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Geosciences LibreTexts

18.0: Introduction

Learning Objectives

After reading this chapter, completing the exercises within it, and answering the questions at the end, you should be able to:

  • Describe the origins of the major topographic features of the sea floor, including continental shelves and slopes, spreading ridges, seamount chains and isolated seamounts, and deep submarine canyons
  • Describe the various components of oceanic crust: pillow basalts, sheeted dykes, gabbro bodies, layered gabbro, and layered ultramafic rock
  • Describe the age distribution of oceanic crust, and explain why all of it is relatively young
  • Summarize the types of sediments and sedimentary rocks that accumulate on the sea floor, and explain why different types of sediment are present in different areas
  • Explain the origins of sea-floor methane hydrates
  • Describe and explain regional variations in the salinity and temperature of ocean water
  • Describe the general nature of major ocean-surface currents and the origins of deep-ocean circulation patterns
  • Explain the importance of ocean currents to our climate

Figure 18.0.1 Oceanic crust (pillow basalt) from the Paleogene Metchosin Igneous Complex, near Sooke, on Vancouver Island. The view is about 1.5 m across. [SE]

Oceans cover 71% of Earth’s surface and hold 97% of Earth’s water. The water they contain is critical to plate tectonics, to volcanism, and of course, to life on Earth. It is said that we know more about the surface of the Moon than the floor of the oceans. Whether this is true or not, the important point is that the ocean floor is covered with an average of nearly 4,000 m of water, and it’s pitch black below a few hundred metres so it’s not easy to discover what is down there. We know a lot more about the oceans than we used to, but there is still a great deal more to discover.

Earth has had oceans for a very long time, dating back to the point where the surface had cooled enough to allow liquid water, only a few hundred million years after Earth’s formation. At that time there were no continental rocks, so the water that was here was likely spread out over the surface in one giant (but relatively shallow) ocean.