Introduction to Earth Science - Chapter 2 PDF

2. PLATE TECTONICS Learning Objectives By the end of this chapter, students should be able to: Describe how the ideas behind plate tectonics started with Alfred Wegener’s hypothesis of continental drift. Describe the physical and chemical layers...

2. PLATE TECTONICS Learning Objectives By the end of this chapter, students should be able to: Describe how the ideas behind plate tectonics started with Alfred Wegener’s hypothesis of continental drift. Describe the physical and chemical layers of the Earth and how they affect plate movement. Explain how movement at the three types of plate boundaries causes earthquakes, volcanoes, and mountain building. Identify convergent boundaries, including subduction and collisions, as places where plates come together. Identify divergent boundaries, including rifts and mid-ocean ridges, as places where plates separate. Explain transform boundaries as places where adjacent plates shear past each other. Describe the Wilson Cycle, beginning with continental rifting, ocean basin creation, plate subduction, and ending with ocean basin closure. Explain how the tracks of hotspots, places that have continually rising magma, is used to calculate plate motion. ,,.. -- \\ ;..----, ·~ v ~- "-.. - PACIFIC "-....... [,.,., "-.. "--.... / I- OCEAN J r AUSTRAL OCEAN AUSTRAL OCEAN ---- Figure 2.1: Detailed map of all known plates, their boundaries, and movements. Revolution is a word usually reserved for significant political or social changes. Several of these idea revolutions forced scientists to re-examine their entire field, triggering a paradigm shift that shook up their conventionally held knowledge. Charles Darwin’s book on evolution, On the Origin of Species, published in 1859; Gregor Mendel’s discovery of the genetic  PLATE TECTONICS | 25 principles of inheritance in 1866; and James Watson, Francis Crick, and Rosalind Franklin’s model for the structure of DNA in 1953 did that for biology. Albert Einstein’s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics. The concept of plate tectonics was just as revolutionary for geology. The theory of plate tectonics attributes the move- ment of massive sections of the Earth’s outer layers with creating earthquakes, mountains, and volcanoes. Many earth processes make more sense when viewed through the lens of plate tectonics. Because it is so important in understanding how the world works, plate tectonics is the first topic of discussion in this textbook. 2.1 Alfred Wegener’s Continental Drift Hypothesis Alfred Wegener (1880-1930) was a German scientist who specialized in meteorology and cli- matology. His knack for questioning accepted ideas started in 1910 when he disagreed with the explanation that the Bering Land Bridge was formed by isostasy, and that similar land bridges once connected the continents. After reviewing the scientific literature, he published a hypothesis stating the continents were originally connected, and then drifted apart. While he did not have the precise mechanism worked out, his hypothesis was backed up by a long list of evidence. 2.1.1 Early Evidence for Continental Drift Hypothesis Wegener’s first piece of evidence was that the coastlines of some continents Figure 2.2: Wegener later in his fit together like pieces of a jigsaw life, ca. 1924-1930. puzzle. People noticed the similarities in the coastlines of South America and Africa on the first world maps, and some suggested the continents had been ripped apart. Antonio Snider-Pellegrini did preliminary work on continental separation and matching fossils in 1858. Figure 2.3: Snider-Pellegrini’s map showing the continental fit and separation, 1858. Figure 2.4: Map of world elevations. Note the light blue, which are continental shelves flooded by shallow ocean water. These show the true shapes of the continents. 26 | PLATE TECTONICS What Wegener did differently was synthesize a large amount of data in one place. He used true edges of the continents, based on the shapes of the continental shelves. This resulted in a better fit than previous efforts that traced the existing coastlines. Wegener also compiled evidence by comparing similar rocks, moun- tains, fossils, and glacial formations across oceans. For example, the ~ ,leaden~ of th "YiUllC l "d reptil fossils of the primitive aquatic reptile Mesosaurus were found on the INOIA i.r,trouuru,. separate coastlines of Africa and South America. Fossils of another reptile, Lystrosaurus, were found on Africa, India, and Antarctica. He pointed out these were land-dwelling creatures could not have swum across an entire ocean. Opponents of continental drift insisted trans-oceanic land bridges allowed animals and plants to move between continents. The land bridges eventually eroded away, leaving the continents permanently separated. The problem with this hypothesis is the improbability of a Figure 2.5: Image showing fossils that connect the land bridge being tall and long enough to stretch across a broad, continents of Gondwana (the southern continents of deep ocean. Pangea). More support for continental drift came from the puzzling evidence that glaciers once existed in normally very warm areas in southern Africa, India, Australia, and Arabia. These climate anomalies could not be explained by land bridges. Wegener found similar evidence when he discovered tropical plant fossils in the frozen region of the Arctic Circle. As Wegener collected more data, he realized the explanation that best fit all the climate, rock, and fossil observations involved moving continents. 2.1.2 Proposed Mechanism for Continental Drift Wegener’s work was considered a fringe science theory for his entire life. One of the biggest flaws in his hypothesis was an inability to pro- vide a mechanism for how the continents moved. Obviously, the conti- nents did not appear to move, and changing the conservative minds of the scientific community would require exceptional evidence that sup- ported a credible mechanism. Other pro-continental drift followers used expansion, contraction, or even the moon’s origin to explain how the continents moved. Wegener used centrifugal forces and preces­ sion, but this model was proven wrong. He also speculated about seafloor spreading, with hints of convection, but could not substanti- Figure 2.6: The basic idea of convection: an uneven heat ate these proposals. As it turns out, current scientific knowledge source in a fluid causes rising material next to the heat reveals convection is one the major forces in driving plate movements, and sinking material far from the heat. along with gravity and density.  