Transcript Plate tectonics lecture, Evidence

+
Evidence for Plate
Tectonics
Chapter 9.4
Objectives
• Summarize
• Evidence
• that led to the theory of plate
tectonics
• the discovery of seafloor
spreading.
• Explain
• Significance of magnetic patterns on
the seafloor
• their relationship to seafloor
spreading
• the theory of platetectonics.
+
Continental Drift - Review
Wegener
- Continental drift hypothesis
 Evidence
Continental
puzzle
Fossil records
Matching mountain ranges
Ancient climate
What part of his hypothesis was not
supported? He had no ?
+
Theory of Platetectonics
The
earths lithosphere is fragmented
into a dozen or more large and small
plates which ride a top the
asthenosphere.
+
Theory of Platetectonics
 Lithosphere
 Earth's
rigid outer shell
• Consists of several plates
 Plates are moving slowly
 Largest plate is the Pacific plate
 Plates are mostly beneath the ocean
 Asthenosphere
• Exists beneath the lithosphere
• Hotter and weaker than lithosphere
• Allows for motion of lithosphere
+
Evidence to support Plate Tectonics
5 main scientific developments led to
the Theory

 1)
Ruggedness and youth of the ocean
floor
 2) Volcanic activity along trenches and
mountain ranges
 3) Paleomagnetism
 4) Seafloor spreading
 5) Hot spots
+
1. Rugged and young ocean floor
Before
the 19th century
The
ocean floor was considered flat
 1955 first soundings chart published (*Sonar)
 Showed underwater mountains
Oceanic
crust less than 200 million
years old (my)

*Improved Technology – Radiometric Dating
Ocean Rocks and Sediments
• Analysis of deep-sea rocks and sediments
produced two important discoveries.
1. The ages of seafloor rocks vary in different places
2. The age of oceanic crust consistently increases with
distance from a ridge.
3.
The thickness of ocean-floor sediment was, in
general, much less than expected
4. The thickness of the sediments increases with
distance from an ocean ridge.
Evidence emerges as Technology improves
• The development of echo-sounding methods
allowed scientists to study the ocean floor in
great detail.
• Sonar uses sound
waves to measure
water depth by
measuring the time it
takes for sound waves
to travel from the
device and back to
a receiver.
Ocean Floor Topography
• The maps made from the data collected by sonar
and magnetometers showed underwater
mountain chains called ocean ridges.
• The same data showed that these underwater
mountain chains have counterparts called
deep-sea trenches.
• These two topographic features of the ocean
floor puzzled geologists for over a decade after
their discovery.
+
2. Earthquakes and volcanic activity
along ocean trenches and mountains
World’s
earthquakes and volcanic activity
 Precise
documentation that it is concentrated along
 Ocean trenches
 Ocean mountain ranges
 *Improved technology - seismology
Greatly
advanced knowledge in 1960’s
 The World
wide standardized seismograph network
(WWSSN)
 Established to monitor activities related to the 1963
treaty banning above ground testing of nuclear
weapons
+
3. Repeated changes of earth’s
magnetic field
Confirmation
of repeated changes of the
earth’s magnetic field in the geologic past
In
the 1950’s scientists using magnetometers
(WWII subs) began noticing odd magnetic
variations across the ocean floor
A
magnetometer is a device that can detect
small changes in magnetic fields, allowing
scientists to construct magnetic maps of the
seafloor.
Magnetism
• Rocks containing iron-bearing minerals provide
a record of Earth’s magnetic field.
Paleomagnetism is the study of Earth’s
magnetic record.
• Basalt, because it is rich in iron-bearing
minerals, provides an accurate record of
ancient magnetism.
Magnetism
The Geomagnetic Time Scale
– Towing magnetometers behind ships to measure the
magnetic field of the ocean floor revealed an
interesting pattern.
• In places where the magnetic readings of the
ocean floor matched Earth’s present field, a
stronger-than-normal reading (+) was recorded.
• In places where the magnetic data were reversed
in relation to Earth’s present magnetic field, a
lower-than-normal reading (–) was recorded.
Magnetism
The Geomagnetic Time Scale
– Studies of continental basalt flows
in the early 1960s revealed a pattern
of magnetic reversals over
geologic time.
– A magnetic reversal is a change in
Earth’s magnetic field.
• A magnetic field that is the same as
the present has normal polarity.
• A magnetic field that is opposite to
the present has reversed polarity.
Seafloor Spreading
Magnetism
Magnetic Symmetry
– The positive and
negative areas of the
seafloor form a
series of stripes that
were parallel to
ocean ridges.
– The magnetic
pattern on one side
of the ridge is a
mirror image of the
pattern on the other
side of the ridge.
Magnetism
Magnetic Symmetry
– The magnetic data collected from the ocean floor
matched the pattern of magnetic reversals that had
been found in basalt flows on land.
– From this match, scientists were able to determine
the age of the ocean floor from a magnetic recording
and quickly create isochron maps of the ocean floor.
– An isochron is a line on a map that connects points
that have the same age.
Magnetism
+
4. Emergence of seafloor
spreading hypothesis
Emergence
of the seafloor spreading
hypothesis and associated recycling
of the ocean crust
The
seafloor spreading hypothesis
was supported by the paleomagnetic
data
Seafloor Spreading
• An American scientist named Harry Hess
proposed the theory of seafloor spreading.
Seafloor spreading states that new ocean crust is
formed at ocean ridges and destroyed at deepsea trenches.
– Magma is forced toward the
crust along an ocean ridge
and fills the gap that is
created.
Seafloor Spreading
Seafloor Spreading
– When the magma hardens, a
small amount of new ocean
floor is added to Earth’s
surface.
– Each cycle of spreading and
the intrusion of magma
results in the formation of
another small section of
ocean floor, which slowly
moves away from the ridge.
+
Intraplate volcanism

Activity within a tectonic plate


Associated with plumes of heat in the mantle
Form localized volcanic regions in the overriding plate called a hot
spot
 Produces basaltic magma sources in oceanic crust (e.g., Hawaii
and Iceland)
 Produces granitic magma sources in continental crust (e.g.,
Yellowstone Park)

Oldest island furthest from hot spot

Helped clear up the mystery of intraplate volcanic activity