Introduction to Ocean Sciences

Chapter 5: Plate Tectonics: History and Evidence

Guide to Reading and Learning

This chapter reviews the history of the plate tectonic theory and the many pieces of scientific evidence that were necessary before the idea that the Earth’s continents were fixed in place and immovable was finally rejected and the theory of plate tectonics became accepted. Did you know that there was substantial evidence that the continents had moved and that scholars as far back as the mid-1500s had proposed that they did indeed move? Did you know that geologists and other scientists scorned the idea that the plates moved and ignored the evidence and the idea that the continents could ever move for hundreds of years, and that this only changed in the early and middle decades of the twentieth century?

Did you know that the methodology of Earth and environmental sciences (ocean science is an Earth and environmental science) is substantially different from the hypothesis-and-experiment approach used in most sciences? In ocean and environmental sciences, we can only rarely use the reductionist approach used in most scientific disciplines. The reductionist approach requires that certain aspects of the system under study be isolated so they can be evaluated in laboratory experiments. However, in Earth and environmental sciences, it is seldom possible to design an experiment that measures the influence of only one or a small number of variables on the parameter of interest, particularly as many of the processes involved take place over tens, hundreds, thousands, or even millions of years. Thus, it is extremely rare that we can prove or disprove a hypothesis with absolute certainty. Instead, we must usually look at many pieces of evidence and integrate this information to approach a hypothesis from several directions before we can make a decision on the hypothesis by the weight of evidence rule. This concept of weight of evidence is important, as many people, including many politicians, seem to believe that, if we just do enough of the right research, we can get unequivocal answers to any environmental problem such as, for example, a perfect forecast of how climate will change due to the enhanced greenhouse effect. Rarely, if ever, is this possible.

The idea that the scientific community is loath to throw out old, partially discredited ideas and that accepted scientific “facts” often prove to be wrong is perhaps new to you. Science teaching and the media tend to foster the idea that science is always either right or wrong and that all that we learn as “scientific facts” are well proven and universally accepted. This is definitely not true. Many critical policy decisions, particularly with regard to environmental management and protection are made because the public accept “facts” without considering alternate explanations. We all are responsible for these decisions so I hope that this chapter will convince you that nobody should either accept or reject the “facts” of science without question. Also, we must recognize that in the future there will be other paradigm shifts in our understanding of the Earth, just like the shift from the view that Earth’s continents were fixed and immovable to our current understanding of plate tectonics.

Chapter 5 Essential to Know 

Critical Concepts used in this chapter

CC.2, CC.3, CC.7

5.1 Methodology of Earth Sciences

  • Earth and environmental sciences rely heavily on integrative methodologies, as opposed to more traditional reductionist approaches to science.

5.2 History of Plate Tectonic Theory

  • The fundamental theory that the continents moved is hundreds of years old, but it was only in 1912 that scientific evidence that supports the theory was first examined. Based on this evidence, a hypothesis that the continents had drifted apart over the past 225 million years was proposed.
  • At that time it was known that ocean fossils could be found at the top of mountains, that many mountains were formed of uplifted seabed, and that fossil distributions indicated that terrestrial species at one time moved freely between the continents. However, the prevailing view of geologists was that the continents were fixed, immobile, and unchanging, and the new theory was rejected.
  • The story of the many pieces of evidence that finally led to acceptance of the plate tectonic theory about fifty years later is a lesson on how difficult it is for science to reject old ideas and accept new ones.

Wegener’s Observations and Theory

  • In 1912, Alfred Wegener proposed that the continents had once all been joined but that they had since moved apart.
  • Between 1915 and the 1950s, the evidence that the continents moved was largely ignored by geologists, in part because Wegener was not recognized because he was not a geologist.

The Evidence Accumulates

  • In the 1950s, Wegener’s theory, by then renamed the continental drift theory, began to be considered seriously.
  • Two key pieces of evidence led to this reconsideration. First, the ocean basins were found to be less than 200 million years old, unlike the continents, where rocks about 4 billion years old had been found. Second, paleomagnetic data showed that the continents must have moved or else there must have been at one time several north magnetic poles.
  • Some of the most important new evidence came from the oceans, especially the discovery of the oceanic ridges that were the sites of many earthquakes and consisted of newly formed rock.