PLATE TECTONICS | 27 2.1.3 Development of Plate Tectonic Theory Wegener died in 1930 on an expedition in Greenland. Poorly respected in his lifetime, Wegener and his ideas about moving continents seemed destined to be lost in history as fringe science. However, in the 1950s, evidence started to trickle in that made continental drift a more viable idea. By the 1960s, scientists had amassed enough evidence to support the missing mechanism—namely, seafloor spreading—for Wegener’s hypothesis of conti­ nental drift to be accepted as the theory of plate tecton­ ics. Ongoing GPS and earthquake data analyses continue to support this theory. The next section provides the pieces of evidence that helped transform one man’s wild notion into a scientific theory. Mapping of the Ocean Floors Figure 2.7: GPS measurements of plate motions. Figure 2.8: The complex chemistry around mid-ocean ridges. In 1947 researchers started using an adaptation of SONAR to map a region in the middle of the Atlantic Ocean with poorly- understood topographic and thermal properties. Using this information, Bruce Heezen and Marie Tharp created the first detailed map of the ocean floor to reveal the Mid-Atlantic Ridge, a basaltic mountain range that spanned the length of the Atlantic Ocean, with rock chemistry and dimensions unlike the mountains found on the continents. Initially scientists thought the ridge was part of a mechanism that explained the expanding Earth or ocean-basin growth hypotheses. In 1959, Harry Hess proposed the hypothesis of seafloor spreading—that the mid-ocean ridges represented tectonic plate factories, where new oceanic plate was issuing from these long volcanic ridges. Scientists later included transform faults perpendicular to the ridges to better account for varying rates of movement between the newly formed plates. When earthquake epicenters were discovered along the ridges, the idea that earthquakes were linked to plate movement took hold. 28 | PLATE TECTONICS One or more interactive elements has been excluded from this version of the text. You can view them online here: https://www.youtube.com/watch?v=TgfYjS0OTWw Video 2.1: Uncovering the secrets of the ocean floor. If you are using an offline version of this text, access this YouTube video via the QR code. Seafloor sediment, measured by dredging and drilling, provided another clue. Scientists once believed sediment accu- mulated on the ocean floors over a very long time in a static environment. When some studies showed less sediment than expected, these results were initially used to argue against continental movement. With more time, researchers discov- ered these thinner sediment layers were located close to mid-ocean ridges, indicating the ridges were younger than the surrounding ocean floor. This finding supported the idea that the sea floor was not fixed in one place. Paleomagnetism The seafloor was also mapped magnetically. Scientists had long known of strange magnetic anomalies that formed a striped pattern of symmetrical rows on both sides of mid-oceanic ridges. What made these features unusual was the north and south magnetic poles within each stripe was reversed in alternating rows. By 1963, Harry Hess and other scientists used these magnetic reversal patterns to support their model for seafloor spreading (see also Lawrence W. Morley). 1608 1706 Oec1i"°\ion (.xg,tt2 ~ t) Oeclinotiof'I (de9tttl ~011) Paleomagnetism is the study of magnetic fields frozen within rocks, basically a fos- Figure 2.9: The magnetic field of Earth, simplified as a bar magnet. silized compass. In fact, the first hard evidence to support plate motion came from paleomagnetism. 1806 Oecfo"IOl.ion H tolt 1900 Oe,clinolion (Oe-gtfft..a.sl) Igneous rocks containing magnetic minerals like mag- netite typically provide the most useful data. In their liquid state as magma or lava, the magnetic poles of the miner­ als align themselves with the Earth’s magnetic field. When the rock cools and solidifies, this alignment is frozen into place, creating a permanent paleomagnetic record that _.. - - ,. ,_.....__, -....,a.'-- QIIOIII. lliil.lM-'- includes magnetic inclination related to global latitude, and declination related to magnetic north. Figure 2.10: How the magnetic poles have moved over 400 years.  PLATE TECTONICS | 29 Scientists had noticed for some time the alignment of magnetic north in many rocks was nowhere close to the earth’s current magnetic north. Some explained this away are part of the normal movement of earth’s magnetic north pole. Eventually, scientists realized adding the idea of continental movement explained the data better than pole movement alone. Wadati–Benioff Zones Around the same time mid-ocean ridges were being investigated, sea-level other scientists linked the creation of ocean trenches and island arcs Figure 2.11: The iron in the solidifying rock preserves continental to seismic activity and tectonic the current magnetic polarity as new oceanic plates form at mid ocean ridges. x lithosphere plate movement. Several inde- xxx pendent research groups recog- x xxxx Beniotf nized earthquake epicenters traced the shapes of oceanic plates sinking into the xxx zone X mantle. These deep earthquake zones congregated in planes that started near x earth quake focus the surface around ocean trenches and angled beneath the continents and island arcs. Today these earthquake zones called Wadati-Benioff zones. Figure 2.12: The Wadati-Benioff zone, showing earthquakes following the subducting slab down. Based on the mounting evidence, the theory of plate tectonics continued to take shape. J. Tuzo Wilson was the first scientist