Chapter 2

Geological studies of the oceans are accomplished with a variety of sophisticated instruments that generate computerized images of the seafloor. These studies have provided insight into how the ocean basins formed, why there is land and ocean, and where there are great deposits of oil and gas.

The oceans cover about 70% of the earth's surface. But why do the oceans exist or for that matter why do we have land and ocean? To understand why there is ocean and land we need to consider:

• the distribution of land and ocean on the planet,
• the composition of these different regions of the earth, and
• the Principle of Isostasy.

Let's look at an exercise developed by D. Reed at San Jose State University. It begins with an explanation of the Hypsographic Curve and then discusses the Principle of Isostasy.

The earth is not static. Current models of the earth describe several plates that are floating on a plastic mantle layer. Convective movements in the mantle lead to movement of these plates. The continents move and oceans expand and contract. The idea of a dynamic earth was embodied in the Theory of Plate Tectonics that was developed in the mid-1950s. Prior to this time no theory could adequately account for the position of the continents.

Plate Tectonics encompasses the concepts of continental drift and seafloor spreading.

• Continental drift
• • Pangaea and the breakup of the continents
  • Short cut to section on magnetic stripes and isotopic clocks for dating the age of rocks
• Sea floor spreading
• • Magnetic anomalies (This page has several short mpeg movies depicting ten million years of seafloor spreading at the axis of the Pacific/Antarctic Ridge. It also has a flyover view of the ridge system).
• What does plate tectonics tell us about:
• • Tsunamis
  • • The Physics of Tsunamis (Excellent animation of 1960 Chilean earthquake that propagated across the Pacific and caused extensive damage. Japan, which is over 17,000 km away from the tsunami's source off the coast of Chile, lost 200 lives to this tsunami. The number of fatalities associated with both the tsunami and the earthquake has been estimated to be between 490 to 2,290. Damage cost estimates were over a half billion dollars. The tsunami struck the Chilean coast within minutes and took approximately 15 hours to reach Hawaii.)
    • How do earthquakes generate tsunamis
    • The Tsunami Warning System
    • The Alaska Tsunami Warning System (Six tsunamis have caused damage in Alaska since 1899 and three of these have caused fatalities. The 1964 earthquake in Prince William Sound generated the costliest tsunami ever to strike the United States. Over $90 million in damage and 119 fatalities resulted.)
    • More on tsunamis
  • Earthquakes
  • • National Earthquake Information Center
    • Florida Seismicity
    • The San Andreas fault and associated quakes
    • The January 17, 1994 Northridge, CA Earthquake (This was the third very destructive earthquake to occur in California in the last 23 years. The first was the San Fernando (Sylmar) Earthquake, affecting the same area in 1971; the second was the, 1989 Loma Prieta Earthquake south of San Francisco. The 1994 event was the most damaging earthquake to strike the United States since the San Francisco Earthquake of 1906. In terms of financial loss, the earthquake is also one of the worst natural disasters in U.S. history, comparable to Hurricane Andrew in 1992.)
    • Susan Rosenberg's Quaking Home Page
    • Seismic Resources on the World Wide Web
    • Plate Tectonics, the Cause of Earthquakes
  • Volcanoes
  • • Ring of Fire
    • Volcano Hazards Program

 Map of the seafloor

Other maps of the seafloor