landforms..this land is our land (teach)
Post on 16-May-2015
2.950 Views
Preview:
DESCRIPTION
TRANSCRIPT
This Land is Our Land
By Moira Whitehouse PhD
Landforms are features that make up the Earth's surface.
Landforms include things like mountains, valleys, plains, plateaus and volcanoes.
Landforms are found on the Earth’s continents.
The abyss contains plains, long mountain ranges, valleys, ocean trenches and volcanoes. Undersea volcanoes, whether active or extinct, are called seamounts. If a seamount grows tall enough to reach above the ocean surface, it forms an island.
Shallow seas surround most continents and cover gently sloping areas called continental shelves. These continental shelves drop off steeply leading to the deepest parts of the ocean called the abyss.
And under the ocean on the seafloor.
Image Creative commons licencehttp://www.bukisa.com/articles/25522_ocean-floor-webquest
sea floor
Plains
Continental shelfSeamounts
Plains
Oceanic ridge
Oceanic ridgeContinental
shelfTrench
Seamounts
Rift valleyMagma
Volcanic island
Mariana Trench http://www.ngdc.noaa.gov
This is an artist’s conception of the deepest known part of any ocean, the Mariana Trench, located in the Western Pacific Ocean. It reaches a depth of 36000 feet below sea level.
However, before studying these forces, we should review some of the more important landforms.
The purpose of this session is to discuss the forces that create landforms.
2. Plains -- an extensive area of flat or rolling, mostly treeless grassland.
http://pics4learning.com/
3. Plateau -- an elevated level expanse of land; a tableland. In this high country plateau, you can also see buttes and mesas.
http://www.flickr.com/puroticorico
4. Mountain -- a high, steep elevation of the earth's surface, higher than a hill.
http://pics4learning.com/
5. Volcano -- an opening in the Earth's crust through which molten lava, ash, and gases are 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 river's 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 or salty water.
http://pics4learning.com/
15. Hill—an elevation in the earth's 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 access from the surface of the ground or from the sea.
Image courtesy of usgs
Now let’s look at them all together on a make believe continent.
The surface of the Earth is constantly changing as new landforms are built and older ones are destroyed by the forces of the Earth.
On the other hand, volcanic eruptions and earthquakes can change the surface of the Earth very quickly.
Some changes happen so slowly that you do not see the differences for a long time—for example the Colorado river carving the Grand Canyon has been happening for millions of years.
How are Landforms Made?• Now let’s examine the forces that create the different landforms. Broadly speaking, there are two kinds:
• Constructive forces—those that build up theland. Included are: 1) plate movement that builds mountains, and 2) deposition that creates landforms such as deltas and layers of sedimentary rock.• Destructive forces—those that wear down the land, like weathering and erosion.
• Landforms such as mountains, volcanoes, and plateaus are built by crustal movement and other tectonic activity inside the Earth.
• Landforms such as deltas, plains and sand dunes are created when rocks and soil resulting from weathering and erosion are carried away and deposited in new areas.
Constructive forces
• Some landforms—canyons, mesas and buttes-- are created by the action of wind, water, and ice—forces of weathering and erosion.
Destructive forces
•These actions physically changes the Earth's surface by carving and eroding land surfaces.
This session will focus on the constructive forces that build up the following landforms—mountains, volcanoes and plateaus.
Another session will deal with weathering, erosion and deposition.
However, in order to understand the constructive forces, we need background information on two topics:
2. the plate tectonic theory.
1. the interior of the Earth.
If you were able to cut the Earth in half, you would find it is made up of different layers.
Each layer has its own characteristics and the rock making up the layers is a mixture of certain minerals.
First let us look at the interior of the Earth.
NASA image
Crust
The thin, outermost layer of the earth is made of solid rock and is called the crust.
All of the landforms on Earth are located on the crust and all life on Earth exists on this top layer.
The Earth’s crust consists of the oceanic and the continental crusts, both of which float on the magma.
• The continental crust is the layer of rock which forms the continents and those areas of shallow seabed close to the shore. The continental crust is much thicker than the oceanic crust--about 19 miles thick.
• The oceanic crust is the layer of rock which forms the floor of an ocean. It is about 4-7 miles thick.
magma
The oceanic crust is made of mostly basalt, a very dense rock that is much heavier than the granite of the continental crust.
