And under the ocean on the seafloor.Shallow seas surround most continents andcover gently sloping areas called continentalshelves. These continental shelves drop offsteeply leading to the deepest parts of theocean called the abyss.The abyss contains plains, long mountainranges, valleys, ocean trenches andvolcanoes. Undersea volcanoes, whetheractive or extinct, are called seamounts. If aseamount grows tall enough to reach abovethe ocean surface, it forms an island.
sea floor Oceanic Continental ridge Trench shelf Seamounts Plains Image Creative commons licence Magma http://www.bukisa.com/articles/25522_ocean-floor-webquest Rift valley Volcanic island Oceanic Continental Seamounts ridge shelf Plains
This is an artist’s conception of the deepestknown part of any ocean, the Mariana Trench,located in the Western Pacific Ocean. Itreaches a depth of 36000 feet below sea level. Mariana Trench http://www.ngdc.noaa.gov
The purpose of this session is to discuss theforces that create landforms.However, before studying these forces, weshould review some of the more importantlandforms.
1. Ocean -- a great expanse of salty water. http://www.flickr.com Repoort
2. Plains -- an extensive area of flat orrolling, mostly treeless grassland. http://pics4learning.com/
3. Plateau -- an elevated level expanse ofland; a tableland. In this high countryplateau, you can also see buttes and mesas. http://www.flickr.com/ puroticorico
4. Mountain -- a high, steep elevation of theearths surface, higher than a hill. http://pics4learning.com/
5. Volcano -- an opening in the Earths crustthrough which molten lava, ash, and gasesare ejected. http://pics4learning.com/
6. Valley--an elongated lowland between ranges of mountains, hills, or other uplands, often having a river or stream running along the bottom. http://pics4learning.com/
7. Canyon -- a narrow chasm with steep cliff walls, cut into the earth by running water. Most canyons were formed by a process of long-time erosion of a plateau. http://www.pdphoto.org
8. Delta -- a place at the rivers mouth, where the river splits into many different sections, forming a marshy triangle. usgs
9. Glacier -- a huge mass of ice slowly flowing over a landmass. http://pics4learning.com
10. Moraine -- an accumulation of boulders, stones, or other debris carried and deposited by a glacier.
11. Mesa--a broad, flat-topped elevation with one or more cliff like sides. http://pics4learning.com/
12. Butte--a hill with a flat top and steep sides rising abruptly from the surrounding area. http://pics4learning.com/
13. Beach--the zone above the water line at the shore of a body of water, marked by an accumulation of sand, stone, or gravel that has been deposited by the tide or waves. http://pics4learning.com/
14. Lake--a large inland body of fresh orsalty water. http://pics4learning.com/
15. Hill—an elevation in the earths surface smaller than a mountain. http://pics4learning.com/
16. Sand dune—a hill of sand created by the wind.
17. Cave -- an underground enclosure with accessfrom the surface of the ground or from the sea.
Image courtesy of usgsNow let’s look at them all togetheron a make believe continent.
The surface of the Earth is constantlychanging as new landforms are builtand older ones are destroyed by theforces of the Earth.
Some changes happen so slowly thatyou do not see the differences for along time—for example the Coloradoriver carving the Grand Canyon hasbeen happening for millions of years.On the other hand, volcanic eruptions andearthquakes can change the surface of theEarth very quickly.
How are Landforms Made?• Now let’s examine the forces thatcreate the different landforms. Broadlyspeaking, there are two kinds:• Constructive forces—those that build up theland. Included are:1) plate movement that builds mountains, and2) deposition that creates landforms such asdeltas and layers of sedimentary rock.• Destructive forces—those that wear downthe land, like weathering and erosion.
Constructive forces• Landforms such as mountains,volcanoes, and plateaus are built bycrustal movement and other tectonicactivity inside the Earth.• 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• Some landforms—canyons, mesas andbuttes-- are created by the action of wind,water, and ice—forces of weathering anderosion. •These actions physically changes the Earths surface by carving and eroding land surfaces.
This session will focus on the constructiveforces that build up the followinglandforms—mountains, volcanoes andplateaus.Another session will deal withweathering, erosion and deposition.
However, in order to understand theconstructive forces, we need backgroundinformation on two topics: 1. the interior of the Earth. 2. the plate tectonic theory.
First let us look at the interior of theEarth.If you were able to cut the Earth in half,you would find it is made up of differentlayers.Each layer has its own characteristics and therock making up the layers is a mixture ofcertain minerals.
