Weathering

True or False: The Earth’s surface has stayed the same for thousands of years

True or False: The Earth’s surface has stayed the same for thousands of years

The Earth’s surface is always changing!

Weathering

takes place as rocks are broken down into smaller pieces over time by the effects of weather

What is Weathering?

•First step to forming soil and sedimentary rock.

•It is the breakdown of rocks into smallerpieces by physical and chemical processes.

•No movement is involved in weathering. •The smaller pieces do not move to a new

location until erosion carries them away.

• Breakdown of rock into smaller pieces without any change in the chemical composition of its minerals

– Sometimes called “physical” weathering– Rock is torn apart by physical force, rather

than by chemical breakdown.

– breakdown of rock by

physical means

What is Mechanical Weathering?

Mechanical Weathering

Ice wedging – water seeps into cracks in rocks, freezes and expands, and eventually cracks the rock

Ice Wedging – Water water seeps into cracks in rocks and

freezes.– Water expands 10% when it freezes, pushing

rock apart.– Repeated

freeze and

thaw cycles

over the

years causes

rock to break

Ice causes weathering

Abrasion – grinding and wearing down of rock surfaces by other rocks

Example: pebbles bump against each other in a river bottom

Example: rock bump against each other at the beach

Wind causes weathering

What evidence of weathering do you see in this

picture?

Why wasn’t this mass of

land weathered

away?

Biotic – means life– Weathering caused by living

organisms – Plant roots act as a wedge

and widen cracks.– Other causes of biotic

weathering:• burrowing animals • microscopic plants • animals• algae• fungi

Mechanical - Biotic

http://commons.wikimedia.org/wiki/File:Wedge_Rock_Turkey_Run_SP,_IN.jpg

Plants –

roots can crack rock

Mechanical Weathering – breakdown of rock by physical means

Animals – human traffic or worms and other animals digging

• Exfoliation or unloading --Rock breaks off into sheets along joints which are parallel to the surface.

-Caused by expansion of rock due to uplift and removal of surface material that originally buried the rock

Exfoliation

http://commons.wikimedia.org/wiki/File:Exfoliation_and_mass_movement.jpg

Thermal Expansion and Contraction – Repeated daily heating and cooling of rock

– Heat causes expansion; cooling causes contraction.

– Different minerals expand and contract at different rates causing the rock to split.

http://commons.wikimedia.org/wiki/File:Penmaen_Dewi-St_David%27s_Head_-_geograph.org.uk_-_1097950.jpg

• Chemical reactions break down the bonds holding the rocks together, causing them to fall apart.

– Chemical weathering occurs in all types of rock.

– Rock reacts with water, gases. and solutions.

What is Chemical Weathering?

Chemical Weathering •Chemical Reactions change rock composition.

They break down rock and minerals into new substances.•Chemical reactions happen

faster in warm, wet conditions

Chemical Weathering -.

4 Big Players1. Water : will dissolve rock, but it takes

1000’s of years.

Grand Canyon National Park, Arizona.

Big Bend National Park, West Texas

breaks down rock and minerals into new substances

Water causes weathering

What evidence of weathering do you see in this

picture?

Chemical Weathering

2. Carbon Dioxide: Dissolves in water to form a weak acid that reacts with calcite,

limestone, and marble

Ex. Acid precipitation (acid rain) breaks down rock.

breaks down rock and minerals into new substances

Carbonation – Carbon dioxide (CO2) is dissolved in water making carbonic acid

Chemical Weathering

There are acids in ground water that dissolve rock underground, creating caves.

Limestone being eroded by water that is high in carbonic acid (formed by carbon dioxide). Natural Bridge Caverns, San Antonio, TX.

breaks down rock and minerals into new substances

Chemical Weathering3. Organic matter decays,

makes water more acidic and reactive.Ex. Living things like algae, lichen, and

humans dissolve rock with the acids they contain.

Chemical Weathering

4. Oxygen: reacts with iron and makes rust (oxidation)

Oxidation – chemical reaction in which an element (iron) combines with oxygen to form an oxide (rust), rust = iron oxide

Monument Valley, Utah.Palo Duro Canyon, Texas