Landforms..this land is our land (Teach)

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.

And under the ocean on the seafloor.
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.
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.

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 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.

Mariana Trench http://www.ngdc.noaa.gov

The purpose of this session is to discuss the
forces that create landforms.

However, before studying these forces, we
should review some of the more important
landforms.

1. Ocean -- a great expanse of salty water.

http://www.flickr.com
Repoort

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.


5. Volcano -- an opening in the Earth's crust
through which molten lava, ash, and gases
are ejected.


6. Valley--an elongated lowland between
ranges of mountains, hills, or other uplands,
often having a river or stream running along
the bottom.


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.


12. Butte--a hill with a flat top and steep sides
rising abruptly from the surrounding area.


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.


14. Lake--a large inland body of fresh or
salty water.


15. Hill—an elevation in the earth's surface
smaller than a mountain.


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.

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.

On the other hand, volcanic eruptions and
earthquakes can change the surface of the
Earth very quickly.

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 the
land. 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.

Constructive forces
• 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.

Destructive forces
• Some landforms—canyons, mesas and
buttes-- are created by the action of wind,
water, and ice—forces of weathering and
erosion.

•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:

1. the interior of the Earth.

2. the plate tectonic theory.

First let us look at 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.

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
Crust
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 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 a
rock called granite.

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.

This rock
is mainly
granite.

This rock
magma is mainly
basalt.

The mantle is the layer directly below the
crust.

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 is
joined to the thin, solid crust forming a solid
layer of rock called the lithosphere.
• 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.

Creative Commons
Wikipedia Commons

The asthenosphere is made of partly molten
rock. The reason rock can be molten is shown
by the temperatures shown on the next slide.

The plates of the
lithosphere float on
this hot, melted
rock.

Creative Commons
Wikipedia 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.

Below the
mantle is
the core,
the center
of the
earth.

http://www.freedigitalphotos.net

The core is also Inner core, solid iron
divided into two and nickel
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

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.

Let’s see how the make-
up of this planetary ball
affects the construction
and destruction of sur-
face land forms.

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.
http://scign.jpl.nasa.gov/

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 Free image from DKImages

jigsaw puzzle.

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 Commons
Wikipedia 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.

The theory of plate tectonics explains how
the movement of the lithospheric plates
and their interaction with each other
produce different landforms.

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.

Wikipedia commons

First of all, notice that most plates have both
Wikipedia Commons

oceanic and continental crust and that few
have only oceanic crust.

Wikipedia Commons
Wikipedia commons

Notice the arrows to see how the plates interact.

You may have noticed that plates can move
in one of three ways:

1. together

2. apart

3. side by side

The margins where plates meet each other
are called boundaries.
Those on the leading
edge 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

Transform
Earthquakes, volcanic activity, mountain
building and the formation of ocean trenches
occur along the boundaries of these plates.

This map of volcano and earthquake activity
mirrors a map of plate boundaries.

Image for educational use http//serc.carleton.educ

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?

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 convergent
boundaries—where two plates collide:
What happens when two plates collide
depends 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.

USGS

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.

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 http://scign.jpl.nasa.gov/

present heights.

Wikipedia Commons
Himalayan Mountains from the air

Himalayan mountains http://www.flickr.com/
Himalayan Trails

As happened with the Himalayas, when
two continental plates collide head on,
the layers of sedimentary rock usually
bend rather than break. The rocks bend in
much the same way a rug wrinkles as it is
pushed across the floor. 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.

Pics4learning

Examples of fault-block mountains include: the
Sierra Nevada mountains in North America and
the Harz Mountains in Germany.

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”.

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 the
http://mapstor.com/
Tibetan plateau

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 formed
the Colorado Plateau about five million years ago.

Colorado Plateau

http://www.flickr.combrianna.lehman

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.

This is an example of basalt lava that
squeezed vertically through the surface
millions of years ago.

U.S. Geological Survey photo by S. R. Brantley.
(fair use policy)

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.
An example is the the Columbia Plateau
which covers parts of the states of Oregon,
Washington, and Idaho.
.

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 extreme heat and
pressure causes the
leading edge
of the oceanic
plate to melt. USGS

The resulting magma rises and gathers in
pools under the continental crust.

As a result:
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.

Image courtesy of FEMA

USGS

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.

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, 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.

Wikipedia commons

Mount St. Helens erupting in 1980

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?

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.

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 http://scign.jpl.nasa.gov/
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.

Volcanoes similarly caused by plate subduction around
the rim of the Pacific ocean are called the Ring of Fire.

Wikipedia Commons

Undersea earthquakes, also common
where two oceanic plates meet, are caused
when these huge masses of earth sliding
past each other get stuck.

Images from usgs

Both plates keep inching along their paths,
but the surface where they meet does not
allow movement. Pressure builds.

With pressure continuing to
build for long periods of
time, everything is under
strain 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” 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.

Recent tsunamis
occurred in Sumatra,
Indonesia in 2004 and
in Samoa in 2009.
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.

Think about the recent devastating
outcomes in Sumatra 2004 and 2009.

Tsunami in 2004 Earthquake in 2009
Wikipedia Commons

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.

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. Image courtesy of USGS

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.

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.

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 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- http://www.navmetoccom.

Atlantic Ridge.

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

Photos from Iceland

javier.losa'sphotostream

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

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 .


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.
ditzy’ girl

Volcanic activity is common here--
Kilimanjaro and Mount Kenya.
schacon

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.

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
NorthAmerican
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.
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.
Aerial view of the fault USGS

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

Wikipedia commons

1994 collapse of Los
Angeles overpass

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.

Red dots are some of the hotspots
found around the world.

usgs

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

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. So as the plate
moves on the present volcano becomes
extinct and a new one develops above the
plume.

Source: Maurice Krafft, Centre de
Volcanologie, France)

Another red dot “hot spot” we are
familiar with is Yellowstone.

usgs

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.

We will use the next few slides to
review the more important concepts
of constructive forces affecting our
planet.

ditzy’ girl

http://www.flickr.com/
puroticorico

Landforms..this land is our land (Teach)

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