Physical Geology lecture outline - Intro to Plate Tectonics

A host of components, exemplars within this paradigm:

Basic definition for plate tectonics: the outer shell of the earth (the lithosphere) consists of distinct curved plates that behave relatively rigidly and move large horizontal distances with respect to each other at average rates of centimeters per year. Oceanic crust is formed at seafloor spreading centers and recycled into the mantle at subduction zones. Plate interiors are relatively unaffected, but plate margins are areas of concentrated geologic activity.

Wegener and continental drift in the 1920s; a failed revolution:

Map from USGS showing a reconstruction of how the continents fit back together. Part of Wegener's supporting evidence for continental drift, and an argument that still holds today, is the similarity of terrestrial plants and large vertebrate terrestrial fossils found on the now widely separate continents. The modern animals and plants on these separate continents are very different. Image source: http://pubs.usgs.gov/gip/dynamic/continents.html .

Seafloor spreading and the revolution succeeds in the 1960s.

Why were many detailed studies of rock magnetism, and particularly that of the ocean basin rocks, being done in the 1950s (definitely not with the intention of figuring out plate tectonics)?

NRM = natural remnant magnetization; this is the ability of some rocks with magnetic material in them (e.g. the mineral magnetite) to have the earth's magnetic field imprinted on them at some point in their history. For basalts this occurs at the time they crystallize and cool below their Curie temperature (the temperature below which the mineral can be imprinted by a magnetic field. The low down - some rocks carry a "fossil" magnetic imprint of the earth's field.

 

A chain of thought: natural remnant magnetization (NRM) -> polarity reversals and magnetostratigraphy -> striped magnetic anomalies on the sea floor -> basic idea of seafloor spreading (good example of an exemplar).

In this schematic diagram colored stripes represent areas of the ocean floor that are magnetized differently because they formed at different times as the earth's field went through polarity reversals. These can be mathematically modeled (the red curve), and given that the history of polarity reversals is given we can then estimate the age of the ocean crust. Image source: http://pubs.usgs.gov/gip/dynamic/stripes.html

Summary: the linear magnetic anomalies are acquired as oceanic crust forms by igneous processess at the spreading ridges with basalts acquiring the magnetic field at that time. When the earth's magnetic field changes and the polarity reverses the oceanic crust that formed after that point has a different fossil field that gets recorded. This evolution is depicted in the diagram to the right, which comes from the USGS Dynamic Earth Site, where you can find more detailed information on the theory of plate tectonics.

Example to the left of actual linear magnetic anomalies on the sea floor off the NW coast of the U.S. color coded by age on the basis of the history of the global magnetic reversals. Image from USGS Dynamic Earth site. You can also see how the spreading ridge (heavy dark dashed lines ) is offset here, along with the magnetic anomaly pattern. This is a transform fault, and is a common configuration in the ocean basins.

Transform ridge geometry = transform faults are the portion that links the spreading ridges. Fracture zones are the portions beyond the fracture zone-ridge juncture.

A schematic model for oceanic crust is captured in diagram above. The small arrows represent the direction of the fossil magnetic field recorded, with the opposing directions due to polarity reversals. Gabbro is the plutonic equivalent of basalt, and pillow basalts are a distinctive form of basalt that forms due to submarine eruptions, and therefore are very common on the ocean floor. Not shown here is the sediment that slowly accumulates on the sea floor on top of the basalts.

In place oceanic crust in the earths ocean basins is all younger than some 200 Ma. Why? Contrast this with continental crust, which can be greater than 3 billion years old.

 

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Cape Disappointment at the very southwest corner of Washington State. The cliffs here are all composed of pillow basalts. The small inset photo in the upper left shows some of the bulbous pillows outlined by their glassy chill margins.

This is a telephoto shot along the same cliff as depicted above that again shows the bulbous forms of the pillow basalts. Notice how there is an asymmetry with rounded edges to the upper left corner and more cuspate (pointed) boundaries. This suggests that up was to the upper left corner. This part of Washington has a slice of crust with oceanic affinities and tectonics has raised it so that these rocks are now exposed..

Close up of some of the pillow basalts exposed in the sea cliff.

This is a cross section of an individual weathered out pillow along the beach. Once can see the outer, dark glassy rind that formed by very quick cooling as the basalt came into the sea water. In the interior you can see radial white features, which are the trails of gas bubbles inside the lava.

YouTube video showing submarine basalt eruption with pillows forming.

YouTube video showing animation and explanation of seafloor spreading.

 

On to other types of plate boundaries.


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