The thin, outermost layer of the earth ismade of solid rock and is called the crust.All of the landforms on Earth are located Cruston the crust and all life on Earth exists onthis top layer.
The Earth’s crust consists of the oceanic andthe continental crusts, both of which float onthe magma. • The oceanic crust is the layer of rock which forms the floor of an ocean. It is about 4-7 miles thick. • The continental crust is the layer of rock which forms the continents and those areas magma of shallow seabed close to the shore. The continental crust is much thicker than the oceanic crust--about 19 miles thick.
The continental crust is mainly made of arock called granite.The oceanic crust is made of mostly basalt,a very dense rock that is much heavier thanthe granite of the continental crust.As a result, the oceanic crust sinks deeperinto the magma (the molten rock) whenthe two formations compete.
This rockis mainlygranite. This rock magma is mainly basalt.
The mantle is the layer directly below thecrust. Earth’s mantle • It is about 1800 miles thick. • The mantle is divided into two regions, the upper and lower sections.
• And here comes the confusing part.• The uppermost part of the mantle isjoined to the thin, solid crust forming a solidlayer of rock called the lithosphere.• The lithosphereincludes the crustand the hard uppermantle and consistsof a series of hugerock plates thatsurround the Earth.
Immediately under the lithosphere is theasthenosphere, the lower part of the mantle. Creative Commons Wikipedia CommonsThe asthenosphere is made of partly moltenrock. The reason rock can be molten is shownby the temperatures shown on the next slide.
The plates of the lithosphere float on this hot, melted rock.Creative CommonsWikipedia Commons
Because of convection,the cooler (heavier)melted rock in theathenosphere sinks asthe hotter (lighter)melted rock risescreating convectioncurrents. Thesecurrents create thetectonic activity thatcauses the crustalplates to slowly move.
The core is also Inner core, solid irondivided into two and nickelregions, the innercore and the outercore. Fromearthquake waves,scientists believethe outer core is aliquid and the innercore is a solid. Outer core, liquid iron
The outer core is made of liquid iron andis very dense. Scientists hypothesize thatthe circulation of the outer core causesthe magnetic field around the Earth.The inner core is made of solid iron andnickel. Many scientists believe it is keptin the solid state because of theextreme pressure from other layers.
Let’s see how the make-up of this planetary ballaffects the constructionand destruction of sur-face land forms.First, recall that landforms arefound on a very thin crustfloating on top of a thick layer ofmolten magma that, because ofconvection, is moving about. http://scign.jpl.nasa.gov/
Next, in addition toknowing that thecrust is part of thelithosphere, we nowunderstand that thelithosphere is brokeninto huge pieces ofrock called plates.These plates fittogether around theglobe like a giant Free image from DKImagesjigsaw puzzle.
Map courtesy NOAAHere are the 7 major plates plus several smaller ones.
• We learned that because of convection themagma below the lithosphere flows, veryslowly, in large patterns. Creative Commons Wikipedia Commons
• As it flows, the magma in the mantle rubson the bottom of the lithosphere and causesthese huge plates to, very slowly, but veryreliably, move.
Now, that we’vereviewed the basicsof the interior of theEarth, let’s look atthe the theory ofplate tectonics andthe constructiveforces that buildnew land—mountains, plateausand volcanoes.
The theory of plate tectonics explains howthe movement of the lithospheric platesand their interaction with each otherproduce different landforms.Remember we said that there are sevenbig lithospheric plates and many smallones and these plates are in constant,albeit very slow, motion.Let’s look at an image showing the platesand see how they move.
Wikipedia commonsFirst of all, notice that most plates have both Wikipedia Commonsoceanic and continental crust and that fewhave only oceanic crust.
Wikipedia CommonsWikipedia commons Notice the arrows to see how the plates interact.
You may have noticed that plates can movein one of three ways: 1. together 2. apart 3. side by side
The margins where plates meet each otherare called boundaries.Those on the leadingedge of moving platesare called leading edgeboundaries.The three types ofboundaries are based onthe three ways platesmove:
1. convergent or collision boundaries—where two plates are colliding Convergent
2. divergent boundaries—where twoplates are moving apart Divergent
3. transform boundaries—where two plates are sliding past another TransformEarthquakes, volcanic activity, mountainbuilding and the formation of ocean trenchesoccur along the boundaries of these plates.
This map of volcano and earthquake activitymirrors a map of plate boundaries. Image for educational use http//serc.carleton.educ
However, in order to make sense of thiswhole thing, we need to pay attention totwo things:1. the direction the plates are moving and2. the boundaries of the plates—doesthe plate have a continental leadingedge or an oceanic leading edge?
