Review Questions and Answers; Plate Tectonics
1. What first led scientists such as Alfred Wegener to suspect that the continents were once joined?
Speculations about the apparent "nice fit" between the west coast of Africa and the east coast of South America date from the sixteenth century, when the first, reasonably accurate maps of the Americas were compiled. Alfred Wegener, early in the twentieth century, was the first, leading, scientific proponent of continental drift. He marshaled geometric, paleontological, paleoclimatic, and geological evidence to support continental drift; but his ideas were largely rejected in England and the United States. They lay dormant until the 1950s, when new studies of the ocean floors gave results that supported the continental drift idea and strongly conflicted with the opposing concept of static continents and eternally old ocean basins.
2. What was Pangaea?
Pangaea (Fig. 19.2) was the supercontinent that existed in late Paleozoic time when the great, southern hemisphere landmass composed of Africa, India, South America, Australia, India, and Antarctica (Gondwanaland) collided with North America, joining most of the world's continents into one, super-large landmass. Pangaea was a relatively short-lived continent; it began breaking up in Triassic time.
3. List the evidence that Wegener and his followers gathered to support the continental drift hypothesis.
If the continents were once together, they must have drifted apart. Thus Wegener (Fig. 19.8) had to prove that now widely separated continents and/or pieces of continents were once close together or contiguous. The evidence included apparent, geometrical fits between edges of continents; similar, late Paleozoic and early Mesozoic, stratigraphic, geologic, and paleoclimatic records from different continents; and organisms, identified by their fossil remains, that would have had serious trouble migrating from one continent to another across oceans the size of those today. These latter organisms include land-dwelling amphibians and reptiles and numerous species of plants.
4. Early in this century, what was the prevailing view of how land animals migrated across vast expanses of ocean?
Animals were thought to have migrated from continent to continent using island chains as stepping stones, floating on logs, crossing on temporary continental links such as today's isthmus of Panama, or swimming (Fig. 19.5). Plants (seeds and spores) floated on currents or were rafted by the wind. In retrospect, these mechanisms were not satisfactory, and oceanographic studies had pretty much debunked them by the early 1950s. For example, Mesosaurus, an early Permian, aquatic reptile known only from South Africa and Brazil, lived in freshwater and coastal, salt water habitats, much like those of the modern crocodile or salt water crocodile. Such animals would have had great difficulty migrating between continents if the Atlantic Ocean was the same size in early Permian time as it is today. It seems more sensible to believe that the land areas were once together and have since drifted apart as the ocean opened and widened.
5. Briefly explain why the recent acceptance of plate tectonics has been described as a scientific "revolution."
The case for plate tectonics as a scientific revolution rests on three, main attributes. First, the idea profoundly changed the ways geologists think about ocean basins and continents, substituting a dynamic, highly imaginative, "fluid", evolutionary vision for a standpat, static one. Second, the plate tectonic concept has spawned numerous hypotheses and ways of testing them. Third, the idea brought together a seemingly endless universe of otherwise unrelated facts and observations from diverse geologic and allied, scientific disciplines. The tremendous, integrative power of the plate tectonic concept justifies characterizing its acceptance as a scientific revolution.
6. How does evidence for a late Paleozoic glaciation in the Southern Hemisphere support the continental drift hypothesis?
A late Paleozoic reconstruction that places the southern hemisphere continents and India around Antarctica and the south pole (Fig. 12.35) satisfies the severe constraints imposed by the common time of glaciation and by movement directions inferred for the late Paleozoic ice sheets on continents that are now widely separated. Without continental drift, glaciers would had to have developed simultaneously on five, distant continents. For Madagascar, India, South America, and Australia, the ice sheets would had to have moved onto the land areas from the ocean. Visualizing these continents as once centered about Antarctica at the south pole and subsequently moving to their present locations by continental drift offers a more convincing, satisfying explanation of the observed facts.
