Chapter 3 Plate Tectonics PDF

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Chapter 3: Plate Tectonics 1.0 Introduction Earth is different from other planets in our solar system because of the unique processes grouped under the term “plate tectonics”. Our whole understanding of the processes that affect Earth suddenly and dramatically changed when we finally realized the significance of the theory of plate tectonics; it was as important to earth scientists as the theory of evolution was to the understanding of processes by biologists. In both cases, what had at first seemed like a diverse set of almost random observations could suddenly be explained by a relatively simple, coherent theory. For geological engineers, plate tectonics is important because it explains, in large part, why and where significant deformation of Earth’s surface occurs (thus where not to build bridges, for instance). To economic earth scientists, it explains the type and location of many metallic mineral deposits. It explains the global distribution of earthquake and volcanic hazards, although the timing and magnitude of natural disasters is a bit trickier! Continents, ocean basins, and mountain ranges – all are produced by plate tectonics. 2.0 Early Development of the Theory 2.1 Continental Drift The theory of plate tectonics was assembled by a number of scientists working over many years. The first steps toward formulation of a theory were made by Alfred Wegener; he grouped his ideas under the heading ‘continental drift’, and published them in 1912. He (like many others) was struck by the ‘jigsaw fit’ of continents across the Atlantic Ocean (Fig.1). He figured the fit was so good, coincidence was impossible. Add to that the wonderful fit of glacial terrain of continents in the southern hemisphere when those land masses were fitted together (Fig.2), the continuity of old geological structures and fossil stratigraphy from continent to continent (Fig.3), and Wegener had some pretty strong ideas to sell. By the way, the ‘super-continent’ that Wegener was picturing in his jigsaw fit, he called “Pangaea”; Figures 1, 2 and 3 show parts of it. We’ll refer to it many times later in the course, and will see better illustrations then and there.

Like most ‘first steps’, Wegener’s was a bit shaky: he had absolutely zero idea of the mechanism required to make continents drift. Fig 1: Wegener's theory of continental drift was based, in part, on the “jigsaw fit” of the continents. Modified from the University of Texas Institute for Geophysics (n.d.), PLATES Project. Retrieved from http://www.ig.utexas.edu/research/projects/plates/images.htm.

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Fig 2: Continental glacial deposits in the Southern hemisphere provide support for continental drift. If tectonic plates did not move that would mean that Paleozoic glaciers grew much closer to the equator and over the oceans. Modified from UC Irvine (n.d.), Continental Drift. Retrieved from http://ocw.uci.edu/cat/oo/getOC WPage.php?course=OC0811004&lesson=003&topic=001&page=9.

Fig 3: Correlation of plant and animal fossils across continents. Note the patterns that form when the continents are rejoined. By the USGS (1999), Historical Perspective. Retrieved from http://pubs.usgs.gov/gip/dynamic/ continents.html.

Nothing exceptional happened toward development of the plate tectonics theory until after the Second World War. In the early 1940s, during the war, it became thoroughly obvious there was a need to detect both sunken ships and lurking submarines, so very sensitive magnetometers (the instruments that measure magnetic field strength) were deployed in ‘pods’

which could be towed behind ships (Fig.4) back and forth across particular sections of ocean. [They were intended to serve as glorified metal detectors that were particularly sensitive to the magnetic fields associated with anomalies of hunks of steel, and to the powerful generators needed to run submarines and ships.] Scientists were pretty amazed to find they could detect curious, repetitive magnetic records in ocean floor rocks (as well as sunken ships and submarines), but there was little opportunity to process these data until the war was over.

Following the war, two lines of research (one to hunt for natural mineral resources, and the second simply to map the geographic and geologic features of ocean floors) began to produce the first comprehensive maps of the ocean floors (Fig.5). This work began in the late 1940s and through the 1950s. In the Atlantic Ocean, the most amazing discovery was the definition of a huge volcanic ridge that extended right from the far north and connected with other similar ridges in the far south. Not only that, it was discovered that the volcanoes defining the ridge were active for almost its whole length – the largest volcanic system anyone had ever seen (these are the high, dark ridges on ocean floors in Fig. 5)!

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Fig 4 (left): Ship towing a magnetometer. Geophysical surveys of the ocean floor record magnetic changes in the oceanic crust. Modified from USGS (1999), Magnetic stripes and isotopic clocks. Retrieved from https://www.e-education.psu.edu/earth520/content/l2_p11.html.

