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
Why?Is it because therock on top isharder than therock below? http://www.flickr.com Wolfgang Staudt
Yes, the rock on the bottom is softer than therock on top of this formation. The top rockweathers more slowly.
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 theycool, 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 togrow, cause the cracks to get bigger andbigger breaking the rock apart.
Here theroots of thetree aregrowing inthe cracks inthe rocksmaking thecracks larger. http://www.flickr.com Chazz Layne
Erosion—the movement of rocks and sedimentfrom one place to another. The main agents are: 1. Water 3. Ice 2. Wind 4. GravityDeposition—the dropping of sediment and/orrocks in another place follows weathering anderosion. Deposition occurs when: 1. Water carrying the sediment slows down. 2. The wind carries the sediment dies down. 3. The glacier carrying the sediment melts.
Here we seethe processesacting ofweathering, Gravityerosion anddeposition Depositionworkingtogether.
Water is the main agent of erosion.Running water carries weathered pieces ofrock from one location to another.It can carry big pieces of rock as well assmaller rock pieces and soil.It can also weather rock at the same time asit is eroding it (carrying it away).
Rock is worn away (weathered) at the sametime that is carried away (eroded) by fastmoving water. Rocks hit one another causing them to break.
Fast moving water canmove (erode) very largerocks transportingthem downhill alongwith smaller rocks. http://www.flickr.comHamed Saber http://www.flickr.com Diego
A slow moving river carries mainly pebbles,sand, silt and clay (sediment. The slowerwater cannot carry bigger rocks. http://www.flickr.com traveling lunas http://www.flickr.comwatchsmart
As soon as the rushing water slows down, the larger pieces of weathered material it had been carrying is deposited.Photo courtesy daneen_vol of Flickr underCreative Commons license
As can be seen by this diagram, as the water slowsdown, first the larger pieces fall out. And then as itflows slower and slower, smaller and smaller piecesare deposited.
Soil and pebbles may be carried for a great distance as sediments in the river. As we said before, a river carrying a lot of sediment looks muddy. Shown here, sediments are being deposited at the mouth of a riverWikipedia Commons in Lake Genoa.
When sediments (gravel and soil) aredeposited at the mouth of a river, a delta maybe formed. It becomes a fertile area for togrow crops such as rice.
Sediment deposited at the mouth of ariver also may build a sandy beach.
Sediment deposited at the mouth of a river also may build a sandy beach. • Some of the sediment is deposited immediately at the mouth of the river.• Most of the sediment becomes suspendedin sea water and is carried along the coast bythe longshore current, a stream of waterflowing parallel to the beach. This current iscreated by waves breaking at an angle toshore.
Wind, the second agent of erosion.Wind picks upsmall pieces ofrock or soil andtransports themfrom theirsource toanother locationwhere they aredeposited. http://www.flickr.com nukeit1
When the wind blows in the desert, sand iscontinuously deposited in a different places.When thewindstopsblowing,new sanddunesmay havebeenformed.
In this section we see will see mesas,buttes, arches, canyons and and otherstrange rock formations that were createdthrough both weathering and erosion.The wind’s remarkable ability to sculptsuch odd and beautiful landforms isexplained by the hardness of the rockinvolved. Some rocks are softer andweather faster than others.
Plateau with mesas, butte and gully http://www.flickr.com / puroticorico
Photo of a mesa—this is what is left of the plateauthat made up this entire landform before weatheringand erosion carried much of it away. http://pics4learning.com/
Photo of buttesWhat could the red line be depicting? http://pics4learning.com/
Photo of archWhy was the middle of the archeroded away and not the supporting sides?. Photo courtesy of USGS
The difference in hardness of the rock composing the “cap” andthe rock below the cap explains why the under rock weatheredfaster than the cap itself. a hoodoo. http://pics4learning.com/
Here we can see that the weathering and erosion from theseboulders is filling in the area beneath them.
Over hundreds, maybe thousands of years,it could have happened something like this.
On their way to the sea, some rivers wind acrossplateaus, carving deep valleys and taking sedimentdown stream with them.Once the valley walls become exposed, theserocks are further weathered by the wind, rainand changing temperatures.Over millions of year, these valleys can becomegiant, majestic chasms called canyons.The Grand Canyon is just such a canyon, it iscontinually being carved by the Colorado River.
Agents of erosion, weathering, and deposition: Ice--Glaciers
Glaciers—great sheets of ice-- createlandforms through both erosion anddeposition.
Glaciers form when, over many years moresnow falls each year than melts. As aresult, a deep layer of compacted snowaccumulates. This layer of snow becomescompressed into a thick sheet of ice.What makes glaciers unique is that theymove. Due to their mass and the force ofgravity, glaciers flow down hill a fewinches or feet per year.
There are two types of glaciers:1. Continental glaciers--glaciers that form over large areas of continents close to the North and South Poles.2. Mountain glaciers--relatively small glaciers that form near the tops of mountains.
Continental glacier in Antarcticahttp://www.coolantarctica.com/gallery2/glaciers/Glacier-10.html
Mountain glacier in the Rocky Mountains http://pics4learning.com
During the past IceAges, very thickcontinental icesheets overlaid muchof the continent ofNorth America asshown in this diagram.These very thickglaciers covered allbut the highestmountains andresulted in significant Grey shaded area—continental glacierserosion.
