Plate Tectonics 3. Where Does All the Extra Crust Go?

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1 Plate Tectonics 3 Where Does All the Extra Crust Go? Unless otherwise noted the artwork and photographs in this slide show are original and by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

2 If oceans spread by making new crust at their centers and moving it outward, where does the crust that already exists go? The Earth is not getting bigger, so it has to go somewhere.

3 FLASHBACK remember that in the Atlantic (and Indian) Ocean the ridge is half-way between two continents that it originally rifted. Not surprisingly, the ocean basin is symmetric. Immediately beside the ridge/rift lie the abyssal plains basaltic oceanic crust that has moved away from the ridge, cooled, and subsided as far as it can. Depths on the abyssal plains run fairly consistently 4-5 km below sea level. Except for the ridge, there is generally little volcanic or seismic activity in such an ocean. The edges of the facing continents are at the shelf break, where the shallow waters (~200m) over continental crust (the shelf) end. The shelf and adjacent coastal plain are geologically identical the position of the shoreline marks where one ends and the other begins. The slope is the original rifted edge of the continent, modified by later deposition, erosion, and mass wasting. The continental rise is the accumulation of sediment that results from the erosion and slumping (plus some sediment from the land). It thins outward as it covers the edges of the abyssal plains. The ridge is the edge of two diverging plates in an ocean like the Atlantic and it is the only plate edge present. Coastal Continental Shelf Plain Shelf Break M.A.R. Rift Mirror image of the other shelf Sea Level Abyssal Plains Continental Rises Continental Slopes

4 Continental Margins such as these are called Atlantic, stable, or trailing, depending on the context of the sentence. Atlantic obviously refers to the typical location where they were first studied. Stable refers to the fact that that there are no volcanoes or seismic zones on them. Trailing means they are on the side of the continent following behind as it drifts away from the ridge. Sea Level

5 The other edges of the plates are equally easy to find in the same way as the ridges by their earthquakes. (Image from Monroe and Wicander, The Changing Earth) Notice how much broader these bands of epicenters are. They are all beside trenches

6 Notice that the ridge/rift and abyssal plain part of the Pacific basin is identical to a basin with stable margins. The processes at the ridge create this regardless of what happens at the far end of the drift system. The continental margins, however, have a much different shape. Instead of a rise/slope/shelf, these margins have a trench/arc configuration, sometimes with a separate marine back-arc basin between the arc and the mainland. With the back-arc basin the margin is a Japanese-type margin. Without one it is and Andean margin. JAPANESE-TYPE MARGIN ANDEAN MARGIN Korea Sea of Japan Japan Abyssal Plain East Pacific Rise Peru-Chile Altiplano (Asia) Japan Trench (note omission of (ridge/rift) Trench Western Eastern (Fujiyama) a long segment) Andes Andes Sea Level Continental Mainland Volcanic Arc Back-arc basin Trench (to ~10 km deep) Trench Volcanic Arc (on mainland no back-arc basin!)

7 DEEPER MOVEMENTS CREATE THE BROAD BENIOFF ZONE OF EARTHQUAKE FOCI Pacific Ocean From East Pacific Rise ANDES VOLCANIC ARC From M.A.R. Note that the Benioff Zone is not simply broad. It also deepens farther from the trench. This is verified by the depths to the foci (point or zone of movement) of the earthquakes. The obvious conclusion is that a piece of oceanic crust is being forced downward and overridden by the converging plate. Each time the plate moves it initiates an earthquake. This process is called subducton. This plate is subducting beneath this one MANTLE

8 Western Andes Altiplano Eastern Andes Andesite Diorite Rhyolite Granite Pacific Ocean From East Pacific Rise From M.A.R. Shallower melting allows less time to differentiate on the way to the surface As the plate moves deeper into the Earth it encounters higher and higher temperatures. Eventually it reaches a temperature where relatively low temp minerals can melt and feed magma to surface volcanoes. The deeper the melting the more time the magma has to differentiate before reaching the volcanoes and erupting. Thus the volcanoes closer to the trench tend to be intermediate (andesite) and the ones farther inland to be felsic (rhyolite). Deeper melting allows more time to differentiate on the way to the surface MANTLE

9 Thus the crust created at the ridges is destroyed (recycled, really, and turned into continental crust) at the subduction zones of the ocean trenches.