Outline of Historical Geology by Ellin Beltz
Introduction, Environment, Stratigraphy
Taxonomy and Taphonomy
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A brief history of Earth
|© 2006 by Ellin Beltz|
Historical Geology - Part IV - Plate Tectonics
Continental drift and plate tectonics
After accurate printed maps became widely available in the late 1600s, many speculated about the close match of the coastlines of South America and Africa.
In 1915 Alfred Wegener [1880-1930], a German meteorologist who had explored widely in Greenland and served in World War I, suggested that continents were adrift on the surface of the earth and had previously joined together. He named the supercontinent formed by the collision of all Earth's landmasses "Pangaea," and analyses rock and fossil data to back up his ideas and proposals.
Readings from Wegener's book, "The Origin of Continents and Oceans," are appended. While not everything Wegener says about the formation and movement of oceanic and continental plates is accurate, it is interesting to see how much information was available in 1929 when the fourth revised edition of his original work was published. He correlated rock formations across oceans, matched continental edges, noted the odd pattern of depth soundings in the oceans, the presence of offshore trenches and island arcs, and fossil plant and animal data to provide evidence in support of his theory of continental drift. Wegener died on an expedition in 1930 to the inland icecap of Greenland. After his death, others championed his suggestions about plate movement. German and Southern Hemisphere geologists accepted his general proposal and wrote papers providing supporting evidence, but English and American geologists vehemently denied the possibility of moving continents.
This attitude persisted until the 1960s when R.S. Dietz proposed a mechanism of "sea floor spreading" and accompanying subduction. Other workers had discovered that igneous rocks had two magnetic polarities, suggesting that the Earth's magnetic field had shifted at specific times in the past. The study of remnant magnetism and accompanying "polar wandering" supported movement of continents over time. Finally, the work of F.J. Vine and D.H. Matthews described magnetic reversals recorded in seafloor igneous rocks. They proved that new rock was constantly being formed at ocean ridges. Remnant paleomagnetism revealed not only the era in which the rock was formed but the rate of spreading during that time. In 1965, J. Tuzo Wilson described transform faults which connect areas of spreading and subduction. Seafloor spreading and transform faulting provided the missing mechanism for Wegener's continental drift. The new theory of plate tectonics incorporates earthquake data, remnant paleomagnetism, mineralogy, seafloor sonar mapping, direct sampling and satellite telemetry.
Seismic data reveals the internal structure of the earth. Earthquakes are vibrations, measured with seismographs, which are caused by rock fracture. Earthquake energy is transmitted in two groups: surface waves (travel on surface) and body waves (travel through Earth's interior). Body waves are divided into primary waves (P-waves) and secondary waves (S-waves). P-waves are faster; their motion is push/pull. S-waves are slower; their motion tends to shake rocks at right angles to direction of travel. P-waves travel through solids, liquids, and gases. S-waves only travel through solids. P-waves travel roughly twice as fast as S-waves (1.7 times as fast). The actual place where the rock breaks is called the focus. The place above the focus on the surface of the Earth is called the epicenter and can be calculated from seismic data by using the difference in velocities (travel time) of P and S waves.
Earthquake epicenters and depth to focus correlate closely with plate boundaries. Earthquake intensity can be measured with various arbitrary systems including the Mercalli intensity scale which assesses damage at a specific place and the Richter scale which determines the magnitude and amplitude of the largest waves. The Mercalli scale can be applied to historic earthquakes by studying reports of damage (church bells rung, chimneys collapsed) while the Richter scale is used only for earthquakes for which seismic data is available. Each notch on the Richter scale represents 30 times more energy than the previous. Earthquakes produce sea-waves called tsunamis, landslides, ground liquefaction in saturated soils (like Mission District, San Francisco), displacement of surface features like fences and streams, and fires in populated areas.
Seismic waves gave us our first look at the interior of the earth. What they revealed has subsequently been improved by other forms of imaging. We now believe that the earth's inner core is a solid iron crystal the axis of which is tilted about 10 degrees from the axis of the whole earth and rotates relative to a point on the surface once every 400 years. The solid iron inner core is 1216 km radius (2432 km diameter). It is surrounded by a liquid and extremely hot outer core which measures 2270 km in radius.
