Download Document

CONTINENTAL DRIFT,
SEA-FLOOR SPREADING,
AND PLATE TECTONICS
An ancient, but amazingly accurate world map!
Early map of trade routes
More modern map of sea
bottom features. Look at
the jigsaw puzzle outlines
of the continents!
Alfred Wegener
• Most people thought the
complementary shapes
were coincidence. Alfred
Wegener was an
exception who sought
more evidence that the
continents had been
joined. He found it in the
rocks & fossils and
proposed the hypothesis
of Continental Drift.
Continental Drift - A Hypothesis
Three important ideas changed our
understanding of the earth:
• Continental drift - a hypothesis
• Sea-floor spreading- a hypothesis
• Plate Tectonics - a theory
What Is This Supercontinent?
Pangaea
Who proposed the hypothesis of continental
drift?
Alfred Wegener in 1912
When did Pangaea break up?
About 180-200 million years ago
The most current theory
shows the development
of modern continental
shape and location
What Kind of Evidence
Supported Continental Drift?
Matching Mountain Belts
• Appalachians and Caledonian Mountains
• Cape Mountains, S. Africa and Sierra of Argentina
• Fossil Evidence
How could fossils of land animals appear on widely
separated continents?
(Evolved independently, On rafts, Swam there,
Land bridges)
• Wegener’s explanation: Continents were all one
Past Climates
• Now-tropical Africa, India, etc., had ice sheets 300
million years ago
• Tillite
• Now-midlatitude North America and Europe, once had
tropical swamps
• Coal
Why Was Continental Drift Not
Accepted?
How could vast continents plow through the
ocean floor?
What was the driving mechanism?
Rotation of earth
Pull of the moon
Inadequate explanation - forces too weak
Wegner’s Problem
• The problem with the Continental Drift
Hypothesis was that Wegener had no
mechanism for moving the continents
through the ocean floor.
• Powerful physical arguments prevented
continental drift from acceptance by the
majority of scientists, particularly those
in the northern hemisphere.
• Wegener died in 1930 with his hypothesis
largely ignored. But despite the problems
and an overwhelming rejection of his
ideas, not everyone forgot about his
work.
• In the 1940’s and 1950’s geophysicists
studying the record of Earth’s magnetic
field began to revive some of the ideas to
explain observed variations in their
observations.
During WWII and with the
initiation of the Cold War,
much research was focused
attention on the ocean floor.
– Researchers were surprised to find
long, large mountain chains and deep
trenches.
– They also found that the rocks which
compose the ocean floor were young,
ranging in age from recent to 200 Ma.
Seafloor Spreading
• Sea Floor Spreading is the hypothesis in
which new ocean floor is created at the
mid-ocean ridges and that plates move
apart at the ridges and the continents
move with them.
Hypothesis of Sea-Floor Spreading
Who proposed this hypothesis?
Harry Hess
What Goes On At Mid-Ocean
Ridges?
Sea-floor spreading
Shallow earthquakes
High heat flow and volcanism
Birthplace of new oceanic crust
Youngest rocks of the sea floor
Magnetic stripes
Thinnest sediment
What Was The Proof Of Sea-Floor
Spreading?
Paleomagnetism of the sea floor
Ancient magnetism records flip-flops of earth’s
magnetic field
North orientation - normal
South orientation - reverse
How Does the Age of the Sea Floor Support Sea-Floor
Spreading?
Oceans are youngest at the Mid-Ocean ridges
Oldest near the continents
Summary of Sea-Floor Spreading
New sea floor is created at mid-ocean ridges.
Sea floor is destroyed in subduction zones.
Seafloor Recycling
• Since new material is created at the midocean ridges and Earth is not expanding,
somewhere material must be removed
from the surface.
• It turns out that old ocean floor is
“subducted” into the mantle at
subduction zones.
Plate Tectonics - What Is This
Grand Unifying Theory?
The lithosphere is cracked into seven or more large plates
with many smaller ones.
The lithosphere moves over a semi-molten asthenosphere.
Most tectonic plates include both continental crust and
oceanic crust.
Map of Tectonic Plates
What global features can now be related to the story of
plate tectonics?
Deep Oceanic Trenches
Mid-Ocean Ridges
Volcanic Activity
Earthquake Epicenters
• By plate we really
mean a “shell” on
Earth’s surface
that is about 100200 km thick and
includes all of the
crust, and the
uppermost part of
the mantle.
• Tectonics is the term we use in geoscience to
represent the formation of folds, fractures,
faults, etc. in rocks.
Major Plates
Newest Design - 2002
Global Seismicity
New Volcanism
Earthquakes
• Most earthquakes occur along plate
margins because plate margins are
relatively weak, but about 10% of
earthquakes occur within the interior of
plates.
• We classify earthquakes depending on
where they are located
• Interplate - between plates
• Intraplate - within plates
Earthquakes Coincide With Plate
Boundaries
Most geological activity occurs along plate
boundaries
Divergent Plate Boundaries
Convergent Plate Boundaries
Transform Plate Boundaries
Divergent Plate Boundaries
Where are most divergent plate boundaries?
In the middle of oceans
Fault Offset the Mid-Ocean Ridges
Shallow earthquakes occur as plates move past
Volcanoes erupt along the rifts
What Kind of Plate Boundary is Iceland?
A divergent plate boundary that forms an island
Do Divergent Plate Boundaries Occur on Land?
East African Rift Valleys
Convergent Plate Boundaries
How many ways can plates converge?
Ocean-Continent
Ocean-Ocean
Continent-Continent
What Goes On At Ocean-Continent
Convergent Plate Boundaries?
Earthquakes occur as the ocean plate subducts
An ocean trench forms as the sea floor bends
Volcanoes form as magma erupts from the
subduction zone
Mountains form as sediments are folded and
faulted
What Kind Of Convergent Plate
Boundary?
Ocean-Ocean Plate Boundary
Earthquakes occur as one ocean plate subducts
An ocean trench forms as the sea floor bends
A volcanic island arc forms as magma rises above
the subduction zone
Continent-Continent Convergent
Plate Boundary
Mountain building occurs as the continental crust
is folded and faulted
Earthquakes occur as two continents collide
NO volcanic activity
India crashes into Asia
Himalayan Mountains and Tibetan Plateau
What Kind Of Plate Boundary?
San Andreas Transform Fault Plate Boundary
Earthquakes only
•What Moves the Plates?
Convection currents in the earth's mantle
Slab-pull in the trenches
Ridge-push at the ridges