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New Zealand Earthquake
• M 6.3 earthquake• Shallow epicenter: 2-
3 miles• Close to Christchurch,
city of 370,000• After shock to the
September 4th M 7.1
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What is the composition of lava formed on oceanic crust?
High percentages Fe, Mg
Low percentages Si, O
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DESCRIBE A CONVERGENT PLATE BOUNDARY
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What is the source of the lava that forms oceanic crust?
the mantle or asthenosphere
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What is the composition of lava associated with
continental crust?
High percentages Si, O
Low percentages Fe, Mg
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The Theory of Plate Tectonics
• Based on two hypotheses– Continental Drift– Seafloor spreading
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Explain the development of the theory of plate tectonics in
terms of the scientific method.
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The Scientific Method
One or more ideas, statements or questions;Single or multiple hypotheses
Collection of data
Observation of data or information
Theory
Not accepted Hypothesis
More Data
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Continental drift hypothesis: evidence?
• Proposed in 1911• All evidence found on the continents• Mechanism for moving the continents was
not known
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• 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
Continental Drift Hypothesis
Glaciers formed at the south pole
Swamps formed in tropical regions
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• The continental drift hypothesis proposes that the continents were assembled to form the super continent Pangaea.
• Moved through time
3:20
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• 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
Fit of Continents
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Fossil Evidence
• Similar terrestrial species were found on many continents now separated by oceans.
• Information collected by paleontologists
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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.
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The Appalachian Mountains
Swamps300 million years ago
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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)
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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
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• 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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Exploration of the Ocean Basins
• Seafloor features
• Composition of the seafloor
• Age of the seafloor
• Hot spots
• Seismicity
• Paleomagnetic pattern produced on the seafloor
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Exploration of Ocean Basins
Ocean basins contain a variety of landforms not discovered until WW II.
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Support
• Echo sounding was used to map the seafloor
Seafloor features were discovered using sonar.
4:26
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Seafloor features
• Mid-oceanic ridges
• Trenches
• Volcanism adjacent to trenches
• Seamounts
• Fracture zones
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Oceanic crust is composed of volcanic rock.
• Basalt is volcanic rock with a mafic composition.• Higher percentages of iron and magnesium, lower
percentages of silicon and oxygen.
Pillow basalts
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Age of Ocean Crust
• Youngest at ridge
• Progressively older away from ridge
• Mirror image across the ridge
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Seismicity
• Earthquakes are concentrated at what is currently known as plate boundaries.
• Explain why there are broad bands of seismicity associated with some areas.
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Earth’s Magnetic Field
Bipolar: acts like a bar magnet
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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.
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A compass has a different declination depending on the
location on the Earth’s surface
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Earth’s Magnetic Field
• Strength varies with location– Red- stronger– Blue weaker
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Iron-rich rocks are magnetized as they cool to the current magnetic
field.
• Basalt= iron rich volcanic rock; oceanic crust
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Paleomagnetic pattern across the mid-oceanic ridge
Mirror image across the ridge and correlative with age
Positive anomalies= normal polarities
Negative anomalies = reverse polarities
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Polar Reversals
• Correlated to time• Scientists dated iron-
rich volcanic rocks on land
• Developed technology to measure the paleomagnetic reading
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Polar Reversals
• Timescale of polar reversals
• Possibly related to flow in the outer core
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Paleomagnetic Evidence
• Positive and negative recordings of the oceanic crust create a symmetrical pattern across the mid-oceanic trench
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• 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
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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
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Driving Mechanisms
Convection cells within the mantle
Pushing at the mid-oceanic ridges
Pulling at the subduction zones
Push
Pull
Convection
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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.
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The Scientific Method
Continental Drift Hypothesis
Collection of data
Observation of data or information
Theory of Plate Tectonics
Mechanism for plate movement
Not accepted
Seafloor Spreading Hypothesis
Associated with the continents
Ocean basins
key
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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.
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Earthquakes and volcanoes outline the tectonic plate boundaries