Chapter 2: The Sea Floor


 The Water Planet
  • The Geography of the Ocean Basins

    The Structure of the Earth

  • Internal Structure

    Continental and Oceanic Crust

  • The Origin and Structure of the Ocean Basins

    Early Evidence of Continental Drift

    The Theory of Plate Tectonics

  • Discovery of the Mid-ocean Ridge

    Significance of the Mid-Ocean Ridge

    Creation of the Sea Floor

    Sea-Floor Spreading and Plate Tectonics

  • The Dynamic Mantle

    Geologic History of the Earth

  • Continental Drift and the Changing Oceans

    The Record in the Sediments

    Climate and Changes in Sea Level

  • The Geological Provinces of the Ocean
  • Continental Margins
  • The Continental Shelf

    The Continental Slope

    The Continental Rise

    Active and Passive Margins

  • Deep-Ocean Basins

    The Mid-Ocean Ridge and Hydrothermal Vents


    Box Reading: "Hot Spots" and the Creation of the Hawaiian Islands



  • Chapter Summary


    Chapter 2 summarizes the basic aspects of the geology of the sea floor and points out its relevance to marine biology:


    Answers to Critical Thinking Questions


    1. The process of plate tectonics is occurring today in the same way as in the past. Can you project future positions of the continents by looking at a map of their present positions and the positions of the mid-ocean ridges (see Fig. 2.6)?


    North and South America will be farther west toward the Pacific, Europe and Asia farther southeast, Australia farther northeast, and Africa farther east. The Pacific is therefore expected to be narrower and the Atlantic wider. The map in Figure 2.10 actually indicates the direction of plate movement but it's better if students deduce these movements by the location of the mid-ocean ridges as indicated on the map in Figure 2.6.


    2. Why are most oceanic trenches found in the Pacific Ocean?


    The Pacific Ocean is shrinking and plates are descending below surrounding plates along its edges, hence the creation of trenches.


    3. Scientists who study forms of marine life that lived more than approximately 200 million years ago usually have to obtain fossils not from the sea floor, but from areas that were once undersea and have been uplifted onto the continents. Why do you think this is?


    Part of what was the sea floor at that time is now above the sea level.


    4. What are some of the major pieces of evidence for the theory of plate tectonics? How does the theory explain these observations?


    Evidence includes: the fitting together of the coasts of the continents on the opposite sides of the Atlantic Ocean, the similarity of geologic formations and fossils found on the opposite sides, a geologically active mid-ocean ridge running along the central Atlantic between the opposite coasts, bottom sediments that get thicker the farther one travels from the ridge, and rocks on the sea floor on one side of the ridge show magnetic bands that are mirror images of rocks found on the opposite side of the ridge. All of these observations are explained by sea-floor spreading from the mid-ocean ridge.