to put the entire picture together by proposing that the opening and closing of the ocean basins. Before long, scientists proposed other models showing plates moving with respect to each other, with clear boundaries between them. Others started piecing together complicated histories of tectonic plate movement. The plate tectonic revolution had taken hold. Figure 2.13: J. Tuzo Wilson. Complete this interactive activity to check your understanding. If you are using an offline version of this text, access this interactive activity via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=139#h5p-8 30 | PLATE TECTONICS Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 2.1 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: ~ https://pressbooks.lib.vt.edu/introearthscience/?p=139#h5p-9 2.2 Layers of the Earth In order to understand the details of plate tectonics, it is essential to first understand the layers of the earth. First- hand information about what is below the surface is very Lithosphere (crust and upper- limited; most of what we know is pieced together from most solid m antle) hypothetical models, and analyzing seismic wave data Mantle and meteorite materials. In general, the Earth can be divided into layers based on chemical composition and physical characteristics. d Core 2.2.1 Chemical Layers Certainly the earth is composed of a countless combina- tion of elements. Regardless of what elements are involved two major factors—temperature and pres- Figure 2.14: The layers of the Earth. Physical layers include lithosphere sure—are responsible for creating three distinct chemical and asthenosphere; chemical layers are crust, mantle, and core. layers. Crust The outermost chemical layer and the one we currently reside on, is the crust. There are two types of crust. Continental crust has a relatively low density and composition similar to granite. Oceanic crust has a relatively high density, especially when cold and old, and composition similar to basalt. The surface levels of crust are relatively brittle. The deeper parts of the crust are subjected to higher temperatures and pressure, which makes them more ductile. Ductile materials are like soft plastics or putty, they move under force. Brittle materials are like solid glass or pottery, they break under force, espe- cially when it is applied quickly. Earthquakes, generally occur in the upper crust and are caused by the rapid movement of relatively brittle materials.  PLATE TECTONICS | 31 The base of the crust is characterized by a large increase o· 110· in seismic velocity, which measures how fast earthquake 60 60' waves travel through solid matter. Called the Mohorovičić Discontinuity, or Moho for short, this zone was discovered by Andrija Mohorovičić (pronounced mo-ho-ro-vee- o· cheech; audio pronunciation) in 1909 after studying earth- quake wave paths in his native Croatia. The change in wave direction and speed is caused by dramatic chemical differ- ences of the crust and mantle. Underneath the oceans, the Moho is found roughly 5 km below the ocean floor. -180' MonoDapthlkfflJ ,o 20 30 40 so eo 10 Under the continents, it is located about 30-40 km below the surface. Near certain large mountain-building events Figure 2.15: The global map of the depth of the Moho. known as orogenies, the continental Moho depth is dou- bled. Mantle The mantle sits below the crust and above the core. It is the largest chemical layer by volume, extending from the base of the crust to a depth of about 2900 km. Most of what we know about the mantle comes from seismic wave analy- sis, though information is gathered by studying ophiolites and xenoliths. Ophi­ olites are pieces of mantle that have risen through the crust until they are exposed as part of the ocean floor. Xenoliths are carried within magma and brought to the Earth’s surface by volcanic eruptions. Most xenoliths are made of peridotite, an ultramafic class of igneous rock (see section 4.2 for explana- tion). Because of this, scientists hypothesize most of the mantle is made of peridotite. Figure 2.16: This mantle xenolith containing Core olivine (green) is chemically weathering by hydrolysis and oxidation into the pseudo-mineral iddingsite, which is a complex The core of the Earth, which has both of water, clay, and iron oxides. The more altered liquid and solid layers, and consists side of the rock has been exposed to the mostly of iron, nickel, and possibly environment longer. some oxygen. Scientists looking at seismic data first discovered this innermost chemical layer in 1906. Through a union of hypothetical modeling, astronomical insight, and hard seismic data, they concluded the core is mostly metallic iron. Scientists studying meteorites, which typically contain more iron than surface rocks, have proposed the earth was formed from meteoric material. They believe the liquid component of the core was created as the iron and nickel sank into the center of the planet, where Figure 2.17: A polished fragment of the it was liquefied by intense pressure. iron-rich Toluca Meteorite, with octahedral Widmanstätten pattern. 2.2.2 Physical Layers The Earth can also be broken down into five distinct physical layers based on how each layer responds to stress. While there is some overlap in the chemical and physical designations of layers, specifically the core–mantle boundary, there are significant differences between the two systems. 