Transcript Plate Tectonics

Chapter 4
Plate Tectonics
Section 4.1 Objectives
1. Explain Wegener’s hypothesis of
continental drift
2. List evidence for Wegener’s
hypothesis of continental drift
3. Describe seafloor spreading
4.1 Continental Drift
In 1912, Alfred Wegener
proposed his continental drift
hypothesis.
It stated that the continents
once formed part of a single
landmass, which he named
Pangaea, which means all
lands.
Wegener thought that
Pangaea began breaking up
into smaller continents about
200 million years ago, and
drifted to their present
locations.
EVIDENCE:
1. Fit of the shapes of the continents
2. Fossils of plants and animals found on different
continents that are separated by oceans
Mesosaurus fossils date back to 270 million years
ago in South America and West Africa. Did it swim that
far? There is not evidence of a land bridge.
EVIDENCE:
3. Glacial grooves show that
glaciers looked like they moved
from sea to land
EVIDENCE:
4. Climatic patterns shown by rock layers:
Some rock types only form in certain climates, for example coal, which forms in warm,
very wet (rainy) environments. If coal is found in a place that is not warm and rainy,
then either the climate has changed or the rock has moved.
The Dance of the Continents…
GO>
Seafloor Spreading Hypothesis
•
In 1947, a group of
scientists set out to map
the Mid-Atlantic Ridge.
This is part of an 80,000
km.-long system of midocean ridges.
•
The oldest rocks found
on the sea floor were
less than 175 million
years old. The oldest
continental rocks are
about 4 billion years old.
WHY IS THIS
INFORMATION
SURPRISING?
Seafloor Spreading Hypothesis
1. The valley at the
center of the MOR is
a rift (a long fracture
in the crust)
2. Magma is coming up
from inside the earth
into the rift.
3. This magma can find
space in the crust
because the ocean
floor is moving away
from both sides of the
MOR
PALEOMAGNETIc REVERSALS
• As magma solidifies
to form rock, the
magnetic fields of
iron-rich minerals
align with the earth’s
magnetic field, just
like a compass.
• Scientists have
discovered that this is
not always the case.
they have found
minerals that point
south.
PALEOMAGNETIc
REVERSALS
• Throughout the earth’s
history, the magnetic
field has reversed itself
many times.
• Such reversals have
come at irregular
intervals, averaging
about every 300,000
years; the last one was
780,000 years ago. Are
we overdue for
another? No one
knows.
PALEOMAGNETISM of THE
OCEAN FLOOR
1. Scientists discovered
magnetic patterns locked
into the rocks of the ocean
floor
2. These patterns showed
alternating bands of normal
and reversed magnetism
3. As molten rock rises from the
rift in an MOR, it quickly
cools and hardens and its
magnetic orientation
becomes fixed.
Paleomagnetism of the ocean floor
Sea Floor
Spreading
Section 4.2 Objectives
1. Summarize the theory of plate tectonics.
2. Compare the characteristic geologic
activities that occur along the three types of
plate boundaries.
3. Explain the possible role of convection
currents in plate movement.
4. Summarize the theory of microplate terranes.
Plate Tectonic Theory: Summary
1. Earth’s outer layer is broken into about 30
sections called plates.
2. The plates are composed of uppermost
mantle and either continental crust or
oceanic crust (lithosphere).
3. The plates ride on the asthenosphere.
4. The plates move because of convection
currents flowing in the mantle below the
plates.
.
ooo
Earth’s Tectonic Plates
Types of Crust
1. oceanic crust
2. continental crust
Lithosphere/Asthenosphere
•
The oceanic and continental crust and the rigid upper mantle make up the
•
The lithosphere forms a thin outer shell that lies above the plastic rock of the
asthenosphere.
lithosphere.
Divergent Boundary
1. The Red Sea
occupies an area
that contains a
divergent
boundary.
2. Seafloor is
pushing the
African and
Arabian plates
away from
each other.
3. A rift valley
runs down the
center of the sea.
Arabian Plate
African Plate
Divergent Boundary
African Plate
Arabian Plate
Red Sea
Boundary
Birth Ocean
Convergent Boundary
• convergent boundary:
the
direct collision of one plate with
another.
•
Three types of convergent boundaries:
1. ocean to ocean
2. continent to ocean
3. continent to continent
ooo
3.
Convergent Boundary
Ocean To Ocean: oceanic plate
subducting under another oceanic
plate
Watch Video
Convergent Boundary
OCEAN TO OCEAN
Convergent Boundary
2. Continent To Ocean:
•
•
•
when the oceanic crust slides beneath continental crust.
this is a process called subduction.
subduction zones create deep trenches
Watch Video
Convergent Boundary
1. Continent to Continent
Watch Video
Transform Boundary
• Transform boundary:
two plates
are grinding side-by-side past each other
Transform Boundary
Plate Boundaries
How many plates?
Convergent boundaries?
Divergent boundaries?
Transform boundary?
4
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Causes Of Plate Motion
• Scientists think that the movement of lithospheric
plates is due to
convection.
Watch Video
Microplate Terranes
• Theory of Microplate Terranes: continents are
actually a patchwork of crustal blocks, called
terranes. Each block has its own distinct geological
history.
• Terranes are regions that are bounded by faults and
have rocks of different character (age, type, fossils)
than in surrounding regions.
• Terranes form in another part of the world and are
moved by plate motion to their present locations
Microplate Terranes of Virginia