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3.18: Seafloor Discoveries in the 20th Century

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    Seafloor Discoveries in the 20th Century

    Although using sound to measure the depth of water was invented early in the 19th century, advanced methods were not widely used to intentionally map the seafloor until WWI and used in association with ship and submarine warfare activities. SONAR (short for SOund NAvigation & Ranging) is a system for detecting objects under water and for measuring the water's depth by emitting sound pulses and detecting or measuring their return after they reflect off the seafloor. Sonar investigation revealed the extent of the Mid-Atlantic Ridge in the center of the Atlantic Ocean basin (Figure 3.30).

    Map of the Atlantic Basin
    Figure 3.30. Seafloor bathymetry of the Atlantic Basin (mapped with SONAR) showing the Mid-Atlantic Ridge, a submarine mountain chain in the middle of the Atlantic Ocean.

    Seismology has revealed important aspects of how lithospheric plates interact with each other, how plates form and are destroyed. In the 1930’s a Japanese scientist, Kiyoo Wadati, thought that deep earthquakes and volcanoes in Japan (and the Pacific Rim) could be explained by continental drift motions. Over time, as earthquake detection equipment (seismographs) were set up around the world and data collections were compiled, it became apparent that there were patterns that showed that nearly all earthquakes occurred in zones where chains of volcanoes and mountain ranges were most actively forming around the Ring of Fire, across southern Europe into east Asia, and along narrow belts beneath the oceans associated with mid-ocean ridges (Figure 5.30). Hugo Benioff (a USGS earthquake scientist) expanded on Kiyoo Wadati's ideas and plotted the location of deep earthquakes to delineated large geologic structures associated with the Pacific's Ring of Fire. It was recognized that earthquakes and volcanoes did not occur at random but at specific and concentrated spots on and within the Earth's crust (Figure 3.31).

    Earthquakes under Japan reveal a pattern.
    Figure 3.31. Earthquakes under Japan revealed a pattern named a Benioff-Wadati Zone.