As a result, the oceanic crust sinks deeper into the magma (the molten rock) when the two formations compete.
The continental crust is mainly made of a rock called granite.
magma
This rock is mainly granite.
This rock is mainly basalt.
The mantle is the layer directly below the crust.
• The mantle is divided into two regions, the upper and lower sections.
• It is about 1800 miles thick.
Earth’s mantle
• The uppermost part of the mantle is joined to the thin, solid crust forming a solid layer of rock called the lithosphere.
• And here comes the confusing part.
• The lithosphere includes the crust and the hard upper mantle and consists of a series of huge rock plates that surround the Earth.
Immediately under the lithosphere is the asthenosphere, the lower part of the mantle.
The asthenosphere is made of partly molten rock. The reason rock can be molten is shown by the temperatures shown on the next slide.
Creative CommonsWikipedia Commons
The plates of the lithosphere float on this hot, melted rock.
Creative CommonsWikipedia Commons
Because of convection, the cooler (heavier) melted rock in the athenosphere sinks as the hotter (lighter) melted rock rises creating convection currents. These currents create the tectonic activity that causes the crustal plates to slowly move.
http://www.freedigitalphotos.net
Below the mantle is the core, the center of the earth.
The core is also divided into two regions, the inner core and the outer core. From earthquake waves, scientists believe the outer core is a liquid and the inner core is a solid. Outer core, liquid iron
Inner core, solid iron and nickel
The outer core is made of liquid iron and is very dense. Scientists hypothesize that the circulation of the outer core causes the magnetic field around the Earth.
The inner core is made of solid iron and nickel. Many scientists believe it is kept in the solid state because of the extreme pressure from other layers.
http://scign.jpl.nasa.gov/
First, recall that landforms are found on a very thin crust floating on top of a thick layer of molten magma that, because of convection, is moving about.
Let’s see how the make- up of this planetary ball affects the construction and destruction of sur-face land forms.
Next, in addition to knowing that the crust is part of the lithosphere, we now understand that the lithosphere is broken into huge pieces of rock called plates. These plates fit together around the globe like a giant jigsaw puzzle.
Free image from DKImages
Map courtesy NOAA
Here are the 7 major plates plus several smaller ones.
• We learned that because of convection the magma below the lithosphere flows, very slowly, in large patterns.
Creative CommonsWikipedia Commons
• As it flows, the magma in the mantle rubs on the bottom of the lithosphere and causes these huge plates to, very slowly, but very reliably, move.
Now, that we’ve reviewed the basics of the interior of the Earth, let’s look at the the theory of plate tectonics and the constructive forces that build new land—mountains, plateaus and volcanoes.
Remember we said that there are seven big lithospheric plates and many small ones and these plates are in constant, albeit very slow, motion.
Let’s look at an image showing the plates and see how they move.
The theory of plate tectonics explains how the movement of the lithospheric plates and their interaction with each other produce different landforms.
Wikipedia CommonsFirst of all, notice that most plates have both oceanic and continental crust and that few have only oceanic crust.
Wikipedia commons
Wikipedia Commons
Notice the arrows to see how the plates interact.
Wikipedia commons
2. apart
3. side by side
1. together
You may have noticed that plates can move in one of three ways:
The margins where plates meet each other are called boundaries. Those on the leadingedge of moving plates are called leading edge boundaries.
The three types of boundaries are based on the three ways plates move:
1. convergent or collision boundaries—where two plates are colliding
Convergent
2. divergent boundaries—where two plates are moving apart
Divergent
3. transform boundaries—where two plates are sliding past another
Earthquakes, volcanic activity, mountain building and the formation of ocean trenches occur along the boundaries of these plates.
Transform
Image for educational use http//serc.carleton.educ
This map of volcano and earthquake activity mirrors a map of plate boundaries.
However, in order to make sense of this whole thing, we need to pay attention to two things:
1. the direction the plates are moving and
2. the boundaries of the plates—does the plate have a continental leading edge or an oceanic leading edge?
