CHAPTER 7: PLATE TECTONICS

1. CONTINENTAL DRIFT: AN IDEA BEFORE ITS TIME: Alfred Wegener was the first serious advocate of continental drift. He coined the term, developed a timetable for drift events, and amassed much of the evidence for it. However, continental drift was thought to be mechanically impossible by many, while others felt that it caused more problems than it solved.
   1. Evidence: The Continental Jigsaw Puzzle: The first person to notice the similarity of the coastlines of western Africa and eastern South America was probably a 16th century mapmaker, or perhaps a geography student of that period. Actually, it is the submerged edges of the continental shelves that would match, and allowances must be made for erosion and deposition after drifting. In Wegener's time all of this was dismissed as coincidence. In 1965, Bullard's computer-generated fit of the continents into the supercontinent of Pangaea captured the attention of many. Today, commercially available microcomputer software allows everyone to try it for themselves.
   2. Evidence: Fossils Match Across the Seas: Wegener offered continental drift as an alternative to land bridges, which seemed necessary to explain how life forms unable to float or swim across an ocean could nevertheless exist on different continents. Wegener's case might have been helped greatly in his time by later discoveries in Antarctica in the 1950's and 1960's, of many of the same flora and fauna he knew from South America, Africa, India, and Australia. Even the present-day distribution of life forms reflects making and breaking of connections by drifting.
   3. Evidence: Rock Types and Structures Match: If the Atlantic Ocean is closed up in the Pangaea configuration, the Appalachian mountain belt of North America lines up with mountain belts in Greenland and Europe, the Ouachita belt of the U.S. lines up with another mountain belt of Europe, and mountain chains in South America line up with trends in Africa. Many other similarities of age and rock type noted by Wegener have been expanded on by later work.
   4. Evidence: Ancient Climates: Evidence of a major continental glaciation about 220 million years ago is found in areas of South America, Africa, Australia, India and Antarctica. However, all of these areas except Antarctica are now too close to the equator and too far apart from each other for a single ice sheet to be feasible. Patterns of ice movement and glacial deposits make much better sense if these areas were fitted together 220 to 300 million years ago.
   
   
2. THE GREAT DEBATE: Controversy raged over Wegener's ideas during the 1920's. In the minds of most earth scientists, the theory of continental drift was declared dead at about the same time that Wegener himself died on an expedition to Greenland in 1930. The mechanisms for drift that Wegener proposed were demonstrated to be inadequate: the mantle and lower crust were considered to be too strong and stiff to allow continental masses to slide through or over them. This weaknesses of the mechanisms Wegener proposed for drift were perhaps so obvious that it their weakness made it easier to dismiss the many lines of evidence. Only a few prominent geologists, most of them outside North America, remained steady as advocates of continental drift from about 1930 to about 1960.
   
   
3. PLATE TECTONICS: A MODERN VERSION OF AN OLD IDEA
   1. Improvements in radiometric dating of rocks and in paleomagnetic measurements led to more definitive results that were difficult or impossible to explain if continents had not shifted relative to each other. Ages of rocks on the sea floor follow a pattern related to the seismic activity and other features. Also, the sea floor is relatively young: no sea-floor rocks older than about 1/25 the age of the earth have yet been found.
   2. It became apparent that seismic, volcanic and tectonic activity on the earth tend to be concentrated in narrow belts of crustal instability. Seismic studies revealed that the motions in these belts fit a global pattern, easily explained by what is now called plate tectonics, in which the earth's crust exists as a group of large, rigid tectonic plates with geologically stable interiors, and most of the geologic instability occurring along their boundaries, where they interact with adjacent plates. The plates (lithosphere, made up of crust and topmost stiff mantle) are able to move over a weak, plastic layer (the asthenosphere) in the upper mantle. In Wegener's time and before, this layer had been theorized but not proven. Also, our measurements show the rates of plate movements (about as fast as one's fingernails grow) to be at most about 1/10 to 1/100 of what Wegener had proposed.
   
