The once hidden half - seafloor spreading.
It can be argued the plate tectonics revolution was in part a consequence of technology allowing us to 'see' the geologic nature of the ocean basins. The geology of the ocean basins is very unlike that of the continents, a 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, which is the process that creates ocean basins.
How do we peer into the watery depths?
Image above is of canyons cutting into the U.S. east coast continental slope. Early on these substantial canyons were enigmatic as to what created them. We now understand that they are formed by turbidity current processes. Image source USGS site : http://soundwaves.usgs.gov/2005/01/research.html. Also underlying the slope is the transition from continental to oceanic crust, a boundary that formed at the transition between continental rifting and seafloor spreading. The transition is buried under a very thick sedimentary cover.
Above: cross section and profile view looking along the axis of spreading of two offset ridges that are linked by a transform. There are 2 drivers of motion of the crust. First is the much larger plate movement rate which is in the range of cms/yr, which is represented by the red arrows. Second there is the smaller rate of subsidence as the oceanic crust formed at the ridge moves away and gets older and colder, which is represented by the blue arrows. This rate is mms/yr or less and will decrease as a square root function with distance from the ridge that crust formed at. For the transform component the strike-slip component will dominate, but for the fracture zone part farther from the crests there will be only the dip component due to the different rates at which the two sides subside.
Image above: Penhyrn Atoll in the Cook Islands, image from NASA. http://daac.gsfc.nasa.gov/oceancolor/shuttle_oceanography_web/oss_28.shtml
Image of bathymetry/topography from http://www.ngdc.noaa.gov/mgg/image/2minrelief.html.
Significance of basalt: One of the most common dredge products from the ocean floor 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.
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.
Diagram above from the USGS showing an actual magnetic profile from a survey across part of the EPR that is then matched to the time scale of polarity reversals to define stripes on the sea floor. The red curve is a model profile produced given the magnetic polarity time scale. Image taken from: http://pubs.usgs.gov/gip/dynamic/stripes.html .
Map diagram to right 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.The color coding shows a model for crust that attained its NRM during a specific polarity reversal as indicated by the polarity reversal time scale in the lower left.
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.
Seafloor spreading: the ridge crests are sites of divergent plate boundaries where geologically continuous igneous activity creates new oceanic crust. This new crust is typically, but not always, added equally to both plates, and it acquires a magnetic imprint reflecting the earth's magnetic polarity when it formed. The spreading ridges occur in segments linked to each other by sub-perpendicular transform faults .
The diagram to the right from the USGS shows the development of the magnetic anomalies in stages as divergence occurs.
YouTube video of Fred Vine explaining the concept of seafloor spreading.
YouTube video showing animation of seafloor spreading, polarity reversals and the creation of the linear magnetic anomalies.
Movements on a sphere (the earth is not flat)
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 globe's 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 for different points of a rigid spherical cap. 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.
Fracture zones and spreading centers as small and great circle paths on a sphere.
Deviations from simple symmetrical spreading patterns:
Seismic refraction and reflection -> allow geophysical imaging/modeling of the rather inaccessible oceanic crust:
Simplified block diagram of seafloor spreading center with sediment and water removed, showing the 3 distinct igneous layers that form. Click on image for a large scale version.
Stream outcrop within the Josephine ophiolite of northern California, where there are several ophiolite bodies. The dark and aphanitic material is basalt and the more phaneritic material is meta-gabbro. There appears to be xenoliths of gabbro in the basalt and fingers of gabbro that are have intruded the basalt, producing complicated and overlapping age relationships. One interpretation is that this is the contact between the overlying extrusive part of the oceanic crust and the underlying magma chamber that fed the extrusions.
In this stream outcrop nearby the one above one can see two basaltic dikes that cut through the gabbros, that represent a magma chamber associated with the seafloor spreading . Out crops like these can lead to discussions on off-axis intrusion, where/when igneous activity is centered on the ridge but also occurs to the sides so that already solidified oceanic crust also gets intruded by dikes. To the degree that ophiolites are oceanic crust they provide the opportunity to study the details of seafloor spreading processes that are inaccessible in in-situ oceanic crust.
A stream boulder of part of the Josephine ophiolite showing a meta-gabbro with rhythmic layering. Mafic magmas can have a low enough viscosity that gravity differentiation occurs as crystallizing grains settle to the chamber floor.
Stream boulder of a meta-ultramafite in the Josephine ophiolite. Note the significant hydrothermal alteration along an array of fractures. Whether this alteration happened during seafloor spreading or afterwards during obduction of the oceanic crust is a worthwhile question to consider. This could either be some of the mantle ultramafite from the very base of the ophiolite, or the ultramafic portion of the magma chamber where olivine and pyroxene settled out.
2011 UNO field trip participants listening to a student given lecture on ophiolites. The brownish boulders are the ultramafites.
Reference on the Josephine ophiolite and an introduction into the ophiolite literature: Alexander, R. J. & Harper, G. D., 1992, The Josephine ophiolite: an ancient analogue for slow- to intermediate-spreading oceanic ridges; Geological Society Special Publication No. 60, pp. 3-38. sp.lyellcollection.org/content/60/1/3.full.pdf
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.