Introduction to Plate Tectonics Courtesy: UCLA, ESS The concept of plate tectonics provides a holistic view of how the Earth works. According to the theory, the outermost division of the Earth consists of cool, rigid lithospheric plates that are in constant motion, being driven by the internal heat of the Earth. These plates are about 100 km thick and float on a hotter, more plastic region of the upper mantle called the asthenosphere (you learned about this in the last lab). New lithospheric material is created at the Oceanic Ridge and Rise System. These are mountainous features on the seafloor where plates move apart (diverge). As the sea floor spreads apart, basaltic magma derived from partial melting of the asthenosphere rises to the surface, solidifies, and becomes new crust at the edge of the lithospheric plates. As a result, the oceanic ridge system typically has high heat flow, volcanic activity and shallow earthquakes (shallow because the rising asthenosphere and magma bring heat close to the surface, allowing rocks below the crust to flow instead of breaking). New lithospheric material created at ridges is eventually destroyed elsewhere by the process of subduction. In regions where plates converge, one lithospheric plate is forced down into the mantle beneath the other plate. These zones of subduction are visible on the seafloor as deep ocean trenches. They are seismically active areas characterized by shallow-, intermediate-, and deep-focus earthquakes (deep earthquakes are possible here because of the cold, brittle lithosphere being pushed deep into the mantle). One consequence of subduction is the generation of magmatic arcs, which are chains of volcanoes that lie parallel to trenches and above subducted slabs of lithosphere. Partial melting of the mantle over subducting lithosphere produces magmas that rise to the surface through fractures in the overriding plate. Compositionally, magmas produced at subduction zones include andesites (diorites) and rhyolites (granites), which are more siliceous in composition (i.e., higher in SiO 2 ) than the basalts produced at mid-oceanic ridges (For an overview see Figure 1). Heat flow is low in trenches, but high at the adjacent magmatic arcs. Fig. 1
Thus far we have described two different types of plate interactions: divergence at mid-ocean ridges, and convergence at ocean trenches. There is a third type of boundary, the transform fault, which occurs where two plates are moving in parallel but opposing directions; in other words, the plates slide past one another. In some cases, transform-fault boundaries are found on continents (such as the San Andreas fault in California, and the Alpine fault system in New Zealand). In such cases, two blocks of continental crust are sliding past one another. These areas are characterized by intermittent shallow seismic activity (which we have all experienced living in Los Angeles!). More commonly, however, transform faults (and associated "fracture zones") are observed on the seafloor as offsets in the axis of ocean ridges (Figure 2). Heat flow is low at transform margins. Outline for Types of Lithospheric Plate Margins I. Divergent -- Plate boundary where two plates are moving away from each other; magmas derived from the asthenosphere rise upward to fill the gap between the diverging lithospheric plates. A. Oceanic Ridge and Rise System (aka Mid-ocean Ridges) -- Linear chains of submerged volcanic mountains and associated central "rift valleys" where basaltic magmas rise to the surface and are extruded as lavas onto the ocean floor. This example represents an advanced stage of rifting and is characterized
by relatively slow rates of spreading (1 to 8 cm/yr, half-rate). Examples include the Mid-Atlantic Ridge and East Pacific Rise. B. Young or 'incipient' ocean basins -- Similar in appearance to mid-ocean ridges but much more limited in extent. These areas represent intermediate stages of continental rifting and ocean-basin formation. Examples include the Gulf of California and Red Sea (See Figure 3). C. On-land rifting -- Continental areas characterized by abnormally high heat flow, extensional (pull apart) faulting and volcanic activity. These areas are thought to represent the initial stages of rifting whereby continents 'drift' apart. A new ocean basin is created between adjacent land masses as they separate. Examples include the East African Rift Valley, the Rhine Valley Graben and the Rio Grande Rift (New Mexico, Colorado). Figure 3. Evolution of an ocean basin from initial rifting to maturity. A and B are similar to the current East African Rift. C is like the Red Sea. D is like the Atlantic Ocean. II. Convergent -- Plate boundary where two plates are moving toward each other, usually one plate is subducted beneath the other. Convergence can be head-on, or oblique. A. Oceanic plate-oceanic plate convergence - Convergence characterized by the subduction of one oceanic plate beneath another oceanic plate, forming deep ocean trenches and their associated volcanic island arcs. Examples include the trench-