32 | PLATE TECTONICS Lithosphere Lithos is Greek for stone, and the lithosphere is the outermost physical layer of the Earth. It is grouped into two types: oceanic and continental. Oceanic lithosphere is thin and relatively rigid. It ranges in thickness from nearly zero in new plates found around mid-ocean ridges, to an average of 140 km in most other loca- tions. Continental lithosphere is generally thicker and consider- ably more plastic, especially at the deeper levels. Its thickness ranges from 40 to 280 km. The lithosphere is not continuous. It is broken into segments called plates. A plate boundary is where two plates meet and move relative to each other. Plate bound- aries are where we see plate tectonics in action—mountain building, triggering earthquakes, and generating volcanic activity. Figure 2.18: Map of the major plates and their motions along Asthenosphere boundaries. The asthenosphere is the layer below the lithosphere. Astheno- means lacking strength, and the most distinctive prop- erty of the asthenosphere is movement. Because it is mechan- ically weak, this layer moves and flows due to convection currents created by heat coming from the earth’s core cause. Unlike the lithosphere that consists of multiple plates, the asthenosphere is relatively unbroken. Scientists have deter- mined this by analyzing seismic waves that pass through the layer. The depth of at which the asthenosphere is found is tem­ perature-dependent. It tends to lie closer to the earth’s surface around mid-ocean ridges and much deeper underneath mountains and the centers of lithospheric plates. Figure 2.19: The lithosphere–asthenosphere boundary changes with certain tectonic situations. Mesosphere The mesosphere, sometimes known as the lower mantle, is more rigid and immobile than the asthenosphere. Located at a depth of approximately 410 and 660 km below the earth’s surface, the mesosphere is subjected to very high pressures and tempera- tures. These extreme conditions create a transition zone in the upper mesosphere where minerals continuously change into various forms, or pseudomorphs. Scientists identify this zone by changes in seismic velocity and sometimes physical barriers to movement. Below this transitional zone, the mesosphere is relatively uniform until it reaches the core. 0 Figure 2.20: General perovskite structure. Perovskite silicates (i.e., Bridgmenite, (Mg,Fe)SiO3) are thought to be the main component of the lower mantle, making it the most common mineral in or on Earth.  PLATE TECTONICS | 33 Inner and Outer Core The outer core is the only entirely liquid layer within the Earth. It starts at a depth of 2,890 km and extends to 5,150 km, making it about 2,300 km thick. In 1936, the Dan- ish geophysicist Inge Lehmann analyzed seismic data and was the first to prove a solid inner core existed within a liquid outer core. The solid inner core is about 1,220 km thick, and the outer core is about 2,300 km thick. It seems like a contradiction that the hottest part of the Earth is solid, as the minerals making up the core should be liquified or vaporized at this temperature. Immense pressure keeps the minerals of the inner core in a solid phase. The inner core grows slowly from the lower outer core solidifying as heat escapes the interior of the Earth Figure 2.21: Lehmann in 1932. and is dispersed to the outer lay- ers. The earth’s liquid outer core is critically important in maintaining a Figure 2.22: The outer core’s spin causes our protective breathable atmosphere and other environmental conditions favorable magnetic field. for life. Scientists believe the earth’s magnetic field is generated by the circulation of molten iron and nickel within the outer core. If the outer core were to stop circulating or become solid, the loss of the magnetic field would result in Earth getting stripped of life-supporting gases and water. This is what happened, and continues to happen, on Mars. Complete this interactive activity to check your understanding. If you are using an offline version of this text, access this interactive activity via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: Im https://pressbooks.lib.vt.edu/introearthscience/?p=139#h5p-10 34 | PLATE TECTONICS 2.2.3 Plate Tectonic Boundaries At passive margins the plates don’t move—the continen­ Paleozooc DFW tal lithosphere transitions into oceanic lithosphere and Hinge Zone forms plates made of both types. A tectonic plate may be 0km l ~ Sealevel made of both oceanic and continental lithosphere con- -5 km nected by a passive margin. North and South America’s Continental Crust -10 km eastern coastlines are examples of passive margins. Active margins are places where the oceanic and continental Thickest section of sediments are deposited lithospheric tectonic plates meet and move relative to adjacent to the continents. above the transitional crust, in the region that becomes each other, such as the western coasts of North and South -40 km _ _ _ _ _ _., the hinge zone. Can be up to 20 km thick. America. This movement is caused by frictional drag cre- Figure 2.23: Passive margin. ated between the plates and differences in plate densities. The majority of mountain-building events, earthquake activity and active volcanism on the Earth’s surface can be attributed to tectonic plate movement at active margins. CONVERGENT TRANSFORM DIVERG EN T CONVE RGENT CONTINENTAL RIFT ZONE PLATE BOU NDARY PLATE BOUNDAR~ PLATE BOUNDAR~ PLATE BOUNDA RV (YOUNO PLATE BOUNDAR VI \ 7REhC H ISLAIIC ARC SHIELD OC[A'JIC 5mEAClhO VOLCANO R OGC f LITHOSPHERE ASTHENOSPHERE SUBOUCTllrn P LATE HOT SPOT Figure 2.24: Schematic of plate boundary types. In a simplified model, there are three categories of tectonic plate boundaries. Convergent boundaries are places where plates move toward each other. At divergent boundaries, the plates move apart. At transform boundaries, the plates slide past each other.  PLATE TECTONICS | 35 Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 2.2 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: ~ https://pressbooks.lib.vt.edu/introearthscience/?p=139#h5p-11 2.3 Convergent Boundaries Convergent boundaries, also called destructive bound- aries, are places where two or more plates move toward each other. Convergent boundary movement is divided into two types, subduction and collision, depending on the density of the involved plates. Continental lithos­ phere is of lower density and thus more buoyant than the underlying asthenosphere. Oceanic lithosphere is more dense than continental lithosphere, and, when old and cold, may even be more dense than asthenosphere. When plates of different densities converge, the higher density plate is pushed beneath the more buoyant plate in Figure 2.25: Geologic provinces with the Shield (orange) and Platform (pink) comprising the craton, the stable interior of continents. a process called subduction. When continental plates converge without subduction occurring, this process is called collision. 