7. Explain how paleomagnetism can be used to establish the latitude of a specific place at some distant time.
Paleomagnetism (Figs. 19.9, 19.10 & 19.11) is the study of remnant magnetic characteristics of rocks and of Earth's magnetic field through geologic time. The inclination or dip of the remnant magnetization gives the latitude at which the magnetization was acquired when the rock originally formed. If the rock is carried to a different latitude by continental drift, its original paleomagnetic inclination (a direct measure of the latitude at which it formed) will not change so long as the rock is not heated above the Curie temperature, about 550 °C.
8. What is meant by seafloor spreading? Who is credited with formulating the concept of seafloor spreading?
Seafloor spreading is the divergent movement of two, oceanic plates away from a mid-ocean ridge, accompanied by addition of new basalt to the trailing edges of the diverging plates. Partial melting of a rising, mantle-peridotite plume supplies the basaltic magma. Professor Harry Hess of Princeton University laid out the basic idea early in the 1960s. New topographic data on the Pacific Ocean floor, acquired while serving as an officer on a U. S. destroyer during World War II, perked his interest in the vast, geologically unknown, oceanic regions. In the decades before and after the war, many English-speaking geologists, especially those in the Northern Hemisphere, supported the then commonly held idea that the continents were static; thus by inference, the ocean basins had to be passive features and very old geologically. Hess adapted his new, seafloor spreading theory to a constant-size Earth in contrast to S. W. Carey, a contemporary geologist from Tasmania, who framed his visionary, global tectonic theories in terms of an expanding Earth.
A few years later, Fred Vine and D. H. Matthews related the relatively simple, magnetic patterns observed in seafloor basalts to results obtained from studies of Earth's paleomagnetic history, including geomagnetic reversals. Their observations and numerous, subsequent studies strongly supported Hess's seafloor spreading concept.
9. Describe how Fred Vine and D. H. Matthews related the seafloor spreading hypothesis to magnetic reversals.
By the early 1970s, age-dating and paleomagnetic studies of basaltic lavas had produced a detailed chronology of Earth's magnetic field from the late Tertiary to the present and had conclusively documented several intervals of reversed polarity. Vine and Matthews recognized two fundamental characteristics of the striplike, seafloor magnetic patterns: they were symmetrically arranged about mid-oceanic ridges (Figs. 19.13, 19.14, 19.15 & 19.16); and the ages, lateral positions with respect to the ridge axis, and remnant magnetic polarities of the seafloor basalts conformed to the polarities and time intervals evident in the newly developed, paleomagnetic time scale (Fig. 19.14). They concluded that the magnetic patterns were acquired when the basalts were erupted along the axis of the mid-ocean ridge. As new magma was erupted, the older basalts split, forming roughly equal-sized strips attached to the trailing edges of diverging, lithospheric plates. These strips retained the rock's original magnetic polarity, accounting for the rough, lateral symmetry observed in the magnetic patterns on both sides of the ridge axis.
10. On what basis were plate boundaries first established?
Volcanic arcs and deep ocean trenches (Figs. 19.23 & 19.27) were found to be associated when the trenches were discovered by oceanographic studies. These arc-trench zones are the loci for great earthquakes and for deep-focus quakes that occur nowhere else. Once the spreading center (mid-ocean ridge) idea was proposed, it became fairly easy to visualize the volcanic arc and trench system as the zone where the oceanic plate is sinking into the mantle. Lithospheric plate boundaries are nicely outlined by earthquake epicenters (Figs. 19.17 & 19.27).
11. Where is lithosphere being formed? Consumed? Why must the production and destruction of the lithosphere be going on at about the same rate?
New, oceanic lithosphere is formed at the mid-ocean ridges, and an equivalent area of old lithosphere sinks into the mantle along a subduction zone. Most geologists believe that Earth's radius and the area of lithosphere have been constant over geologic time; thus the production and subduction rates of lithosphere have to be roughly equal over time.