   
4. PLATE BOUNDARIES: There are three basic ways in which plates can interact: they can diverge, converge, or simply slide past each other. Each type of boundary is characterized by distinctive seismic activity and other geological processes.
   1. Divergent boundaries are where plates move apart, and new crust forms from rising magma. The crests of the mid-ocean ridges are divergent boundaries. Sea-floor spreading is a descriptive term for what goes on in these areas. Radiometric dates showed that rocks at a divergent boundary give young, even recent ages and that progressively older ages occur with increasing distance away from the boundary. The Red Sea (in picture at beginning of chapter) is on a young divergent boundary, and the rift valleys of Africa (also the upper valley of the Rio Grande) are even younger divergent plate boundaries. The margins of continents around the Atlantic Ocean were once in such a configuration, then began to move apart about 165 million years ago. The seismic activity at divergent boundaries consists of shallow-focus earthquakes: the quakes are numerous but the stress environment is tensional rather than compressional.
   2. Convergent boundaries exist where plates are moved together, and lithosphere is in effect destroyed or recycled back into the upper mantle. Because the earth's size and surface area are constant, the average rate of lithosphere destruction must balance with the rate of creation at divergent boundaries. The fault system along which the destroyed slab of lithosphere moves downward is a subduction zone, its surface position marked on the sea floor by a deep-ocean trench. Many of the world's worst seismic-hazard areas (Japan, Alaska, Central America, west coast of South America, northern India, for example) are located over convergent plate boundaries. The seismic activity includes strong earthquakes ranging from shallow-focus to deep-focus (as deep as 700 km). There are three basic types of convergent plate boundary, distinguished by the types of crust involved on either side of the convergence zone:
      1. Oceanic-continental convergence goes on at what are sometimes called Andean-type boundaries, after what is probably the best current example. Oceanic crust, being denser than continental crust, makes up the downgoing slab. A complex mountain range of highly deformed rocks typically develops on the edge of the continent. Also typical is development of a continental volcanic arc fed by material melted from the downgoing slab.
      2. Oceanic-oceanic convergence commonly results in the formation of a volcanic island arc, such as the Aleutian Island chain. Some continental margins contain probable remnants of old arcs that were later pushed against the continent.
      3. Continental-continental convergence produces what are also known as Himalayan-type boundaries, again after the best current example. Extremely thickened continental crust may result in a high mountain range. The Appalachians of the eastern U.S., together with ranges on the other side of the Atlantic Ocean, were formed as a result of collisions of this type, during the formation of Pangaea.
   3. Transform Fault Boundaries are zones along which crust is neither created nor destroyed. Overall, they are lateral faults that allow relief of stress produced by variations in spreading and subduction rates on a round earth. Most transform faults are confined to the sea floor, connecting segments of spreading ridge. Those that cut across continental land areas tend to be notoriously active, such as the well-known San Andreas fault of California.
   
   
5. TESTING THE PLATE TECTONICS MODEL
   1. Evidence: Paleomagnetism: Studies done while Wegener was still alive suggested that when a rock forms, the earth's magnetic field can be copied by iron-bearing magnetic minerals such as magnetite. Basalt, which is abundant on the sea floor, typically enough magnetite to make a good record. During cooling of the basalt from lava, grains of magnetite crystallize, then become magnetized as their temperature drops below the Curie point (about 580°C for magnetite). If the rock is later heated above the Curie point it can be remagnetized. In general, iron-rich igneous rocks such as basalt are the best for paleomagnetic studies, but iron-rich sedimentary rocks can also be used because magnetic mineral grains in them can line up with the prevailing magnetic field when the grains settle during deposition.
      1. Polar wandering: Wegener was aware of early paleomagnetic studies, which suggested that the Earth's magnetic poles had been in different places in the geologic past, and gradually moved (by polar wandering) to their present positions. Later, improved analytical methods and accumulation of more data showed that the apparent polar wandering paths for different continents disagreed with one another, as if each continent had its own magnetic pole. It then became evident that a better explanation was for the continents, not the magnetic pole, to move over the earth, and that each continent apparently moved in a different pattern.
      2. Magnetic reversals and seafloor spreading: Earth's magnetic field periodically reverses itself, and this too can be recorded in iron-bearing rocks. This phenomenon was discovered in the 1920's but poorly understood, and data were scarce. Studies in the 1950's and later revealed a highly consistent, stripe-like pattern of magnetism on the sea floor, centered on the crests of the mid-ocean ridges. Strips of normal-polarity rock (pole aligned with the present magnetic field) alternate with strips of reverse-polarity rock. At about the same time it became evident that ages of sea floor were consistently young at the ridge crests and progressively older to either side with distance from the crests. When Vine and Matthews published an explanation of the magnetic stripes and their connection to the age pattern of the sea floor, the debate over continental drift was emerging from a long dormant period.
   2. Evidence: Earthquake Patterns: Improved equipment and methods allowed seismologists to locate earthquake foci more accurately, and to estimate directions of fault movement from seismograms rather than having to visit the site of the quake. Thus they were able to define plate boundaries, and the relative motions at those boundaries. The results made a good fit with sea-floor spreading, subduction and transform-faulting as defined from other lines of evidence.
   3. Evidence: Ocean Drilling: The Deep Sea Drilling Project (DSDP) used specially-designed drillship, the Glomar Challenger, to collect core samples of sea floor deposits and other types of data. The most striking individual results of DSDP in regard to plate tectonics and continental drift are: 1) the age and thickness pattern of sea-floor sediments, young and thin at the crests of the ridges, progressively older and thicker with distance from the ridge, and absence of sea-floor rocks older than about 180 m.y., slightly after the time when Pangaea began to break up, and about 1/25 the age of the earth.
   4. Evidence: Hot Spots: These are points in the mantle, not necessarily associated with a spreading ridge, where magma is generated at a high rate. They can cause localized igneous activity in the interior of a tectonic plate. As far as we know, hot spots stay fixed at the same place in the mantle, or at least move only very slowly compared to the plates. Thus hot spots leave a distinct trail of igneous activity on the plate moving over them, which is useful in unraveling plate motions. The Hawaiian Islands are the exposed part of a hot-spot trail that extends to the Aleutian trench. Yellowstone National Park marks the site of another hot spot, which has left a track running across Washington state and up the Snake River valley on its way to the park site.
   