A Proposed Mechanism

  • In 1960, Harry Hess, a Princeton geologist, proposed the theory later called seafloor spreading and suggested that the Earth’s outer shell moves in pieces as it floats on the material below. This was critical evidence because it provided for the first time a possible mechanism that could move continents.
  • Hess also proposed that ocean floor was created at the oceanic ridges and destroyed at the trenches.

The Magnetic Anomaly Clue

  • In 1963, Matthews and Vine, two British ocean geophysicists, proposed an explanation for the striped pattern of magnetic anomalies (areas of slightly increased or slightly decreased magnetic field). This magnetic anomaly data had been observed by oceanographers who had collected this data without knowing why (or if) it might be useful. This explanation was the crucial piece of evidence that finally began to convince scientists that the continents did move.

Acceptance of the Theory

  • Only in the mid-1960s was the theory of continental drift, or plate tectonics (as it later became known), generally accepted by the scientific community. Acceptance came only after many pieces of evidence had accumulated and the sum of this evidence could no longer be ignored or explained away.
  • Since the 1960s, scientific studies have led to an ever-improving understanding of plate tectonic processes, and new astronomical measurement techniques and satellite navigation systems have now conclusively proved that the continents are moving.

5.3 Fit of the Continents

  • The earliest evidence that the continents have drifted apart was that the continents’ edges seemed to fit together in a jigsaw puzzle–like arrangement.

Gaps and Overlaps in the Fit

  • Alfred Wegener and others before him noticed that the continents seemed to fit together when reconstructed, especially South America and Africa. However, substantial gaps existed in his reconstruction of this single supercontinent, now named Pangaea, which critics claimed disproved his theory.
  • The fit of the continents improves when the true edges (the edges of the continental shelves) are compared. The true edge is found only by including the continental shelf in the continent, as the continental shelf is submerged continental crust.
  • Reconstructions done at the base of the continental slope (2000 m) or at an intermediate depth (500 m) just beyond the edge of the shelf fill most of the gaps left in Wegener’s reconstruction, leaving only small gaps where the continent edges have been modified by erosion, collapse of small blocks of continental crust, deposition, or growth of coral reefs..

5.4 Geological Similarities between Previously Joined Edges of Continents

  • Many geological formations and fossils on either side of the oceans at locations that would have been joined if the continents were reassembled were identical. This has been known for many decades but was not felt by most geologists to be sufficient evidence that the continents had once been joined.

Mountain Chains      

  • A number of mountain chains, or their eroded remnants, end at the current coastline but are reappear on another continent exactly where they would be joined in the reconstruction of the continents. For example, the Appalachian Mountains run through New England, Canada and Newfoundland, end abruptly at the Atlantic Ocean edge, but reappear on the coasts of Ireland and Brittany

Rocks and Fossils      

  • There are similarities in the composition of 200- to 300-million-year-old rocks and fossils found on separate continents that were thought to have been joined in the reconstruction of Pangaea. However, newer rocks and fossils show substantial differences.

Glaciers

  • Glaciers leave scour marks on the rocks within their valleys that indicate the direction of glacier flows.
  • Glacial scour lines and other evidence of glaciers has been found in South America, southern Africa, India, and Australia, some areas of which are now near the equator, where glaciers were very unlikely to have formed even in an ice age.
  • When the directions of the glacial scour lines on the various continents are placed in the Pangaea reconstruction, their location and direction are all consistent with having been located on a single continent centered at the South Pole.

Reef Corals

  • Tropical reef corals that are about 300 million years old have been found in several continents, some in high latitudes in pieces of what would have been a band within the equatorial region in the reconstruction of Pangaea,. These reefs are consistent with other data that indicates Pangaea had come together from two plates, and that 300 million years ago these two ancient continents were separated by a narrow tropical sea called the Tethys Sea

5.5 Paleoclimate

  • Paleoclimate is the climate that existed at a particular time in the geologic past and can be determined from the nature of the fossils in rocks that were formed at that time.
  • Tropical fossils are found in latitudes that are not now tropical. This can only reasonably be explained if the continents have drifted from their former locations. The only other possible explanation is that 300 million years ago the south polar ice cap reached as far north as the equator and there was also a narrow band of tropical temperatures at about 30oN, a bizarre circumstance believed to be impossible.
  • Additional paleoclimate evidence can be found in the composition of rocks. For example, deposits of coal or desert sand would indicate a warm, dry climate. Again this evidence supports the existence of Pangaea and the relative positions of its parts.

