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10: Plate Tectonics - Geosciences


10: Plate Tectonics - Geosciences

64 10.5 Mechanisms for Plate Motion

It has been often repeated in this text and elsewhere that convection of the mantle is critical to plate tectonics, and while this is almost certainly so, there is still some debate about the actual forces that make the plates move. One side in the argument holds that the plates are only moved by the traction caused by mantle convection. The other side holds that traction plays only a minor role and that two other forces, ridge-push and slab-pull, are more important (Figure 10.28). Some argue that the real answer lies somewhere in between.

Figure 10.28 Models for plate motion mechanisms [SE]

Kearey and Vine (1996) [1] have listed some compelling arguments in favour of the ridge-push/slab-pull model, as follows: (a) plates that are attached to subducting slabs (e.g., Pacific, Australian, and Nazca Plates) move the fastest, and plates that are not (e.g., North American, South American, Eurasian, and African Plates) move significantly slower (b) in order for the traction model to apply, the mantle would have to be moving about five times faster than the plates are moving (because the coupling between the partially liquid asthenosphere and the plates is not strong), and such high rates of convection are not supported by geophysical models and (c) although large plates have potential for much higher convection traction, plate velocity is not related to plate area.

In the ridge-push/slab-pull model, which is the one that has been adopted by most geologists working on plate-tectonic problems, the lithosphere is the upper surface of the convection cells, as is illustrated in Figure 10.29.

Figure 10.29 The ridge-push/slab-pull model for plate motion, in which the lithosphere is the upper surface of the convective systems. [SE]

Although ridge-push/slab-pull is the favoured mechanism for plate motion, it’s important not to underestimate the role of mantle convection. Without convection, there would be no ridges to push from because upward convection brings hot buoyant rock to surface. Furthermore, many plates, including our own North American Plate, move along nicely — albeit slowly — without any slab-pull happening.


Summary

The topics covered in this chapter can be summarized as follows:

Section Summary
10.1 Alfred Wegener: The Father of Plate Tectonics The evidence for continental drift in the early 20th century included the matching of continental shapes on either side of the Atlantic and the geological and fossil matchups between continents that are now thousands of kilometres apart.
10.2 Global Geological Models of the Early 20th Century The established theories of global geology were permanentism and contractionism, but neither of these theories was able to explain some of the evidence that supported the idea of continental drift.
10.3 Geological Renaissance of the Mid-20th Century Giant strides were made in understanding Earth during the middle decades of the 20th century, including discovering magnetic evidence of continental drift, mapping the topography of the ocean floor, describing the depth relationships of earthquakes along ocean trenches, measuring heat flow differences in various parts of the ocean floor, and mapping magnetic reversals on the sea floor. By the mid-1960s, the fundamentals of the theory of plate tectonics were in place.
10.4 Plate, Plate Motions, and Plate Boundary Processes Earth’s lithosphere is made up of over 20 plates that are moving in different directions at rates of between 1 cm/y and 10 cm/y. The three types of plate boundaries are divergent (plates moving apart and new crust forming), convergent (plates moving together and one being subducted), and transform (plates moving side by side). Divergent boundaries form where existing plates are rifted apart, and it is hypothesized that this is caused by a series of mantle plumes. Subduction zones are assumed to form where accumulation of sediment at a passive margin leads to separation of oceanic and continental lithosphere. Supercontinents form and break up through these processes.
10.5 Mechanisms for Plate Motion It is widely believed that ridge-push and slab-pull are the main mechanisms for plate motion, as opposed to traction by mantle convection. Mantle convection is a key factor for producing the conditions necessary for ridge-push and slab-pull.