2.3.1. Subduction Subduction occurs when a dense oceanic plate meets a Accret iona ry prism Vole a nic a re more buoyant plate, like a continental plate or warmer/ Oceanic cru Continental younger oceanic plate, and descends into the mantle. crust The worldwide average rate of oceanic plate subduction is 25 miles per million years, about a half-inch per year. As an oceanic plate descends, it pulls the ocean floor down Sol' Mag ma uppermo chamber into a trench. These trenches can be more than twice as mantl Rising deep as the average depth of the adjacent ocean basin, dia pirs which is usually three to four km. The Mariana Trench, for example, approaches a staggering 11 km. Subduction zone Figure 2.26: Diagram of ocean–continent subduction. 36 | PLATE TECTONICS Within the trench, ocean floor sediments are scraped eac.karc ◄ \/olcank ► ~or rc ► Ocean Front Basin together and compressed between the subducting and overriding plates. This feature is called the accretionary wedge, mélange, or accretionary prism. Fragments of con­ tinental material, including microcontinents, riding atop the subducting plate may become sutured to the accre­ tionary wedge and accumulate into a large area of land called a terrane. Vast portions of California are comprised of accreted terranes. Figure 2.27: Microcontinents can become part of the accretionary prism of a subduction zone. When the subducting oceanic plate, or slab, sinks into the mantle, PACIFIC the immense heat and pressure pushes volatile materials like water PLATE CANADA and carbon dioxide into an area below the continental plate and ~ above the descending plate called the mantle wedge. The ander terrane volatiles are released mostly by hydrated minerals that revert to non-hydrated minerals in these higher temperature and pressure conditions. When mixed with asthenospheric material above the EXPLANATION NORTH plate, the volatile lower the melting point of the mantle wedge, and 0 1s1andarc AMERICAN D Submarine depoaits PLATE through a process called flux melting it becomes liquid magma. - Ancient ocean floor "' ~ Sonoma terrane The molten magma is more buoyant than the lithospheric plate D Attached fragments above it and migrates to the Earth’s surface where it emerges as vol­ D Ancient contlnental ~ \ \ UNITED STATES canism. The resulting volcanoes frequently appear as curved lnte.-ior lcratonl J Divergent boundary... mountain chains, volcanic arcs, due to the curvature of the earth. ~ :;.Salton..t.4. Convergent boundary Trough Both oceanic and continental plates can contain volcanic arcs. -$- Transform boundary MEXICO How subduction is initiated is still a mat- East Pacific Rise ter of scientific debate. It is generally accepted that sub­ duction zones start Figure 2.28: Accreted terranes of western North America. as passive margins, Everything that is not the “Ancient continental interior (craton)” has been smeared onto the side of the continent where oceanic and by accretion from subduction. continental plates come together, and then gravity initiates subduction and converts the passive margin into an active one. One hypothesis is gravity pulls the denser oceanic plate down or the plate can start to flow ductility at a low angle. Scientists seeking to answer this question have collected evidence that suggests a new subduc­ tion zone is forming off the coast of Portugal. Some scientists have pro- posed large earthquakes like the 1755 Lisbon earthquake may even have something to do with this process of creating a subduction zone, although 1755 Lisbon, Portugal Earthquake the evidence is not definitive. Another hypothesis proposes subduction Figure 2.29: Location of the large (Mw 8.5-9.0) 1755 happens at transform boundaries involving plates of different densities. Lisbon earthquake.  PLATE TECTONICS | 37 Some plate boundaries look like they should be active, but show no evidence of subduction. The oceanic lithospheric plates on either side of the Atlantic Ocean for example, are denser than the underlying asthenosphere and are not subducting beneath the continental plates. One hypothesis is the bond holding the oceanic and continental plates together is stronger than the downwards force created by the difference in plate densities. 95 100' 105' 110· Subduction zones are known for having the largest earthquakes 5 5 and tsunamis; they are the only places with fault surfaces large "" -70 enough to create magnitude-9 earthquakes. These subduc­ o·. ,,o tion-zone earthquakes not only are very large, but also are very deep. When a subducting slab becomes stuck and cannot.:;oo descend, a massive amount of energy builds up between the. 5. 5 stuck plates. If this energy is not gradually dispersed, it may force the plates to suddenly release along several hundred kilo- e meters of the subduction zone. Because subduction-zone -10· -10· faults are located on the ocean floor, this massive amount of movement can generate giant tsunamis such as those that fol- lowed the 2004 Indian Ocean Earthquake and 2011 Tōhoku -eoo 95' 100' 105' 110' Earthquake in Japan. A' - 100 g.i= a.200 Q) "O Boeltore 300 ~i1!9centtt I c«41nllool' 0 200 400 600 distance along profile (km) IIIOMlt Figure 2.30: Earthquakes along the Sunda megathrust subduction zone, along the island of Sumatra, showing the 2006 Mw 9.1-9.3 Indian Ocean earthquake as a star. All subduction zones have a forearc basin, a feature of the Back arc overriding plate found between the volcanic arc and convution a:11 oceanic trench. The forearc basin experiences a lot of faulting and deformation activity, particularly within the I accretionary wedge. Figure 2.31: Various parts of a subduction zone. This subduction zone is In some subduction zones, tensional forces working on ocean–ocean subduction, though the same features can apply to continent–ocean subduction. the continental plate create a backarc basin on the inte- rior side of the volcanic arc. Some scientists have pro- posed a subduction mechanism called oceanic slab rollback creates extension faults in the overriding plates. In this model, the descending oceanic slab does not slide directly under the overriding plate but instead rolls back, pulling the overlying plate seaward. The continental plate behind the volcanic arc gets stretched like pizza dough until the surface cracks and collapses to form a backarc basin. If the extension activity is extensive and deep enough, a backarc basin can develop into a continental rifting zone. These continental divergent boundaries may be less symmetrical than their mid- ocean ridge counterparts. In places where numerous young buoyant oceanic plates are converging and subducting at a relatively high velocity, they may force the overlying continental plate to buckle and crack. This is called back-arc faulting. Extensional back- arc faults pull rocks and chunks of plates apart. Compressional back-arc faults, also known as thrust faults, push them together. 