Fig 5: The topography of the world’s oceans. By Tharp and Heezen (1977), World Ocean Floor Panorama. Retrieved from http://www.marietharp.com/wordpress/?page_id= 28.

In 1960 Harry Hess, a professor at Princeton University, conjectured that these ridges represented spreading centers where Earth’s crust was moving in opposite directions like conveyor belts (Fig.6), allowing new ocean floor to be built from volcanic rock at the ridges. He calculated that, because of this activity, the Atlantic Ocean was widening by about 2.5 cm/year (about the same rate that your fingernails grow). If you took that rate and worked backward, North/South America and Europe/Africa were in contact some 180 million years earlier. Unfortunately, poor old Alfred Wegener was already dead, but his skeleton was probably happy because his idea of continental drift was finally finding support!

Fig 6: A model of seafloor spreading. At the ridge, molten rock erupts onto the seafloor and cools into new oceanic lithosphere. Modified from the SAGUARO

Scientists remembered the magnetic patterns discovered by WWII magnetometers, and wondered if there was some important evidence there to help in development of Hess’s theory. Before we can interpret those data, we need to understand Earth’s magnetic field and the magnetic information stored in rocks, because the patterns discovered by those magnetometers were directly a reflection of Earth’s magnetic field.

project (n.d.), Earth Science Investigations using GIS. Retrieved from http://isenm.org/curriculum/saguaro-project.

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3.0 Paleomagnetism and Earth’s Magnetic Field We’ll see, as this chapter develops, that virtually all sceptics of plate tectonics – and there were many initially – were convinced of the theory after the evidence of paleomagnetism (study of Earth’s magnetic field through the analysis of rock magnetism) was made. Earth has a strong magnetic field: if you hold up a compass, one end of the needle points to magnetic north and the other to magnetic south. In fact, any material that’s magnetic will – if allowed to ‘float’ without restriction – take on the same north/south orientation. The simplest expression of Earth's magnetic field is to make a comparison with a simple bar magnet (Fig. 7). Just as you see the lines of magnetic force about a bar magnetic, so we see the magnetic lines of force surrounding Earth, with clearly defined north and south magnetic poles. [The geographic poles are the poles of planet rotation but positions of the magnetic poles are not the same.]

Fig 7: Earth's magnetic field (A) is similar to that of a bar magnet (B). Figure 2-21 in Hyndman and Hyndman (2011), Natural Hazards and Disasters, 3rd edition.

Well, Earth certainly doesn’t have a powerful bar magnet buried beneath surface! In fact, the hypothesis that best fits the facts is something called the dynamo model. In this model, electric currents are generated by enormous ‘dynamos’ driven by circulating hot currents in the liquid metal outer core, and magnetic fields surround those electric currents (Fig. 8).

Fig 8: The Earth dynamo model that creates Earth's magnetic field. By USGS (2015), Geomagnetism FAQ’s. Retrieved from http://www.usgs.gov/faq/?q=categories/9782/2738.

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The earth’s magnetic field is not always stable. When the liquid of the outer core undergoes changes in flow pattern, individual dynamos may change their orientation – in fact, a whole bunch of dynamos could change orientation! If that happens, the direction of the magnetic field that we observe near Earth's surface becomes twisted and tangled. The location of the magnetic poles can change and even appear in unusual places. Recent computer models based upon the absolute best magnetic field data available show a churning, twisting pattern of liquid flow in the outer core at various – unpredictable – times (Fig.9) Fig 9: Computer simulation of Earth's magnetic field between reversals (left) and in the middle of a reversal (right). Modified from Glatzmaier and Roberts (1995), A three-dimensional selfconsistent computer simulation of a geomagnetic field reversal. Nature, 377, 203-209. Retrieved from http://es.ucsc.edu/~glatz/geodynamo.html.

