Theory of plate tectonics [PPT]

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

What is the composition of lava formed on oceanic crust?

High percentages Fe, MgLow 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, OLow 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

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

Continental drift hypothesis: evidence?

• Proposed in 1911• All evidence found on the continents• Mechanism for moving the continents was

not known

• 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

• 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

• The continental drift hypothesis proposes that the continents were assembled to form the super continent Pangaea.

• Moved through time

3:20

• 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

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

Swamps300 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

• 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

Exploration of the Ocean Basins

• 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.

Support

• Echo sounding was used to map the seafloor

Seafloor features were discovered using sonar.

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Seafloor features

• Mid-oceanic ridges

• Trenches• Volcanism

adjacent to trenches

• Seamounts• Fracture zones

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

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

Mirror image across the ridge and correlative with age

Positive anomalies= normal polarities

Negative anomalies = reverse polarities

Polar Reversals

• Correlated to time• Scientists dated iron-

rich 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

• 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

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

Pushing at the mid-oceanic ridges

Pulling at the subduction zones

Push

Pull

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

Continental Drift HypothesisCollection 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

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