2 Plate Tectonics
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
- Understand the layers of the Earth, which can be described by physical and chemical means
- Infer that movement at the three types of plate boundaries is the main cause of earthquakes, volcanoes, and mountain building
- Identify that convergent boundaries are the places where plates come together, and include subduction and collisions as the types of convergent boundaries
- Name divergent boundaries as places where plates separate, and include rifts and mid-ocean ridges as the types of divergent boundaries
- Explain that transform boundaries are places where plates shear past each other, side to side
- Describe the Wilson Cycle as the process of rifting of continents, creating ocean basins, leading to subduction, and finally ending with the closing of the ocean basins
- Explain how the tracks of hot spots, places that have continual rising magma, can be used to calculate rates of plate motion
Revolution is a word usually reserved for significant political or social changes. In science, there have been several revolutions of ideas (paradigm shifts) that have forced scientists to re-examine their entire field. Darwin’s On the Origin of Species in 1859, Mendel’s discovery of genetics in 1866, and the discovery of DNA by James Watson, Francis Crick, and Rosalind Franklin in the 1950s did that for biology. Albert Einstein’s relativity and quantum mechanics concepts in the early twentieth century did the same for Newtonian physics. Plate tectonics was just as revolutionary for geology. Plate tectonics, the idea that the outer part of the Earth moves and causes earthquakes, mountains, and volcanoes, is the lens through which geologic study must be viewed because all earth processes make more sense in this context. Its importance in understanding how the world works is why it is the first topic of discussion in this text.
2.1 Alfred Wegener’s Continental Drift Hypothesis
Alfred Wegener (1880-1930) was a German scientist who specialized in meteorology and climatology. He had a knack for questioning accepted ideas, and this started in 1910 when he disagreed with isostasy (vertical land movement due to weight being removed or added) as the explanation for the Bering Land Bridge . After literary reviews, he published a hypothesis stating the continents had moved in the past . While he did not have the precise mechanism worked out, he had a long list of evidence that backed up his hypothesis.
2.1.1 Early Evidence for Continental Drift Hypothesis
The first piece of evidence is that the shape of the coastlines of some continents fit together like pieces of a jigsaw puzzle. Since the first world map, people have noticed the similarities in the coastlines of South America and Africa , and the continents being ripped apart had even been mentioned as an explanation . Antonio Snider-Pellegrini even did preliminary work on continental separation and matching fossils in 1858.
What Wegener did differently than others was synthesize a large amount of data in one place, as well as use the shape of the continental shelf, the true edge of the continent, instead of the current coastline, which fit even better than previous efforts .
For example, the primitive aquatic reptile Mesosaurus was found on the separate coastlines of the continents of Africa and South America, and the reptile Lystrosaurus was found on Africa, India, and Antarctica. These were land dwellingcreatures that could not have swam across an entire ocean. This was explained away by opponents of continental drift by land bridges. The land bridges, which, in the hypothesis of proponents, had eroded away, allowed animals and plants to move between the continents . However, some of the presumed land bridges would have had to have stretched across broad, deep oceans. Another large piece of evidence was climate anomalies. Late Paleozoic glacial evidence was found in widespread, warm areas like southern Africa, India, Australia, and the Arabian subcontinent. Wegener himself had found evidence of tropical plant fossils in areas north of the Arctic Circle. According to Wegener, the simpler explanation that fit all the climate, rock, and fossil observations, especially as more data were collected, involved moving continents.
2.1.2 Proposed Mechanism for Continental Drift
Wegner’s work was considered a fringe theory for his entire life. One of the biggest apparent flaws and easiest dismissals of Wegener’s hypothesis was a mechanism for movement of the continents. Obviously, the continents did not appear to move, and exceptional evidence would need to be provided to change the minds of the establishment, including a mechanism for movement. Other pro-continental drift followers had used expansion, contraction, or even the origin of the Moon as ideas to how the continents moved. Wegener used centrifugal forces and precession to explain the movement, but that was proven wrong . He had some speculation about seafloor spreading, with hints of convection , but these were unsubstantiated. As it turns out, convection has been revealed as a major force in driving plate movements, according to current knowledge.
