Transcript Theory of plate tectonics

New Zealand Earthquake
• M 6.3 earthquake
• Shallow epicenter: 23 miles
• Close to Christchurch,
city of 370,000
• After shock to the
September 4th M 7.1
What is the composition of
lava formed on oceanic crust?
High percentages Fe, Mg
Low percentages Si, O
DESCRIBE A CONVERGENT
PLATE BOUNDARY
What is the source of the lava
that forms oceanic crust?
the mantle or asthenosphere
What is the composition of
lava associated with
continental crust?
High percentages Si, O
Low percentages Fe, Mg
The Theory of Plate Tectonics
• Based on two hypotheses
– Continental Drift
– Seafloor spreading
Explain the development of the
theory of plate tectonics in
terms of the scientific method.
The
Scientific
Method
Observation of data or information
One or more ideas, statements or questions;
Single or multiple hypotheses
Collection of data
Not accepted
Hypothesis
More Data
Theory
Continental drift hypothesis:
evidence?
• Proposed in 1911
• All evidence found on the continents
• Mechanism for moving the continents was
not known
Continental Drift Hypothesis
• Alfred Wegener
proposed the
hypothesis of
continental drift in
1911
• He gathered
information from many
different sources and
used it as evidence for
his hypothesis
Continental Drift Hypothesis
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Fit of the continents
Fossil evidence
Ancient mountain ranges
Past climate evidence
Swamps formed in tropical regions
Glaciers formed at the south pole
• The
continental
drift
hypothesis
proposes that
the continents
were
assembled to
form the super
continent
Pangaea.
• Moved
through time
3:20
Fit of Continents
• Antonio Snider-Pelligrini
(1858), a geographer cut
out a map of Africa and
South America
suggesting they were
connected at one time
• Other physical evidence
based on observation
was used by Wegener
Fossil Evidence
• Similar terrestrial species were found on many
continents now separated by oceans.
• Information collected by paleontologists
Ancient Mountain Ranges
The same sequence of rocks is found in North America,
Great Brittan, and Norway. The pattern does not make
sense with the continents in their current configuration.
The Appalachian Mountains
Swamps
300 million years ago
Evidence of
Ancient Glaciers
•Glaciers carve the rock as they move.
•Scientists can determine the direction of movement (notice the
direction of movement noted in South America)
•As South America sits today, the pattern would not make sense.
(glaciers do not move from sea level to higher elevations)
Lack of mechanism to explain
continental movement.
• Evidence implied that the continents were at one
time assembled
• Wegener’s idea was not accepted by the
scientific community because he could not
explain how the continents moved
• With advances in technology, new information
was gathered in association with WWII
Exploration of the Ocean Basins
• What information do you know regarding
the seafloor that may be used to help
understand the hypothesis of seafloor
spreading?
Exploration of the Ocean Basins
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•
•
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•
•
Seafloor features
Composition of the seafloor
Age of the seafloor
Hot spots
Seismicity
Paleomagnetic pattern produced on the
seafloor
Exploration of Ocean Basins
Ocean basins contain a variety of landforms not discovered until WW II.
Seafloor
features were
Support
discovered using sonar.
4:26
• Echo sounding was used to map the seafloor
Seafloor
features
• Mid-oceanic
ridges
• Trenches
• Volcanism
adjacent to
trenches
• Seamounts
• Fracture zones
Oceanic crust is composed of
volcanic rock.
Pillow basalts
• Basalt is volcanic rock with a mafic composition.
• Higher percentages of iron and magnesium, lower
percentages of silicon and oxygen.
Age of Ocean Crust
• Youngest at ridge
• Progressively older away from ridge
• Mirror image across the ridge
Seismicity
• Earthquakes are concentrated at what is
currently known as plate boundaries.
• Explain why there are broad bands of
seismicity associated with some areas.
Earth’s Magnetic Field
Bipolar:
acts like
a bar
magnet
Requirements for a magnetic
field
• An interior region of electrically conducting
fluid such as molten metal.
• Convection in that layer of fluid.
• At least moderately rapid rotation.
A compass has a different
declination depending on the
location on the Earth’s surface
Earth’s Magnetic Field
• Strength varies with location
– Red- stronger
– Blue weaker
Iron-rich rocks are magnetized as
they cool to the current magnetic
field.
• Basalt= iron rich volcanic rock; oceanic
crust
Paleomagnetic pattern across the
mid-oceanic ridge
Negative
anomalies =
reverse
polarities
Positive
anomalies=
normal
polarities
Mirror image across the ridge and correlative with age
Polar
Reversals
• Correlated to time
• Scientists dated ironrich volcanic rocks on
land
• Developed
technology to
measure the
paleomagnetic
reading
Polar Reversals
• Timescale of polar
reversals
• Possibly related to
flow in the outer
core
Paleomagnetic Evidence
• Positive and negative recordings of the
oceanic crust create a symmetrical pattern
across the mid-oceanic trench
Seafloor Spreading
• In the 1963, Harry
Hess developed the
idea of seafloor
spreading to explain
the seafloor’s
formation
• Continental
movement is the
result of ocean floor
movement
Seafloor Spreading
• Mafic lava is extruded along the mid-oceanic ridge to
produce ocean floor.
• New material is added, the cooler material is split and is
“pushed” away from the ridge
Driving Mechanisms
Convection cells within
the mantle
Push
Pushing at the midoceanic ridges
Pull
Pulling at the subduction
zones
Convection
The theory of plate tectonics
• Explains that the Earth’s lithosphere is
broken into distinct units that move as a
coherent package. Where the tectonic
plates meet, produces distinct physical
characteristics on the Earth’s surface.
The Scientific Method
Associated
Observation of data or information with the
continents
Continental Drift Hypothesis
Collection of data
Not accepted
Ocean
basins
Seafloor Spreading Hypothesis
key
Mechanism for plate movement
Theory of Plate Tectonics
Information to understand:
• The evidence used by Wegener to describe
the continental drift hypothesis.
• The evidence associated with technology that
was used to collect information about ocean
basins.
• The hypothesis of seafloor spreading
• The accepted tectonic plate driving
mechanism.
Earthquakes and volcanoes outline
the tectonic plate boundaries