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Question of the Day
Question: How do we know that continents
Answer: … … …
Evidence for Plate Tectonics
Shape of Continents
Rock Types
Mountain Chains
Location of Earthquakes
Mid-ocean ridges and ocean floor rock ages
Magnetic Polarity Reversals
Shape of the Continents
• The edges of the continents appear to fit
together like the pieces of a puzzle
Alfred Wegener - Continental Drift
• Noticed that continents appear to fit
• Hypothesized that continents were once
together and had drifted apart
• Idea was called “Continental Drift”
Continental Drift
225 m.y.a.
200 m.y.a.
150 m.y.a.
65 m.y.a.
Present Day
Rock Type Evidence
• Same sequence of rocks of
same age can be found all
over the world
– Indicates that when these rocks
formed all these places were
Fossil Evidence
• Same fossils found on
many different continents
– Fossils of organisms that
could not fly or swim
between continents
– Continents were together when these animals
lived, so they could walk from one continent to
Glacier Evidence
• Glaciers leave marks on
rocks called striations
that show which
direction they move
Glacier Evidence
• Striations have been
found in places too warm
to have glaciers now
• Striations point in
different directions
• If all the continents were
together at the south
pole, striations point in
the same direction
Mountain Chains
• Mountain ranges around
the world appear similar
to each other in
composition and age.
• If we reconstruct the past
arrangement of the
continents, these ranges
formed as one mountain
range and were later split
Mountain Chains
• This is true of
mountain ranges
all over the world
Location of Earthquakes
• Do earthquakes appear randomly on this
• NO!
Location of Earthquakes
• Earthquakes form a definite pattern- we
know now that these are the edges of
“plates” of rock that fit together to form the
earth’s crust
Mid-Ocean Ridges
• First discovered when
scientists started to map
the bathymetry of the
• Long chain of underwater
mountains that stretches
around the entire earth
Ocean Floor Rock Ages
• Rocks all
over the
ocean floor
were dated
Ocean Floor Rock Ages
• Rocks closest to the mid-ocean ridges were
• Rocks farthest from the mid-ocean ridges
were oldest
• Pattern of ages is the same on both sides of
the ridges
• Indicates that new rocks are forming at the
mid-ocean ridges!
The Theory of Plate Tectonics
• Explains all evidence
• Similar to continental drift, but more complex
• Earth’s crust is composed of “plates” that make up
the crust under the ocean and on the continents
• Continents DO NOT float on the oceans
• Plate boundaries do not always occur at the edges of
• Earthquakes and volcanoes occur where two plates
Plate Tectonics Today