2 Plate Tectonics

The rock is getting thinner farther away.
A layer of shallow ocean limestone (white) has been brought to the top of a mountain by the convergent forces of the Sevier Orogeny. Near Sun River Canyon, Montana.

2 Plate Tectonics

KEY CONCEPTS

At 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 hot spots, places that have continually rising magma, is used to calculate plate motion

The map shows many plates.
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 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 movement 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

He is a male in a suit.
Wegener later in his life, ca. 1924-1930.
Alfred Wegener (1880-1930) was a German scientist who specialized in me

teorology and climatology. 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

It shows South America and Africa connected, then apart.
Snider-Pellegrini’s map showing the continental fit and separation, 1858.

Wegener’s first piece of evidence was that the coastlines of some continents fit together like pieces of a jigsaw 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.

The shape of the continents is different than what is seen by just coastlines.
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.

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 .

There are four different fossil organisms that connect South America, Africa, India, Antartica, and Australia.
Image showing fossils that connect the continents of Gondwana (the southern continents of Pangea).
Wegener also compiled evidence by comparing similar rocks, mountains, fossils, and glacial formations across oceans. For example, the fossils of the primitive aquatic reptile Mesosaurus were found on the 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 land bridge being tall and long enough to stretch across a broad, deep ocean.

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

The rising material is drawn red. The cool material is blue.
[Click to Animate] Animation of the basic idea of convection: an uneven heat source in a fluid causes rising material next to the heat and sinking material far from the heat.
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 provide a mechanism for how the continents moved. Obviously, the continents did not appear to move, and changing the conservative minds of the scientific community would require exceptional evidence that supported 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 precession, but this model was proven wrong . He also speculated about seafloor spreading, with hints of convection, but could not substantiate these proposals . As it turns out, current scientific knowledge reveals convection is the major force in driving plate movements.

2.1.3 Development of Plate Tectonic Theory

The map shows many data points all over the world.
GPS measurements of plate motions.
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 continental drift to be accepted as the theory of plate tectonics. 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

The diagram shows water going into the ground and coming out, with many different reactions.
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 .

 

Seafloor sediment, measured by dredging and drilling, provided another clue. Scientists once believed sediment accumulated 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 discovered 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 north end of the magnet is south topographically, and vice versa.
The magnetic field of Earth, simplified as a bar magnet.

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 ).

The poles shift slightly every year.
This animation shows how the magnetic poles have moved over 400 years.

Paleomagnetism is the study of magnetic fields frozen within rocks, basically a fossilized compass. In fact, the first hard evidence to support plate motion came from paleomagnetism.

Igneous rocks containing magnetic minerals like magnetite typically provide the most useful data. In their liquid state as magma or lava, the magnetic poles of the minerals 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 includes magnetic inclination related to global latitude, and declination related to magnetic north.

Animated gif depicting a mid-ocean ridge with two oceanic plates moving away from the center of the ridge. As the movement progresses, symettrical magnetic stripes appear on each side of the ridge.
The iron in the solidifying rock preserves the current magnetic polarity as new oceanic plates form at mid ocean ridges.

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

The earthquakes descend at an angle into the Earth.
The Wadati-Benioff zone, showing earthquakes following the subducting slab down.

Around the same time mid-ocean ridges were being investigated, other scientists linked the creation of ocean trenches and island arcs to seismic activity and tectonic plate movement . Several independent research groups recognized earthquake epicenters traced the shapes of oceanic plates sinking into the mantle. These deep earthquake zones congregated in planes that started near the surface around ocean trenches and angled beneath the continents and island arcs . Today these earthquake zones called Wadati-Benioff zones.

He is an older man in this 1992 image.
J. Tuzo Wilson
Based on the mounting evidence, the theory 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.

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2.2 Layers of the Earth

The crust and lithosphere are on the outside of the Earth and are thin. Below the crust is the mantle and core. Below the lithosphere is the asthenosphere.
The layers of the Earth. Physical layers include lithosphere and asthenosphere; chemical layers are crust, mantle, and core.
In order to understand the details of plate tectonics, it is essential to first understand the layers of the earth. Firsthand information about what is below the surface is very limited; most of what we know is pieced together from hypothetical models, and analyzing seismic wave data and meteorite materials. In general, the Earth can be divided into layers based on chemical composition and physical characteristics.

