The hidden half - seafloor spreading.

It can be argued the plate tectonics revolution was in part a consequence of technology allowing us to 'see' the ocean basins. The geology of the ocean basins is very unlike that of the continents, a very different 'style' of geology. So picking up from our historical approach last time we can consider the evidence that led to the concept of seafloor spreading.

How do we peer into the watery depths?

Figure from portion of a bathymetric map of the world's oceans from Dana's 1894 Manual of Geology. If one takes a closer look at the values and contours, the axial mid-Atlantic Ridge is evident. In addition, in the south Atlantic, some of the oceanic plateaus are also evident. This crude view of the oceans bathymetry was the results of simple soundings. Basically, a line was dropped overboard until the bottom was reached. This involves miles of line! Remember, the fact that the ocean basins were this deep was part of Wegener's arguments in support of continental drift.
Morphology of the oceanic basins.

Image of bathymetry/topography from http://www.ngdc.noaa.gov/mgg/image/2minrelief.html.

Exercise: Self-guided tour of the ocean basins. Use the maps provided to first identify some of the various components listed above, and then produce a short guided tour for your class colleagues of this oceanic basin area. Note any patterns or anomalies you see.

Character of basalt: One of the most common dredge products is basalt. In addition most oceanic islands are composed of basalt. Iceland is a particularly good example. A conclusion is that oceanic crust is much less diverse in composition than continental crust, and basalt is the most common rock type.

NRM, polar wandering, and polarity reversals.

Some basics of NRM:

Two interesting phenomena came to light when looking at the history of the earth's magnetic field as recorded in rocks: polar wandering and polarity reversals.

Magnetic polarity reversals: in a sequence of lavas find NRM vectors for some ages that are subparallel but of opposed polarity to those above and below.

Map diagram from USGS (http://pubs.usgs.gov/publications/text/magnetic.html) of magnetic anomaly pattern off the cost of northwestern U.S., where the Juan De Fuca spreading ridge exists.

 

Striking aspects of seafloor magnetic anomaly patterns.

Vine and Mathews hypothesis of seafloor spreading developed in 1960s on the basis of this kind of data.

The diagram below shows the development of the magnetic anomalies in stages as rifting occurs.

Exercise: Computing seafloor spreading rate histories. Take the sheet identifying the reversal time scale, and the anomaly pattern for the South Atlantic and Pacific spreading ridges. Compute the spreading rate for 10 Ma increments back to 80 Ma. Discuss the results. (Modified from Dallmeyer, 1995, Physical Geology Laboratory Text and Manual; Kendal Hunt).

Fracture zones and spreading centers as small and great circle movements on a sphere.

In class demonstration: movements on a sphere. Using a globe we can track the path of points on the globe. It is obvious that they must follow curved paths to stay on the globes surface as a straight line of movement would cause the point to submerge or elevate above the surface. Using a fixed pole of rotation, small-circle paths can be developed. We can also think of what is required for the plate boundary geometry for pure extension, convergence or strike-slip motion in terms of large and small circles. Moving from the globe we can look at a stereographic project, a mechanism for capturing/describing plate motions on a flat page.

Exercise: Movements on a sphere using a stereographic projection. For the given pole of rotation, and assuming the plate is moving to the left relative to the surrounding plate, what will be the type of plate motion at points A, B, C, D and E. Given the radius of the earth at 1738 km, how long will it take point F to move 10 degrees of small circle arc if the relative motion is 3 cm/yr? What will be the linear velocity of point G in this case?

Some good images of poles of rotation, small-circle paths, and the associated linear velocities.

Deviations from symmetrical spreading patterns:

Simplified block diagram of seafloor spreading center with sediment and water removed, showing the 3 distinct igneous layers that form.

Layers in oceanic crust and a model for the plumbing of spreading.

Ophiolites.

Thermal structure of oceanic crust.

Best fit relationships for topography and age of oceanic crust.

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.