Weathering MechanicalandChemical

What Caused This?

What is Weathering? First step to forming soil and sedimentary rockFirst step to forming soil and sedimentary rock –Breakdown of rock into smaller pieces –The smaller pieces do not move to a new location until erosion carries them away –Many types of weathering

What is Mechanical Weathering? What is Mechanical Weathering? Breakdown of rock into smaller pieces without any change in the chemical composition of its mineralsBreakdown 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 –Smaller pieces do not move to a new location until erosion carries them away

Mechanical - Ice Wedging  Ice Wedging –Water fills joints of rocks and freezes –Water expands 10% when it freezes, pushes rock apart –Repeated freeze and thaw cycles over the years causes rock to break along joint

Mechanical - Exfoliation Exfoliation or unloadingExfoliation 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

Mechanical - Thermal Thermal expansionThermal expansion –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

Mechanical - Biotic Biotic – means lifeBiotic – means life –Weathering caused by living organisms –Plant roots act as a wedge and widen cracks –Other causes of biotic weathering are digging animals, microscopic plants and animals, algae and fungi.

What is Chemical Weathering? Breakdown of rock into smaller pieces because of change in the chemical composition of its mineralsBreakdown of rock into smaller pieces because of change in the chemical composition of its minerals –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 - Oxidation Oxidation - oxygen combines with other elements in rocks to form new types of rockOxidation - oxygen combines with other elements in rocks to form new types of rock –New substances are usually much softer than original, easier for other forces to break rock apart –Causes a “rusting” of the rock, often causes a color change in the rock

Chemical - Carbonation Carbonation – Carbon dioxide (CO 2 ) is dissolved in water making carbonic acidCarbonation – Carbon dioxide (CO 2 ) is dissolved in water making carbonic acid –Weak acid is formed when CO 2 in the air mixes with rain. This is the same acid found in soft drinks. –Weak acid is formed when CO 2 in the air mixes with rain. This is the same acid found in soft drinks. –Acid is too weak to harm plants and animals but slowly causes feldspars and limestone to decompose

Erosion and Deposition Agents, Forces, and Results

What Caused This?

What is Erosion? Erosion - moving of rock material from one place to a new locationErosion - moving of rock material from one place to a new location –For erosion to occur three processes must take place: lifting them, detachment of particles, and transporting them –Many agents of erosion - flowing water, moving ice, waves, gravity, or wind –Sand consists of small pieces of rock that have been weathered from a parent rock, eroded, and deposited somewhere else

What Is Wind Erosion? Wind - responsible for wearing away rocks and creating great deserts like the Sahara Desert and GobiWind - responsible for wearing away rocks and creating great deserts like the Sahara Desert and Gobi –Most effective in moving loose material –Two main effects: (1) Wind causes small particles to be lifted and moved away. (2) Suspended particles may impact on solid objects causing erosion by abrasion (rubbing). –Occurs in areas where there is not enough rainfall to support vegetation

What Is Water Erosion? Water - most influential force in erosion –Ability to move materials from one location to another over long distances –The faster water moves in streams the larger objects it can pick up and transport –Responsible for wearing away of rocks in rivers, lakes, and the oceans

What Is Wave Erosion? Waves - relentless pounding –Erodes the softer, weaker parts of the rock first, leaving harder, more resistant rock behind –Can take over 100 years to erode a rock to sand –Energy of waves along with the chemical content of the water erodes the rock off the coastline

What Is Gravitational Erosion? Mass movement - downward movement of rock and sediments, mainly due to the force of gravity. –Moves material from higher elevations to lower elevations where streams and glaciers can pick up the material and move it to lower elevations –Process is occurring continuously on all slopes, some act very slowly while others occur very suddenly until equilibrium is reached

What Is Glacial Erosion? Ice - moves and carries rocks, grinding the rocks beneath the glacierIce - moves and carries rocks, grinding the rocks beneath the glacier –Glaciers pluck and abrade to cause erosion –Plucking occurs when water enters cracks under the glacier, freezing, and breaking off pieces of rock that are then carried by the glacier. –Abrasion cuts into the rock under the glacier, smoothing and polishing the rock surface

What is Deposition? Deposition - laying down of sediment that has been transported by a medium such as wind, water, or iceDeposition - laying down of sediment that has been transported by a medium such as wind, water, or ice –Process of erosion stops when the moving particles fall out of the transporting medium and settle on a surface. This settling is deposition. –If the speed of the medium slows or the resistance of the particles increases, the balance changes and causes deposition. –Speed can be reduced by large rocks, hills, vegetation, etc.

Deposition - Wind Wind speed can be related to variations in heating and coolingWind speed can be related to variations in heating and cooling –Wind can transport fine particles in suspension hundreds of km from its original source in the desert. –Heavier material may be blown along the ground. –Material is eventually deposited when the wind changes direction or loses its strength. –Obstacles, whether natural of man-made, will often decide where the deposition occurs and the nature of the feature formed.

Deposition - Water Running water enters a large, fairly still body of water and its speed decreases.Running water enters a large, fairly still body of water and its speed decreases. –As speed decreases, water's ability to carry sediments decreases –Sediments carried by running water are deposited where the slowing water can no longer move them. Largest particles are deposited near the shore. Increasingly smaller particles settle out farther from the shore where the water is calmer –Occurs in streams, rivers, oceans, etc.

Deposition - Ice Glacial flows of ice - become slower when the ice begins meltingGlacial flows of ice - become slower when the ice begins melting – Deposits left by glaciers are called moraines and outwashes outwashes – Moraines are large chunks of broken rock left at the base and sides of the glacier as it melts and recedes and sides of the glacier as it melts and recedes –Finer material is carried in the rivers that form when the glacial ice melts. The deposits of these rivers look similar to normal river deposits and are called outwashes

What’s the Difference? WEATHERING - think weather wearing rock down EROSION - think of a road and traveling DEPOSITION – think of depositing money in a bank