2.2.1 Chemical Layers

Certainly the earth is composed of a countless combination of elements. Regardless of what elements are involved two major factors—temperature and pressure—are responsible for creating three distinct chemical 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, especially when it is applied quickly. Earthquakes, generally occur in the upper crust and are caused by the rapid movement of relatively brittle materials.

Places with mountain building have a deeper moho.
The global map of the depth of the moho.
The base of the crust is characterized by a large increase in seismic velocity, which measures how fast earthquake 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-cheech; audio pronunciation) in 1909 after studying earthquake wave paths in his native Croatia . The change in wave direction and speed is caused by dramatic chemical differences of the crust and mantle. Underneath the oceans, the Moho is found roughly 5 km below the ocean floor. Under the continents, it is located about 30-40 km below the surface. Near certain large mountain-building events known as orogenies, the continental Moho depth is doubled .

Mantle

The xenolith sits on top of a basalt rock. It has three sides like a pyramid; one of the sides is more altered to iddingsite.
This mantle peridotite xenolith containing olivine (green) is chemically weathering by hydrolysis and oxidation into the brown pseudo-mineral iddingsite, which is a complex of water, clay, and iron oxides. The more altered side of the rock has been exposed to the environment longer.

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 analysis, though information is gathered by studying ophiolites and xenoliths. Ophiolites 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 chapter 4.2 for explanation). Because of this, scientists hypothesize most of the mantle is made of peridotite .

Core

The meteorite is polished showing the Widmanstätten Pattern.
A polished fragment of the iron-rich Toluca Meteorite, with octahedral Widmanstätten Pattern.
The core of the Earth, which has both liquid and solid layers, and consists mostly of iron, nickel, and possibly 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 it was liquefied by intense pressure .

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.

Lithosphere

There are about 10 major plates
Map of the major plates and their motions along boundaries.

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 locations. Continental lithosphere is generally thicker and considerably 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 boundaries are where we see plate tectonics in action—mountain building, triggering earthquakes, and generating volcanic activity.

Asthenosphere

It is thin at a mid-ocean ridge, thick under collisions
The lithosphere-asthenosphere boundary changes with certain tectonic situations.
The asthenosphere is the layer below the lithosphere. Astheno- means lacking strength, and the most distinctive property of the asthenosphere is movement. Because it is mechanically 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 determined this by analyzing seismic waves that pass through the layer. The depth of at which the asthenosphere is found is temperature-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.

Mesosphere

The atoms are arranged.
General perovskite structure. Perovskite silicates (e. g. 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.
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 temperatures. 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.

Inner and Outer Core

Is shows her as a young woman
Lehmann in 1932

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 Danish 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 and is dispersed to the outer layers .

The Earth is cut out with the core being shown.
The outer core’s spin most likely causes our protective magnetic field.

The earth’s liquid outer core is critically important in maintaining a breathable atmosphere and other environmental conditions favorable 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 .

2.2.3 Plate Tectonic Boundaries

The plate thins from continent to ocean
Passive margin
At passive margins the plates don’t move—the continental lithosphere transitions into oceanic lithosphere and forms plates made of both types. A tectonic plate may be made of both oceanic and continental lithosphere connected by a passive margin. North and South America’s eastern coastlines are examples of passive margins. Active margins are places where the oceanic and continental lithospheric tectonic plates meet and move relative to each other, such as the western coasts of North and South America. This movement is caused by frictional drag created 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.

It shows all the types
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.

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2.3 Convergent Boundaries

The legend shows shields, platforms, orogens, basins, large igneous provinces, and extended crust.
Geologic provinces of Earth. Orogenies are labeled light blue.
Convergent boundaries, also called destructive boundaries, 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 lithosphere 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 a process called subduction. When continental plates converge without subduction occurring, this process is called collision.