The speed of the core's independent spin is between 0.4 and 1.8 degrees a year which works out to a full lap relative to a fixed point on the surface about every 400 years. The leading suggestion for the cause of the independent motion is an interaction between magnetic fields generated by fluids moving in the Earth's outer core which is molten iron and the inner, solid crystal. The implications of differential rotation between the core and the surface are hot topics at professional meetings and in various physical and geophysical journals. The iron crystal in the core is longer from north to south, but it does not exactly match Earth's poles. As a result of the core's orientation and rotation, "magnetic north" moves over time. The physical pole of the planet also wobbles over time, a process called the "precession of the equinoxes."
The next layer out from the core is the mantle which behaves in a plastic manner, convecting heat with slow moving "blobs" of colder rock at a depth of 600 km. The colder rock is apparently remnants of previously subducted slabs. The mantle measures 2885 km in radius and is ultramafic in composition Finally, the crust of the earth is composed of two kinds of material: dense, dark mafic oceanic crust; and lighter (both in color and density), felsic continental crust. Oceanic crust is approximately 5 km thick, while average continental crust thickness is about 40 km.
Earth's crust and tectonic boundariesThe crust of the earth is divided into polygonal plates. The thirteen large plates shown on Figure 3 have been named and are divided into two types: Plates with only oceanic crust and the occasional island (Pacific plate, Juan de Fuca plate, Cocos plate, Nazca plate, Caribbean plate, Scotia plate, Philippine plate); and plates with both continental and oceanic crust (Eurasian plate, African plate, Australian-Indian plate, North American plate, South American plate, Antarctic plate). The "Physiographic Chart of the Sea Floor" handout shows major underwater structures on a Mercator Projection and labels major features of the sea floor including trenches, ridges, and fracture zones. A corresponding map of the Earth's surface, such as found in any atlas indicates and names the mountain ranges, volcanos, rift valleys/major river systems, and other non-tectonic surface features.
Whether on land or under the water, three types of stress (compression, tension and shear) result in three types of plate boundaries (convergent, divergent and transform). Boundaries tend to join in threes; these locations are called triple junctions. An equatorial cross-section of the Earth shows the distribution of spreading ridges and subduction zones as well as continental masses and island arcs. The ridges and trenches do not alternate in a regular pattern, suggesting that the underlying mantle is not regular.
Plate tectonic theory is supported by paleomagnetic data which shows that the Earth's magnetic field polarity has reversed at intervals in the past. Igneous rocks record paleomagnetism which can be recorded and mapped to show the changing positions of the continents and oceanic plates though time. But since we have found no seafloor older than 180 Mybp, continental reconstructions before that time are based only on continental rocks and the occasional bit of ancient seafloor pasted onto the continents during collision. Another support for the movement of crustal plates comes from the study of hotspots. The most studied hot-spot is that which underlies the Hawaiian Islands. Heat from that mantle plume domes and fractures the lithosphere through which hot mafic magma erupts forming an island. Over time, as the plate shifts position relative to the hot spot, a chain of islands is formed which shows the distance in both space and time from the hot spot.
Present day patterns show shallow earthquakes at divergent and transform boundaries and deep-focus quakes along subduction trenches at convergent boundaries. The centers of the major plates are usually seismically inactive - however, major earthquakes have occurred in these presumed stable continental interiors. The strongest earthquakes recorded in North America occurred in the winter of 1811 to 1812 centered around New Madrid, Missouri. Reelfoot lake was created during this seismic event which also reversed the flow of the Mississippi for several days and rang church bells in New York and Boston. Fortunately, the area was not greatly populated. Another New Madrid earthquake of the same magnitude would devastate St. Louis and its effects might extend to Chicago. Other earthquakes have been recorded from "stable continental cratonic areas." A December, 1993 quake in central India killed thousands of people.