38 | PLATE TECTONICS The dual spines of the Andes Mountain range include a example of compressional thrust faulting. The western spine is part of a volcanic arc. Thrust faults have deformed the non-volcanic eastern spine, pushing rocks and pieces of continen­ tal plate on top of each other. There are two styles of thrust fault deformation: thin-skinned faults that occur in superficial rocks lying on top of the continental plate and thick-skinned faults that reach deeper into the crust. The Sevier Orogeny in the western U.S. is a notable thin-skinned type of deformation created during the Cretaceous Period. The Laramide Orogeny, a thick- skinned type of deformation, occurred near the end of and slightly after the Sevier Orogeny in the same region. Flat-slab, or shallow, subduction caused the Laramide Orogeny. When the descending slab subducts at a low angle, there is more contact between the slab and the overlying continental plate than in a typical subduction zone. The shallowly-subducting slab pushes against the overriding plate and creates an area of deformation on the overriding plate many kilometers away from the subduction zone. Figure 2.32: Shallow subduction during the Laramide Oceanic–Continental Subduction orogeny. Oceanic-continental subduction occurs when an oceanic plate dives below a continental plate. This convergent boundary has a trench and mantle wedge and frequently, a volcanic arc. Well- known examples of continental volcanic arcs are the Cascade Mountains in the Pacific Northwest and western Andes Mountains in South America. Oceanic–Oceanic Subduction................ Oceanlc -contlnental conver9ence Figure 2.33: Subduction of an oceanic plate beneath a continental plate, forming a trench and volcanic arc. The boundaries of oceanic-oceanic subduction zones show very different activity from those involving oceanic–continental plates. Since both plates are made of oceanic lithosphere, it is usually the older plate that subducts because it is colder and denser. The volcanism on the overlying Ouanlc-oceanlc convergence oceanic plate may remain hidden underwater.. If the volcanoes rise high Figure 2.34: Subduction of an oceanic plate beneath enough the reach the ocean surface, the chain of volcanism forms an another oceanic plate, forming a trench and an island island arc. Examples of these island arcs include the Aleutian Islands in the arc. northern Pacific Ocean, Lesser Antilles in the Caribbean Sea, and numerous island chains scattered throughout the western Pacific Ocean.  PLATE TECTONICS | 39 2.3.2. Collisions When continental plates converge, during the closing of an ocean basin for example, subduction is not possible between the equally buoyant plates. Instead of one plate descending beneath another, the two masses of continental lithosphere slam together in a process known as collision. Without subduction, there is no magma formation and no volcanism. Col­ lision zones are characterized by tall, non-volcanic mountains; a broad zone of frequent, large earthquakes; and very little volcanism. Con tine nt a l-co n tlnental conve rgence When oceanic crust connected by a passive margin to continental crust Figure 2.35: Two continental plates colliding. completely subducts beneath a continent, an ocean basin closes, and continental collision begins. Eventually, as ocean basins close, continents join together to form a massive accumulation of continents called a supercontinent, a process that has taken place in ~500 million year old cycles over earth’s history. The process of collision created Pangea, the supercontinent envisioned by Wegener as the key component of his continental drift hypothesis. Geologists now have evi- dence that continental plates have been continuously converging into superconti­ nents and splitting into smaller basin-separated continents throughout Earth’s existence, calling this process the supercontinent cycle, a process that takes place in approximately 500 million years. For example, they estimate Pangea began separat- ing 200 million years ago. Pangea was preceded by an earlier supercontinents, one of which being Rodinia, which existed 1.1 billion years ago and started breaking apart 800 million to 600 million years ago. A foreland basin is a feature that.. develops near mountain belts, as the combined mass of the mountains Figure 2.36: A reconstruction of forms a depression in the lithospheric Pangaea, showing approximate plate. While foreland basins may positions of modern continents. occur at subduction zones, they are most commonly found at collision boundaries. The Persian Gulf is possibly the best modern example, created entirely by the weight of the nearby Arabian Plate Zagros Mountains. If continental and oceanic Figure 2.37: The tectonics of the Zagros Mountains. lithosphere are fused on Note the Persian Gulf foreland basin. the same plate, it can par- tially subduct but its buoy- ancy prevents it from fully descending. In very rare cases, part of a continental plate may become trapped beneath a descending oceanic plate in a process called obduction. When a portion of the continental crust is driven down into the subduction zone, due to its buoyancy it returns to the surface relatively quickly. As pieces of the continental lithosphere break loose and migrate upward Figure 2.38: Pillow lavas, which only form under through the obduction zone, they bring along bits of the mantle and ocean water, from an ophiolite in the Apennine Mountains floor and amend them on top of the continental plate. Rocks composed of central Italy. of this mantle and ocean-floor material are called ophiolites and they pro- vide valuable information about the composition of the mantle. 