12. Why is the oceanic portion of a lithospheric plate subducted while the continental portion is not?
In general, continental lithosphere is not dense enough to sink into the mantle and be subducted. However, small pieces or chips of continents may be subducted if they are carried down with unusually old, dense, oceanic lithosphere.
13. In what ways may the origin of the Japanese Islands be considered similar to the formation of the Andes Mountains? How do they differ?
The Japanese Islands, like the Andes, owe their history of active volcanism and tectonism to their location on a continental margin, above an oceanic plate that is sinking into a subduction zone. The Andes are built on fairly old, thick, continental crust along the western edge of South America; the Japanese Islands are built on much thinner, younger, continental crust along the eastern edge of Eurasia. In general, thicker crust is more buoyant than thinner crust, thus accounting for the higher elevations of the Andean zone.
14. Differentiate between transform faults and the two other types of plate boundaries.
Transform boundaries are long, vertical, deep faults along which two plates move in opposite directions horizontally and parallel to the boundary. Plate motions at divergent and convergent plate boundaries have their major components of motion perpendicular to the boundary. Volcanism is prominent along convergent and divergent plate boundaries but is generally absent along transform fault boundaries.
15. Some people predict that California will sink into the ocean. Is this idea consistent with the concept of plate tectonics?
No! The sliver of California west of the San Andreas fault is slowly moving northwestward with the Pacific plate; movement is essentially horizontal. Far in the geologic future, the sliver may eventually arrive in Alaska or the Aleutian Islands.
16. Applying the idea that hot spots remain fixed, in what direction was the Pacific plate moving while the Emperor Seamounts were being produced? (Figure 19.30) While the Hawaiian Seamounts were being produced?
The Emperor Seamounts are aligned roughly north-south and are older to the north, proving that when they formed, the Pacific plate was moving northward with respect to a deep, stationary, hot spot. The Hawaiian Islands are oriented west-northwest and east-southeast and are youngest to the southeast; thus the plate was moving WNW over the same hot spot when the Hawaiian Seamounts were growing. Loihi (Box 4.3), a rapidly growing seamount off the southern coast of Hawaii, is the newest active volcano to develop over the hot spot. Its location supports continuing northwest movement of the Pacific plate.
17. With what type of plate boundary are the following places or features associated (be as specific as possible): Himalayas, Aleutian Islands, Red Sea, Andes Mountains, San Andreas fault, Iceland, Japan, Mount St. Helens?
Himalayas - These have formed along a convergent, continent-continent, collisional boundary. The Indian subcontinent (riding on the Australian-Indian plate) is sliding at a low angle beneath the southern edge of the Eurasian continent on the Eurasian plate.
Aleutian Islands - These islands are the oceanward part of a volcanic island arc situated on the northwestern margin of the North American plate; the volcanoes lie above the subducting Pacific plate.
Red Sea - The Red Sea occupies a major rift zone and very young, seafloor spreading center that has opened between Africa and the Arabian block.
Andes Mountains - The Andes are a volcanic and plutonic arc resting on the western margin of the South American continent and plate; they lie above subducting, oceanic lithosphere of the Nazca and Antarctic plates.
San Andreas fault - This is a transform fault that forms the boundary between the North American and Pacific plates. The crustal sliver composed of westernmost California and the Baja California peninsula on the eastern edge of the Pacific plate is moving northwestward with respect to North America.
Iceland - Iceland and nearby, smaller islands comprise a major zone of basaltic volcanism that probably overlies a mantle hot spot located directly beneath the Mid-Atlantic Ridge, the divergent boundary between the Eurasian and North American plates.
Japan - The Japanese Islands lie on the eastern margin of the Eurasian plate, above subducting parts of the Pacific and Philippine oceanic plates.
Mount St. Helens - This is a very young stratovolcano in the state of Washington; it is part of the Cascade Range, a continental-margin, volcanic arc extending from the Canadian border to northern California. The volcanoes are situated above a small, subducting fragment of oceanic lithosphere called the Juan de Fuca plate (Fig. 19.26).