In 1831 the HMS Beagle set sail " to complete the survey of Patagonia and Tierra del Fuego, commenced under Captain King in 1826 to 1830, -- to survey the shores of Chile, Peru, and of some islands in the Pacific -- and to carry a chain of chronometrical measurements round the World". Dr. Charles Darwin was a member of the scientific company on board. read a selection from Darwin's "Voyage of the Beagle". Read the first few paragraphs or feel free to spend some time poling around in the other texts that this link offers. I would be interested in whether you find reading a book on the Internet a worthwhile activity or does the absence of a hard copy make it difficult to read?
Today modern oceanographic vessels attempt to study the same phenomena but have the advantage of modern technology. Echo sounders are used to measure the depth to the sea floor. Seismic profiling involves detonating a charge (which is often a pulse of compressed air). This energy penetrates the rocks beneath the sea floor and are analyzed in the same way that seismologists analyze naturally occurring Earthquakes. Gravimeters and Magnetometers are used to detect positive and negative gravity and magnetic anomalies.
It is convenient to classify continental margins into:
Beneath a spreading center is a characteristic set of features:
As the oceanic crust ages and begins to spread laterally the deep sea sediment layer thickens. In deeper water most of the sediment is enriched in silica-rich material (recall that calcium carbonate dissolves in deep sea water).
This sequence is termed an ophiolite sequence . Ophiolite sequences have been recognized on continental areas (for example, along the west coast of Newfoundland). These sequences are interpreted to be the remnants of old oceanic crust. Ophiolites are usually emplaced when an ancient ocean basin is closed along a convergent margin.
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Copyright by John C. Butler, July 29, 1995