5.6 Isostasy and the Mohorovičič Discontinuity

  • There is a discontinuity in earthquake wave velocity at a depth of 30–40 km beneath the continents and 6 km beneath the seafloor.
  • Calculations based on the mean density of oceanic and continental crust showed that the mean levels of the seafloor and continent surface were consistent with the theory that the crust “floated” on the mantle material below.
  • The finding that the mantle behaved like a fluid provided the first evidence of a process that could cause or allow the continents to be able to move.

5.7 Earthquakes

  • Earthquakes occur when accumulated stress in solid rock leads to its fracture.

Earthquake Locations

  • The locations of earthquakes are not random. They are concentrated in bands that follow the edges of some continents and other bands that pass through the middle of the ocean basins—locations eventually identified as plate margins.

Depth of Earthquake Foci

  • Deep-focus earthquakes occur only near the deep-ocean trenches where the seafloor is being subducted; only shallow-focus earthquakes occur at the oceanic ridges where seafloor is being created.

Directions of Motion

  • Originally seismographs could only record the depth and general location of earthquakes, but more recent designs that use computers to analyze their data can not only establish the precise locations of earthquakes but also can describe directions and types of movement of each side of the earthquake fault.
  • Earthquake locations and characteristics are consistent with movements hypothesized by the plate tectonic theory. For example, earthquakes on the oceanic ridges are almost all located exactly at the middle of the ridge and involve both vertical motions and horizontal separation of the two sides of the fault. Also, numerous earthquakes are located on transform faults but very few on the adjacent fracture zones.

Characteristics of Subduction Zone Earthquakes

  • The distribution and characteristics of earthquakes in subduction zones studied by sophisticated modern seismographs are consistent with the motions (bending, compression, stretching) that would be expected if the oceanic plate was descending into the mantle and being heated.
  • The locations of earthquake foci are found at progressively deeper depths, with distance from the subduction zone consistent with movements associated with the subducting tectonic plate descending at an angle under the nonsubducting plate.

Computer Tomography

  • Computer tomography produces three-dimensional images of the Earth’s interior using computer analysis of earthquake waves from a single earthquake that are received by many seismometers at different measurement sites.
  • Computer tomography has begun to reveal many features of the mantle and core that are consistent with the existence of convection motions within the core and at least the upper mantle.
  • It has also revealed deep mantle upwelling plumes beneath some hot spots and blocks of oceanic crust that are apparently descending into the mantle deep beneath subduction zones.

5.8 Volcanoes

  • Volcanoes are located at hot spots and on the oceanic ridges where mantle upwelling creates oceanic crust, as well as near subduction zones where descending blocks of oceanic crust and sediments are heated, melt, and rise.

Oceanic Ridge and Hot-Spot Volcanoes 

  • During the 1950s and 1960s, the oceanic ridges were found to be composed of volcanic rocks of very recent origin, indicating that these ridges were the regions where magma upwells from below to form new oceanic crust.

Subduction Zone Volcanoes

  • Most of the world’s volcanoes, other than those on the oceanic ridges and at hot spots, are within 100–200 km of the deep ocean trenches on the continent at pleat boundaries where the trench is next to a continent, and as a chain of volcanic island and seamounts parallel to the trench at plate boundaries where the trench is not next to a continent.
  • The locations of these volcanoes are consistent with plate tectonic theory, as they are evidence of melting of oceanic crust from an oceanic crust tectonic plate as the edge of that plate is progressively subducted, descending at an angle beneath the nonsubducting plate.