Eventually the dynamos start up again in a uniform pattern, but the orientation of the magnetic poles may be flipped 180° so that the north and south poles are flipped. Following these ‘flips’, the magnetic field may remain stable for many millions of years – but in what we call a ‘reverse’ polarity as opposed to the polarity of today, which we choose to call ‘normal’. We have evidence preserved in ancient rocks that these flips in the magnetic poles have occurred fairly regularly. It is easiest to understand the magnetic properties of rocks by looking at igneous rocks such as basalt. Basalt (the most common volcanic rock) contains enough iron that, when the magma cools, some number of small magnetite (iron oxide) mineral crystals will form. The magnetite crystals are magnetic and when they are floating in a liquid magma the will orient themselves in a direction parallel to Earth's magnetic lines of force (Fig. 10). Not only will the little magnetite crystals orient themselves N-S in a horizontal plane, but they will also show the Fig 10 (above): As magnetite minerals cool they adopt a position same degree of tilt up or down parallel to the external magnetic field at the time of cooling. Modified that the magnetic force field from Nelson (2012), Continental Drift, Sea Floor Spreading and Plate Tectonics. Retrieved lines have at the location of from http://www.tulane.edu/~sanelson/eens1110/pltect.htm. magma eruption

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For example, in Hawaii, as basalt lava solidifies, little magnetite crystals will orient themselves N-S and the north poles of the crystals will point down about 20° from horizontal – marking the fact that Hawaii is located at about the 20o north latitude position. Only exactly on the equator will the magnetite crystal ‘needles’ be exactly horizontal; only exactly at the north magnetic pole and the south magnetic pole will the magnetite crystals be vertical (Fig.11). So if we read the magnetic properties of basalt properly, we can tell fairly well where the lava eruption was relative to the north magnetic pole of the time.

Fig 11: Orientation of magnetite at different latitudes. Modified from USGS (2012), Paleomagnetism: Finding a Rock ’s Place of Birth. Retrieved from http://geomaps.wr.usgs.gov/parks/noca/sb10paleomag.html.

4.0 Sea-Floor Spreading: The Evidence from Paleomagnetism Remember those bits of puzzling data collected during and right after World War II by submerged magnetometers? There were parallel strips of ocean floor (made of basalt) that had alternating magnetic field orientation signatures: a strip with ‘normal’ orientation, then a strip of ‘reverse’ and back to ‘normal’. Well, in 1963 the answer to that puzzling pattern was found by a couple of Brits called Vine and Matthews.

Fig 10: Magnetic striping pattern of the sea floor due to normal and reversed periods of magnetic polarity. By Tivey (2004), Woods Hole Oceanographic Institute, Paving the Seafloor-Brick by Brick. Retrieved from https://www.whoi.edu/page.do?pid=10367&tid=3622&cid=2506.

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They carried out a survey of an underwater volcano (near the Seychelles Islands). They confirmed that magnetite crystals in the basalt flows from the volcano oriented themselves according to magnetic north-south. Upon dating the rocks they found that the pattern of alternating strips of orientation was agerelated. Amazingly, a perfectly symmetrical pattern of magnetic field orientation/age was discovered (Fig. 12). The volcanic ridges mark where the lithospheric plates on either side are slowing moving apart (i.e. they are spreading centers, just as Harry Hess said)

Magma erupts from undersea volcanoes at the ridges Magnetite crystals orient themselves according to Earth’s magnetic field at the time of eruption (thus preserving evidence within the rock whenever the field flips) As spreading continues, part of each flow ends up on either of the plates moving apart, thus creating the symmetrical pattern of alternating N/S strips of ocean floor. The plates move apart from ocean ridges as though they were behaving like conveyor belts heading off in opposite directions! 6

At this point we can calculate spreading rates for any ridge system: measure the distance from ridge center to some particular strip of basalt off to one side, take a chunk of that basalt and measure its age…and voila: distance divided by time = rate of spreading. Also, we can use GPS (Global Positioning System) to watch the plates move in real time. A very accurate GPS unit is fixed onto a continent and the location of the GPS unit (as measured by satellite) will shift at a rate of a few cm per year. OK…we have proof that Earth’s plates move apart at the spreading centers in mid-oceans and that new ocean crust is produced there. So that must mean Earth’s surface area is increasing, right? Well, of course, we now know it isn’t (although there were some pretty hot arguments about that in the late1960s and early 70s), so if new crust is being produced in some places, old crust must be destroyed in others. And this brings us to subduction zones. 5.0 Subduction Zones: Plate Compression Certain regions of ocean floors are marked by very deep trenches; those regions are also marked by numerous earthquakes. Since earthquakes are produced by movement along fractures (faults) in the crust, then there’s certainly movement going on in these regions. Take, for example, a series of curved trench features that more or less ring the Pacific Ocean (Fig.13) - marked here by earthquake foci. While we can’t do the same sort of measurement of plate motion here that was possible on spreading centers, it’s pretty obvious these trenches (and all the earthquakes associated) mark the regions where lithosphere (crust) is consumed. Certainly, they mark regions of great compression.