Various mechanisms for the driving forces of plate tectonics have been proposed. Similarities between the proposals include suggestions that heat convection in the mantle influences movement of the plates and that the whole process repeats at about a 300-400 million year cycle (supercontinent, rifting, spreading, subducting, new supercontinent). Most geologists accept that there was a supercontinent about 600 million years ago (the old red sandstone continent of early geologists), and another at about 300 million years ago (Wegener's Pangaea). At present the continents are fairly widespread, although there is a concentration of continents in the northern hemisphere which may be the beginning of the next supercontinent.
Composition and movement of tectonic platesThere are two kinds of tectonic plates which form the surface of the Earth: oceanic plates and continental plates.
Each type of plate is the opposite of each other, one is heavy - the other is light, one preserves old rocks - the other is but recently formed, one is submerged beneath deep water - the other is generally exposed to the air or only shallowly submerged below the sea. Together they form the crust of the Earth which moves slowly in response to forces from the mantle and the core below.
Oceanic plates are composed of mafic rocks such as gabbro and basalt erupted or intruded as lava from a direct mantle-derived source; have a specific gravity of 3.0 and up; are approximately 5 kilometers thick (3.1 miles); are covered by ocean water; and are all younger than 180 million years with the exception of small bits of older oceans which have been welded onto continents. Oceanic plates occasionally contain basalt platforms and submerged islands which can make sections of these plates have a thickness of up to 40 kilometers (24.8 miles). Dredging and drilling reveal that many of these platforms and submerged islands (seamounts) have had full suites of flora and fauna prior to being lost below the seas. Rocks of the oceanic plates preserve a record of remanent magnetism for the last 180 million years and permit accurate continental reconstructions from then to now.
Continental plates are composed of intermediate and felsic rocks such as rhyolite, andesite, granite and diorite which have specific gravities less than 3.0 and usually less than 2.5; are composed of recycled and reworked older rocks; are between 25 and 40 kilometers thick (15.5 to 24.8 miles); and are considerably older than the oceans. The oldest continental rocks have radiometric ages in excess of three billion years. Continental plates may be exposed to weathering under air, fresh water, or shallowly below sea-level. The depth of "epicontinental seas" or "flooded continents" rarely exceeds several hundred feet. In contrast, the abyss of the deep ocean may extend to 15,000 feet below sea level. We now understand that the cratons, or core portions, of the continents are complex, not a simple bit of volcanic rock representing the oldest continental crust.
It appears that the first crust was all of a type similar to, but heavier than, oceanic crust. It was a thin, cold layer on top of the seething heat of the core and mantle of the coalescing Earth. At this time, the atmosphere was not dominated by oxygen, so the entire geochemistry of the earliest times is vastly different than now. Iron precipitated from sea water and was laid down in banded layers as sediment. If life was present at this time, it was in the form of single-celled or multicelled bacteria such as the 3.2 billion year old bacteria found by Dr. Schopf in Australian sediments. Communal bacteria formed pillow-like structures called "stromatolites" beginning about 2.6 billion years before the present.
Archean tectonics is still poorly understood, but it appears that the archaeocean plates collided with each other to produce early island arcs. This is where the first continental crust formed, as reworked and recycled igneous rocks intruded and erupted, while heavier fractions of the melted rocks descended back to the mantle from which they originally formed. Erosion from the island arcs produced the first continent-derived sediments which later tectonic activity deformed to metasedimentary rocks.
Plate boundariesCompressive force, tensional force and shear force produce the three major types of plate boundaries called convergent boundaries, divergent boundaries and transform boundaries.
Compressional stress first deforms a crustal plate, then produces a plate boundary. During deformation, rocks may be folded, thrust faulted, or thickened. One plate may drop below another, or collision may result in huge folded mountain ranges. There are three general forms of convergent boundaries, ocean to continent, ocean to ocean and continent to continent. Convergent boundaries are compressional environments where subduction usually produces both igneous and metamorphic rocks.