  1. List some of the evidence used by Wegener to support his idea of moving continents.
  2. What was the primary technical weakness with Wegener’s continental drift theory?
  3. How were mountains thought to be formed (a) by contractionists and (b) by permanentists?
  4. How were the trans-Atlantic paleontological matchups explained in the late 19th century?
  5. In the context of isostasy, what would prevent an area of continental crust from becoming part of an ocean?
  6. How did we learn about the topography of the sea floor in the early part of the 20th century?
  7. How does the temperature profile of the crust and the mantle indicate that part of the mantle must be convecting?
  8. What evidence from paleomagnetic studies provided support for continental drift?
  9. Which parts of the oceans are the deepest?
  10. Why is there less sediment in the ocean ridge areas than in other parts of the sea floor?
  11. How were the oceanic heat-flow data related to mantle convection?
  12. Describe the spatial and depth distribution of earthquakes at ocean ridges and ocean trenches.
  13. In the model for ocean basins developed by Harold Hess, what took place at oceanic ridges and what took place at oceanic trenches?
    Figure A
  14. What aspect of plate tectonics was not included in the Hess theory?
  15. Figure 10.36 shows the pattern of sea-floor magnetic anomalies in the area of a spreading ridge. Draw in the likely location of the ridge.
  16. What is a mantle plume and what is its expected lifespan?
  17. Describe the nature of movement at an ocean ridge transform fault (a) between the ridge segments, and (b) outside the ridge segments.
  18. How is it possible for a plate to include both oceanic and continental crust?
  19. What is the likely relationship between mantle plumes and the development of a continental rift?
  20. Why does subduction not take place at a continent-continent convergent zone?
  21. Divergent, convergent, and transform boundaries are shown in different colours on Fiugre 10.37. Which colours are the divergent boundaries, which are the convergent boundaries, and which are the transform boundaries?
    Figure B [Image Description]
  22. Name the plates on this map and show their approximate motion directions.
  23. Show the sense of motion on either side of the plate boundary to the west of Haida Gwaii (Queen Charlotte Islands).
  24. Where are Earth’s most recent sites of continental rifting and creation of new ocean floor?
  25. What is likely to happen to western California over the next 50 million years?
  26. What geological situation might eventually lead to the generation of a subduction zone at a passive ocean-continent boundary such as the eastern coast of North America?

Image Descriptions

Figure B image description: A black line with triangles pointing towards the coast stretches from the Oregon and Washington state up just past Vancouver Island to the southern tip of Haida Gwaii. This line also appears along the Alaskan coast and stretches part way down the Alaskan Pan-Handle. A thin red line stretches from the Alaskan Pan-Handle down just past the southern tip of Haida Gwaii. From that point, it alternates from being a thin red to a thick blue line to form uneven angles zig zagging south past Oregon state. [Return to Figure B]


10.2 Global Geological Models of the Early 20th Century

The untimely death of Alfred Wegener didn’t solve any problems for those who opposed his ideas because they still had some inconvenient geological truths to deal with. One of those was explaining the distribution of terrestrial species across five continents that are currently separated by hundreds or thousands of kilometres of ocean water (Figure 10.2), and another was explaining the origin of extensive fold-belt mountains, such as the Appalachians, the Alps, the Himalayas, and the Canadian Rockies.

Before we go any further, it is important to know what was generally believed about global geology before plate tectonics. At the beginning of the 20th century, geologists had a good understanding of how most rocks were formed and understood their relative ages through interpretation of fossils, but there was considerable controversy regarding the origin of mountain chains, especially fold-belt mountains. At the end of the 19th century, one of the prevailing views on the origin of mountains was the theory of contractionism — the idea that since Earth is slowly cooling, it must also be shrinking. In this scenario, mountain ranges had formed like the wrinkles on a dried-up apple, and the oceans had submerged parts of former continents. While this theory helped to address the dilemma of the terrestrial fossils, it came with its own set of problems, one being that the amount of cooling couldn’t produce the necessary amount of shrinking, and the other being the principle of isostasy (which had already been around for several decades), which wouldn’t allow continents to sink. (See Section 9.4 for a review of the important principle of isostasy.)

Another widely held view was permanentism, in which it was believed that the continents and oceans have always been generally as they are today. This view incorporated a mechanism for creation of mountain chains known as the geosyncline theory. A geosyncline is a thick deposit of sediments and sedimentary rocks, typically situated along the edge of a continent (Figure 10.5).

Figure 10.5 The development of a geosyncline along a continental margin. (Note that a geosyncline is not related to a syncline, which is a downward fold in sedimentary rocks.) [SE]

The idea of geosynclines developing into fold-belt mountains originated in the middle of the 19th century, proposed first by James Hall and later elaborated by Dwight Dana, both of whom worked extensively in the Appalachian Mountains of the eastern United States. The process of converting a geosyncline into a mountain belt was never really adequately explained, although it was widely believed that mountain belts formed when geosynclines were compressed by forces pushing from either side. The problem is that, without the lateral forces related to plate tectonics, no one was able to adequately describe what would do the pushing. The sediments that accumulate within a geosyncline are derived from erosion of the adjacent continent. Geosynclinal sediments — which eventually turn into sedimentary rocks — may be many thousands of metres thick. As they accumulate, they push down the pre-existing crustal rocks. Extensive geosynclinal deposits exist around much of the coastline of most of the continents there is a large geosyncline along the eastern edge of North America.

