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PLATE TECTONICS
Continental Drift
&
Seafloor Spreading
Continental Drift
• German scientist Alfred
Wegener proposes the
theory of Continental
Drift in 1912
What does Continental Drift mean?
• It means exactly what is sounds like!
• Continental Drift = continents (land masses)
drifting (moving)
• How many continents are there?
• There are 7 continents.
• Wegener hypothesized that all the present
continents once formed a single landmass,
which he named Pangaea.
• Pangaea means “all lands”
• Panthalassa was the huge ocean surrounding
Pangaea.
The break up of Pangaea
Evidence of Continental Drift
• 1. Similarities in the
coastlines
Evidence of Continental Drift
• 2. Fossils of the same plants and
animals found in areas that had been
adjoining part of Pangaea.
Mesosaurus
• Small extinct land reptile
• Lived 270 million years ago
• Fossils found in eastern South
America and western Africa.
Glossopteris
• Fossilized fern found on 4
continents
• Continents with very different
modern climates, led
Wegener to believe these
continents were once
connected having a similar
climate
Evidence of Continental Drift
• 3. Geologic Evidence
•
Age and types of rocks
•
Mountain age and structure
•
Climatic pattern
•
Debris form glaciers
•
Coal deposits
Did people accept Wegener’s theory?
• NO!
• The evidence Wegener had to support his theory was not
enough to convince many people during his lifetime.
• Not until the 1960’s were his beliefs accepted by the
scientific community.
• The truth was out there!
Seafloor Spreading
• In the 1960’s, Harry Hess developed his now famous and
accepted theory of Seafloor Spreading.
• Hess proposed that hot, less dense material in the mantle
is forced upward to the surface at a mid-ocean ridge.
• Example: Mid-Atlantic Ridge
Harold Hess
Evidence of Seafloor Spreading
• 1. Age evidence
• Studies by the Glomar Challenger
in 1968 discovered that the
youngest rocks were found at the
mid-ocean ridges and rocks
became increasingly older farther
from the ridges on both sides.
Evidence of Seafloor Spreading
• 2. Paleomagnetism (Magnetic clues)
• Important evidence that led to the absolute belief in seafloor
spreading
• Scientist found that rocks on the ocean floor showed many
magnetic reversals.
• Reverse back and forth in strips parallel to the mid-ocean ridge.
• Evidence needed to solidify Wegener’s original hypothesis.
Theory of Plate Tectonics
• By the 1960’s Continental Drift was widely accepted and this led
to the rise of another theory known as Plate Tectonics.
• The term tectonic comes from the Greek work tektonikos meaning
“construction”
• Plate tectonics not only describes continental movement but also
proposes a possible explanation of how and why the continents
move.
• The theory of Plate Tectonics states
Earth’s crust and upper mantle are
broken into sections.
• These sections, called plates, move around
on the mantle.
• About 30 lithospheric plates have been
identified
• Some of these plates are moving towards each
other, some are moving away from each other,
and some are sliding past one another.
How fast do plates move per year?
Plate movements vary, one example:
Lithospheric Plate Boundaries
• The boundaries or edges of the plates have been divided
into three types of plate boundaries. They are:
• 1. Divergent Plate Boundaries
• 2. Convergent Plate Boundaries
• 3. Transform Fault Boundaries
1. Divergent Plate Boundaries
• 2 plates moving away from
each other
• Examples:
• Mid-Atlantic Ridge
• The Great Rift Valley in
eastern Africa
2. Convergent Plate Boundaries
• Plate colliding (pushing in to) with
neighboring plates.
• There are 3 types of Convergent Plate
Boundaries, They are as follows:
1. Ocean-Continent
• The area where the oceanic plate descends into the upper mantle is
called a subduction zone.
• The oceanic crust subducts (moves under another) because it is
less dense than the continental crust.
• Oceanic crust is generally basaltic in nature as continental crust
generally granitic.
• Ocean trenches generally form along subduction zones.
• Many volcanic mountains also form along subduction zones.
• Examples: The Andes Mountains in South America where
Nazca and South American Plates are colliding.
• Where does this happen in the U.S.?
2. Continent-Continent
• When two colliding plates have the same density neither plate is
subducted, the plates experience uplifting instead.
• Example:
• The Himalayan Mountains
• Formed when the IndoAustralian Plate crashed into
the Eurasian Plate.
• This is still going on today as
the Himalayas are growing.
3. Ocean-Ocean
• Part of the subducted plate melts, and the resulting molten rock rises to the surface along
the trench to form a chain of volcanic islands, called an island arc.
• Example:
• The islands of Japan
are volcanic island
arcs formed when
two oceanic plates
collided.
Review
The Three types of Convergent Boundaries
• Ocean-Ocean (Subduction occurs)
• Ocean-Continent (Subduction occurs)
• Continent-Continent (Uplifting occurs)
3. Transform Fault Boundaries
• Forms when two plates are sliding past one another.
• Scrape and move in series of sudden spurts of activity
separated by periods of little or no movement.
• Example: San Andreas Fault where the Pacific Plate is
sliding past the North American Plate
Causes of Plate Tectonics
• Convection Currents
• The entire cycle of heating, rising, cooling, and sinking is
called a convection current.
• The same process occurring in the mantle is thought to be
the force behind plate tectonics.
• Convection occurs in a heated water filled beaker.
•
The same process is thought to happen in the mantle as it is also experiencing heat gain rising from the
mantle.
Effects of Plate Tectonics
• 1. Tension forces
• Rift Valleys form where plates
are diverging.
• Examples:
• Great Rift Valley of Africa
• Earthquakes
• East Pacific Rise
• Tension also forms normal
faults.
Effects of Plate Tectonics
• 2. Compression Forces
• Mountain Building
• Island Arcs and Volcanoes
form
• Reverse Faults also occur.
• Earthquakes
Effects of Plate Tectonics
• Shearing forces create
strike-slip faults
• Example:
• San Andreas Fault,
California
• Earthquakes