There are two general effects of compression on tectonic plates: subduction and continental suture. Usually a continental suture begins with subduction of one plate below another. Sediments and some parts of the subducting plate are scraped off and welded (obducted) to the overriding plate. A deep trench forms as one plate plunges below the other. Subduction zones are more than just the trench. As the subducting plate begins its descent, small step faults compensate for the bending in the crust. A wedge of sediments is deformed onto the leading edge of the overriding plate forming a sedimentary arc or an accretionary wedge.
As subduction continues, the descending plate is subject to great heat and pressure which softens and melts its rocks, causing underplating on the overriding plate and finally the rise of recycled magma through the upper plate. Erupted magma forms volcanos; intrusive magma forms plutons (batholiths, stocks and laccoliths) and may intrude country rock to form sills and dikes.
The two compressional environments which result in subduction produce vastly different geomorphology ("earth-shapes"). Oceanic-to-oceanic collisions are marked by volcanos and island arc volcanism. Oceanic-to-continental collision results in a deep-ocean trench at the leading edge of the subduction zone (such as the Marianas Trench), and volcanic arcs (Caribbean Islands, Andes Mountains and/or Cascades volcanos) with violent pyroclastic eruptions of andesitic lavas, rising batholiths of felsic granitic rocks.
Continental-continental collisions, like that of India with China, result in low angle thrust faults, the partial subduction of one plate under the other (India under Tibet), and the production of huge folded mountain ranges (Himalayas, Alps, Urals and Appalachians). Shallow and deep focus earthquakes outline the edges of active collision zones and provide information about the depth of the subducted plate.
There are three proposed phases of Indian/Tibetan collision. In the first phase, the whole ocean has been subducted and thrust faults form on the Indian plate as it approaches the accretionary wedge on the Asian plate. It has been proposed that the descending plate broke away, permitting more heat to reach the lower part of the lithosphere producing large granite intrusions. Meanwhile, the Indian plate wedged under the Asian plate, doubling the thickness of the lithosphere and raising the Himalayas. Some geologists suggest that the overlapping plates maintain a double thickness of lithosphere through which the heat of the mantle and core are less able to diffuse. The huge height of the Himalayas in this model results from doming under the doubled plates as well as from compressional mountain building.
Other compressional mountain ranges include the Archean-age Laurentides, the Paleozoic Taconics, Caledonians, Acadians, Klamaths, Hercynians and the terminal-Paleozoic Appalachian/Atlas mountains which formed during the final accretion of the supercontinent named "Pangaea" by Wegener. During the Mesozoic in North America, the Nevadan and Laramide Orogenies produced the Sierra Nevadas and the Rocky Mountains and finally, the Himalayas and the Alps formed from collisions during the last 65 million years of Earth history, the Cenozoic.
Structural features of compressive mountain ranges include thrust faults, normal faults, uplifted blocks, overriding blocks, folding of former sediments into anticlines and synclines. In addition, the compressional forces are so great that structures can be formed at a great distance from the area where mountains arise. For example, the LaSalle Anticline in Illinois appears to have resulted from the terminal Paleozoic formation of Pangaea (from about 360 to 250 million years before the present). The regional jointing of the midcontinent, following a pair of joints (SE/NW and NE/SW) also results from the collision of the North American and African plates, while the Ouachita uplift of Arkansas is the result of the contemporary North American/South American collision. Tensional stress produces divergent boundaries where plates rift and spread apart. Divergent boundaries can change over time. For example, when rifting began to separate the supercontinent Pangaea about 180 million years ago, rift valleys formed along the edge of what are now the North American plate and the African plate. One failed rift valley with associated intermediate lava ("aulacogen") is marked by the Palisades columnar diorite/diabase formation along the Hudson River in New York. Successful rifting forced the continents apart and formed a seafloor of mafic oceanic crust at the Mid-Atlantic Ridge. This spreading continues today: New York and London are separating at about one centimeter per year.
Hot spots have been suggested as an initial cause of divergent rifting, curiously, the Hawaiian islands formed as the Pacific plate moved over a hot spot without rifting the plate. Other known hot spots include one under Yellowstone National Park which previously was responsible for the Columbia flood basalts and the Snake River flood basalts but did not succeed in splitting western North America. We know that hot mantle material is rising in the East African Rift Valley where a divergent boundary is forming; in some way this may be different than hot spots which do not break plates.
