Transcript deep-ocean trench

Why did some people of the past
believe the continents were once
connected?
Many people had noticed the shapes of
the continents seemed to fit together like
the pieces of a jigsaw puzzle.
As long ago as 1620, the English thinker
Francis Bacon noticed just how similar the
coasts of South America and Africa are.
Who was Alfred Wegener?
A German astronomer and a
meteorologist, Alfred
Wegener, proposed the
theory of Continental Drift to
explain why the shapes of
the continents are similar.
What is a Scientific Theory?
A well tested concept that
explains a wide range of
observations.
What is the Theory of Continental Drift?
The theory of Continental Drift states
that the current continents were once
connected together in a supercontinent
and have since broken apart and
drifted to their current positions.
What did Alfred Wegener name
the supercontinent mentioned in
his theory of Continental Drift?
PANGAEA -- Greek meaning “all lands”
Stretched from pole to pole and centered where
Africa is today
Continental Drift
What evidence did Alfred
Wegener have to support his
theory of Continental Drift?
Evidence Supporting Continental Drift
Landform Evidence
 The coast of different continents are
similar (especially the east coast of South
America and the west coast of Africa).
Evidence Supporting Continental Drift
Landform Evidence
 The coast of different continents are
similar (especially the east coast of South
America and the west coast of Africa).
 Similar rock formations and coal deposits
found on different continents
Evidence Supporting Continental Drift
Fossil Evidence
 Similar fossils on different continents
Evidence Supporting Continental Drift
Climate Evidence
 Evidence of past tropical climates in the
Antarctic.
– Fossils of Glossopteris, a fern-like tropical plant
found in colder areas

Arctic climates (glacier marks) in the tropics.
Why did scientist of Alfred
Wegener’s day (early 1900’s)
not accept his theory?
He could not provide the force that
caused the continents to move.
They felt it would have been impossible for huge
solid chunks of land to have plowed through the
ocean floor.
What is the Theory of Plate Tectonics?
• The geological theory that
states the pieces of Earth’s crust
are in constant slow motion;
driven by convection currents in
the mantle.
• This explains the formation,
movement, and subduction of
Earth’s plates
How do the plates of the crust affect each
other?
They
collide.
They pull apart.
They grind past each
other.
The movements of these types of
plates produce changes in the
Earth’s surface.
volcanoes/earthquakes
mountain
ranges/valleys
mid-ocean ridges/deepsea trenches
What are plates & plate
boundaries?
•Plates are the pieces the
crust is broken into.
•The edges of these pieces
are called boundaries .
•Cracks in the crust along
these boundaries are called
faults.
What are the three
kinds of plate boundaries?
Transform
Boundaries
Divergent Boundaries
Convergent Boundaries
Transform plate boundary
A
place where two plates grind
past each other, moving in
horizontal opposite directions.
 Crust is neither created (cooled
magma) nor destroyed (melted)
Example: San Andreas Fault
What are major feature formed by
transform boundaries?
 Most
transform boundaries
connect sections of ridges under
the ocean
 Responsible for creating most
earthquakes.
Transform Fault Boundary
San Andreas Fault
Divergent plate boundary
A place where two
plates move apart or
Divide.
What geologic features form at
divergent plate boundaries?
• Mid-Ocean Ridge
•separating plates beneath the ocean
•the longest chain of mountains (or
volcanoes) in the world.
•It circles the Earth like an endless seam on a
baseball.
Example: Mid-Atlantic Ridge
Eldfell in Heimaey
October 1973. Eldfell volcano is erupting.
What geologic features form at
divergent plate boundaries?
•Rift Valleys – separating plates on land
•Examples of rift valleys:
Rio Grande Rift, Great Rift Valley in
East Africa
East African Rift Valley
Describe a convergent plate boundary.
•A place where two plates come
together.
•Collisions occur between:
•Oceanic crust & continental crust
•Oceanic crust & oceanic crust
•Continental crust &
continental
crust
What geologic
features
form at convergent
plate boundaries
when…
Collision between oceanic crust &
continental crust

Form deep ocean trenches
– The oceanic crust is more dense than continental crust.
– When the two collide, a deep underwater canyon forms where
the ocean crust plunges back into the mantle.
– The place where it plunges back into the mantle is called a
deep-ocean trench.
– This sinking of the older more dense crust is called subduction.
Examples include Andes, Cascades, Sierra Nevadas

Form volcanoes on continental crust
– The subducting plate melts, the melted rock is less dense and
rises to create volcanoes
DEEP-OCEAN TRENCH
Link to
Simulation
oceanic crust & oceanic crust


Form deep ocean trenches
Form arc islands (volcanic)
Examples include Aleutian,
Tonga, Mariana Islands
Link to
Simulation
Collisions occurring between
continental crust & continental crust
form
mountain ranges
Link to
Simulation
Continental crust & continental crust
Collision of India and Asia