Proponents of the geosyncline theory of mountain formation, and there were many well into the 1960s, also had the problem of explaining the intercontinental terrestrial fossil matchups. The simple explanation was that there were “land bridges” across the Atlantic along which animals and plants could migrate back and forth. One proponent of this idea was the American naturalist Ernest Ingersoll. Referring to evidence of past climate changes, Ingersoll contributed the following to the Encyclopedia Americana in 1920: “The most interesting feature of these changes, however, is that by which, now and again, the Old World was connected with the New by necks or spaces of land, known as “land-bridges” especially as these permitted an interchange of plants and animals, giving to us many new ones from the other side of the ocean, including, finally, man himself.” [1]

There are many problems with the land-bridge theory, one being that it is completely inconsistent with isostasy, and another that there is no evidence of the remnants of the land bridges. The Atlantic Ocean is several thousand metres deep over wide areas, and so the underwater slopes leading up to a land bridge would have to have been at least tens of kilometres wide in most places, and many times that in others. A land bridge of that size would certainly have left some trace.

Exercise 10.1 Fitting the Continents Together

The main continents around the Atlantic Ocean are depicted here in the shapes that they might have had during the Mesozoic, including the extents of their continental shelves. Cut these shapes out and see how well you can fit them together in the positions that these areas occupied within Pangea. You can refer to a map of Pangea to help you make the fit.


10.3 Geological Renaissance of the Mid-20th Century

As the mineral magnetite (Fe3O4) crystallizes from magma, it becomes magnetized with an orientation parallel to that of Earth’s magnetic field at that time. This is called remnant magnetism . Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite (up to 1 or 2%), are particularly susceptible to being magnetized in this way, but even sediments and sedimentary rocks, as long as they have small amounts of magnetite, will take on remnant magnetism because the magnetite grains gradually become reoriented following deposition. By studying both the horizontal and vertical components of the remnant magnetism, one can tell not only the direction to magnetic north at the time of the rock’s formation, but also the latitude where the rock formed relative to magnetic north.

In the early 1950s, a group of geologists from Cambridge University, including Keith Runcorn, Ted Irving, [1] and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting paleomagnetic data. They found that rocks of different ages sampled from generally the same area showed quite different apparent magnetic pole positions (Figure 10.3.1). They initially assumed that this meant that Earth’s magnetic field had, over time, departed significantly from its present position—which is close to the rotational pole.

Figure 10.3.1 Apparent polar-wandering paths (APWP) for Eurasia and North America. The view is from the North Pole (black dot) looking down. The outer circle is the equator. In the diagram to the right the curve locations have been corrected taking continental drift into account.

The curve defined by the paleomagnetic data was called a polar wandering path because Runcorn and his students initially thought that their data represented actual movement of the magnetic poles (since geophysical models of the time suggested that the magnetic poles did not need to be aligned with the rotational poles). We now know that the magnetic data define movement of continents, and not of the magnetic poles, so we call it an apparent polar wandering path (APWP).

What is a polar wandering path?

At around 500 Ma, what we now call Europe was south of the equator, and so European rocks formed then would have acquired an upward-pointing magnetic field orientation (see Figure 9.3.2 and Figure 10.3.2). Between then and now, Europe gradually moved north, and the rocks forming at various times acquired steeper and steeper downward-pointing magnetic orientations.When researchers evaluated magnetic data in this way in the 1950s, they plotted where the North Pole would have appeared to be based on the magnetic data and assumed that the continent was always where it is now. That means that the 500 Ma “apparent” north pole would have been somewhere in the South Pacific, and that over the following 500 million years it would have gradually moved north.Of course we now know that the magnetic poles don’t move around much (although polarity reversals do take place) and that the reason Europe had a magnetic orientation characteristic of the southern hemisphere is that it was in the southern hemisphere at 500 Ma.Runcorn and colleagues soon extended their work to North America, and this also showed apparent polar wandering, but the results were not consistent with those from Europe. For example, the 200 Ma pole for North America plotted somewhere in China, while the 200 Ma pole for Europe plotted in the Pacific Ocean. Since there could only have been one pole position at 200 Ma, this evidence strongly supported the idea that North America and Europe had moved relative to each other since 200 Ma. Subsequent paleomagnetic work showed that South America, Africa, India, and Australia also have unique polar wandering curves. In 1956, Runcorn changed his mind and became a proponent of continental drift.This paleomagnetic work of the 1950s was the first new evidence in favour of continental drift, and it led a number of geologists to start thinking that the idea might have some merit. Nevertheless, for a majority of geologists working on global geology at the time, this type of evidence was not sufficiently convincing to get them to change their views.

