Page 399 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 12 The Dynamic Planet 363
Two cyclic systems drive the rock cycle. At and above Earth’s surface, the hydrologic cycle, fuelled by solar energy, drives the exogenic processes. Below Earth’s surface and within the crust, the tectonic cycle, powered by internal heat, drives the endogenic pro- cesses. We now discuss plate tectonics theory and the tectonic cycle.
Plate Tectonics
Have you ever looked at a world map and noticed that a few of the continental landmasses appear to have match- ing shapes like pieces of a jigsaw puzzle—particularly South America and Africa? The reality is that the con- tinental pieces once did fit together. Continental land- masses not only migrated to their present locations but are continuing to move today at speeds up to 6 cm per year. We say that the continents are adrift because con- vection currents in the asthenosphere and upper mantle provide upwelling and downwelling forces that push and pull portions of the lithosphere. Thus, the arrangement of continents and oceans we see today is not permanent, but is in a continuing state of change. This theory was at first controversial, but it is now accepted and is the underly- ing foundation for much of Earth systems science. Let us trace the discoveries that led to the theory we now know as plate tectonics.
Continental Drift
As early mapping gained accuracy, some observers no- ticed the agreement in the profiles of the continents, par- ticularly those of South America and Africa. Abraham Ortelius (1527–1598), a geographer, noted the appar- ent fit of some continental coastlines in his Thesaurus Geographicus (1596). In 1620, English philosopher Sir Francis Bacon noted gross similarities between the edges of Africa and South America (although he did not suggest that they had drifted apart). Benjamin Franklin wrote in 1780 that Earth’s crust must be a shell that can break and shift by movements of fluid below. Others wrote—unscientifically—about such apparent relation- ships, but it was not until much later that a valid expla- nation emerged.
In 1912, German geophysicist and meteorologist Alfred Wegener presented an idea that challenged long- held assumptions in geology, and three years later pub- lished his book Origin of the Continents and Oceans. After studying the geologic record represented in the rock strata, Wegener found evidence that the rock assem- blages on the east coast of South America were the same as those on the west coast of Africa, suggesting that the continents were at one time connected. The fossil record provided further evidence, as did the climatic record found in sedimentary rocks. Wegener hypothesized that the coal deposits found today in the midlatitudes exist because these regions were at one time nearer the equator
and were covered by lush vegetation that later became the lithified organic material that forms coal. Wegener concluded that all landmasses migrate, and that approxi- mately 225 m.y.a. they formed one supercontinent that he named Pangaea, meaning “all Earth.”
Today, scientists regard Wegener as the father of plate tectonics, which he first called continental drift. However, scientists at the time were unreceptive to We- gener’s revolutionary proposal. A great debate began with Wegener’s book and lasted almost 50 years. As modern scientific capabilities led to discoveries that built the case for continental drift, the 1950s and 1960s saw a revival of interest in Wegener’s concepts and, fi- nally, confirmation. Although his initial model kept the landmasses together too long and his proposal in- cluded an incorrect driving mechanism for the moving continents, Wegener’s arrangement of Pangaea and its breakup was correct.
Figure 12.14 shows the changing arrangement of the continents, beginning with the pre-Pangaea configura- tion of 465 m.y.a. (during the Middle Ordovician Pe- riod; Figure 12.14a) and moving to an updated version of Wegener’s Pangaea, 225–200 m.y.a. (Triassic–Jurassic Periods; Figure 12.14b); followed by the configuration that occurred by 135 m.y.a., with the continents of Gondwana and Laurasia (the beginning of the Cretaceous Period; Figure 12.14c); the arrangement 65 m.y.a. (shortly after the beginning of the Tertiary Period; Figure 12.14d); and finally the present arrangement in modern geologic time (the late Cenozoic Era; Figure 12.14e).
The word tectonic, from the Greek tektonikùs, mean- ing “building” or “construction,” refers to changes in the configuration of Earth’s crust as a result of internal forces. Plate tectonics is the theory that the lithosphere is divided into a number of plates that float independently over the mantle and along whose boundaries occur the formation of new crust, the building of mountains, and the seismic activity that causes earthquakes. Plate tecton- ics theory describes the motion of Earth’s lithosphere; we discuss the various principles of this theory ahead in this section and in Chapter 13.
Seafloor Spreading
The key to establishing the theory of continental drift was a better understanding of the seafloor crust. As scientists acquired information about the bathymetry (depth variations) of the ocean floor, they discovered an interconnected worldwide mountain chain, forming a ridge some 64000 km in extent and averaging more than 1000 km in width (see the opening map in Chapter 13). The underwater mountain systems that form this chain are termed mid-ocean ridges (Figure 12.15).
In the early 1960s, geophysicist Harry H. Hess pro- posed that these mid-ocean ridges are new ocean floor formed by upwelling flows of magma from hot areas in the upper mantle and asthenosphere and perhaps from