Plate Tectonics opens by examining the lines of evidence that Alfred Wegener used in the early 1900s to support his continental drift hypothesis. This evidence included matching the outlines of the shorelines of continents that are now separated by vast ocean basins, fossils, rock types and structural similarities between continents, and paleoclimates. Also presented are the main objections to Wegener’s ideas.
Following a brief overview, the theory of plate tectonics is discussed in detail. The movement of lithospheric plates and different types of plate boundaries are examined extensively. Paleomagnetism (polar wandering and magnetic reversals), the distribution of earthquakes, ages and distribution of ocean basin sediments, and hot spots are used to provide additional support for plate tectonics. The formation and eventual breakup of Pangaea are also investigated. The chapter closes with comments about the driving mechanism of plate tectonics.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•List the evidence that was used to support the continental-drift hypothesis.
•Describe the theory of plate tectonics.
•Explain the differences between the continental-drift hypothesis and the theory of plate tectonics.
•List and describe the evidence used to support the plate tectonics theory.
•Explain the difference between divergent, convergent, and transform plate boundaries.
•Discuss the formation and breakup of Pangaea.
•Describe the models that have been proposed to explain the driving mechanism for plate motion.
Chapter Summary
•In the early 1900s Alfred Wegener set forth his continental-drift hypothesis. One of its major tenets was that a supercontinent called Pangaea began breaking apart into smaller continents about 200 million years ago. The smaller continental fragments then “drifted” to their present positions. To support the claim that the now-separate continents were once joined, Wegener and others used the fit of South America and Africa, ancient climatic similarities, fossil evidence, and rock structures.
•One of the main objections to the continental-drift hypothesis was its inability to provide an acceptable mechanism for the movement of continents.
•The theory of plate tectonics, a far more encompassing theory than continental drift, holds that Earth’s rigid outer shell, called the lithosphere, consists of about twenty segments called plates that are in motion relative to each other. Most of Earth’s seismic activity, volcanism, and mountain building occur along the dynamic margins of these plates.
•A major departure of the plate tectonics theory from the continental-drift hypothesis is that large plates contain both continental and ocean crust and the entire plate moves. By contrast, in continental drift, Wegener proposed that the sturdier continents “drifted” by breaking through the oceanic crust, much like ice breakers cut through ice.
•The three distinct types of plate boundaries are 1) divergent boundaries—where plates move apart, 2) convergent boundaries—where plates move together as in oceanic-continental convergence, oceanic-oceanic convergence, or continental-continental convergence, and 3) transform boundaries—where plates slide past each other.
•The theory of plate tectonics is supported by 1) paleomagnetism, the direction and intensity of Earth's magnetism in the geologic past, 2) the global distribution of earthquakes and their close association with plate boundaries, 3) the ages of sediments from the floors of the deep—ocean basins, and 4) the existence of island groups that formed over hot spots and provide a frame of reference for tracing the direction of plate motion.
•The gross details of the migrations of individual continents over the past 500 million years have been reconstructed. Pangaea began breaking about 200 million years ago. North America separated from Africa between 200 and 165 million years ago. Evidence indicates that prior to the formation of Pangaea the landmasses had probably gone through several episodes of fragmentation similar to what we see happening today.
•Several models for the driving mechanism of plates have been proposed. One model involves large convection cells within the mantle carrying the overlying plates. Another proposes that dense oceanic material descends and pulls the lithosphere along. No single driving mechanism can account for all of the major facets of plate motion.