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Lecture 6: PLATE TECTONICS & SEA-FLOOR SPREADING

Sea-floor spreading - More detail
Plate tectonics - detailed description
Evidence in support of those theories
PROVINCES OF THE SEA FLOOR -- DESCRIPTION, ORIGIN
Mid-ocean ridges

Sea Floor Spreading

Review of concepts

Mantle Convection as the driving force- previously proposed but not accepted

Harry Hess provided important synthesis and a detailed theory- 1960

1) First to propose that old oceanic crust returns to mantle at trenches

2) Recognized the Mid-Ocean Ridges as spreading sites

 

Plate Tectonics- The complete theory

Lithosphere consists of rigid plates (slabs).

Plates move over the soft asthenosphere.

Thus, continents move about ("drift") as passengers on plates.

Plate boundaries - where plates interact

"Divergent" - plates move apart.

• New oceanic lithosphere is created.
• Sea-floor "spreading centers" (Hess).

"Convergent," Collision zones" -- plates move together.

What happens depends on "leading edges" of plates.

• Oceanic plate -- "subducted" into mantle.
• Continental plate -- deformed and thickened (too light and thick
  to be subducted)

"Transform" - plates slide past one another.

Completion of the plate tectonics revolution: Hypotheses made, tested, confirmed-

Evidence for sea-floor spreading and plate tectonics:
Distribution of earthquakes
Heat flow from the sea floor
Thickness and age of sediments on the sea floor
Magnetization of ocean crust

PROVINCES OF THE SEA FLOOR -- DETAILED DESCRIPTION, ORIGIN

Mid-ocean ridges
Length, elevation from ocean basins, depth to crest
Width -- varies from ocean to ocean
Central "rift valley"
Important geologic phenomena
Shallow, weak earthquakes
Recent formation of basalt (volcanic activity)
Hot-springs and geysers in rift valleys

Detailed notes begins here

Sea Floor Spreading

Review of concepts- SEE NOTES FROM LAST TIME

 

• Mantle Convection as the driving force
• Harry Hess was not the first person to propose that continental drift was real and was driven by convective overturn of the mantle- Arthur Holmes (Edinburgh) had proposed this in the 1920's. But the idea did not take hold because there were not enough corroborating data.
• Hess was the first to add the idea that old oceanic crust is transport back into the mjantle at the trenches, AND he recognized the role of mid-oceans ridges as the spreading sites where new oceanic crust is made and developed this idea further (1960). Thus, Hess gets credit for synthesizing the concept of sea floor spreading just as much evidence was mounting that would eventually support it.

Plate Tectonics

... followed closely on the heels of sea-floor spreading in 1968. The important assumptions of the theory are as follows:

The rigid outer shell of the Earth (termed the "lithosphere") is broken into a series of large slabs, or plates, that move at rates of a few centimeters per year and interact with one another at their boundaries.

Rigid plates move over a soft, underlying layer in the upper mantle termed the "asthenosphere." Global plate motion is probably driven by convection in the mantle (as Hess proposed in 1962).

Plates can consist of either (1) oceanic lithosphere only or (2) oceanic and continental lithosphere. Thus, the movement of plates provides a mechanism for continental drift -- continents can move about as "passengers" on larger, lithospheric plates.

What happens at the boundary between to plates depends on their relative motion

o Plates move apart - "divergent" boundaries. These are regions where new oceanic lithosphere is being created (or will soon be created). Hess called these type boundaries "spreading centers," sites where sea-floor spreading begins.

o Plates move together - "convergent" boundaries or collision zones. What happens at these boundaries depends on the type of lithosphere (oceanic or continental) that are colliding. An oceanic plate will dive into the mantle, a process called "subduction." A continental plate cannot be subducted because it is too light and thick; it will be deformed and become thicker as collision continues.

o Plates slide past one another - "transform fault" boundaries. Plates are neither produced or destroyed at these boundaries, but there is a good deal of friction between the sliding plates.

Over the next several lectures, we will be interpreting the features of the sea floor. In particular, we will identify particular features with plate boundaries. As part of this, we will develop the evidence from the sea-floor that lead to the acceptance of plate tectonics and sea-floor spreading (as well as continental drift) as important theories.

