Weathering 1

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

The Earth’s surface is always changing! True or False: The Earth’s surface has stayed the same for thousands of years False 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 Weathering causes soil formation

What is Weathering? First step to forming soil and sedimentary rock. It is the breakdown of rocks into smaller pieces 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. 9

What is Mechanical Weathering? 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

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: rock bump against each other at the beach Example: pebbles bump against each other in a river bottom

Wind causes weathering Why wasn’t this mass of land weathered away? What evidence of weathering do you see in this picture?

Mechanical - Biotic 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 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 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 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

What is Chemical Weathering? 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.

Chemical Weathering Chemical Reactions change rock composition. They break down rock and minerals into new substances. Chemical reactions happen faster in warm, wet conditions

Big Bend National Park, West Texas breaks down rock and minerals into new substances Chemical Weathering -. 4 Big Players 1. Water : will dissolve rock, but it takes 1000’s of years. Big Bend National Park, West Texas Grand Canyon National Park, Arizona.

Water causes weathering What evidence of weathering do you see in this picture?

Chemical Weathering Acid precipitation 2. Carbon Dioxide: breaks down rock and minerals into new substances 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. Carbonation – Carbon dioxide (CO2) is dissolved in water making carbonic acid

Chemical Weathering breaks down rock and minerals into new substances 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.

Chemical Weathering 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 (oxidation) 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 Palo Duro Canyon, Texas Monument Valley, Utah.