Download PLATE TECTONICS

Mr. Alfonso 12-2004
PLATE TECTONICS - A POWERFUL
UNIFYING THEORY
Plate tectonics is a relatively new
scientific concept, introduced some 30
years ago, but it has revolutionized our
understanding of the dynamic planet
upon which we live.
The theory has unified the study of the
earth by drawing together many
branches of the earth sciences.
It has provided explanations to
questions that scientists had speculated
upon for centuries.
Alfred Wegener in the
early 1900’s proposed
that the continents
about 200 million years
ago were joined
together in a single
large land mass he
called Pangaea
(meaning “all land” in
Greek). He proposed
that Pangea had split
apart and the
continents had moved
gradually to their
present positions - a
process that became
known as Continental
Drift.
CONTINENTAL DRIFT
Pangea - Alfred Wegner
contended that around 200-250 million years ago the
supercontinent Pangea began splitting apart
and since then the continents have moved to their
present positions.
Wegner’s Evidence for Continental Drift
Fossils of plants and animals of the same
species found on different continents.
Wegner’s Evidence for Continental Drift
Mesosaurus
a freshwater
reptile fossil
found in
Africa and
South
America.
Glossopteris; a fern that
requires warm climates
was found on Antarctica,
Southern South America,
Australia, Southern Africa
and India.
Wegner’s Evidence for Continental Drift
The distribution of climate sensitive sedimentary rocks
on the different continents.
This suggests that the Artics were not always COLD!
Wegner’s Evidence for Continental Drift
Although today we know that Alfred Wegener was correct about
continental drift, at the time his theory was not widely accepted. In spite of
the evidence that the continents had once been joined, few scientists could
understand how the massive continents, weighing billions of tons, could
actually move. Wegener never satisfactorily explained this problem.
He suggested the centripetal force of the earth rotating about its axis slung
the continents around, and that when the continents moved, they plowed
through the ocean floor. Contemporary calculations did not support this.
And without a mechanism Wegner did not have support from his
colleagues!!!!
Confirmation of continental drift would have to wait until the 1960’s, when
a better understanding of the ocean floor lead to the concept of sea floor
spreading. Confirmation of sea floor spreading, and additional types of
evidence, would eventually vindicate Wegener and lead to the most
important unifying concept in geology: the theory of plate tectonics.
The earth is layered
with respect to
physical properties.
The crust and the
upper part of the
mantle is called the
LITHOSPHERE and
it is RIGID, BRITTLE
and behaves
ELASTICALLY.
The part of the mantle
directly below the
lithosphere is
termed the
ASTHENOSPHERE
which behaves like a
PLASTIC - a solid
that flows.
Why plates move…
CONVECTION
Density differences causes warmer part of the
plasti-like mantle to rise and the cooler to sink
caussing the “wax on–wax off” circular motion
PLATE TECTONICS INTRODUCED
Earth’s lithosphere, which consists of the earth’s crust and
upper mantle, is cut up into roughly 20 plates that move
relative to one another atop of the asthenosphere.
Two Types of Plates
1.Oceanic
2. Continental
• Under oceans
• Under continents
• Thin
• Thick
• Dense and Heavy
• Lighter
• Always Sinks under
• Never Sinks
So plates both exist and move
And the consequences are:
– Earthquakes
– Volcanic activity
– Mountains
Types of plate Boundaries
Transform, Divergent, Convergent
Types of boundaries
Divergent
Convergent
(sea floor spreading) (with subduction)
Transform
(San Andreas Fault)
Boundaries Summarized
Divergent
Convergent
Transform
•plates are moving
apart
•plates are coming
together
•plates are slipping
past each other
(normal)
(reverse)
•new crust is created
(strike-slip)
•crust is not created
or destroyed
•Magma is coming to
the surface
• No volcanism
•crust is returning
to the mantle
Sea Floor Spreading
Divergent: Sea Floor Spreading
• Mid Atlantic Ocean Ridge
• longest topographic feature on Earth (70,000
km!)
• 2-3 km above ocean basins
Divergent: Sea Floor Spreading
Pillow Lava rocks
Evidence that
the ocean floor is spreading
Magnetic Bands Reversals
– Lava comes up along ridge lines (mostly underwater) as
plates separate.
– In 76 million years there’ve been 171 reversals of the
earth’s magnetic field.
– Lava contains iron.
– Cooling lava locks in the prevailing magnetism.
– The ocean floor near the ridges has the prevailing field
and the floor further from the ridges shows field
reversals: evidence that the ocean floor is spreading.
Rocks and fossils dating
– Older as one moves away from ridges
– Youngest rock is next to the ridge
Magnetic field reversal
• Proves the Earth magnetic field reverses itself
every 27000 years
• Another proof that the sea floors are spreading
Age of sea floor as measured by fossils
- Older as one moves away from ridges
- Youngest rock is next to the ridge
Transform Faults
Plates move past
each other
strike slip faults
Example:
The San Andreas Fault
-California
3 Types of plate Collisions
Oceanic
Vs
Continental
Oceanic subducts
or Sinks
Continental
Vs
Continental
No Subduction
(mountain
formed)
Oceanic
Vs
Oceanic
One will subducts
or Sinks
(heaviest)
Fig. 2.11
Convergent Plate Boundaries
Ocean-Continent
collision
oceanic always
subducts or sinks
under continental
Examples:
Nazca plate
vs.
South American plate
(forming the Andes)
Convergent Plate Boundaries
Continent-Continent
collision
No Subduction
Example:
Indian plate
v.s.
Eurasian plate
(forming the Himalayas,
Mt. Everest)
Convergent Plate Boundaries
Ocean-Ocean collision
One of them always subducts or sinks
The Pacific Ring Of Fire
• A collection of earthquakes and volcanoes that make
a ring around the Pacific ocean
• It shows the inter-relation of plate tectonics
This map, which shows 20th-century earthquakes
(in red), illustrates how they cluster on the edges of
the major tectonic plates (outlined in yellow).
Hot Spot Formation of Hawaii
Plates Map