Lava Types

  • The types of lava erupted at volcanoes are also consistent with plate tectonic theory, which requires that hot spot and oceanic ridge volcanoes erupt magma from the molten mantle, while subduction zone volcanoes also erupt melted oceanic crust, sediments, and water.
  • Hot spot and oceanic ridge volcanoes erupt almost exclusively basaltic lava, which is characteristic of molten mantle material.  In contrast, subduction zone volcanoes erupt both basaltic lava (characteristic of melted magma) and andesitic lava (characteristic of melted oceanic crust and sediments).
  • Subduction zone volcanoes located on the continents erupt both basaltic and andesitic lava, but also rhyolitic lava, which is characteristic of melted continental crust (created as the molten lava from the subducting plate rises through the continental rocks).

5.9 Paleomagnetism

  • When volcanic rocks first solidify, they retain a small residual magnetic field that remains fixed afterward. If the direction of this field is measured later, the direction degree of rotation of the rock with respect to the magnetic field since the rock was first formed can be deduced.

Reading the Paleomagnetic Record

  • Rocks can be rotated and tilted by earthquakes, but not all of the rocks of a continent will have been rotated or tilted. Therefore, measurement of the paleomagnetic field direction in many samples of rocks of the same age from within an individual continent can indicate which direction the magnetic north pole was with respect to the continent at the time those rocks were formed.
  • Examination of rocks from each of the continents shows that the either the location of the magnetic pole must have moved or the continent must have been rotated.
  • Examination of rocks from several different continents reveals that if the rotated paleomagnetic field was evidence that the magnetic pole had moved, there must have been more than one such pole in the past, as each continent’s rocks point to a different location. However, if there has always been only one magnetic north pole—which physicists believe is not only true but necessary to comply with the laws of physics—the continents must have both rotated and moved across the planet’s surface. When movements and rotation of the continents are pieced together from the paleomagnetic data they show the locations of continents to have been consistent with the existence of Pangaea and its breakup.

Dip Angle and Paleolatitude

  • When volcanic rocks first solidify their residual magnetic field is aligned not only horizontally toward the pole in the Earth’s magnetic field but also at an angle to the vertical that depends on the latitude where the rocks formed (dip angle). At the pole, a compass needle turned on its side will point straight down, while at progressively lower latitudes it points at a progressively smaller angle until, at the equator, it is parallel to the Earth’s surface.
  • Dip angle data from rocks from the various continents show that the continents must have changed latitude in the past and, again, the changes are consistent with the existence of Pangaea and its breakup.

5.10 Ocean Floor Magnetic Anomalies

Confusion and Enlightenment

  • Anomalies occur in the magnetic field measured over the ocean. Some areas have a slightly higher magnetic field and others a slightly lower magnetic field.
  • Oceanographers found that there were stripes of slightly higher and lower magnetic field aligned with the center of the oceanic ridges. The field alternated between a high and a low anomaly with each stripe at increasing distances from the oceanic ridge. For a period of years this strange anomaly pattern was recognized, but its existence was not understood.
  • The striped pattern of magnetic anomalies was eventually explained in 1963 by Matthews and Vine, two British ocean scientists. They concluded that the anomalies were caused by the small magnetic field of oceanic crust rocks below and that successive stripes represented times in the past when the Earth’s magnetic field was either normal or reversed (so the north magnetic pole was located at the south geographic pole). Thus, each stripe was ocean floor created at the oceanic ridge at some time in the past and then moved progressively away from the ridge as newer crust formed behind it.
  • Matthews and Vine supported their conclusion by pointing out that the symmetry in the striped pattern on the two sides of the ridge was created because new seafloor is created and then split by newer seafloor, with the two spilt halves moving off the ridge axis in opposite directions.
  • This explanation of the seafloor magnetic anomaly data was a revelation that finally provided proof that the continents drift.

A Lesson about “Useless” Data

  • The magnetic anomaly data that Matthews and Vine used to finally provide conclusive evidence that plate tectonics occurred was collected by oceanographers without any immediately known purpose or application. The lesson is that all data are valuable because even seemingly useless data can sometimes be the key to solving a major scientific mystery.

5.11 Age of the Ocean Floor

  • The age of sediments and rocks can now be dated from drill core samples. However, these are very expensive to obtain so very few are available even today, and none were available until after the plate tectonics theory became generally accepted. Thus, most seafloor age measurements have been obtained by indirect means.