2.3.1. Subduction

Video showing continental-oceanic subduction, causing volcanism. By Tanya Atwater and John Iwerks.

Subduction occurs when a dense oceanic plate meets a more buoyant plate, like a continental plate or warmer/younger oceanic plate, and descends into the mantle . 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 into a trench. These trenches can be more than twice as deep as the average depth of the adjacent ocean basin, which is usually three to four km. The Mariana Trench, for example, approaches a staggering 11 km .

Many features are labeled on the diagram, but the main idea is the ocean plate descending below the continental
Diagram of ocean-continent subduction.
This drawing depicts a microcontinent riding with a subducting plate, and not being subductable, becoming accreted to the melange.
Microcontinents can become part of the accretionary prism of a subduction zone.

Within the trench, ocean floor sediments are scraped together and compressed between the subducting and overriding plates. This feature is called the accretionary wedge, mélange, or accretionary prism. Fragments of continental material, including microcontinents, riding atop the subducting plate may become sutured to the accretionary wedge and accumulate into a large area of land called a terrane . Vast portions of California are comprised of accreted terranes .

Map showing large areas of the western North American continent that are accreted.
Accreted terranes of western North America. Everything that is not the “Ancient continental interior (craton)” has been smeared onto the side of the continent by accretion from subduction.

When the subducting oceanic plate, or slab, sinks into the mantle, the immense heat and pressure pushes volatile materials like water and carbon dioxide into an area below the continental plate and above the descending plate called the mantle wedge. The 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 plate, the volatile lower the melting point of the mantle wedge, and through a process called flux melting it becomes liquid magma. The molten magma is more buoyant than the lithospheric plate above it and migrates to the Earth’s surface where it emerges as volcanism. The resulting volcanoes frequently appear as curved mountain chains, volcanic arcs, due to the curvature of the earth. Both oceanic and continental plates can contain volcanic arcs.

It is large and offshore.
Location of the large (Mw 8.5-9.0) 1755 Lisbon Earthquake.

How subduction is initiated is still a matter of scientific debate . It is generally accepted that subduction zones start as passive margins, where oceanic and 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 subduction zone is forming off the coast of Portugal . Some scientists have proposed large earthquakes like the 1755 Lisbon earthquake may even have something to do with this process of creating a subduction zone , although the evidence is not definitive. Another hypothesis proposes subduction happens at transform boundaries involving plates of different densities .

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.

The earthquakes follow the slab down.
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.

Subduction zones are known for having the largest earthquakes and tsunamis; they are the only places with fault surfaces large enough to create magnitude-9 earthquakes. These subduction-zone earthquakes not only are very large, but also are very deep. When a subducting slab becomes stuck and cannot descend, a massive amount of energy builds up between the stuck plates. If this energy is not gradually dispersed, it may force the plates to suddenly release along several hundred kilometers of the subduction zone . Because subduction-zone faults are located on the ocean floor, this massive amount of movement can generate giant tsunamis such as those that followed the 2004 Indian Ocean Earthquake and 2011 Tōhoku Earthquake in Japan.

It shows backarc, forearc, and arc.
Various parts of a subduction zone. This subduction zone is ocean-ocean subduction, though the same features can apply to continent-ocean subduction.

All subduction zones have a forearc basin, a feature of the overriding plate found between the volcanic arc and oceanic trench. The forearc basin experiences a lot of faulting and deformation activity, particularly within the accretionary wedge .

In some subduction zones, tensional forces working on the continental plate create a backarc basin on the interior side of the volcanic arc. Some scientists have proposed 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.

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 continental 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.

The subducting plate goes right under the overriding plate
Shallow subduction during the Laramide Orogeny.

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 .

Oceanic-Continental subduction

The thinner ocean plate is going under the thicker continental plate.
Subduction of an oceanic plate beneath a continental plate, forming a trench and volcanic arc.