Possibilities are:1. A continental plate moves into a continental plate.2. An oceanic plate moves into a continental plate.3. An oceanic plate moves into an oceanic plate.4. An oceanic plate moves away from an oceanic plate.5. A continental plate moves away from a continental plate.6. Two plates slide by one another.
Let’s look at that image of the plates again.Wikipedia Commons
We will first consider convergentboundaries—where two plates collide:What happens when two plates collidedepends on whether the two plates are: • both continental plates • both oceanic plates or • an oceanic and continental plate. Let’s look at each of these situations.
USGSWhen two continental plates collide, the rock isuplifted and compressed causing the land to rise,crumple and buckle. Mountain ranges and highplateaus result.
These collisions produce Earth’s mostspectacular mountain ranges and deepestvalleys.Mountain ranges that were formed in thisway include the Alps, the Appalachians,the Urals, and the most striking example,the Himalayas. The Himalayas are the highest mountains in the world, towering as high as 29,000 feet.
Millions and millions of years ago thecontinental plate carrying the continent of Indiamoved north and collided with the Euroasiancontinental plate.The slow continuousgrinding of the twoplates pushed up theHimalayan Mountainsand the TibetanPlateau to their http://scign.jpl.nasa.gov/present heights.
Wikipedia CommonsHimalayan Mountains from the air
As happened with the Himalayas, whentwo continental plates collide head on,the layers of sedimentary rock usuallybend rather than break. The rocks bend inmuch the same way a rug wrinkles as it ispushed across the floor. A bend in a rockis called a fold and the resultingmountains are calledfolded mountains.
Examples of folded mountains. The Himalayas, the Andes in South America, the Alps, the Rocky Mountains and the Appalachian Mountains are all folded mountains.Pics4learning
Sometimes the stress of the twocontinental plates moving together causethe rocks to break rather than fold.A break in the Earth’s crust is called a fault.The blocks of rock along the fault can slideup, down or sideways forming anothertype of mountain.
Mountains formed in this way are called fault-block mountains. Pics4learningExamples of fault-block mountains include: theSierra Nevada mountains in North America andthe Harz Mountains in Germany.
When continental plates meet and pushup “new” mountains, the land behind themountain chain often is also up lifted.However, it doesn’t break or fold. As aresult a high flat area is formed---aplateau.The Tibetan Plateau was created when theIndian Plate and Eurasian Plate collided.The Himalayan Mountains formed alongthe edge of the collision, and the unbrokenplateau behind them rose as a “flat table”.
Tibetan plateau with the Himalayan Mountains in the background.http://www.ram.org Ganges River flood plain to the left, then Himalayan Mts. covered with snow, then thehttp://mapstor.com/ Tibetan plateau
Another way that a plateau is formed occurs whentwo continents meet but the magma does notcollect in a chamber. Instead it rises beneath a large,stable landmass.If the magma is unable to break through anycracks or vents, it exerts pressure on the land,causing it to rise upward in one piece.Geologists believe this uplifting process formedthe Colorado Plateau about five million years ago.
If the magma is able to break through cracks orvents, plateaus are formed by repeated flows ofmolten rock over millions of years on the surface ofthe Earth. The magma can squeeze throughvertically or horizontally as can be seen by thefollowing pictures.
This is an example of basalt lava thatsqueezed vertically through the surfacemillions of years ago. U.S. Geological Survey photo by S. R. Brantley. (fair use policy)
The lava that oozes on the Earth’s surfacethrough cracks or vents sometimesspreads out over large areas filling invalleys and covering hills. This processrepeats itself many times over the years.The hardened lava sheets pile up and forma raised plateau called a lava plateau.An example is the the Columbia Plateauwhich covers parts of the states of Oregon,Washington, and Idaho..
The Columbia Plateau Wikipedia CommonsWhile standing on one, a plateau may look a lot like aplain, a broad flat area. However, a plateau hasexperienced some kind of uplift, it is tectonically active.A plain is not.
We have discussed what happens when twocontinental plates collide: tall mountainchains, deep valleys and high plateaus.Now we will consider what happenswhen an oceanic and continental plateconverge collide?
When an oceanic plate moves into a continental plate, it slides under because it is denser and thus, heavier.The extreme heat andpressure causes theleading edgeof the oceanicplate to melt. USGSThe resulting magma rises and gathers inpools under the continental crust.
