, solubility, zones of weakness • Climate – Wide variations in temperature and moisture accelerate weathering • Soil and vegetation – Exposes rock to variations in moisture and chemistry • Length of exposure

Weathering Changes the Surface Area to Volume Ratio Rock corners are eroded more rapidly than sides

(more surface area at corners)

Physical Weathering

• Physical forces break rock into smaller pieces w/out changing mineral composition Types of physical weathering • Frost wedging • Unloading • Expansion and contraction • Biological activity

Frost wedging

Repeated cycles of freezing & thawing can break rock into smaller fragments How?

Water expands ~ 9% upon freezing (Other minerals also expand/contract) Where?

Regions w/ daily freeze/thaw cycles

Frost wedging

Frost wedging

Talus slope

Frost wedging

Talus “cones”

Unloading

Expansion of rock from removal/erosion of overlying rock (“sheeting”) Why?

Reduction in pressure (since less overlying weight) Manifestation: Slab-like layers/sheets break loose

Unloading

Continued weathering causes rock slabs to separate & fall

HALF DOME YOSEMITE NATIONAL PARK (California)

“exfoliation domes”

Another “sheeting” example

“Devil’s Marbles” Australia

Spheroidal weathering

Thermal expansion

Thermal expansion of rock can also occur without water or minerals present Expansion / contraction of rock from heating / cooling

Biological activity

Weathering of rock from activities of

   

organisms plants burrowing animals humans

Chemical Weathering

• Processes that break rock components and internal structures of minerals, making new minerals Ways to chemically weather rock • Oxidation (add oxygen) • Dissolution (dissolve) • Hydration (add water)

Chemical Weathering

• All minerals are chemically unstable once removed from where they were formed • Water is the primary agent of chemical

weathering

Products of Chemical Weathering

• Clays • Oxides • Salts – Halite • Silica and quartz sand

Oxidation

Chemical reaction that causes loss of electrons

Example

Iron (Fe) + water (H 2 O)

oxygen (O) combines with Fe to form hematite (Fe 2 O 3 )

Oxidation decomposes important minerals:

mineral formula

olivine pyroxene

(Mg, Fe ) 2 SiO 4 (Mg, Fe )SiO 3

hornblende Ca

2 ( Fe ,Mg) 5 Si 8 O 22 (OH) 2 where

Earth’s mantle Earth’s mantle Continental rocks

Dissolution

Dissolving minerals by a liquid agent (such as water)

How?

Many minerals are water-soluble

example: Halite (salt)

Adding acid (H + ) increases corrosiveness

example: carbon dioxide and rain

Dissolution

• Rainfall in the atmosphere produces carbonic acid (H 2 CO 3 ) which can dissolve carbonates and decompose minerals – The pH of “natural” rain is 5.6 to 5.8

• pH of 7 is neutral • pH of acid rain can be as low as 4.5

(10-15 times more acidic than natural rain) • pH of ketchup is ~ 3.9

• pH of Coke ® is ~ 2.7

Hydration

The reaction of any substance w/ water.

Example

Silicates

primarily decomposed by hydration Water molecules break down from H 2 O into H

+

and (OH

-

)

Hydration Example

Decomposition of granite

Granite (mainly quartz

+

carbonic acid ( water + + potassium feldspar carbon dioxide ) )  Liberates potassium from feldspar important plant nutrient  makes kaolinite this left over mineral is clay - very stable - main inorganic part of soil  quartz very stable, glassy appearance - carried to sea: beach sand, sand dunes - cements to form sandstone

Formation of Soil

• Product of both physical and chemical weathering

“good soil:”

Made of:

decomposed rock + decayed animal/plant life (“humus”) + water + air

45% 5% 25% 25%

Important Soil-Forming Factors

• Climate – Temperature & precipitation • Time – Longer time = thicker soil • Plants/Animals – Organic matter • Slope – If too steep, little/no soil

Soils and Climate

• Soil formation is directly linked to climate that soil forms in Three major groups • Laterite – Wet climate • Pedalfer – Temperate climate • Pedocal – Dry climate

Soil Profile

A horizon

Laterite

C horizon

Pedalfer

Pedocal

Soil Erosion

• The biggest problem facing the developing world because of deforestation – Much of Madagascar’s soils has been lost due to deforestation • Read Box 6.1 (pp. 132-133)