GEOLOGY 10    Web Notes #2. Plate Tectonics, Earthquakes, and Volcanoes
(note: LT = Lutgens & Tarbuck, Foundations of Earth Science, 4th edition)


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Note: Because the printable notes contain so many images,
I have only included additional Web links here.
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U.S. Navy: Bathymetry, sampling, and magnetic anomalies
In the 1940s and 1950s, the U.S. Navy funded mapping of the seafloor using echo sounding, or downward-directed sonar, in order to find or hide submarines. Generalized maps were released to the public via the National Geographic Society [Map of the seafloor].

Among the remarkable features:
submerged mountain ranges, called oceanic ridges
cracks perpendicular to the ridges, called fracture zones
narrow, deep gashes, called trenches
vast flat areas, called abyssal plains
drowned undersea islands, called seamounts and oceanic plateaus
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Continental magnetic anomalies are splotchy and irregular, but marine magnetic anomalies form long, parallel bands (not stripes painted on the seafloor!) of alternating positive and negative anomalies [see this Web site]; i.e., about half the rocks are reversely magnetized (N arrow points S). But for several years they didn’t know how to interpret these patterns.
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USGS: origin of the Himalayas