1. A continental plate moves into a continental plate.
6. Two plates slide by one another.
5. A continental plate moves away from a continental plate.
3. An oceanic plate moves into an oceanic plate.
2. An oceanic plate moves into a continental plate.
Possibilities are:
4. An oceanic plate moves away from an oceanic plate.
Wikipedia Commons
Let’s look at that image of the plates again.
We will first consider convergent boundaries—where two plates collide:
• both continental plates
What happens when two plates collide depends on whether the two plates are:
• an oceanic and continental plate.• both oceanic plates or
Let’s look at each of these situations.
Continental plates converging
Wikipedia commons
When two continental plates collide, the rock is uplifted and compressed causing the land to rise, crumple and buckle. Mountain ranges and high plateaus result.
USGS
These collisions produce Earth’s most spectacular mountain ranges and deepest valleys.
Mountain ranges that were formed in this way 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 the continental plate carrying the continent of India moved north and collided with the Euroasian continental plate.
The slow continuous grinding of the two plates pushed up the Himalayan Mountains and the Tibetan Plateau to their present heights.
http://scign.jpl.nasa.gov/
Wikipedia CommonsHimalayan Mountains from the air
The rocks bend in much the same way a rug wrinkles as it is pushed across the floor.
As happened with the Himalayas, when two continental plates collide head on, the layers of sedimentary rock usually bend rather than break.
A bend in a rock is called a fold and the resulting mountains are called folded 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 two continental plates moving together cause the 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 slide up, down or sideways forming another type of mountain.
Mountains formed in this way are called fault-block mountains.
Examples of fault-block mountains include: the Sierra Nevada mountains in North America and the Harz Mountains in Germany.
Pics4learning
When continental plates meet and push up “new” mountains, the land behind the mountain chain often is also up lifted. However, it doesn’t break or fold. As a result a high flat area is formed---a plateau.
The Tibetan Plateau was created when the Indian Plate and Eurasian Plate collided. The Himalayan Mountains formed along the edge of the collision, and the unbroken plateau behind them rose as a “flat table”.
USGS
USGS
http://mapstor.com/
Ganges River flood plain to the left, then Himalayan Mts. covered with snow, then the Tibetan plateau
http://www.ram.org
Tibetan plateau with the Himalayan Mountains in the background.
Another way that a plateau is formed occurs when two continents meet but the magma does not collect in a chamber. Instead it rises beneath a large, stable landmass.
If the magma is unable to break through any cracks 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 or vents, plateaus are formed by repeated flows of molten rock over millions of years on the surface of the Earth. The magma can squeeze through vertically or horizontally as can be seen by the following pictures.
U.S. Geological Survey photo by S. R. Brantley. (fair use policy)
This is an example of basalt lava that squeezed vertically through the surface millions of years ago.
An example is the the Columbia Plateau which covers parts of the states of Oregon, Washington, and Idaho. .
The lava that oozes on the Earth’s surface through cracks or vents sometimes spreads out over large areas filling in valleys and covering hills.
This process repeats itself many times over the years. The hardened lava sheets pile up and form a raised plateau called a lava plateau.
The Columbia Plateau
Wikipedia Commons
While standing on one, a plateau may look a lot like a plain, a broad flat area. However, a plateau has experienced some kind of uplift, it is tectonically active. A plain is not.
We have discussed what happens when two continental plates collide: tall mountain chains, deep valleys and high plateaus.
Now we will consider what happens when an oceanic and continental plate converge collide?
Continental and oceanic plates converging
When an oceanic plate moves into a continental plate, it slides under because it is denser and thus, heavier.
The resulting magma rises and gathers in pools under the continental crust.
The extreme heat and pressure causes the leading edge of the oceanic plate to melt. USGS
First, a deep ocean trench forms where the oceanic plate moves under the continental plate.
Second, when enough magma collects in the pools under the continental plate, and enough pressure develops, a volcano erupts.
As a result:
USGS
Image courtesy of FEMA
An example of an oceanic plate subducting under a continental plate would be on the western coast of South America.
•The Nasca Plate (oceanic plate) is moving under the South American Plate. Result: the Andes Mountains.
Many volcanoes and earthquakes occur in this region.
Wikipedia commons
http://pubs.usgs.gov/
The convergence of the Nazca and the South American Plate
Andes Mountains
Image courtesy of National Geographic
Andes Mountains
Another place, closer to home, where subduction is occurring is found on the west coast of United States.