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

The ocean plate subducts beneath a different ocean plate.
Subduction of an oceanic plate beneath another oceanic plate, forming a trench and an island 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 oceanic plate may remain hidden underwater.. If the volcanoes rise high enough the reach the ocean surface, the chain of volcanism forms an island arc. Examples of these island arcs include the Aleutian Islands in the northern Pacific Ocean, Lesser Antilles in the Caribbean Sea, and numerous island chains scattered throughout the western Pacific Ocean .

2.3.2. Collisions

The two continental plates stay up.
Two continental plates colliding.

In places where two continental plates converge toward each other, subduction is not possible. This occurs where an ocean basin closes and a passive margin is attempted to be driven down with the subducting slab. Instead of subducting beneath the continent, the two masses of continental lithosphere slam into each other in a process known as a collision . Collision zones are known for tall mountains and frequent, large earthquakes, with little to no volcanism. With subduction ceasing with the collision, there is not a process to create the magma for volcanism.

The rock is cray with many circles inside
Pillow lavas, which only form under water, from an ophiolite in the Apennine Mountains of central Italy.

Continental plates are too low density to subduct, which is why the process of collision occurs instead of subduction. Unlike the dense subducting slabs that form from oceanic plates, any attempt to subduct continental plates is short lived. A very rare exception to this is obduction, in which a part of a continental plate is caught beneath an oceanic plate, formed in collision zones or with small plates caught in subduction zones. This imbalance in density is solved by the continental material buoying upward, bringing oceanic floor and/or mantle material to the surface, and is the main source of ophiolites. An ophiolite consists of rocks of the ocean floor that are moved onto the continent, which can also expose parts of the mantle on the surface.

The mountains are loading the crust down, leading to a depressed basin, which is the Persian Gulf
The tectonics of the Zagros Mountains. Note the Persian Gulf foreland basin.
Foreland basins can also develop near the mountain belt, as the lithosphere is depressed due to the mass of the mountains themselves. While subduction mountain ranges can cause this, collisions have many examples, with possibly the best modern example being the Persian Gulf, a feature only there due to the weight of the nearby Zagros Mountains. Collisions are powered by the subducting oceanic lithosphere, and eventually stop as the continental plates combine into a larger mass. In truth, a small portion of the continental crust can be driven down into the subduction zone, though due to its buoyancy, it is (relatively) quickly returned to the surface . Because of the relative plastic nature of continental lithosphere, the zone of deformation is much more broad. Instead of earthquakes located along a narrow boundary, collision earthquakes can be found hundreds of miles from the suture between the land masses.

Pangaea has a crescent shape.
A reconstruction of the supercontinent Pangaea, showing approximate positions of modern continents.
The best modern example of this process occurs concurrently in many locations across the Eurasian continent, and includes mountain building in the Pyrenees (Iberian Peninsula converging with France), Alps (Italy converging into central Europe), Zagros (Arabia converging into Iran), and Himalayan (India converging into Asia) ranges. 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.

Animation of India crashing into Asia, by Tanya Atwater.

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2.4 Divergent Boundaries

Divergent boundaries (sometimes called constructive boundaries) are places where two or more plates have a net movement away from each other. They can occur within a continental plate or an oceanic plate, though the typical pattern is for divergence to begin within continental lithosphere in a process known as “rift to drift,” described below.

2.4.1. Continental Rifting

While the area extends, individual grabens drop down relative to the horsts.
Faulting that occurs in divergent boundaries.

Because of the thickness of continental plates, heat flow from the interior is suppressed. The shielding that supercontinents provide is even stronger, eventually causing upwelling of hot mantle material. This material uplifts, weakens overlying continental crust, and as convection beneath naturally starts pulling material away from the area, the area starts to be deformed by tensional stress. This forms a valley feature known as a rift. These features are bounded by normal faults and include tall shoulders called horsts and deep basins called grabens (or half-grabens when only one-sided). When rifts form, they can eventually causes lakes and even oceans to form as divergent forces continue (see below).

The branches of the plate boundaries are 120 degrees apart.
The Afar Triangle (center) has the Red Sea ridge (center to upper left), Gulf of Aden ridge (center to right), and East African Rift (center to lower left) form a triple junction that are about 120° apart.

