ELEMENTS OF OCEANOGRAPHY

 

TOPIC 5 - PLATE TECTONICS
[Continetal Drift Lecture ppt | pdf]
[Plate Tectonics Lecture ppt | pdf]

Lecture 1:

Topic 5: Plate Tectonics - Historical Foundations
Plate Tectonics is a paradigm developed recently (1960's) to explain many aspects of Earth (or any planetary) dynamics - volcanoes, earthquakes, mountain building, etc.

Evolution of Views about the Origin of the Ocean Basins
Before the 19th Century AD, the most common view was that all Earth features formed during the origin of planet and are mostly unchanged since then.
But, even the Ancient Greeks were aware of the fossil remains of marine organisms near the tops of mountains. How did they get there?

19th Century Views of Ocean Basin Origins
The Pacific Ocean basin formed when the Moon was ripped away from the Earth. (The size of the Pacific Ocean Basin is just larger than the diameter of the Moon.)
Vertical Tectonics - The surface of the Earth goes up and down (over long time intervals) due to unknown processes acting deep in the Earth. One part of this theory was isostacy.

Theory of Continental Drift
Since the 16th Century as better and better maps were being developed, people began to notice that the eastern margins of the Americas looked very much like the western margins of Europe/Africa. Could they have once been joined?

Alfred Wegener's Synthesis of Continental Drift
In 1912, Alfred Wegener proposed that several continents now separated by major Ocean basins had been much closer together about 300 Ma and had been part of a single super continent called Pangaea
About 200 Ma, Pangaea split into two new continents called Laurasia and Gondwana.
Laurasia = North America, Europe, and Asia
Gondwana = South America, Africa, India, Australia, Antarctica

Evidence for Continental Drift (Available to A. Wegener)
Puzzle-like fit of the continents
Distinctive rock units on different continents match when put together
Mountain ranges on different continents match when put together
Distinctive fossils occur on several different continents
Permain glacial deposits only 'make sense' if Pangaea existed

Problem with Continental Drift
No reasonable, geological way to do it!
The theory envisioned the continents sliding over the mantle.
Geophysical data indicated crustal rocks would shatter if pushed.
Result: Continental Drift was not generally accepted

Development of Paleomagnetism (1950-1970) - A new way to study Earth History and Tectonics
Average magnetic field directions at one time all point toward the North Pole.
Sequence of paleomagnetic directions shows evidence of magnetic polarity reversal.

Magnetic Field Apparent Polar Wander Paths (APWPs)
Recent measurements of field always point to North Pole and estimate site latitude
Paleomagnetic studies of some older rocks suggest pole positions have changed systematically over time
Either continents have moved Or North Pole has moved

North American and European Apparent Polar Wander Paths for the last 300 million years
Apparent polar wander path (APWP) for each continent is generally different - but some segments do match if continents are moved.
If North America is rotated back to a position next to Europe, the APWPs are the same for time when they were both part of Pangaea.

Magnetic polarity
In the course of measuring paleomagnetism, it became clear that the earth's magnetic field has flipped polarity in the past: Seems to take ~4000 years.
Field intensity decreases before it flips.

Another Development in Geomagnetism - Magnetic Stripes
on the Ocean Floor
Marine Geophysical surveys during and since WWII have consistently noted linear magnetic anomalies in the oceanic crust termed Marine Magnetic Anomalies
Marine magnetic anomalies always run parallel to mid ocean ridges
Marine magnetic anomaly patterns are symmetric on both sides of each ridge

Theory of Sea Floor Spreading ­ I
Mid ocean ridges are chains of volcanoes which receive magma from the mantle
As new magma is injected, oceanic crust on either side of the volcano chair moves away to make room
Mid Ocean ridges are topographically high because the crust is hot and less dense
Older oceanic crust (deep ocean basins) is colder and denser (and thus sinks deeper)

Theory of Sea Floor Spreading ­ II
Magnetic stripes reflect times when Earth's magnetic field is alternatively normal and reversed polarity - locked into rocks as they form
Fracture zones relive stress of oceanic crust as it moves away from the ridges
Trenches are places where Oceanic crust is being pushed back down into the mantle

Lecture 2:

The Unifying Theory of PLATE TECTONICS
The Earth's surface is made up of seven large lithospheric plates (North America, South America, Africa, Eurasia, Australia, Antarctica) and smaller plates.
Three types of plate boundaries
Constructive (divergent) = mid ocean ridges
Destructive (convergent) = trenches
Conservative = transform faults
Plates move smoothly on the top of aesthenosphere
Plate movements are driven by gravity and mantle convection(?)

Three Types of convergent Plate Boundaries
Continent/Ocean plate convergence
Continental arc volcanism
Trench adjacent to continent
Ocean/Ocean plate convergence
Island arc volcanism
Trench adjacent to island arc
Continent/Continent plate convergence
Massive mountain building
No visible trench

Implications of Plate Tectonics for Western North America
Cascades volcanoes are a continental arc
The San Andreas fault is a plate boundary separating Pacific and North American plates
Subduction occurs off Washington/Oregon and Central American
Gulf of California and Salton Sea is ocean ridge forming new ocean crust

Hot Spots
One type of geological feature is not due to plate tectonics - Hot Spot volcanoes like HAWAII
These volcanoes are due to excess heat and magma rising from deep in the Earth's interior below lithosphere plates
Hot Spots don't move; as plates move over them they produce lines of volcanoes - only one active at any time

Driving Mechanisms
Mantle Convection.
Recall that the interior of the earth is much hotter than the surface, allowing for convection
Mantle Convection.
Recall that the interior of the earth is much hotter than the surface, allowing for convection
Ridge Push.
Slab Pull.
Slab Rollback.

 

Return to Table of Contents