There are two general types of divergent boundaries, oceanic divergent boundaries and continental divergent boundaries. Both form as a result of rising heat. The crust first domes, then rifts apart.
Oceanic divergent boundaries such as the Mid-Atlantic Ridge and the East Pacific Rise have huge underwater canyons bounded on both sides by new oceanic crust composed of mafic rocks (pillow basalt, basalt, and gabbro). Black smokers, piles of basalt rubble and exceptional mineralization are associated with MORBs (Mid-Ocean Ridge Basalt) environments. When obducted onto continents MORBs are often mined for their exceptionally pure mineral deposits (e.g. Cyprus = copper). MORBs may have been an early cradle of life, certainly they are fertile areas of the Earth's surface today.
Researchers using special submersible vehicles capable of withstanding immense abyssal pressures have discovered entire ecosystems of life surrounding and on MORBs. Familiar forms are bacteria and worms, less familiar forms fit into common orders, but all are based on a chemical system which does not require photosynthesis as light does not penetrate deep into water. Rather the organisms derive energy from organic chemical reactions powered by geothermal energy.
Cooled pillow lava forms fine-crystal basalt, while coarser basalt formed in the vertical "sheeted dikes" originally described from exposures obducted onto continents. Below this some cuts showed lower layers of gabbro and ultramafic peridotite. The whole structure is referred to as an "ophiolite suite" in some geology books. The origin of this odd term is as follows. The rocks were mafic and so looked green to early workers. This resulted in their being given the rock name "ophiolite" or "serpent-rock." The "suite" was tacked on by later workers who realized that there are no pillow lavas without associated tubes, so no pillow basalt without sheeted dike lava. The term "sheeted dike" refers to the tendency of these dikes to line up adjacent to each other like the pages, or sheets, of a book. Ophiolite suites have been widely studied due to the economic importance of their associated mineral deposits.
Studying ophiolite suites on land prompted some workers to correlate ophiolite suites with the layers of the oceanic crust seen by seismologists. Reading earthquake (and nuclear testing) wave data, seismologists had proposed that oceanic crust was formed of four distinct layers: an upper layer of sediment, a slow zone where waves travel at 6 kilometers/second (km/s), a middle 7.2 km/s zone, and the bottom zone where waves move at 8.1 km/s. Geologists proposed that the 6 km/s zone was the basalts in both pillows and the top portion of the sheeted dikes, the 7.2 km/s zone was the gabbro and that the 8.1 km/s zone was the peridotite. Drilling into the deep sea floor confirmed these speculations.
MORBs are usually associated with oceanic fracture zones. Geologists first observed fracture zones because the magnetic data showed offsets - some small, some major. J. Tuzo Wilson first proposed that shear stress was responsible for the production of fracture zones and suggested that the fractures transformed the forces of subduction and spreading, permitting the Earth's plates to slip past each other. There are two general types of transform boundaries, oceanic transform boundaries and continental transform boundaries.
An example of an active oceanic transform boundary is found where the Juan de Fuca Plate and the Pacific Plate are sliding past each other near Cape Mendocino, California.
Continental transform boundaries include the San Andreas Fault which runs from southern California north to the Cape Mendocino triple junction and the Dead Sea Fault Zone in the Middle East. Crust is not created or destroyed at transform boundaries, but metamorphic rocks form along the faults.
Wilson's original proposal included his analysis of the geometry of dextral transform faults. Each of these can also occur in a left-handed (sinistral) form.
Transform faults result from shear force, and unlike the other two boundary types, produce no igneous volcanic or plutonic rocks. However, the bump-and-grind of the transform fault produces metamorphic rocks like "migmatite" and "fubarite." The fault may have scarp or trace that can be seen at the surface or by satellite imagery.
Wilson also named the point where ridges (rifts/spreading centers) meet trenches (subduction zones) and/or transform faults "triple junctions." They always occur with three arms, have about 120 degrees between the arms, and may have an inactive (or apparently inactive) arm. An analysis of the geometry and stability of all possible triple junctions shows that not all possible configurations commonly occur on Earth; indeed some are more common than others.
