Space travelers seeing our beautiful, blue planet would likely call it the water planet. The hydrosphere, notably the ocean, dominates the surface of Earth with over 70% of the surface covered by oceans. Clearly, the part of the geosphere covered by the oceans is important to our understanding of the Earth's systems, even though most of the seafloor is not readily accessible to direct observation.
The hydrosphere is, of course, vital to the biosphere. Primitive life began in the sea and evolved over billions of years into the rich and diverse plant and animal life that we see today. Life on land is a relative newcomer. It has been only a few hundred million years since the first creatures ventured out of the sea and land-dwelling life evolved and flourished.
Most of what we know about the seafloor has been discovered since the second half of the twentieth century. Because of the difficulty in accessing the deep ocean floor, our maps of its surface are not as complete as those of some of our neighboring planets.
We do know that the rocks and topography of the sea floor are different from those on land. To understand the evidence for plate tectonics in the next chapter, you need to understand the nature of major sea-floor features such as mid-oceanic ridges, oceanic trenches, and fracture zones, as well as the surprisingly young age of the sea-floor rocks.
The material discussed in this chapter and the next are an excellent example of how the scientific method works. This chapter is concerned with the physical description of most sea-floor features - the data gathering part of the scientific method. The next chapter shows how the theory of plate tectonics explains the origin of many of these features. Geologists generally agree upon the descriptions of features but often disagree on their interpretations. As you read, keep a clear distinction in your mind between data and the hypotheses used to explain the data.
