Transcript File - Geography World of Jaydeep Mehta

• Some questions we will answer today:
– How is the earth always changing?
– What forces inside the earth create and change landforms
on the surface?
– What is the theory of plate tectonics and how does it
work?
– What two theories help make up the theory of plate
tectonics?
– What is continental drift and sea floor spreading?
– What happens when the plates crash together, pull apart,
and slide against each other?
Different theories about
phenomena on earth’s surface
• Most of them contradict each other
• Not reliable
• Eg.
–
–
–
–
Roman Church
Copernicus
Aristotle
Galileo
Bases of studies of every
discipline
Physics
Chemistry Biology
Gravity
Atom
Cell
Earth
Science
Plate
tectonic
The Plate Tectonic Theory
• One single theory
• explains almost all phenomena prevalent in
the lithosphere
–
–
–
–
–
–
volcanism,
mid-oceanic ridges,
deep sea trenches,
earthquakes,
mineralization,
mountain-building and many more.
• “Paradigm Shift” Yet tip of the iceberg
To really understand how the earth became to look as it
does today, and the theory of plate tectonics, you also
need to become familiar with two other ideas:
Continental Drift
and
Seafloor Spreading.
The idea of ‘moving’ continents
Continental Drift Theory
• In the early 1900s a German explorer and
scientist proposed the continental drift theory.
He proposed that there was once a single
“supercontinent” called Pangaea.
Continental drift: An
idea
before its time
• Alfred Wegener
• Proposed hypothesis in
1915
• Published The Origin of
Continents and Oceans
• Continental drift hypothesis
• Supercontinent Pangaea
began breaking apart
about 200 million years
ago
CONTINENTAL DRIFT
THEORY
• Alfred Wagener- 1912, revised in 1924.
• He was a meteorologist studying global
climatic changes in the geologic past.
• ‘jigsaw fit’ of continents
– Brazil- Gulf of Guinea
– North American coastline- Scandinavian
coastline
– Ethiopia- Western India and Pakistan
– Australia- Bay of Bengal
THE THEORY
•
•
•
•
•
Super continent- Pangaea
Covered by huge water body- Panthalassa
Pangaea- Laurasia & Gondwanaland
Seperated by a narrow sea- Tethys Sea
250-300 mya, both started coming closer, Indian
peninsula started drifting north-eastwards, Tethys
Sea became smaller
• Himalayas and Alps were formed
• 180 mya, Americas started drifting westwards,
forming Rockies and Andes
Evidence used in support of
continental drift hypothesis
•Fit of the continents
•Fossil evidence
•Matching rock type and
mountain belts
•Paleoclimatic evidence
Evidence for Continental Drift
• Jigsaw Puzzle fit of
continents
Alfred Wegener
during Greenland
expedition
Glacial Deposits
More evidence
• Matching geologic
structures including:
– Mountain chains
– Ore deposits
– Same rocks of same age
The Evidence for Continental Drift
• Fossil Evidence
–
–
–
–
Glossopteris
Cynognathus
Mesosaurus
Lystrosaurus
Wegener not believed
• Why? – What could possibly force the continents to
move across the ocean floor in this way. They
would be crushed.
– He was a meteorologist, not a geologist
Sea-Floor Spreading
• Sea-floor spreading: The process by
which molten material adds new
oceanic crust to the ocean floor
Seafloor Spreading
• Emerged from the study of the ocean
floor.
• Series of mountains that extend around
the world, stretching more than 64
thousand kilometers (40 thousand
miles).
Developments 50s and 60s
• World war 2 submarines found
mountains under the oceans – the
mid-ocean ridges
• Theory of seafloor spreading
suggested by Princeton professor Dr.
Harry Hess
Seafloor spreading
First look at the earth’s layers as shown here.
Evidence for Sea-Floor
Spreading
Trend of the Mid-Atlantic Ridge
• Similar to the coastlines
of adjacent continents
• Europe and Africa to
the east
• North and South
America to the west
Oceanic Crust is thin
• Both sides of the ridge, oceanic crust is
hardly 4 to 5 km thick
• Rest all oceans –between 10 to 20 km
Volcanoes
• Continuous chain of volcanoes all along the
mid-Atlantic ridge
• What would be the effect of this at the
ridge?
Age of rocks
• Youngest near the ridge
• As we move away from the ridge, rocks are
older in age
• No rock in the Atlantic ocean is more than
200 million years old ( The crust is not
more than 200 million years old)
How are these matching stripes
evidence of sea-floor spreading?
• The pattern of
stripes is the same
on both sides of
the ridge,
indicating that the
sea floor has
spread from the
mid-ocean ridge
What happens to the rock along
the ridge when new molten
material erupts?
• The spreading
molten material
pushes the older
rock to both sides
of the ridge
Where is the oldest part of the
ocean floor?
• The oldest part of
the ocean floor is
farthest from the
mid-ocean ridge
Pangaea revisited
• By piecing together this
information, we can see
how the continents have
moved over the past 200
million years, due to
seafloor spreading
If new crust develops from midoceanic ridges……….
• Where does the old
crust go?
• Why the amount of
lava coming out
and forming crust
does not reduce?
Look at the observations made by
• Hugo Benioff &
Vine and MathewsAmerican
GEOLOGISTS
• They observed
certain phenomena
in the eastern coast
of Eurasia
Deep sea coast
• The sea near the east coast of China was
very deep.
• Yellow Sea
• South China Sea
• East China Sea
Ring of Fire
• Chain of volcanic islands running parallel to
the east coast of Eurasia
– Aleutian
– Japan
– Phillipines
Volcanic activity
• Intense
• Explosive.