There a small oceanic plate called the Juan de Fuca Plate is subducting under the North American Plate.
This subduction is occurring on the coast of Washington state, Oregon and northern California. The Juan de Fuca Plate, a vestige oceanic plate, is pushing under the North American Plate.
This subduction results in the building of the Cascade Mountain Range. Well-known volcanoes in this range are Mount St. Helens, Mount Adams and Mount Hood.
Mount St. Helens erupting in 1980Wikipedia commons
So far we have explored what happens when:
1. Two continental plates converge.
2. An oceanic and continental plate converge.
What happens then when two oceanic plates collide?
Two oceanic plates converging
When two oceanic plates converge, one of the plates subducts under the other. The plate descending into the asthenosphere is heated to the point that it becomes semi liquid magma, which rises to the surface, thereby creating an island arc or island chain.
An example:
Japan.
Wikipedia commons
http://scign.jpl.nasa.gov/
An example is in the northwest, where the Pacific plate plunges under the North American plate. As the crust is pushed deep into the earth by the relentlessly shoving Pacific plate, it starts to melt and some of the melted crust rises back to the surface in volcanic eruptions. These volcanoes form an arc of volcanic islands called the Aleutian Islands.
Wikipedia Commons
Volcanoes similarly caused by plate subduction around the rim of the Pacific ocean are called the Ring of Fire.
Undersea earthquakes, also common where two oceanic plates meet, are caused when these huge masses of earth sliding past each other get stuck.
Both plates keep inching along their paths, but the surface where they meet does not allow movement. Pressure builds.
Images from usgs
When finally, the pressure is strong enough to overcome the resistance to movement, the plate becomes violently “unstuck”—an earthquake occurs. Images from usgs
With pressure continuing to build for long periods of time, everything is under strain and distortion occurs.
With the instantaneous “readjustment” of the seafloor around the subduction zone, a huge amount of water is displaced causing a giant swell in the ocean– a tsunami.
Images from usgs
When a tsunami reaches an island beach it forms an enormous wave which can cause great destruction.
Wikipedia Commons
In the southeastern Indian Ocean, the Indo-Australian and Eurasian Plates collide resulting in frequent large undersea earthquakes, many causing tsunamis.
These huge waves were caused by undersea earthquakes where oceanic plates converge. Here you can see the Eurasian Plate and Australian Plate (both with leading oceanic plates) colliding producing an earthquake.
Recent tsunamis occurred in Sumatra, Indonesia in 2004 and in Samoa in 2009.
Think about the recent devastating outcomes in Sumatra 2004 and 2009.
Tsunami in 2004 Earthquake in 2009
Wikipedia Commons
Plates diverging
Now we will consider divergent boundaries—where two plates pull apart.
We will look at two divergent boundary situations:
When two oceanic plates diverge
When two continental plates move apart.
Oceanic plates diverging
When two oceanic plates diverge (move apart), magma from the mantle flows upward filling the gap between the two plates. When the lava hits the cold water it solidifies as basalt rock. If this process occurs over a long, long time, a new mountain range is built. This type of mountain chain is called a midoceanic ridge.
Image courtesy of USGS
Wikipedia commons
Wikipedia Commons
Here we see magma building up to form a chain of mountains as two oceanic plates diverge (pull apart).
As a result of this process, new oceanic crust is continuously being created between the diverging plates. As new crust is built, the older crust is migrating away from the fault.
usgs
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.
Magma oozing out of these “pull aparts” over millions and millions of years has built and is still building an underwater mountain range down the middle of the Atlantic Ocean called the Mid-Atlantic Ridge.
http://www.navmetoccom.
The tremendous forces involved in this mountain building process often fracture the crust resulting in volcanoes and earthquakes.
When the volcanoes along the ridge erupt, new land is formed. Sometimes the “new land” rises above the surface of 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
javier.losa's photostream
http://www.flickr.com
Photos from Iceland
Image from NASA
Here we see a chain of mountains being built under all Earth’s oceans where ocean plates diverge.
As a result of this activity all the oceans are getting wider, albeit a few centimeters each year.
However, not all divergent boundaries are found in the middle of large oceans.