Euler polesLeonhard Euler [1707-1783] (pronounced "Oil-er") was a Swiss mathematician who studied under the Bernoullis and received his master's degree at age 16 from the University of Basel. He followed the Bernoullis to Russia and was professor of mathematics in at the Petersburg Academy founded by the Empress Catherine I. Isaac Azimov called Euler "the most prolific mathematician of all time." He wrote on every branch of mathematics and left intricately detailed arguments which any student of the discipline can follow. He published 800 papers while alive and left enough papers behind that his citations follow his death by 35 years. He was the first to propose that light was a wave form and that color was a function of wavelength.
In the Earth Sciences, he is recognized for his work on the movement of the surface of spheres. His theorem states that the movement of a rigid surface of any sphere over the sphere itself is defined by a single angular rotation about a pole of rotation (and, by definition, its anti-pole, or antipode). The theorem is used to relate the motion between two plates to each other. There are three ways to find a Euler pole. One way to determine the true tangential motion between two plates is by tracing the transform faults on their common boundary. These will follow small circles centered on the pole of relative motion. However, ocean ridges are commonly offset by transform faults and would give an inaccurate pole. Scientists then use statistics to predict a circle within which it is most likely that the pole lies. Another way to find the Euler pole is to determine the spreading rates from magnetic data, find a point which formed at the same time and is now on two plates and measure the distance between the plates. The final way is the least reliable and is based on earthquake data.
Finding the Euler pole for diverging plate boundaries is fairly straightforward, but to do so for converging boundaries requires vector algebra and a few assumptions. What is important for the scope of this class is to understand the method can be, and has been, applied to the complete mosaic of plates which cover the Earth. Resolving the mechanical forces which rotate the plates and actually drive plate tectonics is the focus of the next section.
Plate boundaries show different amounts of movement along their divergent, convergent or transform boundaries. The orientation of divergent and convergent boundaries with respect to small-circle movements results in varying movement relative to the adjoining plate. We are helped (for the last 180 million years) by having magnetic records from the seafloor. Prior to the assembly of the end-Paleozoic Pangaea, relative movement calculations are usually based on geological data primarily fossils, rock types and remnant magnetism.
Rates of movement of the various plates are determined by analyzing magnetic data. For seafloors along spreading ridges, magnetic data can be correlated from side-to-side of the ridge, while along island chains the lavas of successive islands and seamounts can be dated and rate of motion calculated. The Atlantic Ocean is opening at an approximate rate of one centimeter per year (1 cm/y) so each side is growing at a half cm/y. In contrast the Pacific is practically whizzing along at about twice that rate, reaching the astonishing velocity of 1 cm/y along some of its margins. One of the slowest spreading centers is that under the East African Rift, which is moving at only 0.06 cm/y. The rates of movement along many plate margins have been calculated.
Mechanisms driving plate tectonicsThe major argument against Wegener by English and American geologists was that he provided no convincing mechanism to move his proposed plates across the surface of the Earth. Curiously, biologists accepted Darwin's Theory of Evolution without a mechanism. In much the same way that some people still reject evolution, Northern Hemisphere English-speaking geologists rejected continental drift for four generations. Techniques and resources initially developed for World War II and the Space Race were adapted to study Earth. Actual measurements by geostationary satellites have proved beyond any possible doubt that the continents and ocean floors are moving relative to each other, but the final word on the actual mechanisms of movements remains to be written.
What we do know is that the forces driving the plates are complex. The energy which drives the system is indisputably the heat generated in the core and mantle brought to the surface by convection from the mantle, but the forces at work on each and every plate are various, variable and difficult to resolve. These forces include the push of new seafloor (ridge push), buoyancy and negative buoyancy of the plates relative to the mantle viscosity, the pull of descending slaps, the suction applied by descending plates (trench suction), the resistance along the ridges, the resistance of slabs to bending, the resistance of slabs to moving, overriding plate resistance, and mantle drag under oceans and continents.