2.1.3 Development of Plate Tectonic Theory
Wegener died in 1930 on an expedition in Greenland. In his lifetime, he was poorly respected and his ideas of moving continents seemed destined to be lost to history as a fringe idea. However, starting in the 1950s, evidence started to trickle in that made continental drift more viable. By the 1960’s, there was enough evidence supporting Wegener’s missing mechanism – seafloor spreading. This allowed for the hypothesis of continental drift to develop into the Theory of Plate Tectonics. Widespread acceptance among scientists has transformed Wegener’s hypothesis to a Theory. Today, GPS and earthquake data continue to back up the theory. Below are the pieces of evidence that allowed the transformation.
Mapping of the Ocean Floors
Starting in 1947 and using an adaptation of SONAR, researchers began to map a poorly-understood topographic and thermal high in the mid-Atlantic . Bruce Heezen and Marie Tharp were the first to make a detailed map of the ocean floor, and this map revealed the mid-Atlantic Ridge, a basaltic feature unlike the continents . Initially, this was thought to be part of an expanding Earth , or a mechanism for the growth of the ocean . Transform faults were also added to explain movements more completely . When it was later realized that earthquake epicenters were also located within this feature , the idea that this was part of continental movement took hold.
Another way the seafloor was mapped was magnetically. Scientists had long known of strange magnetic anomalies (magnetic values that differ from expected values) associated with the ocean floor. This tool was adapted by geologists later for further study of the ocean depths, including strange alternating symmetrical stripes on both sides of a feature (which would be discovered later as the mid-ocean ridge) showing reversing magnetic pole directions . By 1963, these magnetic stripes would be explained in concordance with the spreading model of Hess and others ( , see also for information on Lawrence W. Morley’s contribution). This animation of paleomagnetism and sea-floor spreading illustrates this effect.
Seafloor sediment was also an important feature that was measured in the oceans, both with dredging and with drilling. Sediment was believed to have been piling up on ocean floors for a very long time in a static model of accumulation. Initial studies showed less sediment than expected, and initial results were even used to argue against continental movement . With more time, researchers discovered thinner sediment close to ridges, indicating a younger age . The seafloor spreading model will be discussed further below in chapter 2.4.2.
Around the same time that mid-ocean ridges were being investigated, ocean trenches and island arcs were also being linked to seismic action, thus explaining the opposite sides of the movement of plates . A zone of deep earthquakes that lay along a plane trending from the surface near the trenches to inside the Earth beneath the continents and island arcs were recognized independently by several scientists . Today called the Wadati-Benioff zone, it was an important piece of the puzzle.
Magnetic field mapping, as mentioned above, was not the only way paleomagnetism was used in the development of plate tectonics. In fact, the first new hard evidence that supported plate motion came from paleomagnetism. Paleomagnetism is the study of magnetic fields frozen within rocks, basically a fossil compass. This is typically most useful with igneous rocks where magnetic minerals like magnetite crystallizing in the magma align with the Earth’s magnetic field and in the solid rock point to the paleo-magnetic north. The earth’s magnetic field creates flux lines surrounding the magnetic north and south poles (like a bar magnet) which are both close to the Earth’s rotational north and south poles. In igneous rocks, magnetic minerals align parallel with these flux lines as shown in the figure. Thus both magnetic inclination, related to latitude, and declination related to magnetic north are preserved in the rocks.
Scientists had noticed for some time that the magnetic north to which many rocks pointed was nowhere close to current magnetic north. This was explained away by implying the magnetic north pole moved over time (which it does, as shown in the animation). Eventually, scientists started to realize that moving continents explained the data even better than moving the pole around alone .
Using all of the evidence mentioned above, the idea of plate tectonics took shape. J. Tuzo Wilson was the first scientist to put the entire picture together of an opening and closing ocean . Before long, models were proposed showing the plates moving with respect to each other with clear boundaries between them , and scientists had also started to piece together complicated tectonic histories . The plate tectonic revolution had taken hold.
2.2 Layers of the Earth
In order to understand the details of plate tectonics, one must first understand the layers of the Earth. Humankind has very limited first-hand information regarding what is below; most of what we know is pieced together from models, seismic waves, and assumptions based on meteorite material. In general, the Earth can be divided into layers based on chemical composition and physical characteristics.