Sometimes continental plates move apart.
Continental plates diverging
Image courtesy of National Geographic
Where two continental plates separate, a rift valley is formed. If this movement occurs over a very long period of time, one continent can break apart and become two .
Wikipedia Commons
Let’s look at that image of the plates again.
In East Africa a smaller plate called the African Somalian Plate is pulling away from the African Nubian Plate. These two plates are moving away from each other and also away from the
Arabian plate to the north. The result is a huge valley called the East African Rift Zone.
The east African rift valleys is a good example and it represents the initial stage in the breakup of the African continent.
http://www.flickr.comditzy’ girl
http://www.flickr.comschacon
Volcanic activity is common here-- Kilimanjaro and Mount Kenya.
Tectonic plates with transform movement
Now we will consider transform boundaries—where two plates slide side by side:
This type of movement commonly produces earthquakes.
Wikipedia Commons
Let’s look at that image of the plates again.
As we saw with the subducting ocean plates, plates sliding by one another do not always move evenly and smoothly. Sometime the touching
surfaces get stuck. But as we saw before, the movement of the plates continues and pressure along the fault line builds up. When pressure to move exceeds the force holding the surfaces still, a sudden violent thrust occurs. This is an earthquake.
Earthquakes are common along transform faults.
http://www.arthursclipart.org/
We have our own transform fault. Along the west coast of North American, the Pacific Plate is sliding past the North America Plate creating a fault called the San Andreas Fault.
In fact, the Pacific Plate is very gradually carrying the western-most part of California northward.
The city of Los Angeles rides on top of the oceanic Pacific plate.
Here you can see the Pacific Plate moving northeast and the North American Plate sliding southwest creating the San Andreas Fault.
The San Andreas Fault in red, extends near the border with Mexico to the south through the city of San Francisco and continues on and off shore to the coast of northern California.
In some parts of California, you can actually see the San Andreas Fault line where the two plates are sliding by one another.
Aerial view of the fault USGS
The land to the west of the San Andreas Fault is slowly moving north. The land to the east of the fault is moving south.
The great 1857 earthquake is estimated to have moved some of the ground shown here sideways about 10 meters.
Photo courtesy Alisha Vargas of Flickr under Creative Commons license
1906 San Francisco earthquake
1994 collapse of Los Angeles overpass
Wikipedia commons
http://wapedia.mobi/en/Northridge_earthquake
“Hot spot” volcanic activity
Most earthquakes and volcanic eruptions occur near plate boundaries. However, there are few areas far from the plate boundaries where volcanoes erupt.
usgs
Red dots are some of the hotspots found around the world.
For example, the Hawaiian Islands, which are entirely of volcanic origin, have formed in the middle of the Pacific Ocean more than 3,200 km from the nearest plate boundary.
How do the Hawaiian Islands and other volcanoes that form in the interior of plates fit into the plate tectonics picture?
http://www.flickr.com/ mccum934
USGS
So as the plate moves on the present volcano becomes extinct and a new one develops above the plume.
Scientists believe that below the crust in these areas, a hot plume of magma rises from deep within the Earth.
When the plumes breaking through the Earth’s surface a volcano erupts. These plumes are thought to be stationary relative to the lithospheric plates that move over them.
Source: Maurice Krafft, Centre de Volcanologie, France)
Image courtesy of National Geographic
usgs
Another red dot “hot spot” we are familiar with is Yellowstone.
Geologists believe that a few hotspots exist below the North American Plate. The best known is the hotspot under the continental crust of Yellowstone National Park in northwestern Wyoming. http://www.flickr.com/ jimbowen0306
In Yellowstone, you can find several calderas (large craters formed by the ground collapse accompanying explosive volcano eruptions).
These were formed by three gigantic eruptions that occurred in the past two million years. The most recent one occurred about 600,000 years ago.
Ash deposits from these powerful eruptions have been found as far away as Iowa, Missouri, Texas, and even northern Mexico.
http://www.nps.gov/
We will use the next few slides to review the more important concepts of constructive forces affecting our planet.
USGS
http://pubs.usgs.gov/
http://pics4learning.com/
http://www.arthursclipart.org/
Image courtesy of National Geographic
top related