As a result:First, a deep ocean trench forms where theoceanic plate moves under the continentalplate.Second, when enough magma collects in thepools under the continental plate, andenough pressure develops, a volcano erupts.
An example of an oceanic platesubducting under a continental platewould be on the western coast of SouthAmerica. •The Nasca Plate (oceanic plate) is moving under the South American Plate. Result: the Andes Mountains. Many volcanoes and earthquakes occur in this region.
The convergence of the Nazca and the South American Plate Andes Mountains http://pubs.usgs.gov/
Image courtesy of National Geographic Andes Mountains
Another place, closer to home, wheresubduction is occurring is found on thewest coast of United States.There a small oceanic plate called the Juande Fuca Plate is subducting under theNorth American Plate.
This subduction isoccurring on thecoast of Washingtonstate, Oregon andnorthern California.The Juan de FucaPlate, a vestigeoceanic plate, ispushing under theNorth AmericanPlate.
This subductionresults in thebuilding of theCascade MountainRange. Well-known volcanoesin this range areMount St. Helens,Mount Adams andMount Hood.
Wikipedia commons Mount St. Helens erupting in 1980
So far we have explored what happenswhen:1. Two continental plates converge.2. An oceanic and continental plateconverge. What happens then when two oceanic plates collide?
When two oceanic plates converge, one of the platessubducts under the other. The plate descending intothe asthenosphere is heated to the point that itbecomes semi liquid magma, which rises to thesurface, thereby creating an island arc or islandchain. An example: Japan.
An example is in thenorthwest, where thePacific plate plungesunder the NorthAmerican plate. As thecrust is pushed deepinto the earth by the http://scign.jpl.nasa.gov/relentlessly shovingPacific plate, it startsto melt and some of the melted crust risesback to the surface in volcanic eruptions.These volcanoes form an arc of volcanicislands called the Aleutian Islands.
Volcanoes similarly caused by plate subduction aroundthe rim of the Pacific ocean are called the Ring of Fire. Wikipedia Commons
Undersea earthquakes, also commonwhere two oceanic plates meet, are causedwhen these huge masses of earth slidingpast each other get stuck. Images from usgsBoth plates keep inching along their paths,but the surface where they meet does notallow movement. Pressure builds.
With pressure continuing tobuild for long periods oftime, everything is understrain and distortion occurs. When finally, the pressure is strong enough to overcome the resistance to movement, the plate becomes violently “unstuck”—an earthquake Images from usgs occurs.
With the instantaneous “readjustment” ofthe seafloor around the subduction zone, a huge amount of water is displaced causing agiant swell in the ocean– a tsunami. Images from usgsWhen a tsunami reaches an island beach itforms an enormous wave which can causegreat destruction.
Wikipedia CommonsIn the southeastern Indian Ocean, the Indo-Australianand Eurasian Plates collide resulting in frequent largeundersea earthquakes, many causing tsunamis.
Recent tsunamisoccurred in Sumatra,Indonesia in 2004 andin Samoa in 2009.These huge waves werecaused by underseaearthquakes whereoceanic plates converge.Here you can see the Eurasian Plate andAustralian Plate (both with leading oceanicplates) colliding producing an earthquake.
Think about the recent devastatingoutcomes in Sumatra 2004 and 2009.Tsunami in 2004 Earthquake in 2009 Wikipedia Commons
When two oceanicplates diverge (moveapart), magma fromthe mantle flowsupward filling the gapbetween the twoplates. When the lavahits the cold water itsolidifies as basalt rock. Image courtesy of USGSIf this process occurs over a long, long time, anew mountain range is built. This type ofmountain chain is called a midoceanic ridge.
Wikipedia CommonsHere we see magma building up to form achain of mountains as two oceanic platesdiverge (pull apart).As a result of this process, new oceanic crust iscontinuously being created between thediverging plates. As new crust is built, the oldercrust is migrating away from the fault.
This is what is happening in the middle of the Atlantic Ocean. The North American plate and the Eurasian plates are pulling apart in the North Atlantic and the South American plate and the African plate in the South Atlantic.usgs
Magma oozing outof these “pullaparts” overmillions and millionsof years has builtand is still buildingan underwatermountain rangedown the middle ofthe Atlantic Oceancalled the Mid- http://www.navmetoccom.Atlantic Ridge.
The tremendous forces involved in thismountain building process often fracturethe crust resulting in volcanoes andearthquakes.When the volcanoes along the ridgeerupt, new land is formed. Sometimesthe “new land” rises above the surfaceof the ocean and becomes an island.
