Download Continental Drift

Chapter 9
PLATE TECTONICS
9.1: Continental Drift
• An example of the scientific method in
geology:
OBSERVATION
OBSERVATION
FORM
HYPOTHESIS
COLLECT many forms
of
DATA/EXPERIMENT
OBSERVATION
CHECK OUT
PREDICTIONS
THROUGH…
MAKE PREDICTIONS
DEVELOP and
EXPAND THEORY
Continental Drift
• EARLY OBSERVATIONS:
– 1.) “Shape” of continents: many
scientists observed that several of
the continents appeared to “fit
together” like puzzle pieces
South America and Africa
appear to have “matching”
coastlines
– 2.) Rock and Fossil Observations:
Similar rock formations and
fossils of several different plants
and animals were found on widely
separated continents
– 3.) Climate Observations:
scientists found features such as
glacial deposits in warm locations
(like Africa) and tropical reef
deposits in cold locations (like
Greenland)
GREENLAND?
AFRICA?
OBSERVATIONS --> HYPOTHESIS
– HYPOTHESIS: Educated guess
that attempts to explain an
observation
• Continental Drift: Alfred Wegener’s
hypothesis of early 1900’s that the
continents move around on top of
ocean crust
Scientists laughed and asked Wegener, “What forces would cause
continents to move around on Earth’s surface??”
HYPOTHESIS --> COLLECT DATA
• Wegener searched for data that
might explain what forces would
make continents move until his
death in 1930
Alfred Wegener
COLLECTING DATA
• Data collected in the 1940’s and
1950’s led scientists to believe the
sea floor was actually spreading
apart
– Technology such as sonar
allowed scientists to make maps
of the underwater topography of
the ocean floor
– SONAR: send sound waves from
ship, measure the time it takes
for them to return, determine
DEPTH of water!
COLLECTING DATA
• DATA Point 1: There is a gigantic mountain
range in the middle of the major oceans,
and deep trenches along the coastlines of
the continents!
COLLECTING DATA
• DATA Point 2: After collecting samples,
scientists found that ocean crust near the
ridges (in the middle of the ocean) is much
younger than the ocean crust near the
continents (at the edges of the ocean)
Youngest rocks
Older
rocks
COLLECTING DATA
• DATA Point 3: Paleomagnetic Data
– Earth has a magnetic field, and new rocks
align themselves to this field as they cool
and crystallize, serving as a “compass
needle” record of the magnetic field at that
time
– Every few million years or so, a magnetic
reversal occurs, where Earth’s magnetic
field “flip-flops” (so that our compasses
would instead point SOUTH)
– The new rocks suddenly crystallize with an
opposite “compass needle” alignment
(called “reversed”)
COLLECTING DATA
– Scientists noticed that the record of
magnetic reversals in the rocks on
either side of the mid-ocean ridge was
identical
• All of this data pointed to one
conclusion - the sea floor is spreading
apart in the middle of each ocean!
• Seafloor Spreading: ocean crust is
forming at ocean ridges and is being
destroyed in deep-sea trenches
• REMEMBER: the development of Plate
Tectonics Theory is an example of the
scientific method in geology
OBSERVATION
OBSERVATION
FORM
HYPOTHESIS
COLLECT DATA
OBSERVATION
CHECK OUT
PREDICTIONS
THROUGH
MAKE PREDICTIONS
DEVELOP and EXPAND
a THEORY
A NEW THEORY IS BORN
• THEORY: A broad explanation for
many observations, supported by a
large body of evidence
• A theory is NOT a “guess” or “hunch”!!
A NEW THEORY IS BORN: Continental
Drift Theory
• “Seafloor spreading” explained WHY
continents may have moved over time
and moved CONTINENTAL DRIFT
from a hypothesis to a theory
CONTINENTAL DRIFT
PLATE TECTONICS THEORY IS BORN
• The Continental Drift Hypothesis was
replaced by PLATE TECTONICS THEORY
– The continents have shifted over time, but
they do not “drift” on top of ocean crust
– Large blocks of crust (called PLATES), not
the continents, are moving around the
planet over time
– Most plates include some continental and
some ocean crust locked together.
PLATE TECTONICS THEORY