2.2.1 Chemical Layers
The Earth has three main divisions based on their chemical composition, which means chemical makeup. Certainly, there are countless variations in composition throughout the Earth, but it appears that only two major changes take place, leading to three distinct chemical layers.
The outermost chemical layer, and the layer you currently reside on, is known as the crust. The crust has two types: continental crust, which is relatively low density and has a composition similar to granite, and oceanic crust, which is relatively high density (especially when it is cold and old) and has a composition similar to basalt. In the lower part of the crust, rocks start to be more ductile and less brittle, because of added heat. Earthquakes, therefore, generally occur in the upper crust.
At the base of the crust is a large change in seismic velocity called the Mohorovičić Discontinuity, or moho for short. This was discovered by Andrija Mohorovičić (pronounced mo-ho-ro-vee-cheech) in 1909 by studying earthquake wave paths in his native Croatia . It is caused by the dramatic change in composition that occurs between the mantle and the crust. Underneath the oceans, the moho is about 5 km down. Under continents, the average is about 30-40 km, except near a large mountain-building event (known as an orogeny), where that thickness is about doubled .
The mantle is the layer below the crust and above the core, and is the largest 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 waves, though some direct information can be gathered from ophiolites (see ch. 2.3.2). Also, carried within magma are xenoliths, which are small chunks of lower rock carried to the surface by eruptions. These xenoliths are mainly made of the rock peridotite, which on the scale of igneous rocks is ultramafic (see ch. 4.2). We assume the majority of the mantle is made of peridotite .
The core of the Earth, which has both liquid and solid components (see below), is made mostly of iron and nickel and possibly minor oxygen . First discovered in 1906 by looking into seismic data, it took the union of modeling, astronomical insight, and seismic data to arrive at the idea that the core is mostly metallic iron . Meteorites contain much more iron than typical surface rocks, and if meteoric material is what made the Earth, the core would have formed as dense material (including iron and nickel) sank to the center of the earth via its own weight as the planet formed, heating the Earth intensely .
2.2.2 Physical Layers
The Earth can also be broken down into 5 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.
The lithosphere is the outermost physical layer of the Earth. Including the crust, it has both an oceanic component and a continental component. Oceanic lithosphere, ranging from a thickness of zero (at the forming of new plates about the mid-ocean ridge) to 140 km, is thin and relatively rigid. Continental lithosphere is considerably more plastic in nature (especially with depth) and is overall thicker, from 40 to 280 km thick . Most importantly, the lithosphere is not continuous. It is broken into several segments that geologists call plates. A plate boundary is where two plates meet and move relative to each other. It is at and near plate boundaries where the real action of plate tectonics is seen, including mountain building, earthquakes, and volcanism (see below).
The asthenosphere is the layer below the lithosphere. The most distinctive property of the asthenosphere is movement. While still solid, over geologic time scales it will flow and move because it is mechanically weak . It is in this layer that movement, partly driven by convection of intense interior heat, allows the lithospheric plates to move. Since certain types of seismic waves pass through the asthenosphere, we know that it is solid, at least at the very short time scales of the passage of seismic waves . The depth (and occurrence) of the asthenosphere is dependent on heat, and can be very shallow at mid-ocean ridges and very deep in plate interiors and beneath mountains.
The mesosphere, or lower mantle as it is sometimes called, is more rigid and immobile than the asthenosphere, though still hot. This can be attributed to increased pressure with depth. Between approximately 410 and 660 km depth, the mantle is in a state of transition as minerals with the same composition are changed to various forms (pseudomorphs) dictated by the conditions of increasing pressure . This is shown by changes in seismic velocity, and this zone also can be a physical barrier to movement . Below this zone, the mantle is relatively uniform and homogeneous, as no major changes occur until the core is reached.
Inner and Outer Core
The outer core it the only entirely liquid layer found within Earth. It starts at 2,890 km (1,795 mi) depth and extends to 5,150 km (3,200 mi). Inge Lehmann, a Danish geophysicist, was the first to prove that there was an inner core that was solid within the liquid outer core based on analyzing seismic data . The solid inner core is about 1,220 km (758 mi) thick, and the outer core is about 2,300 km (1,429 mi) thick .
