In this part of the book we have considered tools and concepts essential for our examination of the history of Earth and other planets. We turn next to that history, starting at the beginning of our solar system and its central star, the Sun.
Summary
Plate tectonics is a description of the way in which the Earth’s crust moves in response to the heating of the crust from below.
The development of plate tectonics is an excellent example of how diverse kinds of evidence can challenge existing theo- ries and force their replacement by new, initially controversial ones. Although the shapes of the continents and the corre- spondence of fossil species long suggested the idea that they might have once been joined together, only after the topogra- phy of the seafloor was revealed, along with the presence of stripes of alternating magnetic polarity revealing the presence of seafloor spreading, was the idea taken seriously. It is along the mid-oceanic ridges that new seafloor is produced from the rising and solidification of magma, leading to the spreading of the seafloor. As the seafloor moves away from the spreading centers, it cools and becomes denser. Eventually the cooling seafloor becomes denser than the underlying mantle on which
the crust rides, and it founders and sinks in the form of subduct- ing slabs. The zones of subduction corresponding to deep-sea trenches, usually (but not always) found along the margins of continents. The continents represent a different type of crust, less dense and hence more buoyant than oceanic crust, which with only minor exceptions is never subducted into the man- tle. The overall movement of the plates on a spherical Earth also requires places where plates are moving laterally relative to each other. The patterns of earthquakes and volcanism are strongly correlated with the edges of plates, and different types of volcanism occurring at spreading centers, mid-ocean ridges, and in the interiors of plates reveal the various processes by which melting occurs associated with plate tectonics. Among Earth, Venus, and Mars, only the Earth seems to have plate tectonics today, although Venus might have experienced it in the past.
Questions
1. Why do you suppose geologists of the early twentieth cen- tury were so reluctant to consider continents moving across the globe, in view of the fact that they accepted as plausible largeverticalmovements?
2. The technologies available to geologists after World War II provide an excellent example of how military technology can create scientific revolutions. What other areas of sci- ence or medicine were revolutionized as a result of military developments in World War II?
3. How is the geology of the area where you live related to the global pattern of plate tectonics? Has the geologic nature of your area been determined mostly by subduction, by spread- ing, by transform motion, or by hot spot volcanism (for example)?
4. In what ways does the lateral motion of plates depend on the Earth being a sphere; that is, how would plate tectonics be different were the Earth a flat sheet in which the plates were small compared to the size of the sheet?
General reading
Cloud, P. 1988.Oasis in Space: Earth History from the Beginning.
W. W. Norton, New York.
National Research Council. 2008. Origin and Evolution of the Earth. The National Academies Press, Washington DC.
References
Browne, M. W. 1995. Experts ponder causes of breakup of ancient supercontinent.New York Times, Oct. 3, p. B5.
Cloud, P. 1988.Oasis in Space: Earth History from the Beginning.
W. W. Norton, New York.
Dalziel, I. W. D. 1997. Neoproterozoic–Paleozoic geography and tectonics: review, hypothesis, environmental speculation.Geo- logical Society of America Bulletin109, 16–42.
Gutierrez-Alonso, G., Fern´andez-Su´arez, J., Weil, A. B.et al. 2010.
Self-subduction of the Pangaean global plate.Nature Geo- science1, 549–53.
Isacks, B., Oliver, J., and Sykes, L. R. 1968. Seismology and the new global tectonics.Journal of Geophysical Research 73, 5,855–99.
Morgan, W. J. 1968. Rises, trenches, great faults and crustal blocks.
Journal of Geophysical Research73, 1,959–82.
Smith, W. H. F. and Sandwell, D. T. 1997. Global sea floor topogra- phy from satellite altimetry and ship depth soundings.Science 277, 1956–62.
van Andel, T. H. 1992. Seafloor spreading and plate tectonics. In Understanding the Earth: A New Synthesis (G. C. Brown, C. J. Hawkesworth, and R. C. L. Wilson, eds). Cambridge University Press, Cambridge, UK, pp. 167–86.
Wyllie, P. 1971.The Dynamic Earth. John Wiley and Sons, Inc., New York.
PART III
The historical planet:
Earth and solar system through time
97
10
Formation of the solar system
Introduction
Having dealt with some of the tools and key concepts to which we will return as we develop the history of Earth and the other planets, we are ready now to consider that history. Five cen- turies after the beginning of the European Renaissance, human- ity’s explorations of Earth and the cosmos have exposed an intriguing, perhaps profound, paradox. Earth and the other planets of the solar system seem to be explainable as manifes- tations of common physical processes that have operated over very small and very large scales, to produce a range of cosmic phenomena. In this sense we are neither special nor particularly important in the grand scheme of things.
On the other hand, in our own solar system, we now under- stand Earth as the one planet with a uniquely stable climate on its surface, equable for liquid water over the billions of years
required to bring forth intelligent life. Although Mars may have come close to this state at one time, the surface appears life- less today. Europa may have a habitable oceanic environment beneath its icy crust. An intriguing possibility is that Saturn’s moon Titan may have had a stable “hydrosphere” over its his- tory, but one in which methane substitutes for water: whether such an environment could be habitable for a very exotic form of life is not known (Chapter 12). Other solar systems may be common and life may flourish elsewhere, but it is also possible, with what we know today, that we are a rare or even unique speck in the cosmos. We will know more over the decades to come, but for now we seek to understand how this planet came to be, and how physical processes have operated to make it habitable for billions of years.