• Lava different than that of the Mid-Atlantic
ridge
Frequent earthquakes
• Japan gets at least 2 earthquakes every day
Conclusion
• As Eurasia is pushed from Mid-Atlantic
Ridge,
– It pressed against and subsided under the crust
which make up the floor of the Pacific Ocean
– Constant rubbing of both the plates melts some
rocks, magma forms and erupts out forming the
islands.
Process known as??????????????????????????
Subduction
• Subduction: The
process by which
oceanic crust sinks
through a deepocean trench and
back into the
mantle; a
convergent plate
boundary
Subduction at Deep-Ocean
Trenches
Deep-Ocean Trenches
• Deep-Ocean Trenches: A deep valley
along the ocean floor through which
oceanic crust slowly sinks towards
the mantle
What happens to the ocean
floor at deep ocean trenches?
• At deep-ocean trenches, subduction
allows part of the ocean floor to sink
back into the mantle, over tens of
millions of years
Plate Tectonics
•
•
•
•
•
•
Unifying concept
Sea floor spreading
Continental Drift
Earthquakes
Volcanoes
Mountains
Sea islands
According to the theory of plate tectonics, the earth’s
outer shell is not one solid piece of rock. Instead the
earth’s crust is broken into a number of moving
plates. The plates vary in size and thickness. They
keep moving in different directions
2 Types of Plates
• Ocean plates - plates below the
oceans
• Continental plates - plates
below the continents
What is the Asthenoshere?
• The plastic layer below the
lithosphere = asthenosphere
• The plates of the lithosphere
float on the asthenosphere
Drifting of continents
• When the tectonic plates under the
continents and oceans move, they carry the
continents and oceans with them.
Sea-Floor Spreading
•
•
•
•
•
Mid oceanic ridges
Magma comes out
Forms new crust
Pushes existing crust on both sides
Drifting continents
Subduction
• Heavier plate subsides under lighter plate.
• Magma erupts over the thinner plate
• Sometimes magma may pile over on the
ocean to form islands
Plate Tectonic Theory
• Plates of rigid lithosphere (oceanic and continental) move from the energy of
heat transfer below
• Their interactions define divergent, convergent, and transform boundaries
and control many surface processes
Plate tectonics
• Sea floor spreading provides the driving
mechanism for movement
• However, it is not the continents that are
moving, but the “plates” of lithosphere
“floating” in effect on the asthenosphere
• The lithosphere is made up of about 20 plates
which move relative to each other in several
ways
• Let’s look at a generalized sketch
The Plates
Types of Plate Margins
• Plate boundaries
• Interactions among individual plates
occur along their boundaries
• Types of plate boundaries
– Divergent plate boundaries
– Convergent plate boundaries
– Transform fault boundaries
DIVERGENT PLATE
BOUNDARY
• At a divergent plate
boundary lithospheric
plates move away from
each other.
• The mid-Atlantic Ridge, a
topographically high area
near the middle of the
Atlantic Ocean, is an
example of a divergent
plate boundary.
• New crustal material
Features of Divergent
Boundaries
• Mid-ocean ridges
• rift valleys
• fissure volcanoes
They’re Pulling Apart!
• When plates pull away from
one another they form a
diverging plate boundary, or
spreading zone.
Thingvellir, the spreading zone in Iceland between the North American (left
side) and Eurasian (right side) tectonic plates. January 2003.
Divergent: Atlantic Ridge
LAVA FOUNTAINS
KRAFLA VOLCANO
ICELAND
Divergent boundaries in Continents
• Continental rifts
• Splits landmasses into two or more
smaller segments
The East African Rift
CONVERGENT PLATE
BOUNDARY
• At a convergent plate
boundary, lithospheric
plates move toward each
other.
• The west margin of the
South American continent,
where the oceanic Nazca
Plate is pushed toward and
beneath the continental
portion of the South
American Plate, is an
example of a convergent
plate boundary
Convergent plate boundaries
• Types of convergent boundaries
• Oceanic-oceanic convergence
– When two oceanic slabs converge, one
descends beneath the other
– Often forms volcanoes on the ocean floor
– If the volcanoes emerge as islands, a volcanic
island arc is formed (Japan, Aleutian islands,
Tonga islands)
– Subducting plate bends downward forming
an oceanic trench
ure 12.16
Oceanic-oceanic Collision
Oceanic-Continental Collision
ANDES
• The Andes Mountain
Range spans the
entire length of South
America, along the
western coast. During
this subduction some
Nazca crust is
scraped off along
base of the Andes,
adding height to the
entire range.
RING OF FIRE
OCEANIC-CONTINENTAL COLLISONS SUSTAIN MOST
VOLCANIC ACTIVITY AROUND THE PACIFIC OCEAN
Convergent plate boundaries
• Types of convergent boundaries
• Continental-continental convergence
– Continued subduction brings continents together
– Less dense, buoyant continental lithosphere does
not subduct
– Result is a collision between two continental
blocks
– Process produces mountains (Himalayas, Alps,
Appalachians)
The collision of India and Asia
produced the Himalayas
Transform fault boundaries
• Third type of plate boundary
• Plates slide past one another and no new
lithosphere is created or destroyed
Transform Boundaries
• Transform faults mark fractures in the crust where plates slide
laterally past each other
• The San Andreas fault separates the Pacific plate from the
North American plate
• These areas are likely
to have a rift valley,
earthquake, and
volcanic action.
San Andreas Fault, CA
Hot spots
• Caused by rising plumes of mantle
material
• Volcanoes form over them (Hawaiian
Island chain)
• Mantle plumes are long-lived structures
and originate at great depth, perhaps at
core-mantle boundary
The Hawaiian Islands form over
stationary hot spot
OTHER HOTSPOTS
Plate motions also can be looked at into the future, and we can have a stab at
what the geography of the planet will be like. Perhaps in 250 million years
time there will be a new supercontinent.