[ ŏf′ē-ə-līt′, ō′fē- ]
A sequence of rocks consisting of deep-sea marine sediments overlying (from top to bottom) pillow basalts, sheeted dikes, gabbro, dunite, and peridotite. Ophiolite sequences are indicators of sea-floor spreading where the oceanic crust is thinned by the forces of plate tectonics, and magma rises to the sea's floor and generates new crust. As lava spills out into the cold ocean water it forms pillow basalts; magma that cools before reaching the surface forms sheeted dikes (dikes that appear to be vertically stratified because of the way the magma cools). Magma cooling deeper in the crust forms gabbro, dunite, and peridotite.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.