The earliest proposals for a plate tectonic mechanism suggested that heat rising from the core, through the mantle caused plate movements directly. In this model, rising heat generated spreading centers, cooler material beginning to descend into the mantle dragged the slabs down convection centers. The uneven distribution of ridges and trenches shown in figure 4 was one of the major arguments which refuted this simple model. The irregular edges of the plates also argues against a simple model, as one would assume that convection cells in the mantle would behave like convection cells in any other material - being fairly regular and evenly distributed in the material being heated. The constant geometry of convection cells is insufficient to explain the irregularity of the resulting system, so some irregular forces are needed to resolve the angular motions and rates observed in nature.
The dynamic gravitational model takes into account mantle convection, but primarily to maintain mass balance, provide energy and thermal bulges from which plates slide under the force of gravity. The greater density of oceanic plates causes them to descend in subduction. Note that no continental plates are subducting. Their lighter density prevents them from descending. Sliding off a thermal bulge is called ridge push and sliding into a subduction zone is called slab pull. Of the two, slab pull is considered to be the more powerful force, confirmed by data which shows that the longer the subduction zone the faster the movement (e.g. Pacific Plate).
The second model is more satisfactory than mere mantle convection cells (mantle drag model) but it must not be thought that the effect of convection cells has been limited or removed by the dynamic gravitational model. The action of convection cells can be seen below oceanic regions, particularly the Pacific and Indian oceans, where their heat is required to drive the system. Some geologists feel that convection is less effective below the continents where heat tends to build up due to the greater thickness of continental crust. This heat is released in hotspot activity, finally resulting in rifting. Convection cells are mapped by studying gravity anomalies below the seafloor. What emerges is a three-dimensional image of the crust/mantle system showing the immense complexities still to be resolved before any model can be accepted as the driving force of plate tectonics.
Another proposal put forward exists side-by-side with mantle convection and gravitational effects. It is known as the mantle-plume hypothesis. It suggests that material subducted eventually arrives at a transition zone in the mantle where the density of the descending material prevents it from being carried any farther down. These cold blobs persist at the transition zone (600 km discontinuity) until eventually heated enough to rise back up through the mantle. These plumes of superheated, lighter density material rise through the mantle and eventually reach the surface where they are erupted as magma from hotspot volcanos. Discrete bubbles of superheated material may help explain why hotspot islands like the Hawaiian islands form as separate entities rather than an unbroken ridge of material. Additional work has suggested that the plumes have a top blob and a bottom blob which reach the surface at intervals. This model might explain double eruptions of flood basalts such as those which created the Columbia Plateau in western North America.
Mapping modern plate tectonic boundariesThe boundaries presently active on Earth's surface have been mapped and their relative motions to each other resolved such that the absolute motion of the plates relative to the core of the Earth can be understood. The ridge system represents spreading centers where new seafloor rises and hardens forming new oceanic crust. Subduction zones are the areas where oceanic crust plunges below other crust, whether oceanic or continental. The difference between these two forces is adjusted by transform faults. Where three plate boundaries meet, the point is called a triple junction. Some plates are moving faster than others. Some plates do not appear to be moving much relative to each other and have formed continental sutures. The areas under the poles are less well understood due to the ice caps.
However, it is known that the Antarctic plate is moving north into the Pacific while the Indian/Australian plate and Pacific plate are moving faster northward while the circum-Antarctic ridge system lengthens. Africa is essentially fixed in place, but is rotating counterclockwise relative to the plates which surround it. The American plate is being pushed rapidly westward where it subducts the Pacific plate and may have remnant former oceanic plates buried beneath it. A counterclockwise rotation in the North Atlantic accommodates the slower northerly advances of the Eurasian plate.
North America and Eurasia are welded together along a suture line represented by the unlabeled line extending from practically the North Pole down and through the islands of Japan. This weld may or may not be tectonically active at present. Some geologists have proposed that it is a former collision site which is now functioning as a very slow moving transform fault, but more work is needed to resolve the motions in this area.
Outline of Historical Geology by Ellin Beltz
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©2008 by Ellin Beltz -- January 10, 2008