This breakup via rifting, while initially seeming random, actually has two influences that dictate the shape and location of rifting. First of all, the stable interiors of some continents, called a craton, are seemingly too strong to be broken apart by rifting. Where cratons are not a factor, rifting typically occurs along the patterns of a truncated icosahedron, or “soccer ball” pattern. This is the geometric pattern of fractures that requires the least amount of energy when expanding a sphere equally in all directions . Taking into account the radius of the Earth, teach side of the soccer ball pattern creates ~110 km long lines of faulting and volcanism, which have 120° angles between them. As the rift goes forward and expands in one location, it often fizzles out in other directions. This is a main factor in forming something known as failed rift arms. Even if the motion stops, a minor basin can develop in this weak spot called an aulacogen, which can form long-lived basins well after tectonic processes stop. These are places where extension started but did not continue. One famous example is the Mississippi Valley Embayment, which forms a depression through which the upper end of the Mississippi River flows. In places where the rift arms do not fail, for example the Afar Triangle, three divergent boundaries can develop near each other forming a triple junction.

There is a series of mountains and valleys
NASA image of the Basin and Range horsts and grabens across central Nevada.

Rifts come in two types: narrow and broad. Narrow rifts contain a concentrated stress or divergent action. The best active example is the East African Rift Zone, where the horn of Africa near Somalia is breaking away from mainland Africa. Lake Baikal in Russia is also an active rift. Broad rifts distribute the deformation over a wide area of many fault-bounded locations, like in the western United States in a region known as the Basin and Range. The Wasatch Fault, which created the Wasatch Range in Utah, marks the eastern edge of the Basin and Range (Animation 1 and Animation 2).

The rift is a series of valleys in eastern Africa.
The narrow East African Rift.
Earthquakes, of course, do occur at rifts, though not at the severity and frequency of some other boundaries. Volcanism is also common in the extended, faulted, and thin lithosphere found at rift zones due to decompressional melting and faults acting as conduits for the lava reaching the surface. Many relatively young volcanoes dot the Basin and Range, and very strange volcanoes occur in East Africa like Ol Doinyo Lengai in Tanzania, which erupts carbonatite lavas, relatively cold liquid carbonate .

South America and Africa rift, forming the Atlantic. Video by Tanya Atwater.

2.4.2. Mid-ocean ridges

The ocean starts as a valley and then gets wider and wider.
Progression from rift to mid-ocean ridge.
As rifting and volcanic activity progress, the continental lithosphere 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 process that gives birth to a new 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.

A mid-ocean ridge, also known as a spreading center, has many distinctive features. They are the only places on Earth where new new oceanic lithosphere is being created, via a slow oozing volcanism. As the oceanic lithosphere spreads apart, rising asthenosphere melts due to decreasing pressure (just like at rifts) and fills in the void, making the new lithosphere and crust. These volcanoes produce more lava than all the other volcanoes on Earth combined, and yet are not usually listed on maps of volcanoes due to the vast majority of mid-ocean ridges being underwater. Only rare locations, such as Iceland, are the volcanism and divergent characteristics seen on land. Technically, these places are not mid-ocean ridges, because they are above the surface of the seafloor.

The map shoes colors that represent different ages.
Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.

This concept of mid-ocean ridges was even hypothesized by Alfred Wegener . Because the lithosphere is very hot at the ridge, it has lower density. This lower density allows it to isostatically ‘float’ higher on the asthenosphere. As the lithosphere moves away from the ridge by continued spreading, the plate cools and starts to sink isostatically lower, creating the surrounding abyssal plains with lower topography . Age patterns also match this idea, with younger rocks near the ridge and older rocks away from the ridge. Sediment patterns also thin toward the ridge, since the steady accumulation of dust and biologic material takes time to accumulate.

Video of spreading along several mid-ocean ridges, showing magnetic striping symmetry. By Tanya Atwater.