Sediment Thickness

  • Because ocean sediments accumulate continuously layer upon layer, older oceanic crust should be covered by deeper sediments than younger crust. Oceanographers were able to determine that the oceanic ridges had little or no sediment and that the sediments became thicker with increasing distance from the ridges.  This was another key piece of evidence supporting plate tectonics.

Magnetic Anomalies

  • The magnetic anomaly stripes on the seafloor are like tree rings, each ring representing a period of normal or reversed magnetic field in the Earth’s history. Once the chronological record of reversals of the Earth’s magnetic field had been read by analysis of the paleomagnetism and fossils in many different rocks of different ages, it became relatively simple to estimate seafloor age by simply counting the stripes outward from the oceanic ridges.
  • From magnetic anomaly data ocean scientists were able to determine that the oldest surviving oceanic crust was about 170 million years old.

Deep-Sea Drilling    

  • The first deep sea drilling research vessel began operation in 1968.
  • During the next several years this vessel drilled a line of holes in the North Atlantic Ocean floor from the oceanic ridge toward the North American continent.
  • Samples of bedrock or sediments immediately above the bedrock from these North Atlantic samples were dated by accurate radioisotope techniques and shown to be progressively older with distance from the ridge. This data was probably the final piece of evidence leading to universal acceptance of plate tectonics

5.12 Direct Measurements of Plate Movement

  • Direct measurements of plate motions have now been made despite the fact that these motions are extremely slow.

Ground-Based Measurements

  • The first direct measurements of plate motions were made by placing a laser on one side of a plate boundary and a mirror on the other and measuring the time taken for light to travel across and back.
  • These measurements showed rates of plate motion that were consistent with plate tectonics theory, but because they were only made at a few locations, some claimed that they might only represent local motions.

Satellite Measurements

  • Highly sensitive and precise measurements of tectonic plate motions are now made using data from a worldwide network of high-precision GPS monitoring systems that is calibrated to improve its accuracy and sensitivity to very small differences in locations by data from two other systems. The first uses lasers bounced off mirrors on satellites and the second uses an array of radio telescopes simultaneously observing quasars. This quasar-based system is capable of measuring plate motions as small as a few millimeters per year.

Emerging Details of Plate Movements

  • Within the past decade more has been learned about the current velocities and directions of motion of the tectonic plates. Figure 4.6 shows measured rates and directions of motion for all the major plates. It was not possible to measure the rate and direction of motion as shown in Figure 4.6 until the early 2000s.
  • Plate motions have been found to occur as a combination of slow creep and much faster sharp movements, which are usually associated with earthquakes.

Critical Concept Reminders:

CC.2 Isostasy, Eustasy, and Sea Level (p. 113)

  • Earth’s crust floats on the plastic asthenosphere. Sections of crust rise and fall isostatically as temperature changes alter their density and as their mass loading changes due to melting or to the formation of ice stemming from climate changes. This causes sea level to change on the coast of that particular section of crust. Sea level can also change eustatically when the volume of water in the oceans increases or decreases due to changes in water temperature or changes in the amount of water in glaciers and ice caps on the continents. Eustatic sea level change takes place synchronously worldwide and much more quickly than isostatic sea level changes. To read CC2 go to page 5CC.

CC.3 Convection and Convection Cells (p. 117)

  • Fluids that are heated from below, such as the Earth’s mantle, rise because their density is reduced. They continue to rise to higher levels until they are cooled sufficiently, at which time they become dense enough to sink back down. This convection process establishes convection cells in which the heated material rises in areas of upwelling, spreads out, cools, and then sinks at areas of downwelling. Areas of downwelling are the subduction zones, while areas of upwelling include the oceanic ridges and hot spots. To read CC3 go to page 10CC.

CC.7 Radioactivity and Age Dating (pp. 108, 124, 125, 126)

  • Some elements have naturally occurring radioactive (parent) isotopes that decay at precisely known rates to become different (daughter) isotopes, which are often isotopes of another element. Measurement of the concentration ratio of the parent and daughter isotopes in a rock or other material can be used to calculate its age, but only if none of the parent or daughter isotopes have been gained or lost from the sample over time. Radioisotope age dating is used in conjunction with other dating methods including variations in fossil assemblages and magnetic field properties. To read CC7 go to page 18CC.

 

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