It seems like a contradiction that the hottest part of the Earth is solid, as high temperatures usually lead to melting or boiling. The solid inner core can be explained by understanding that the immense pressure inhibits melting , though as the Earth cools by heat flowing outward, the inner core grows slightly larger over time . As the liquid iron and nickel in the outer core moves and convects, it becomes the most likely source for Earth’s magnetic field . This is critically important to maintaining the atmosphere and conditions on Earth that make it favorable to life. Loss of outer core convection and the Earth’s magnetic field could strip the atmosphere of most of the gases important to life and dry out the planet, much like what has happened (and continues to happen) to Mars .
2.2.3 Plate Tectonic Boundaries
Places where oceanic and continental lithospheric plates meet and move relative to each other are called active margins (e.g. the western coasts of North and South America). A location where continental lithosphere transitions into oceanic lithosphere without movement is known as a passive margin (e.g. the eastern coasts of North and South America). This is why a plate may be made of both oceanic and continental lithosphere. In the process of plate tectonics, the lithospheric plates movement is the main force that causes the majority of features and activity on the Earth’s surface that can be attributed to plate tectonics. This movement occurs (at least partially) via the drag of motion within the asthenosphere and/or because of density.
As they move, the plates interact with each other at the boundaries between the plates. These interactions are the primary drivers of mountain building, earthquakes, and volcanism on the planet. In a simplified plate tectonic model, plate interaction can be placed in one of three categories. In places where plates move toward each other, the boundary is known as convergent. In places where plates move apart, the boundary is known as divergent. In places where the plates slide past each other, the boundary is known as transform. The next three subchapters will explain the details of the movement at each type of boundary.
2.3 Convergent Boundaries
Convergent boundaries (sometimes called destructive boundaries) are places where two or more plates have a net movement toward each other. Convergent boundaries, more than any other, are known for orogenesis, the process of building mountains and mountain chains (orogeny). The key to convergent boundaries is understanding the density of each plate involved in the movement. Continental lithosphere is always lower density and is buoyant when compared to the asthenosphere. Oceanic lithosphere, on the other hand, is more dense than continental lithosphere and, when old and cold, may even be more dense than the asthenosphere. When plates of different density converge, the more dense plate sinks beneath the less dense plate. This process is called subduction.
Subduction is when oceanic lithosphere descends into the mantle due to its density . The average rate of subduction of oceanic crust worldwide is 25 miles per million years , about a half inch per year. Continental lithosphere can partially subduct if attached to sinking oceanic lithosphere, but its buoyancy does not allow it to fully subduct. As the plate descends, it also pulls the ocean floor down in a feature known as a trench. On average, the ocean floor is around 3-4 km deep. In trenches, the ocean can be more than twice as deep, with the Mariana Trench approaching a staggering 11 km .
Within the trench is a feature called the accretionary wedge (sometimes known as melange or accretionary prism), which is a mix of ocean floor sediments that are scraped and compressed at the boundary between the subducting plate and the overriding plate. Sometimes pieces of continental material, like microcontinents, riding with the subducting plate will become sutured to the accretionary wedge, forming a terrane . In fact, large portions of California are comprised of accreted terranes .
When the subducting plate (known as a slab) submerges into the depths of the mantle, the heat and pressure are so immense that lighter materials (known as volatiles) like water and carbon dioxide are pushed out of the subducting plate into an area called the mantle wedge above. The volatiles are released mostly via hydrated minerals that revert to non-hydrated forms in these conditions. These volatiles, when mixed with asthenospheric material above the plate, lower the melting point of the material; at the temperature of that depth, the material melts to form magma. This process of magma generation is called flux melting. Magma, because of its lower density, migrates toward the surface, creating volcanism. This forms a curved chain of volcanoes (due to many boundaries being curved on a spherical earth), a feature called an arc. The overriding plate (which contains the arc) can be either oceanic or continental, where some features are different, but the general architecture remains the same. Below is a description of each type of subduction.