Many lakes in North America including the GreatLakes, were created by glacial moving over therock and gouging out deep “holes” which filledwith water when the glaciers melted. Wikipedia Commons
In the last glaciation period, which endedapproximately 10,000 years ago, 32 percent ofEarths land area was covered with glaciers.Glaciers now cover only about 10 percent ofthe land area.That glacial ice is found mainly over Antarctica.Most of the other glaciers overlie Greenland; theremaining small percentage are mountain glaciersfound in places such as Alaska, the CanadianArctic, Patagonia, New Zealand, the HimalayanMountains, the Rocky Mountains, and the Alps.
That glacial ice is found mainly over Antarctica.Most of the other glaciers cover Greenland; theremaining small percentage are mountain glaciersfound in places such as Alaska, the CanadianArctic, New Zealand, the HimalayanMountains, the Rocky Mountains and the Alps.
Glaciers are not landforms. The action ofglaciers, however, creates landforms.Glaciers move, and as they do, they scourthe landscape, "carving" out landformsand eroding material.This happens because friction breaks thebedrock into pieces of smaller rock andsoil. This debris becomes embedded inthe bottom of the moving glacier and iscarried downhill.
Like a big bull dozer, glaciers drag and push rocky debris downhill to the end of their travel for that winter (or for that ice age).As theymove, the rocksand soil stuck inthe bottom ofthe glacierscrape andscratch the landunderneaththem.In the photo above, you can see these “scratches”.
Imagine that scrapping continuing for millionsof years and you can see how glaciers turnV shaped valleys into U shaped valleys.
http://www.flickr.com/theslowlaneAs the glacier moves through a V-shaped valley,it sculpts a “U”.
A horn is apyramid-shapedmountain peakcreated byseveral glacierseroding away atdifferent sides ofthe samemountain. http://www.geology.wisc.edu
Glaciers leave a predictable mark on thelandscape through their deposition.As the weather warms, glaciers pullback, leaving deposits of the rockymaterial along the sides and at theend of their travels.This deposition creates landformfeatures called moraines.
Rocks and sediment deposited on the sides or the endsWikipedia Commons of a glacier create a landform called a moraine. Wikipedia Commons
When sedimentsare deposited by ariver over a longperiod of time intoa shallow ocean orlake, layers ofsedimentary rockmay be formed.Just what issedimentary rock? http://www.texasbeyondhistory.net/
One of the three major rocktypes, sedimentary rock is formed whensediments such as tiny fragments of rockincluding pebbles, gravel, sand, silt andclay are naturally glued together undergreat pressure.The next few slides will help us betterunderstand just how sedimentary rock isformed.
The first steps in theformation ofsedimentary rock areweathering, erosion anddeposition of fragmentsof rock in a river.
The fast moving river carries the pieces ofrock. In the process, the rocks hit oneanother and are weathered further andfurther, forming tiny pieces of sediment.
Here you can see the pieces of rockbecoming smaller and smaller as the riverflows towards the sea.
As it nears the sea the river gradually slowsdown, depositing bigger pieces of rock. Bythe time it reaches the sea it is mainlycarrying small pebbles and soil. As it entersthe sea it slows down even more anddeposits the sediment it is carrying.
The river deposits the larger heavierpebbles first and then gradually the smallerand lighter sediments. In this way layers ofsediment are deposited on the floor of thesea. The upper layers of sediment areheavy and push the sediments beneathcloser together.
Then the process of lithification occurs. Theseawater between the grains of sedimentevaporates leaving behind the minerals thatwere dissolved in the water. These mineralsglue the tiny fragments of sediment togetherto form a type of rock called sedimentary rock.
Over millions of years, the sea fills upwith sediment and the water evaporates.The result--layers of sedimentary rock. http://www.knockan-crag.co.uk/ written permission to use
Five types of sedimentary rock areformed in this way:• conglomerate• sandstone• shale • siltstone • limestoneThe type of rock depends on what type ofsediment is cemented together.
Particles of pebblesor gravel cementedtogether becomeconglomerate.Particles of sandcemented togetherbecome sandstone.
Particles of siltcemented togetherbecomes siltstone.Particles of mudcemented togetherbecomes shale ormudstone.
Rocky beaches are shaped by theweathering, erosion and desposition by theof waves (water) and wind.The constant beating of the cliffs and seastacks weather and erode the rock.Eroded material is then depositedbecoming available to form sand bars andbarrier islands.
Because of the severe pounding ofboth wind and waves, theseformations erode relatively quickly.Over a long period of time sea stacksometimes completely weather and erodeaway.The following series of pictures shows thesame sea stack photographed over a 100year period.
Sea arches and seacaves are createdwhen part of the rockmaking up a cliff isharder than otherparts. The softersection can not standthe erosive powers ofthe waves for as longas the harder sectionand erodes faster.
The softer material in this span has alreadyeroded, leaving the harder rock spanning the arch. http://www.flickr.com mikebairdThe softer rock in this cliff hasalready eroded, leaving a sea cave.
Sandy Beaches• Are deposition landforms, meaningthat they are formed by deposits ofsediment.•The sediments deposited are sand, silt andpebbles– materials carried by a river orstream from the inland into the ocean;•River sediments are the source of most ofthe sand on beaches.
Washed to sea by streams and rivers, twoseparate processes result in the deposit ofthis sand and sediment on the shore.• Some of the sediment is depositedimmediately at the mouth of the river oftenforming a delta.• Most of the sediment becomes suspendedin sea water and is carried along the coast bythe longshore current, a stream of waterflowing parallel to the beach. This current iscreated by waves breaking at an angle toshore.
Rocks and sediment deposited at the footof a glacier where the glacier melts createa landform called a moraine. THE END