The older stripes are farther from the ridge.
A time progression (with “a” being first and “c” being last) showing a spreading center getting wider while recording changes in the magnetic field of the Earth.
Another distinctive feature around mid-ocean ridges is magnetic striping. Called the Vine-Matthews-Morley Hypothesis , it states that as the material moves away from the ridge, it cools below the Curie Point, which is the temperature at which the magnetic field is imprinted on the rock as the rock freezes. Over time, the Earth’s magnetic field has flipped back and forth, and it is this change in the field that causes the stripes . This pattern is a great record of past ocean-floor movements, and can be used to reconstruct past tectonics and determine rates of spreading at the ridges .

Video of the breakup of Pangea and formation of the northern Atlantic Ocean. By Tanya Atwater.

There is a large build up of minerals around the vent
Black smoker hydrothermal vent with a colony of giant (6’+) tube worms.
Mid-ocean ridges also are home to some of the most unique ecosystems ever discovered, found around hydrothermal vents that circulate ocean water through shallow oceanic crust and send it back out rich with chemical compounds and heat. While it was known for some time that hot fluids could be found on the ocean floor, it was only in 1977 when a team of scientists using the Diving Support Vehicle Alvin discovered a thriving community of organisms , including tube worms bigger than people. This group of organisms is not at all dependent on the sun and photosynthesis, but instead relies on chemical reactions with sulfur compounds and heat from within the Earth, a process known as chemosynthesis. Before this discovery, the thought in biology was that the sun was the ultimate source for energy in ecosystems; now we know this to be false. Not only that, some have suggested it is from this that life could have started on Earth , and it now has become a target for extraterrestrial life (e.g. Jupiter’s moon Europa) .

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2.5 Transform Boundaries

Sinistral moves to the left, dextral moves to the right.
The two types of transform/strike slip faults.
A transform boundary (sometimes called a strike slip or conservative boundary) is a place where the motion is of the plates sliding past each other. They can move in either dextral fashion (with the side opposite moving toward the right) or a sinistral fashion (with the side opposite moving toward the left). Most transform boundaries can be viewed as a single fault or as a series of faults. As stress builds on adjacent plates attempting to slide them past each other, eventually a fault occurs and releases stress with an earthquake. Transform faults have a shearing motion, and are common in places where tectonic stresses are transferred. In general, transform boundaries are known for only earthquakes, with little to no mountain building and volcanism (see exceptions below).

The fault runs through California.
Map of the San Andreas fault, showing relative motion.
The majority of transform boundaries are associated with mid-ocean ridges. As spreading centers progress, these aseismic fracture zone transform faults accommodate different amounts of spreading due to Eulerian geometry that a sphere rotates faster in the middle (Equator) than at the top (Poles) than along the ridge. However, the more significant transform faults (in the eyes of humanity) are the places where the motion occurs within continental plates with a shearing motion. These transform faults produce frequent moderate to large earthquakes. Famous examples include California’s San Andreas Fault, both the Northern and Eastern Anatolian Faults in Turkey, the Altyn Tagh Fault in central Asia, and the Alpine Fault in New Zealand.

2.5.1. Transpression and Transtension

The fault is dextral, and has a leftward bend, causing uplift.
A transpressional strike-slip fault, causing uplift called a restraining bend.
In places where transform faults are not straight, they can create secondary faulting. Transpression is defined as places where there is an extra component of compression with shearing. In these restraining bends, mountains can be built up along the fault. The southern part of the San Andreas Fault has a large area of transpression known as the “big bend” and has built, moved, and even rotated many mountain ranges in southern California .

The fault is dextral, and has a rightward bend, causing a valley.
A transtensional strike-slip fault.
Transtension is defined as places where there is an extra component of extension with shearing. In these releasing bends, depressions (and sometimes volcanism) are formed along the fault. The Dead Sea and California’s Salton Sea are examples of basins formed by transtensional forces.

