Plate Tectonics KEY WORDS: continental drift, seafloor spreading, plate tectonics, mid ocean ridge (MOR) system, spreading center, rise, divergent plate boundary, subduction zone, convergent plate boundary, suture zone, deep ocean trench, conservative boundary, transform fault, strike slip fault, fracture zone, earthquakes, oceanic crust, continental crust, upwelling, convection. Introduction Geologists have hypothesized that the outer 60-100 km of the Earth are composed of approximately seven principal slabs of rigid lithospheric (rock) material. These slabs, known as plates, account for most of the dynamic features of the crust of the Earth. This includes earthquakes, mountain chain building, and volcanic eruptions. The hypotheses developed to explain the presence of and interactions between these plates have been widely tested and are generally accepted today. They were, however, quite controversial when they were first proposed some thirty-five years ago. The concepts behind these hypotheses are now collectively known as plate tectonics and have been advanced from hypotheses to the level of a theory. The theory of plate tectonics evolved from the hypothesis of continental drift, proposed by Alfred Wegener in the early part of the 20 th century. Wegener hypothesized that continental masses plow through oceanic rock. Because oceanic rock is more rigid than continental rock, and because he failed to provide an acceptable mechanism to account for how continents could have drifted great distances (not to mention the fact that Wegener was a meteorologist) his ideas were widely rejected. In the subsequently developed hypotheses of seafloor spreading and plate tectonics, however, there was no problem of continents plowing through oceanic rock. Instead, rigid plates, including those with continents, simply move about relative to one another. Much evidence acquired in the early 1960 s and subsequently provided strong support for the new hypothesis, and plate tectonics enjoyed wide-ranging support from earth scientists. It is now regarded as a theory. The boundaries between lithospheric plates are zones of considerable deformation caused by the motions of the plates. These zones are characterized by abundant earthquakes. Boundaries between crustal plates There are three basic types of plate boundaries: 1. DIVERGENT. The first type of plate boundary is found on the mid ocean ridge system (e.g., the Mid Atlantic Ridge, the East Pacific Rise), where lava of basaltic composition upwells from mantle and flows out onto the ocean floor where it cools and solidifies to create new oceanic crust. The young crust is then pulled apart as additional lava comes up and newer crust is formed along the center of the ridge. As a result of this process, oceanic crust moves outward (horizontally) on both sides of an oceanic ridge, and new crust is continually added to older oceanic crust. This type of plate boundary is known as 1
a divergent plate boundary, ridge, or rise where the dominant force is tension (i.e., PULLING apart). 2. CONVERGENT. Creation of crust along ridges must be balanced by consumption of crust elsewhere. Many plates are bounded by zones where rigid crust descends deep into the mantle and the crust is ultimately reabsorbed. These plate boundaries are known as subduction zones. The dominant force is compression (i.e., PUSHING together). There are three sub-types of convergent boundaries: a) where dense oceanic crust converges with less dense continental crust. The greater density oceanic crust descends below the continental crust. b) where oceanic crust collides with oceanic crust from another plate. The older (cooler and denser) plate descends and the younger (warmer and less dense) plate overrides. c) where two continental slabs encounter each other, a more complex convergent plate boundary occurs. The interaction between these slabs causes intense deformation and mountain building, leading to the formation of a suture zone. 3. CONSERVATIVE. The third type of plate boundary occurs where compensating motion takes place without creating or consuming crust and plates slide past each other. These conservative plate boundaries are characterized by long faults between the adjoining plates that are known as transform faults (i.e., one feature is transformed into another). They occur where new crust zones (i.e., ridges or rises) abruptly end. These plate boundaries are called transform plate boundaries. The physiographic feature formed by the transform fault and their extensions beyond the region connecting two sections of ridge are called fracture zones. Note that the extensions of the fracture zones beyond the active ridge segments DO NOT separate different plates but are embedded within a single plate. Major lithospheric plates of the Earth There are seven major lithospheric plates on the rigid outer surface of the Earth (Fig. 1). Several smaller plates, as well as regions of extremely complex tectonic morphology exist and complete the puzzle of the Earth s plate tectonic structure. The latter will not be considered in this laboratory exercise. In the attached diagram of the Earth s plates and that showing the location of major earthquake epicenters (Fig. 2), note that the zones of earthquakes coincide with plate boundaries. There is, however, a major distinction between the various zones in terms of the depth of occurrence of the earthquakes. Shallow earthquakes occur at the MOR and along transform faults, whereas deeper earthquakes occur at subduction zones, with the deepest ones located at substantial distances beyond where the descending slab begins to be subducted. For example, the deepest earthquakes associated with the Peru-Chile trench occur well eastward of the Andes mountains of South America (see Fig. 2). 2