40 | PLATE TECTONICS Figure 2.39: India crashing into Asia. The area of collision-zone deformation and seismic activity usually covers a broader area because continental lithos­ phere is plastic and malleable. Unlike subduction-zone earthquakes, which tend to be located along a narrow swath near the convergent boundary, collision-zone earthquakes may occur hundreds of kilometers from the boundary between the plates. The Eurasian continent has many examples of collision-zone deformations covering vast areas. The Pyrenees mountains begin in the Iberian Peninsula and cross into France. Also, there are the Alps stretching from Italy to central Europe; the Zagros mountains from Arabia to Iran; and Himalaya mountains from the Indian subcontinent to central Asia. Take this quiz to check your comprehension of this section. l"I'.iol!l If you are using an offline version of this text, access the quiz for section 2.3 via the QR code. [!l\f\·. An interactive H5P element has been excluded from this version of the text. You can view it online here: ~ https://pressbooks.lib.vt.edu/introearthscience/?p=139#h5p-12 2.4 Divergent Boundaries At divergent boundaries, sometimes called constructive boundaries, lithospheric plates move away from each other. There are two types of divergent boundaries, categorized by where they occur: continental rift zones and mid-ocean ridges. Continental rift zones occur in weak spots in the continental lithospheric plate. A mid-ocean ridge usually origi- nates in a continental plate as a rift zone that expands to the point of splitting the plate apart, with seawater filling in the gap. The separate pieces continue to drift apart and become individual continents. This process is known as rift-to-drift.  PLATE TECTONICS | 41 2.4.1. Continental Rifting Horst Graben In places where the continental plates are very thick, they reflect so much heat back into the mantle it develops strong convection cur- rents that push super-heated mantle material up against the overly- ing plate, softening it. Tensional forces created by this convective upwelling begin to pull the weakened plate apart. As it stretches, it becomes thinner and develops deep cracks called extension or normal faults. Eventually plate sections located between large faults drop into deep depressions known as rift valleys, which often con- tain keystone-shaped blocks of down-dropped crust known as Figure 2.40: Faulting that occurs in divergent boundaries. grabens. The shoulders of these grabens are called horsts. If only one side of a section drops, it is called a half-graben. Depending on the conditions, rifts can grow into very large lakes and even oceans. While seemingly occurring at random, rifting is dictated by two factors. Rifting does not occur in continents with older and more stable interiors, known as cra­ tons. When continental rifting does occur, the break-up pattern resembles the seams of a soccer ball, also called a truncated icosahedron. This is the most common surface-fracture pattern to develop on an evenly expanding sphere because it uses the least amount of energy. Using the soccer ball model, rifting tends to lengthen and expand along a par- ticular seam while fizzling out in the other directions. These seams with little or no tectonic activity are called failed rift arms. A failed rift arm is still a weak spot in the continental plate; even without the presence of active extension faults, it may develop into a called an aulacogen. One example of a failed rift arm is the Mississippi Valley Embayment, a depression through which the upper end Figure 2.41: The Afar Triangle (center) has the of the Mississippi River flows. Occasionally connected rift arms do develop con- Red Sea ridge (center to upper left), Gulf of currently, creating multiple boundaries of active rifting. In places where the rift Aden ridge (center to right), and East African arms do not fail, for example the Afar Triangle, three divergent boundaries can Rift (center to lower left) form a triple junction that are about 120° apart. develop near each other forming a triple junction. Rifts come in two types: narrow and broad. Narrow rifts are characterized by a high density of highly active divergent boundaries. The East African Rift Zone, where the horn of Africa is pulling away from the mainland, is an excel- lent example of an active narrow rift. Lake Baikal in Russia is another. Broad rifts also have numerous fault zones, but they are distributed over wide areas of deformation. The Basin and Range region located in the western United States is a type of broad rift. The Wasatch Fault, which also created the Wasatch Mountain Range in the state of Utah, forms the eastern divergent boundary of this broad rift (animation 1 and animation 2). Figure 2.42: NASA image of the Basin and Range horsts and grabens across central Nevada. 