2.5.2. Piercing Points

The offset is to the left.
Wallace (dry) Creek on the Carrizo Plain, California. Note as the creek flows from the northern mountainous part of the image, it takes a sharp right (as viewed from the flow of water), then a sharp left. This is caused by the San Andreas Fault cutting roughly perpendicular to the creek, and shifting the location of the creek over time. The fault can be seen about halfway down, trending left to right, as a change in the topography.
A piercing point is a feature that is cut by a fault, and thus can be used to recreate past movements along the fault. While this can be used on all faults, transform faults are most adapted for this technique. Normal and reverse faulting and/or divergent and convergent boundaries tend to obscure, bury, or destroy these features; transform faults generally do not. Piercing points usually consist of unique lithologic, structural, or geographic patterns that can be matched by removing the movement along the fault. Detailed studies of piercing points along the San Andreas Fault has shown over 225 km of movement in the last 20 million years along three different active traces of the fault.

Video of the origin of the San Andreas fault. As the mid-ocean ridge subducts, the relative motion between the remaining plates become transform, forming the fault system. Note that because the motion of the plates is not exactly parallel to the fault, it causes divergent motion in the interior of North America. By Tanya Atwater.

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2.6 Wilson Cycle & Hot Spots

The diagram shows the last 1000 million years.
Diagram of the Wilson Cycle, showing rifting and collision phases.
The Wilson Cycle is named for J. Tuzo Wilson who first described it in 1966. The Wilson Cycle outlines the origin and subsequent breakup of supercontinents. This cycle has been clearly operating for the last billion years with supercontinents Pangaea and Rodinia, and possibly billions of years before that . The driving force of this is two-fold. The more straightforward mechanism arises from the fact that continents hold the Earth’s internal heat much better than the ocean basins . When continents congregate together, they hold more heat in which more vigorous convection can occur, which can start the rifting process. Mantle plumes are inferred to be the legacy of this increased heat and may record the history of the start of rifting . The second mechanism for the Wilson Cycle involves the destruction of plates. While rifting eventually leads to drifting continents, does their continued movement result from a continuation of the ridge spreading and underlying convection (known as ridge push), or do the plates move because of the weight of the subducting slab sinking via its density (known as slab pull) or the height of the ridge pushing down (known as gravitational sliding) ? To be sure, both are factors in plate movement and the Wilson Cycle. It does appear, in the current best hypothesis, that there is a larger component of slab pull than ridge push .

2.6.1. Hot Spots

The plate is moving to the left, the magma stays in the center am makes a chain of volcanoes.
Diagram showing a non-moving source of magma (mantle plume) and a moving overriding plate.
While the Wilson Cycle can give a general overview of plate motions in the past, another process can give more precise (but mainly recent) plate movement. A hot spot is an area of rising magma, causing a series of volcanic centers which form volcanic islands in the ocean or craters/mountains on land. There is not a plate tectonic process, like subduction or rifting, that causes this volcanic activity; it seems as if totally disconnected to plate tectonics processes. Also first postulated by J. Tuzo Wilson, in 1963, hot spots are places that have a continual source of magma with no earthquakes, besides those associated with volcanism. The classic idea is that hot spots do not move, though some evidence has been suggested that the hot spots do move as well . Even though hot spots and plate tectonics seem independent, there are some relationships between them, and they have two components: Firstly, there are several hot spots currently and several others in the past that are believed to have begun at the time of rifting. Secondly, as plate tectonics moves the plates around, the assumed stationary nature of hot spots creates a track of volcanism that can measure past plate movement. By using the age of the eruptions from hot spots and the direction of the chain of events, one can identify a specific rate and direction of movement of a plate over the time the hot spot was active.

Hotspots are scattered around the world.
Map of world hotspots. Larger circles indicate more active hotspots.
Hot spots are still very mysterious in their exact mechanism of magma generation. The main camps on hot spot mechanics are diametrically opposed. Some claim deep sources of heat, from as deep as the core, bring heat up to the surface in a structure called a mantle plume . Some have argued that not all hot spots are sourced from deep within the planet, and are sourced from shallower parts of the mantle . Others have mentioned how difficult it has been to image these deep features . The idea of how hot spots start is also controversial. Usually, divergent boundaries are tabbed as the start, especially during supercontinent break up, though some question whether extensional or tectonic forces alone can explain the volcanism . Subducting slabs have also been named as a cause for hot spot volcanism . Even impacts of objects from space have been used to explain plumes . However they are formed, there are dozens found throughout the Earth. Famous examples include the Tahiti, Afar triangle, Easter Island, Iceland, the Galapagos Islands, and Samoa. The United States has two of the largest and best-studied examples: Hawai’i and Yellowstone.