Iceland is an example of an island formed by magma that came from between diverging oceanic plates. It sits on top of the Mid- Atlantic Ridge.USGS
Photos from Iceland http://www.flickr.comjavier.losasphotostream
Image from NASAHere we see a chain of mountains being builtunder all Earth’s oceans where ocean plates diverge.
As a result of this activity all the oceansare getting wider, albeit a few centimeterseach year.However, not all divergent boundaries arefound in the middle of large oceans.Sometimes continental plates move apart.
Where two continental plates separate, a riftvalley is formed. If this movement occursover a very long period of time, onecontinent can break apart and become two . Image courtesy of National Geographic
Let’s look at that image of the plates again.Wikipedia Commons
In East Africa a smaller plate called the African Somalian Plate is pulling away from the African Nubian Plate. These two plates are movingaway from each other and also away from theArabian plate to the north. The result is ahuge valley called the East African Rift Zone.
The east African rift valleys is a goodexample and it represents the initial stage inthe breakup of the African continent. http://www.flickr.com ditzy’ girl
Volcanic activity is common here-- Kilimanjaro and Mount Kenya.http://www.flickr.comschacon
Now we will consider transformboundaries—where two plates slide sideby side: This type of movement commonly produces earthquakes.
Let’s look at that image of the plates again.Wikipedia Commons
As we saw with the subducting ocean plates,plates sliding by one another do not always moveevenly and smoothly. Sometime the touchingsurfaces get stuck. But as we saw before, themovement of the plates continues andpressure along the fault line builds up. Whenpressure to move exceeds the force holdingthe surfaces still, a sudden violent thrustoccurs. This is an earthquake.Earthquakes are common along transformfaults.
We have our own transform fault. Along thewest coast of North American, the PacificPlate is sliding past the North America Platecreating a fault called the San Andreas Fault.In fact, the Pacific Plate is very graduallycarrying the western-most part ofCalifornia northward.The city of Los Angeles rides on top ofthe oceanic Pacific plate.
Here you cansee the PacificPlate movingnortheast andtheNorthAmericanPlate slidingsouthwestcreating the SanAndreas Fault.
The San AndreasFault in red,extends near theborder withMexico to thesouth throughthe city of SanFrancisco andcontinues on andoff shore to thecoast of northernCalifornia.
In some parts of California,you can actually see theSan Andreas Fault linewhere the two plates aresliding by one another.The land to the west of theSan Andreas Fault is slowlymoving north. The land tothe east of the fault ismoving south. Aerial view of the fault USGS
The great 1857 earthquake is estimated tohave moved some of the ground shownhere sideways about 10 meters. Photo courtesy Alisha Vargas of Flickr under Creative Commons license
1906 San Francisco earthquake Wikipedia commons 1994 collapse of Los Angeles overpasshttp://wapedia.mobi/en/Northridge_earthquake
Most earthquakes and volcanic eruptionsoccur near plate boundaries. However,there are few areas far from the plateboundaries where volcanoes erupt.
Red dots are some of the hotspots found around the world. usgsFor example, the Hawaiian Islands, which areentirely of volcanic origin, have formed in themiddle of the Pacific Ocean more than 3,200km from the nearest plate boundary.
How do theHawaiian Islandsand othervolcanoes thatform in theinterior of platesfit into the platetectonicspicture? http://www.flickr.com/ mccum934
Scientists believe that below the crust inthese areas, a hot plume of magma risesfrom deep within the Earth. When the plumesbreaking through the Earth’s surface a volcanoerupts. These plumes are thought to bestationary relative to the lithospheric platesthat move over them. So as the platemoves on the present volcano becomesextinct and a new one develops above theplume.
Source: Maurice Krafft, Centre deVolcanologie, France)
Another red dot “hot spot” we arefamiliar with is Yellowstone. usgs
Geologists believethat a few hotspotsexist below theNorth AmericanPlate. The bestknown is thehotspot under thecontinental crust ofYellowstoneNational Park innorthwesternWyoming. http://www.flickr.com/ jimbowen0306
In Yellowstone, you can find several calderas (largecraters formed by the ground collapseaccompanying explosive volcano eruptions).These were formed by three gigantic eruptions thatoccurred in the past two million years. The mostrecent one occurred about 600,000 years ago.Ash deposits from these powerful eruptionshave been found as far away as Iowa, Missouri,Texas, and even northern Mexico.