   
6. PANGAEA: BEFORE AND AFTER
   1. Breakup of Pangaea: Wegener's names for the major supercontinental masses are still used:
      1. Pangaea ("all-earth") existed about 200 m.y. ago. Wegener had no idea what the configuration of the continents was like before then, but work in the last 20 years has yielded a reasonably good history of much older plate movements.
      2. Gondwanaland (or just Gondwana, a word that itself means "land") was the southern portion of Pangaea, consisting of South America, Africa, Antarctica, Australia, and the subcontinent of India.
      3. Laurasia, the northern portion of Pangaea, consisted of North America (Laurentia, with Greenland as part of it), Europe and Asia.
      4. The sequence and timing of the events in the breakup of Pangaea are briefly summarized in the Dietz/Holden reconstructions shown in the text (Figs. 7-26A through 7-26E, pp. 206-207).
   2. Before Pangaea: The lack of ocean floor older than Pangaea makes study of earlier drift more difficult, but evidence left on the continents can still be used. Old mountain belts, such as the inner parts of the Appalachians in the U.S. and the Urals in the Russia, are records of ancient continental collisions and other drifting events. Still older mountain belts exist deeper in continental interiors, and record earlier events.
   3. A look into the Future: Projection of current plate motions allows us to speculate on how the continents may be arranged in the future. However, the prediction shown in the text is based on the assumption that plate velocities will stay the same for the next 50 million years or so, and we have no guarantee that they will.
   
   
7. THE DRIVING MECHANISM: Lack of a suitable mechanism was the major point of objection to Wegener's original theory of continental drift. Today, the weight of the favorable evidence prods us to seek a mechanism instead of rejecting the theory. If the debate were managed like a murder trial, the prosecution would be able to document "means" and "opportunity" quite well, but would have only relatively weak, circumstantial evidence for a "motive." Some of the best current candidates for a mechanism are given below. All of these have been developed with the aid of mechanical models and mathematical analyses to the extent that they are plausible, but as yet the Earth has yielded little decisive evidence that any of them is correct.
   1. The best-known mechanism , the convection current hypothesis, involves thermal convection cells below the lithosphere, with hot material rising under the spreading ridges along with magma, the cold limbs sinking at trenches and promoting subduction, and the lateral motions of the cells dragging lithosphere plates over the surface. In the somewhat similar hot plumes hypothesis, convection is strongest near what amount to powerful hot spots. By making assumptions about the nature of rocks in the mantle, convection cells can be mathematically proven to exist. The problem has been to catch real mantle-rock in the act of convection.
   2. Another possibility is "slab-push", or the hydraulic wedging apart of lithosphere by forcible injection of magma at ridge crests, plus gravity-induced sliding of lithosphere down the slopes of the mid-ocean ridge. The existence of such forces can hardly be doubted, but their effectiveness is debated.
   3. "Slab-pull" from the sinking edge of a subducted plate also probably exists but its effectiveness also is uncertain.

Last revised June 3, 2001 by MAJordan