Hawaiian Hot Spot

There are a series of island and seamounts in the Pacific Ocean, with a bend in the middle.
The Hawaii-Emperor seamount and island chain.
The big island of Hawai’i is the active end of the Hawaiian-Emperor seamount chain, which stretches across the Pacific for almost 6000 km. The evidence for this hot spot goes back at least 80 million years, and presumably the hot spot was around before then, but rocks older than that in the Pacific Plate had already subducted. The most striking feature of the chain is a large bend that occurs about halfway through the chain that occurred about 50 million years ago . The change in direction has been more often linked to a plate reconfiguration , but also to other things like plume migration . While it is often assumed that mantle plumes do not move, much like the plumes themselves, this idea is under dispute by some scientists.

The islands get older to the left.
Diagram of the Hawaiian hotspot and islands that it formed.
3D seismic imaging (called tomography) has mapped the Hawaiian mantle plume at depths including the lower mantle . Within the Hawaiian Islands, there is clear evidence of the age of volcanism decreasing, including island size, rock age, and even vegetation. Hawai’i is one of the most active hot spots on Earth. Kilauea, the main active vent of the hot spot eruption, has continually erupted since 1983.

Yellowstone Hot Spot

The hotspot started near the Idaho-Oregon-Nevada boarder, then moved toward its present location neat the Wyoming-Idaho-Montana boarder.
The track of the Yellowstone hotspot, which shows the age of different eruptions in millions of years ago.
The Yellowstone Hot Spot is formed from rising magma, much like Hawai’i. The big difference is Hawai’i sits on a thin oceanic plate, which makes the magma easily come to the surface. Yellowstone, however, is on a continental plate. The thickness of the plate causes the generally much more violent and (thankfully) less frequent eruptions that have carved a curved path in the western United States for over 15 million years (see figure). Some have speculated an even earlier start to the hot spot, tying it to the Columbia River flood basalts and even 70 million-year-old volcanism in Canada’s Yukon .

The eruptions trend eastward due to prevailing winds.
Several prominent ash beds found in North America, including three Yellowstone eruptions shaded pink (Mesa Falls, Huckleberry Ridge, and Lava Creek), the Bisho Tuff ash bed (brown dashed line), and the modern May 18th, 1980 ash fall (yellow).

The most recent large eruption formed the current caldera and the Lava Creek tuff. This eruption threw into the atmosphere about 1000 cubic kilometers of magma erupted 631,000 years ago . Ash from the eruption has been found as far away as Mississippi. The next eruption, when it occurs, should be of similar size. This would be a large calamity to not only the western United States, but also the world. These so-called “supervolcanic” eruptions have the potential for volcanic winters lasting years. With so much gas and ash filling the atmosphere, sunlight is blocked and unable to reach Earth’s surface as well as normal, which could drastically alter global environments and send worldwide food production into a tailspin.

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Summary

Plate tectonics is a unifying theory in geology, because it can explain almost all of Earth’s geologic activity. Since its inception in the 1950s and 1960s, all work done by geologists has been using this new perception of the world, and research before then must be considered either obsolete, misguided, or useful only for a focus on the first-level observations. Plate tectonics states the Earth’s surface is broken into several plates. Plates are composed of a solid and relatively brittle lithosphere, and a mobile and ductile asthenosphere below, which drives the above plates to move, somewhat similarly to objects sitting on a conveyor belt. Where these plates meet, they can move together convergently, apart divergently, or shear in a transform boundary. Earthquake and volcanoes are mainly formed by these interactions. The main exception is hot spots, which are zones of rising magma that are not caused by plate movement.

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References