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Weather & Erosion

Mr. Litaker

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Weathering When Rocks undergo changes at or near the Earth’s

surface it is called

Weathering

What type of rock weathers the fastest?ans. Sedimentary

There are 2 Types of Weathering:1. Mechanical or Physical 2. Chemical or Decomposition

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Mechanical

Mechanical – A type of Physical Weathering A type of Physical Weathering in which no new substance is formed. Rock is in which no new substance is formed. Rock is only broken down only broken down physicallyphysically into smaller into smaller pieces of rocks.pieces of rocks. – Occurs mostly in temperate zones where you have

alternating freezing and thawing.– Occurs when plants take root in cracks and break

rock up.– Occurs when animals trample rocks and break them

down

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Types of Mechanical Weathering (Disintegration)

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1. Joints

Unloading or Joints– Mass of rock Mass of rock is pushed upward at the surface is pushed upward at the surface reducing the pressure on the rock reducing the pressure on the rock layer. layer.

Result = slabs of rock break free and Result = slabs of rock break free and gravity pulls them down slope. gravity pulls them down slope. (Talus)(Talus)

When granite expands into long When granite expands into long curved cracks parallel to the surface.curved cracks parallel to the surface.

When joints develop on the surface of When joints develop on the surface of the rock, the rock breaks into curved the rock, the rock breaks into curved sheets that peel away from the sheets that peel away from the underlying rock in a process calledunderlying rock in a process called Exfoliation.

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example: Stone Mountain, Georgia.

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•T h e r m a l E x p a n s I o n - •repeated daily heating and cooling of rock; •heat causes e x p a n s i o n; cooling causes contraction.

•different minerals expand and contract at different rates causing stresses along mineral boundaries.

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2. Ice Wedging

A type of mechanical weathering where water

seeps into cracks or joints and freezes.

When water freezes it expands by 10%. This process widens cracks in the rock.

Over time the fracture will expand as the freeze/thaw cycle continues. Can result in the formation of a talus slope

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Rock Fall - the freefall of rock from a cliff. Rock falls are often the result of physical weathering such as ice wedging.

The rock typically accumulates at the base of the cliff in the form of talus (loose rock). Rock falls are often triggered by earthquakes.

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3. Organic Activity

Root wedging occurs when a plant, especially trees, sink root systems into existing joints and fractures. As the root grows it forces the fracture to expand. Sometimes this cracks rocks into smaller pieces.

Animals that burrow in the ground expose rocks and cause them to be weathered.

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4. Abrasion

Moving material can impact or abrade on a surface. The surface can fracture, flake or loose small particles.

Rounded rocks have been exposed to more mechanical weathering; they have traveled further, they have been around for longer, or existed in areas with more rubbing and abrasion

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Types of Chemical Weathering (Decomposition)

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Decomposition

Decomposition – A type of Chemical Weathering in which new substances are formed and the rocks are altered in composition.– Occurs mostly in warm climates – Typically replacing strong minerals with weaker

minerals– 3 common chemical reactions associated with

chemical weathering are dissolution, hydrolysis, and oxidation.

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1. Dissolution

Dissolution occurs when rocks and/or minerals are dissolved by water. The dissolved material is transported away leaving a space in the rock. One consequence of this process is the formation of caves in limestone areas.

rain + carbon dioxide (from air) carbonic acid (reacts with rocks)

H2O + CO2 H2CO3

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The details of marble statues and tombstones will be gradually worn away by dissolution.

Image courtesy of Dr. Annabelle Foos.

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 Dissolution

Acidic waters (from pollution or natural) dissolve limestone allowing for additional water to gain entrance.  Can cause sinkholes and karst features as well as dissolution of statutes and grave stones.

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1. Carbonation (Dissolution)

Carbon dioxide (CO2) from the air is

dissolved in rainwater to create a weak acid, carbonic acid (H2CO3), that preferentially

dissolves certain rocks and minerals, e.g. limestone, marble. All rain is mildly acidic (average pH ~5.6, compared with neutral fluids [pH 7] and highly acidic [pH 1] substances).

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Different effects of chemical weathering according torock type: limestone in foreground; granite in leftbackground, Vermont.

Dissolution

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Cavern Systems

Caves form when dissolution occurs along a series of fractures in limestone to create a larger opening. Water passing through the rock enlarges the cave and associated re-precipitation can form a variety of features. The dissolved limestone is transported through the cave and may be precipitated to form new features such as stalagtites that grow downward from the cave ceiling and stalagmites that grow up from the floor. If they meet they form a compound cave formation such as a column.

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Sinkholes

Not all the products of dissolution are below ground. Sinkholes form at the surface from the collapse of the roof of an underlying cavern or by dissolution of rock along a series of fracture surfaces.

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Image below shows a sinkhole, (Watlings Blue Hole) in the Bahamas.

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2. Spheroidal Weathering

When water enters along cracks or joints in a rock, it weathers the corners and edges very fast because they have a greater surface area. This is called Spheroidal weathering. This happens when minerals in rocks turn to clay.