island arc systems of the Aleutians, Puerto Rico-lesser Antilles, and Tonga. In oceanic convergence, the more dense (older, and therefore colder) plate is subducted, with the less dense (younger, warmer) crust forming the overriding plate. B. Oceanic plate - continental plate convergence - Characterized by subduction of high-density oceanic plate beneath a lower-density continental plate, leading to the formation of an ocean trench adjacent to the continent with a chain of volcanic mountains on the continent. Examples include the Andes (South America), the Cascade Range of Oregon and Washington, and Peninsular Ranges of Central America. C. Continental plate-continental plate convergence - In this case, neither of the low density continental plates can be subducted into the relatively dense mantle. The intense compression of pre-existing continental rocks forms linear belts of folded mountains along the boundary (suture zone) where convergence occurs. In some areas, pieces of ocean floor are caught up between colliding continents. Examples of mountains formed by continental collisions include the Zagros (active; Iran), the Himalayas (active; Nepal), the Alps (older; Italy/Switzerland), the Urals (ancient; Russia, Kazakistan), and the Appalachians (ancient). Figure 4. Three types of convergent plate boundaries. III. Transform - Margins where two plates neither converge or diverge, but slide past each other in parallel, but opposing directions. When scientists first mapped the mid-ocean ridge in the Atlantic Ocean, it seemed like a fairly regular mountain range except for being submerged below sea level. Then in
1956, Marie Tharp, a geologist at the Lamont Geological Observatory, discovered that the Mid-Atlantic Ridge was split down the middle by a central rift valley (Fig. 2A). Further mapping showed an even more curious thing - the ridge isn't a continuous line of mountains. At many places, the ridge line is broken by offsets, along linear zones of fracturing that extend for long distances away from the ridge axis at right angles to the central rift valley. In 1965, J. Tuzo Wilson, a Canadian geophysicist, combined information on earthquakes and seafloor spreading and showed that active faulting along ridges was likely to occur in only a portion of the fracture zone: the portion lying between offset ridge segments. This part of a fracture zone is called a ridge transform fault (Fig. 2B). A. Ridge transforms - faults oriented perpendicular to the axes of mid-ocean ridges, offsetting the ridge axes. The transform fault between offset ridges is seismicallyactive. Seismically inactive fracture zones extend away from the ridge segments. Ridge transforms are associated with shallow-focus seismic activity (Fig. 2B). B. On-land transforms - Transform faults that develop where two continental blocks slide past each other. An example of this is the San Andreas fault in California (Figure 4).
IV. Intraplate Regions and Hotspots It should be apparent from the preceding discussion that a very strong correlation exists between the locations of plate boundaries and tectonic activity such as volcanism. Volcanic and earthquake activity are not typical within the interiors of plates (intraplate regions). However an important exception to this where large pulses, or plumes, of basaltic magmas rise up from deep mantle sources. These mantle plumes, or hotspots, can produce a stationary source of volcanism for millions of years. Such a hotspot is responsible for the Hawaiian Island chain of volcanoes and the Emperor Seamount chain in the North Pacific. These chains of volcanic islands and seamounts form as the lithosphere passes over the stationary volcanic source at the hotspot (Figure 5). Several island-seamount chains can be found in all ocean basins. The large (4x5 foot) tectonics maps in the lab have nice illustrations of the Hawaiian hot spot, located on the back of the map. A small bathymetry map of the Hawaiian Island Chain is shown in Figure 6, below. Figure 6 Bathymetry of the Hawaiian Islands and Emperor Seamounts in the central Pacific Ocean. Light shades of gray indicate shallow seafloor. (USGS base map)