Fig 11. Earthquake foci around the globe – many are concentrated around the Pacific Ring of Fire (highlighted). Modified Figure 2-16b in Hyndman and Hyndman (2011) Natural Hazards and Disasters, 3rd edition.

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A Californian by the name of Hugo Benioff worked on the development and application of instruments (seismographs) to accurately measure earthquake locations; his co-worker (and the guy who eventually perfected the instrument) was Charles Richter. By mid-1950s the seismometer was sufficiently developed that Benioff was using it to examine these trenches in the ocean floor. When he traced the locations of earthquakes originating in those trench zones, here is what he found (Fig.14): a series of earthquake foci defining a thin zone from the trench surface sloped downward and away from the ocean side toward the continental plate side. It didn’t take any imagination at all to appreciate that this marked the descending slab of ocean floor.

Fig. 14: The Wadati-Benioff zone near the Japan Trench. Earthquakes are produced at different depths as the stillrigid, descending plate interacts with the overriding lithosphere. Modified from figure 12.17 in Tarbuck et al. (2012), Earth: An Introduction to Physical Geology, 3rd Canadian edition.

Today, instruments are very much more sensitive, and we can image the whole sinking slab seismically. Seismic waves travel more efficiently (faster) through colder material than through warmer material, so using a very sensitive technique called seismic tomography we can define the relatively cooler ocean floor slabs as they fall from Earth’s surface toward the core-mantle boundary (Fig.15; the cooler material is blue). Fig. 15: Seismic tomography of a subducting plate in the Pacific. By Zhao et al. (1997), Depth extent of the Lau back-arc spreading center and its relation to subduction processes. Science 278: 5336, pg. 254-257.

6.0 Plate Boundaries So far we have discussed plate boundaries that are undergoing tensional stress (divergent boundary = spreading centers) and boundaries formed by plates under compression stress (convergent boundary = subduction zones). But because we are dealing with motion of segments on a sphere, we have three (not two) types of plate interaction. A quick look at Fig.16 8

convinces us; if we move Plate A to the left, we are obligated, to get rid of the same area on the left (grey overlap) as we created on the right (yellow gap) because we can’t arbitrarily make Earth larger. Obviously, at the bottom and top of Plate A, we are producing breaks (or faults) which simply have a shearing motion; we call those transform plate boundaries where one plate slips more or less horizontally past the other, and no production or consumption of crust is involved. Sometimes we might call the breaks formed ‘strike-slip faults’ because the chunks on either side of the fault just slip past each other along the direction the fault is striking.

Fig 16: Tectonic plate movement on Earth and three types of plate boundaries. By Encyclopaedia Britannica (2015), Plate tectonics: theoretical diagram of plate movement effects. Retrieved from http://www.britannica.com/EBchecked/topic/175962/E arth/images-videos.

Now we’ve explained all of the different types of plate boundaries satisfactorily, but we still have the problem that Wegener, Hess, and all their successors had: no mechanism! So let’s return to the development discussion and see what’s missing. 7.0 The Driving Forces At the centre of the Earth is a solid metal (mostly iron) ball, about as hot as the surface of the Sun (estimated between 5000° and 7000°C). This is the inner core. It spins at its own rate, at least 0.2° of longitude per year faster than Earth’s current rotation rate, inside a very thick layer of liquid metal (again, mostly iron) known as the outer core. Outward from the core is the mantle and crust; the crust and the outer, brittle portion of mantle are together called lithosphere and the hot, mobile part of the mantle beneath that is called the asthenosphere. 7.1 Is Convection Enough? With a really hot interior and a cold surface, it is inevitable that significant heat transfer processes are in constant action inside Earth’s hot asthenosphere – and they are the driving forces of plate tectonics. In the early days scientists devised a simply convective overturn process to explain the heat transfer from core to surface, and they based it upon the simple model of water overturning by convection in a saucepan over a flame (Fig. 17). The result was the sort of model illustrated in Fig. 18 – and that’s about as wrong as you can get!

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Fig. 17: Simple convection model in a saucepan. By John Wiley and Sons, Inc. (1998). Retrieved from http://www.geo.arizona.edu/xtal/nats101/s04-08.html.

Fig. 18: Convection model of the Earth. By Surachit (2007), ...