42 | PLATE TECTONICS Rifts have earthquakes, although not of the magnitude and frequency of other EU R AS I A N P~ A T E boundaries. They may also exhibit volcanism. Unlike the flux-melted magma found in subduction zones, rift-zone magma is created by decompression melting. As the continental plates are pulled apart, they create a region of low pressure that melts the lithosphere and draws it upwards. When this molten magma reaches the weak- ened and fault-riddled rift zone, it migrates to surface by breaking through the plate or escaping via an open fault. Examples of young rift volcanoes dot the Basin and Range region in the United States. Rift-zone activity is responsible for generating some unique volcanism, such as the Ol Doinyo Lengai in Tanzania. This volcano erupts lava consisting largely of carbonatite, a relatively cold, liquid carbonate min­ eral. 2.4.2. Mid-ocean Ridges As rifting and volcanic activity Rift Valley progress, the continental lithos­ (Ahic:an rm n lley) phere becomes more mafic (see chapter 4) and thinner, with the eventual result transforming the plate under the rifting area into oceanic lithosphere. This is the Figure 2.43: India colliding into Eurasia to create the modern day Himalayas. process that gives birth to a new New Ocean Basin {Red Sea) ocean, much like the narrow Red Sea emerged with the movement of Arabia away from Africa. As the oceanic lithosphere continues to diverge, a mid-ocean ridge is formed. Mid-ocean ridges, also known as spreading centers, have several dis- tinctive features. They are the only places on earth that create new Mature Ocean oceanic lithosphere. Decompression melting in the rift zone changes (,\tlAOUc) asthenosphere material into new lithosphere, which oozes up through cracks in oceanic plate. The amount of new lithosphere being created at mid-ocean ridges is highly significant. These undersea rift volcanoes produce more lava than all other types of volcanism combined. Despite this, most mid-oceanic ridge volcanism remains unmapped because the volcanoes are located deep on the ocean floor. Figure 2.44: Progression from rift to mid-ocean ridge. In rare cases, such as a few locations in Iceland, rift zones display the type of volcanism, spreading, and ridge formation found on the ocean floor.  PLATE TECTONICS | 43 The ridge feature is created by the accumulation of hot lithosphere material, which is lighter than the dense underlying asthenosphere. This chunk of isostatically buoyant lithosphere sits partially submerged and partially exposed on the asthenosphere, like an ice cube floating in a glass of water. As the ridge continues to spread, the lithosphere material is pulled away from the area of volcanism and becomes colder and denser. As it continues to spread and cool, the lithosphere settles into wide 0 10 20 30 40 50 60 70 80 90 l 00 110120130 140 150160 170180280 swathes of relatively featureless topography called abyssal plains with Age of Oceanic lithosphere [m.y.) lower topography. Figure 2.45: Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along This model of ridge formation suggests the sections of lithosphere the coasts of the continents. furthest away from the mid-ocean ridges will be the oldest. Scientists have tested this idea by comparing the age of rocks located in various locations on the ocean floor. Rocks found near ridges are younger than those found far away from any ridges. Sediment accumulation patterns also confirm the idea of sea-floor spreading. Sediment layers tend to be thinner near mid-ocean ridges, indicating it has had less time to build up. No rma l magnetic As mentioned in the section on paleomagnetism and the development of plate polarity tectonic theory, scientists noticed mid-ocean ridges contained unique mag- D Reversed netic anomalies that show up as symmetrical striping on both sides of the ridge. The Vine-Matthews-Morley hypothesis proposes these alternating reversals are created by the earth’s magnetic field being imprinted into magma after it emerges from the ridge. Very hot magma has no magnetic field. As the oceanic plates get pulled apart, the magma cools below the Curie point, the tempera­ ture below which a magnetic field gets locked into magnetic minerals. The alternating magnetic reversals in the rocks reflects the periodic swapping of Figure 2.46: A time progression (with “a” being earth’s magnetic north and south poles. This paleomagnetic pattern provides a youngest and “c” being oldest) showing a spreading center getting wider while recording great historical record of ocean-floor movement, and is used to reconstruct past changes in the magnetic field of the Earth. tectonic activity and determine rates of ridge spreading. One or more interactive elements has been excluded from this version of the text. You can view them online here: ~ https://www.youtube.com/watch?v=6o1HawAOTEI Video 2.2: Pangea breakup and formation of the northern Atlantic Ocean. If you are using an offline version of this text, access this YouTube video via the QR code. 44 | PLATE TECTONICS Thanks to their distinctive geology, mid-ocean ridges are home to some of the most unique ecosystems ever discovered. The ridges are often studded with hydrothermal vents, deep fissures that allow seawater to circulate through the upper portions of the oceanic plate and interact with hot rock. The super- heated seawater rises back up to the surface of the plate, carrying dissolved gasses and minerals, and small particulates. The resulting emitted hydrother­ mal water looks like black underwater smoke. Scientists had known about these geothermal areas on the ocean floor for some time. However, it was not until 1977, when scientists piloting a deep sub- mergence vehicle, the Alvin, discovered a thriving community of organisms clustered around these hydrothermal vents. These unique organisms, which include 10-foot-long tube worms taller than people, live in the complete dark- ness of the ocean floor deprived of oxygen and sunlight. They use geothermal energy provided by the vents and a process called bacterial chemosynthesis to feed on sulfur compounds. Before this discovery, scientists believed life on earth could not exist without photosynthesis, a process that requires sunlight. Some scientists suggest this type of environment could have been the origin of Figure 2.47: Black smoker hydrothermal vent life on Earth, and perhaps even extraterrestrial life elsewhere in the galaxy, such with a colony of giant (6’+) tube worms. as on Jupiter’s moon Europa. Take this quiz to check your comprehension of this section. If you are using an offline version of this text, access the quiz for section 2.4 via the QR code. An interactive H5P element has been excluded from this version of the text. You can view it online here: https://pressbooks.lib.vt.edu/introearthscience/?p=139#h5p-13 2.5 Transform Boundaries A transform boundary, sometimes called a strike-slip or conservative Si nistral (left-lateral) De>