How subduction initiates is still a matter of some debate . Presumably, this would start at passive margins where oceanic and continental crust meet. At the current time, there is oceanic lithosphere that is denser than the underlying asthenosphere on either side of the Atlantic Ocean that is not currently subducting. Why has it not turned into an active margin? Firstly, there is strength in the connection between the dense oceanic lithosphere and the less dense continental lithosphere it is connected to, which needs to be overcome . Gravity could cause the denser oceanic plate to force itself down or the plate can start to flow ductility at a low angle . There is evidence that new subduction is starting off the coast of Portugal . Large earthquakes, like the 1755 Lisbon Earthquake, may even have something to do with this process of creating a subduction zone , though it is not definitive. Transform boundaries that have brought areas of different densities together are also thought to possibly start subduction .
Besides volcanism, subduction zones are also known for the largest earthquakes in the world. In places, the entire subducting slab can become stuck, and when the energy has built up too high, the entire subduction zone can slide at once along a zone extending for hundreds of kilometers along the trench, creating enormous earthquakes and tsunamis . The earthquakes can not only be large, but they can be deep, outlining the subducting slab as it descends. Subduction zones are the only places on Earth with fault surfaces large enough to create magnitude 9 earthquakes. Also, because the faulting occurs beneath seawater, subduction also can create giant tsunamis, such as the 2004 Indian Ocean Earthquake and the 2011 Tōhoku Earthquake in Japan.
Subduction, which is a convergent motion, can have varying degrees of convergence. In places that have a high rate of convergence, mostly due to young, buoyant oceanic crust subducting , the subduction zone can create faulting behind the arc area itself, known as back-arc faulting. This faulting can be tensional (see below, in chapter 2.4), or this area is subject to compressional forces. A modern example of this occurs in the two ‘spines’ of the Andes Mountains. In the west, the mountains are formed from the volcanic arc itself; in the east, thrust faults have pushed up another, non-volcanic mountain range still part of the Andes. This type of thrusting can typically occur in two styles: thin-skinned, which only faults surficial rocks, and thick-skinned, which thrusts deeper crustal rocks. Thin-skinned deformation notably occurred in the western U.S. during the Cretaceous Sevier Orogeny . Later (or near the end of the Sevier Orogeny), thick-skinned deformation also occurred in the Laramide orogeny . The Laramide Orogeny is also known for another subduction feature: flat slab subduction. When the slab subducts at such a low angle, there is interaction between the slab and the overlying continental plate. Magmatic activity can give rise to mineral deposits, and deformation can occur well into the interior of the overriding plate . All subduction zones have a forearc basin, which is an area between the arc and the trench. This is an area of a high degree of thrust faulting and deformation, seen mostly within the accretionary wedge . There are also places where the convergence shows the results of tensional forces. A variety of causes have been proposed for this, including slab roll-back due to density or ridge migration . This causes extension behind the volcanic or island arc, known as a back-arc basin. These can have so much extension that rifting and divergence can develop, though they can be more asymmetric than their mid-ocean ridge counterparts .
Oceanic-continental subduction occurs when an oceanic plate dives below continental plates. This boundary has a trench and mantle wedge, but the volcanoes are expressed in a feature known as a volcanic arc. A volcanic arc is a chain of mountain volcanoes, with famous examples including the Cascades of the Pacific Northwest and the Andes of South America .
Oceanic-oceanic subduction zones have two significant differences from boundaries that have continental lithosphere. Firstly, each plate in an ocean-ocean plate boundary is capable of subduction. Therefore, it is typical that the denser (and therefore older and colder) of the two plates is the one that subducts. Secondly, since both plates are oceanic, volcanism creates islands instead of continental mountains. This chain of active volcanoes is known as an island arc. There are many examples of this on Earth, including the Aleutian Islands off of Alaska, the Lesser Antilles in the Caribbean , and several island arcs in the western Pacific.
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.
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.
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.
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.
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
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).
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, this includes ~110 km segments of deformation and volcanism which have 120° turns, 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.
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).
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 .
2.4.2. Mid-ocean ridges
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.
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.
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 .
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) .
2.5 Transform Boundaries
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 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
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 .
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
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.
2.6 Wilson Cycle & Hot Spots
The Wilson Cycle, named for J. Tuzo Wilson who first described it , describes 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
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 described by J. Tuzo Wilson, 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, you can identify a specific rate and direction of movement of a plate over the time the hot spot was active.
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
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.
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 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 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, which could send worldwide food production into a tailspin.
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.