Exercises During this exercise you will use a map of the ocean floor to carry out a series of observations and calculations about plate motions. The observations will help you recognize and familiarize yourself with the primary physiographic features associated with plate tectonics (i.e., plate boundaries). The calculations will illustrate some of the concepts of seafloor spreading and plate tectonics and will demonstrate how application of basic principles using very simple approximations allows you to draw relatively accurate conclusions. Certain island chains, those created by hot spots, represent an aid to understanding plate motions as you will discover in the exercises below. 1. Refer to Figures 1 and 2 and carry out the following: Draw a dark (or red) line along all plate boundaries that show evidence of subduction (convergence). These can be identified by the existence of deep earthquakes. The remaining plate boundaries in Figure 1 are either divergent plate boundaries or conservative (transform) plate boundaries. Keep in mind that plate motion is towards subduction zones, and away from (perpendicular to) ridges. Noting where you just marked the location of subduction zones, use a different color pen to mark small double arrows along the MOR system to show the relative spreading direction. Transform faults are zones where there is neither tension nor compression. These are strike-slip faults and must trend parallel to the directions of plate motion. Using this as a guide, draw blue lines along transform faults. Now that you have outlined the three types of boundaries along the major lithospheric plates, you are prepared to consider the actual motions of these plates. Because it is possible that all plates are moving relative to a stationary point on Earth, there is a potential for great complexity in the absolute plate motions. It is, however, much simpler to consider that one plate is fixed and that all the others move relative to it. For the sake of this exercise let us assume that the American plate is stationary. Now draw a single large arrow on each plate to show its motion RELATIVE to the (stationary) American Plate. Fill out Table 1. 3
Figure 1: Major Lithospheric Plates of the World. Figure 2: Locations of epicenters of eathquakes throughout the world 4
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Table 1. Classification of Geologic Processes Geologic Process Creation of oceanic crust Volcanism Creation of fold-fault mountain ranges Compression Tension Shear Type of plate boundary Divergent (ridges Convergent or rises; zones of (subduction zones; seafloor spreading) colliding plates) Conservative (transform faults) 2. In this exercise you will calculate the age of various portions of the seafloor (and features thereupon) based on their current location relative to the Mid Ocean Ridge (MOR) system (i.e., their distance from the MOR) and the appropriate present-day MOR spreading rate. The following spreading rates are given. Note that these are half spreading rates, i.e., the rate at which one side of the MOR spreads. The full spreading rate is, of course, twice these values because, as new seafloor is formed, it spreads on both sides of the MOR. East Pacific Rise 8 cm/yr Mid Atlantic Ridge 1 cm/yr Indian Ocean Ridge 2.5 cm/yr Assuming that the Hawaiian Islands have been carried along with the motion of the Pacific Plate, calculate the age difference between the (Big) island of Hawaii and the bend in the Hawaiian Archipelago-Emperor Seamount chain. Estimate the age of the seafloor at Cape Hatteras, North Carolina. Knowing that the Himalaya represent a suture zone formed by the collision of India with Asia, estimate when India first began colliding with Asia using the spreading rate of the Indian Ocean Ridge. Assuming you have lived in the same house all your life and you have not changed the configuration of your bedroom since birth (unlikely, but assume this is the case) calculate how far your bed has moved since you were born. 3. Compare the ages obtained above with radiometric age given below and discuss the 6
agreement or disagreement with your calculations. If there is a substantial disagreement between your calculation and the data provided, speculate what could have caused the discrepancy in age. Ages of various features in the World Ocean: Hawaiian Archipelago-Emperor Seamount chain bend: 42 Ma Initial collision of India with Asia: 53 Ma Opening of North Atlantic Ocean: 165 Ma 4. You have noted in exercise two above that the Hawaiian-Emperor trend has a break in trend slightly to the west of Midway Island. What could have caused this change in orientation of the islands and seamounts? 7