WEATHERING Destructive Forces in Nature.. Those that destroy landforms.By Moira Whitehouse PhD
How are Landforms Made?• The forces that create the differentlandforms are, broadly speaking: • Constructive forces • Destructive forces
Constructive forces• Are those that build up the land.• Landforms such as mountains ranges,volcanoesand plateaus are built by themovement of the Earth’s plates• Landforms such as deltas, plains and sanddunes are created when rocks and soilresulting from weathering and erosion arecarried away and deposited in new areas.
Destructive forces• Those that wear down the land, like weatheringand erosion.(Don’t let the name “destructive forces” mislead you.Destructive forces create landforms like canyons,valleys, etc. but to do so they first had to destroysome other landforms, mountains, plateaus, etc.)
The two players in the destructive process are: Weathering and Erosion
All rocks do not weather at the same rate.Further we will find that parts of somerocks weather faster than other parts ofthe same rock. Why?Whatever the reason, one can find somevery odd looking weathered rock:
Why?Grand canyon Image courtesy of National Geographic
Yes, the rock on the bottom is softer than therock on top of this formation. The top rockweathers more slowly.
Why?Is it because therock on top isharder than therock below? http://www.flickr.com Wolfgang Staudt
What causes weathering; that is, whatcauses rocks to break into smaller andsmaller pieces?
• Nonliving things and living things canbreak bigger rocks into smaller pieces.We will look first at non living things thatbreak up rock.
Nonliving things that break rocks into pieces. 1. Water running over the rock 2. Water freezing in cracks in the rocks 3. The temperature of rocks changing from hot to cold 4. The abrasion of rock by the blowing wind carrying sand 5. Water with acid in it
Rapidly moving water particularly high up inthe mountains or a canyon can break off piecesof rock. http://www.ngu.no/en-gb/hm/
Seen here, a small,fast runningstream of water isdramaticallycutting throughthe rock in the sideof this mountain.
Rocks carried by fast moving water hit other rocksbreaking them into pieces. Moving sand acts likesandpaper on the larger rocks in the river bedrubbing off pieces of rock. These smallerpieces are then carried downstream by thefast movingriver. http://www.flickr.com kia4067
Here you can see pieces of rock created byfast moving water. http://www.flickr.com Randy OHC
Rocks that have been tumbled for a long time in rivers and streams become smooth and rounded.http://www.flickr.comDawn
3. Changing temperature also causesweathering. As rocks heat up, they expand. As they cool,they contract.This process is repeated over and over againin nature. Eventually this process causes them tobreak apart.
Here we see large rock in a desert environment that has probably been exposed to the freeze- melt cycle.http://www.flickr.comHoggheff aka Hank Ashby aka Mr. Freshtags
Carbonic acid is very common in nature. It isproduced when carbon dioxide combineswith water.When this weak carbonic acid tricklesinto cracks in limestone, it dissolves therock and eats “holes” in it.
The mildly acidic rainwater flows intocracks in the ground.Sometimes it eatshuge holes in therock--caves. http://www.esi.utexas.edu
The same acid that made this rock “holy”when it was buried in the ground, alsoworks to make caves
stalactites stalagmitesThis is a picture of a cave with stalactitesand stalagmites.
When the acid water dissolved the rockevaporates, crystals of calcite are left behind.When the water from many, many drips at thetop of a cave evaporates, a stalactite forms. (theone on the ceiling stuck tight ...stalactite)Drips that fall on the cave floor cause stalagmitesto grow. (The stalagmitesmight have stuck to theceiling but they didn’t.)
More pictures ofstalactites andstalagmites, doyou rememberwhich is which?
Sometime other minerals in rocks react withthe weak acid in water to form other weakersubstances.These weaker substances are then more easilyworn away by weathering. Feldspar changes to clay.
The roots of plants, particularly tree roots,are amazingly strong. When they startgrowing as tiny root hairs they can fit intothe smallest of cracks.As these tree roots continue to grow,cause the cracks to get bigger and biggerbreaking the rock apart.
Here theroots of thetree aregrowing inthe cracks inthe rocksmaking thecracks larger. http://www.flickr.com Chazz Layne
3. Burrowing animalsWhen animals burrow in rocks or betweenthe rocks, they carry seeds which germinatein the cracks in the rocks.
How about a little review. Do you remember what the word weathering of rock means?Yes, it means breaking rock into smaller and smaller pieces.But, with weathering, it is not a hammer or man whobreaks the rock. It is forces in nature.
What are the main forces in nature that break rock intosmaller and smaller pieces? Yes, there is: • fast running water • wind carrying sand • water in the cracks freezing, melting and freezing • rocks heating up in daytime and cooling down at night • acid in water “eating” holes in rock and forming caves • plant roots • burrowing animals
What force in nature iscausing the rock toweather in the pictureson the left forming adeeper and deepervalley?
In the desert, thereis very little runningwater. What forcein nature causesrock formations likethe one seen in thepictures on theleft?
What force innature has causedthe rock in thepicture on the leftto break into twopieces?
What will happento this rock if watergets into the cracksand freezes, melts,freezes and meltsover and overagain?
After millions andmillions of yearsthe mountains onthe left will finallylook like the onesshown below.Weathering isconstantly changingthe surface of theEarth.