Evidence that supported the theory of plate tectonics and completed the scientific revolution

Simply having the idea of sea-floor spreading was not enough to convince the scientific community that continents drifted and that most of the earth's features are related to plate tectonic processes. Other evidence was mounting that eventually was fit into the new paradigm with stunning results. In addition, testable hypothese were generated and oceanic expeditions were undertaken to test them. Here is some of the evidence:

Distribution of earthquakes: Earthquake epicenters do not occur randomly; most are located in long narrow zones at (a) mid-ocean ridges and (b) marginal trenches in the oceans. Since earthquakes are generated by storing and then releasing fractional energy, these zones must be places where lithsopheric plates are moving and interacting with one another -- in other words, major earthquake zones define lithospheric plate boundaries!

Heat flow from the sea floor: Because the interior of Earth is hot, heat continually escapes through the crust. This heat flow is determined by the increase in temperature with depth. The greater the "temperature gradient,' the greater the heat flow. Heat flow measurements on the sea floor show that it is very high at mid-ocean ridges. This observation is consistent with the emplacement of new oceanic crust by rising convection currents. As sea floor spreads away from ridges, it cools and subsides.

Thickness and age of sediments on the sea floor: Sea-floor spreading predicts that oceanic crust becomes older with distance from mid-ocean ridge spreading centers. Therefore, the thickness of sediments should increase with distance also, because there is more time for sediments to accumulate on older crust. In addition, the geologic age of the oldest sediment immediately above the basalt crust should increase with distance. We'll look at the evidence later.

Magnetization of ocean crust: The regular magnetic patterns of the sea floor were first observed in the 40's and 50's. The interpretation of those patterns - resulting from periodic "reversals" of Earth's magnetic field and the continuous creation of oceanic crust - became perhaps the most convincing evidence in support of sea-floor spreading. However, when proposed in 1963, this was outlandish idea. We'll examine the evidence and interpretation later.

Provinces of the Sea Floor

Description, Origin from Sea-Floor Spreading and Plate Tectonics

Mid-ocean ridges -- Description

Mid-ocean ridges (MORs) are a 65,000 km-long continuous submarine moutain chain. They rise 1-3,000 meters above the adjacent ocean-basin floor. The width of MORs varies - they are relatively narrow in the Pacifc, and relatively wide in the Atlantic.

The depth of the crest of MORs is about 2-3,000 meters Nearly all ridges have a central "rift valley," a down-dropped region. This central region is the site of important geologic phenomena:

• Weak (low-magnitude) earthquakes with a shallow focus.
• Evidence for recent formation of basalt (the shape of basalt boulders are similar to those that form in Hawaii when lava flows enter cold sea water).
• Submarine hot-springs and geysers (hydrothermal activity). These extraordinary phenomena were first discovered in 1977 in the central rifts of mid-ocean ridges in the Pacific. They probably form as sea water enters oceanic crust through cracks and fissures. This water is heated to 360 deg C when it encounters basalt magma chambers at depth. It then rapidly emerges at ridge crests as extremely hot geysers. The geysers often contain suspended mineral matter -- "black smokers" have dark sulfide minerals; "white smokers have light-colored minerals. Bacteria who live on the sulfur compounds from the geysers provide the food for a bizarre biological community. The finding of hydrothermal communities living off bacteria was completely unexpected!

Video tape (GV#16, Miracle Planet #2, East Pacific Rise, black smokers, white smokers, topography of mid-ocean ridge 16:20-19:30)

Study/Review Questions:

6-1. Briefly describe the relationship between the lithosphere and asthenosphere of Earth. How do they differ in their physical properties?
6-2. Name a plate that is made up of oceanic lithosphere only. Name a plate that is made up of oceanic and continental lithosphere. Can a plate be made up of continental lithosphere only? (yes or not).
6-3. Why did the theory of plate tectonics vindicate, in a sense, Wegener's proposal of continental drift?
6-4. What are the three types of plate boundaries? Describe the relative motion of plates at those boundaries. At what type of boundary is new oceanic lithosphere created? At what type of boundary is older oceanic lithosphere destroyed?
6-5. Briefly describe the distribution of earthquakes. How is this pattern explained by plate tectonics?
6-6. Describe the relationship between heat flow from the sea floor and distance from mid-ocean ridges. How is this relationship explained by sea-floor spreading and plate tectonics?
6-7. Describe the relationship between (a) the age and thickness of sediments on the sea floor and (b) distance from mid-ocean ridges. Is this relationship consistent with the predictions of sea-floor spreading (and hence plate tectonics)?
6-8. Draw a topographic cross-section across a typical mid-ocean ridge. Include the central rift valley. In general, how high are ridges above the adjacent deep-sea floor, and how far below the sea surface are they at their crests?

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