Weathering and Soils

Weathering & Soils 1.Weathering vs. Erosion 2.Joints: Setting the Stage 3.Physical (Mechanical) Weathering 4.Chemical Weathering 5.Weathering Products 6.Weathering Landscapes 7.Soils: Introduction

1. Weathering vs. Erosion Common Error Made in K-12 Earth Science Teaching: these are synonyms No! Weathering – chemically dissolving candy or physically crunching candy – breakdown in place Erosion – moving pieces (dissolved or as fragments)

1 st - Weathering = decay in place 2 nd - Detach = break off 3 rd Erode = move

1 st – decayed 2 nd – detached 3 rd – eroded

The balance between weathering and erosion defines the landscape

In deserts – transport is faster

Humans can upset the balance and accelerate erosion. So when transport (detachment and erosion) becomes faster than weathering, landscapes are not sustainable.

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2. Joints: Setting the Stage Joints – fractures (that allow water to penetrate and weather the rock) Many ways to make joints: Cooling & contraction Tectonic Stresses

Very different!

Columnar Jointing From contraction after lava flow cools

or faulting/folding stresses the rock

Joints can even fracture sedimentary strata from regional tectonic pressures

3. Physical (Mechanical) Weathering Frost weathering Pressure release weathering Salt Weathering Thermal Expansion/Contraction Wetting/Drying Root pressure

Frost Weathering

Classroom Resources Clip of freezing exploding metal Spin-around showing fractured rock in Antarctica

Breaks rock along small fractures

Breaks rock along large fractures & produces jagged alpine topography

Aesthetic, so used in commercials

Pressure Release Sometimes called “exfoliation” (like exfoliating skin) – incorrect usage pressure release shells

Erosion removes overburden & shells pop off as pressure is released

Glacial Erosion Great Way to Generate Pressure Release

Salt Weathering Salt is common in deserts Salt is common along coasts

Mechanics of Salt Weathering Salt Crystal Growth: Extreme pressures in cracks and rock pores are caused by salt crystal growth from solution. There are varying causes of and extents to which salt growth occurs. Hydration: The hydration of various salts causes expansion & contraction, pushing apart the silicate host minerals Thermal Expansion: During temperature fluctuations, salts trapped in pores may expand to a greater degree than the surrounding rock minerals.

Caverns (tafoni) Base of rock notching (basal weathering) Wedging

Caverns (tafoni – larger cavernous forms)

(Goudie and Viles, 1997:168)

Caverns (alveoli – smaller cavernous forms)

Weathering along bases of rocks

Wedging

Thermal Expansion/Contraction

A bit of moisture & sudden heat makes the rock pop

Vermillion Cliffs Wetting/Drying

Root Pressure

Online Animations Physical Weathering Visualizations ualization/collections/physical_weathering. html

Physical weathering “sets up” chemical weathering

Why do you ground coffee? to increase surface area

Classroom Resource Animation of physical weathering exposing more surface area for chemical weathering

4. Chemical Weathering Egyptian Obelisk – chemically weathered when brought to wetter environment, so water matters!

Classroom Resource Clip introducing chemical weathering

Dissolution Sugar & Salt Dissolves – so do rocks Best example: limestone

Other rocks dissolve too, but slower than limestone

Acid Rain Accelerates Decay Crosses political boundaries …

Taj Mahal Athens

Classroom Resources Dissolution from dropping a weak acid on limestone Close-up of dissolving mineral in electron microscope

Oxidation

Hydrolysis Water molecules at the mineral surface dissociate into H+ and OH- and the mobile H+ ions penetrate the crystal lattice, creating a charge imbalance, that causes cations (important nutrients) such as Ca 2+, Mg 2+, K + and Na + to diffuse out. For example, the feldspar reacts to decay and leaves a residue of clay mineral. H+H+ OH - H+H+

Hydration Water alters structure Complexation Metals released from primary minerals such as iron and manganese build complexes with organic components, such as fulvic acids and humic acids, causing an imbalance between cations and anions – that leads to mineral decay

Online Animations Chemical Weathering Visualizations ualization/collections/chemical_weathering.html

5. Weathering Products Quartz Sand: quartz is one of the last minerals to decay – it survives weathering & erosion to be deposited in

Rock Coatings

Nutrients - released from mineral weathering Calcium Sodium Magnesium Potassium

Clay Minerals Formed

Clays represent Earth’s ultimate decay of rock

If have too much clay, it shrinks & swells

Classroom Resources Feldspar turns to clay

6. Weathering Landscapes Consider a common rock – granitic rocks (granite, granodiorite, tonalite, diorite …) made up of interlocking minerals

Decay of weak minerals (biotite, feldspar) separates grains and makes granite sand called - GRUS

Grus produced most rapidly where joints intersect

Grus erosion from joints creates rounded forms at Mt Rushmore

Core stones made when corners of granite blocks weathered into grus

Core stones in subsurface are “emerge” onto the surface as the grus washes away with rain and flowing water, because they are too big to be carried by water

Grus washes away easily with rain, leaving piles of core stones - tors

Tors (piled up core stones) very common in the Sonoran Desert

Tors often take on significance to people

Granite weathering took a long time in the subsurface (from groundwater) – spheroidal forms were then exposed by erosion of grus

Dome forms produced the same way: subsurface weathering in joints Granite that is not heavily joined becomes domes after grus washed away Rio de Janeiro - Sugar Loaf

Half dome was made in the subsurface in tropical times and exposed by erosion of grus

Karst Topography: entire landscape made by dissolution weathering

Other rocks can also dissolve to form karst (gypsum, rock salt)

If exposed see grooves (karren)

Solution doline – dissolve fastest in joints

“Sinkhole” (doline)

Can also create doline by collapse

Florida – lots of groundwater pumping & roof of cave collapses Before Development After solution doline

Sinkholes merge to form Uvale valley

“Blind” rivers flow down sinkholes into cavern systems

Caves Formation Limestone Cave

Caves Features

Stalagtite Stalagmite Speleothems: Cave formations

Limestone Caves Step 1: Groundwater dissolves limestone, most aggressively at the water table. Also, groundwater follows lines of weakness in the limestone enlarging caves. Step 2. When the water table drops, stalactites and stalagmites can form on the roof and floor, respectively.

The water table usually drops when the stream has “cut down” to a lower level

Stalagmite – requires lots of time with water table much lower

Classroom Resources Cave Formation “Spin Around” Blind River

The southwest China karst region has “tower” karst” forms

7. Soils: Introduction Soils are more than just weathered rock

Soils organize themselves into layers

Online Free Resource URL

Classroom Resources Important in making humus & aerating soil Biomantles show Dynamic nature of soils

Classroom Resources: Soils Soil Horizons Visualizations horizons.html Soil Orders Visualizations orders.html Physical Properties of Soils physical_properties.html

Online Resources

Imagery seen in this presentation is courtesy of Ron Dorn and other ASU colleagues, students and colleagues in other academic departments, individual illustrations in scholarly journals such as Science and Nature, scholarly societies such as the Association of American Geographers, city,state governments, other countries government websites and U.S. government agencies such as NASA, USGS, NRCS, Library of Congress, U.S. Fish and Wildlife Service USAID and NOAA.