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Spheroidal Weathering

Caption: Caption: Spheroidal weatheringSpheroidal weathering

(a) The rectangular blocks outlined by joints are (a) The rectangular blocks outlined by joints are attacked by chemical weathering processes,attacked by chemical weathering processes,

(b) but the corners and edges are weathered (b) but the corners and edges are weathered most rapidly. most rapidly.

(c) When a block has been weathered so that it is (c) When a block has been weathered so that it is spherical, its entire surface is weathered spherical, its entire surface is weathered evenly, and no further change in shape occurs.evenly, and no further change in shape occurs.

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Cracks in the diabase break up the rock into adjoining blocks. Water seeping along the cracks weathers (decomposes) each isolated block from the outside in toward the center. The weathered rock peels off the unweathered core like onion skin.

Pictured here is a phenomenon referred to as spheroidal weathering.

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3. Hydrolysis (Granite Weathering)

Hydrolysis occurs when minerals react with water to form other products. Feldspar, the most common mineral in rocks on the earth's surface, reacts with water to form a secondary mineral such as kaolinite (a type of clay) and additional ions that are dissolved in water.

The weaker clay is readily worn away by physical weathering.

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Feldspar alters to clay (kaolinite) plus dissolved materials

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Leaching Leaching is the process where

minerals that are soluble and dissolved are carried down through the soil by the movement of water.

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4. Acid Precipitation

The term acid precipitation is used to specifically describe wet forms of acid pollution that can be found in rain, sleet, snow, fog, and cloud vapor.

Precipitation is considered to be acidic when its pH falls below 5.6

(which is 25 times more acidic than pure distilled water).

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Nitrogen oxidesNitrogen oxides (NOx) (NOx) and and sulfur dioxide sulfur dioxide Form ACID PRECIPITATIONForm ACID PRECIPITATION

Most wet acid deposition forms when Most wet acid deposition forms when nitrogen oxides (NOx) and sulfur dioxide nitrogen oxides (NOx) and sulfur dioxide (SO(SO22) are converted to ) are converted to nitric acidnitric acid (HNO (HNO33) )

and and sulfuric acidsulfuric acid (H (H22SOSO44) through ) through oxidation oxidation

andand dissolution dissolution..

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How do we measure pH?Acid rain is measured using a scale called "pH." The lower a substance's pH, the more acidic it is.

Pure water has a pH of 7.0. Normal rain is slightly acidic because carbon dioxide dissolves into it, so it has a pH of about 5.5. As of the year 2000, the most acidic rain falling in the US has a pH of about 4.3.

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Acid deposition has a variety of effects including, Damage to forests and soils, fish and other living things, materials, and human health. Acid rain also reduces how far and how clearly we can see through the air, an effect called visibility reduction.

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Chemical Weathering – e.g Dissolution by acid rain

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Picture of acid-generating, iron sulfate salts on the New Albany Shale

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5. Plant Acids

Lichens and Mosses grow on rocks and produce weak acids that dissolve the surface of rocks.

Example: Humic Acid

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Plant Acids – Eating away at rock chemically.

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6. OxidationOxygen, the second most

common element in the air we breathe, reacts with iron in minerals to form iron oxide minerals, e.g. hematite (rust). As many minerals contain iron, it is not unusual to see red-colored rocks like the example from Arches National Park.

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Weathering rind in granite produced primarily byoxidation.

National Arches State Park

Oxidation is occurring

Oxidation Oxidation occurring on the occurring on the door and top of door and top of this buildingthis building

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Identify what type of

Weathering is occurring in the following

slides.

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Identify the Following Pic.

A.

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Identify the Following Pic.

B.

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Identify the Following Pic.

C.

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Identify the Following Pic.

This Arrow is pointing at __________ type of weathering

D.

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Identify the Following Pic.

E.

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Identify the Following Pic.

F.

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Identify the following Pic

G.

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New

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Erosion

Erosion – A process by which weathered products are picked up and carried away and re-deposited in a different area.

Agents of Erosion:1. Gravity 2. Wind 3. Water 4. Glaciers/Ice

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Rates of Weathering

Three Factors affect the rate of weathering:

1. Rock Characteristics2. Climate3. Surface Area

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Rock Characteristics

Cracks – allow rocks to weather because they influence the ability of water to penetrate rock.

Mineral composition – can have a big effect on weathering. Ex. – Granite – weathers slow and resistant to

chemical weathering– Marble – undergoes rapid chemical weathering

Ex. Gravestones

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ClimateClimatic features, especially

TEMPERATURE & MOISTURE, have a strong effect on the rate of weathering.

Why?Controls Freeze-thaw cycles – which in turn controls amt. of Frost Wedging.Controls amt. of Vegetation – which in turn affects amt. of humic acid released by plants.

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What is the BEST climate?

High TemperaturesHigh Temperatures

and

Abundant Moisture

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Differential WeatheringDifferent parts of rock masses will

weather at different rates. Why

Because of different mineral compositions

Different layers of the rock are weathered at different rates depending on the minerals within the rock. The layers sticking up most are the most resistant

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Rates of weathering will be influenced by...

Surface area exposed - weathering occurs on the surface. More surface exposed, the faster the weathering will occur.