During the 20th century, our knowledge and understanding of the ocean basins and their geology increased dramatically. Before 1900, we knew virtually nothing about the bathymetry and geology of the oceans. By the end of the 1960s, we had detailed maps of the topography of the ocean floors, a clear picture of the geology of ocean floor sediments and the solid rocks underneath them, and almost as much information about the geophysical nature of ocean rocks as of continental rocks.

Up until about the 1920s, ocean depths were measured using weighted lines dropped overboard. In deep water this is a painfully slow process and the number of soundings in the deep oceans was probably fewer than 1,000. That is roughly one depth sounding for every 350,000 square kilometres of the ocean. To put that in perspective, it would be like trying to describe the topography of British Columbia with elevation data for only a half a dozen points! The voyage of the Challenger in 1872 and the laying of trans-Atlantic cables had shown that there were mountains beneath the seas, but most geologists and oceanographers still believed that the oceans were essentially vast basins with flat bottoms, filled with thousands of metres of sediments.

Following development of acoustic depth sounders in the 1920s (Figure 10.3.3), the number of depth readings increased by many orders of magnitude, and by the 1930s, it had become apparent that there were major mountain chains in all of the world’s oceans. During and after World War II, there was a well-organized campaign to study the oceans, and by 1959, sufficient bathymetric data had been collected to produce detailed maps of all the oceans (Figure 10.3.4).

Figure 10.3.3 Depiction of a ship-borne acoustic depth sounder. The instrument emits a sound (black arcs) that bounces off the sea floor and returns to the surface (white arcs). The travel time is proportional to the water depth. Figure 10.3.4 Ocean floor bathymetry (and continental topography). Inset (a): the mid-Atlantic ridge, (b): the Newfoundland continental shelf, (c): the Nazca trench adjacent to South America, and (d): the Hawaiian Island chain.

The important physical features of the ocean floor are:

  • Extensive linear ridges (commonly in the central parts of the oceans) with water depths in the order of 2,000 to 3,000 m (Figure 10.3.4, inset a)
  • Fracture zones perpendicular to the ridges (inset a)
  • Deep-ocean plains at depths of 5,000 to 6,000 m (insets a and d)
  • Relatively flat and shallow continental shelves with depths under 500 m (inset b)
  • Deep trenches (up to 11,000 m deep), most near the continents (inset c)
  • Seamounts and chains of seamounts (inset d)

Seismic reflection sounding involves transmitting high-energy sound bursts and then measuring the echos with a series of geophones towed behind a ship. The technique is related to acoustic sounding as described above however, much more energy is transmitted and the sophistication of the data processing is much greater. As the technique evolved, and the amount of energy was increased, it became possible to see through the sea-floor sediments and map the bedrock topography and crustal thickness. Hence sediment thicknesses could be mapped, and it was soon discovered that although the sediments were up to several thousands of metres thick near the continents, they were relatively thin — or even non-existent — in the ocean ridge areas (Figure 10.3.5). The seismic studies also showed that the crust is relatively thin under the oceans (5 km to 6 km) compared to the continents (30 km to 60 km) and geologically very consistent, composed almost entirely of basalt.

Figure 10.3.5 Topographic section at an ocean ridge based on reflection seismic data. Sediments are not thick enough to be detectable near the ridge, but get thicker on either side. The diagram represents approximately 50 km width, and has a 10x vertical exaggeration.

In the early 1950s, Edward Bullard, who spent time at the University of Toronto but is mostly associated with Cambridge University, developed a probe for measuring the flow of heat from the ocean floor. Bullard and colleagues found the rate to be higher than average along the ridges, and lower than average in the trench areas. Although Bullard was a plate-tectonics sceptic, these features were interpreted to indicate that there is convection within the mantle — the areas of high heat flow being correlated with upward convection of hot mantle material, and the areas of low heat flow being correlated with downward convection.

With developments of networks of seismographic stations in the 1950s, it became possible to plot the locations and depths of both major and minor earthquakes with great accuracy. It was found that there is a remarkable correspondence between earthquakes and both the mid-ocean ridges and the deep ocean trenches. In 1954 Gutenberg and Richter showed that the ocean-ridge earthquakes were all relatively shallow, and confirmed what had first been shown by Benioff in the 1930s — that earthquakes in the vicinity of ocean trenches were both shallow and deep, but that the deeper ones were situated progressively farther inland from the trenches (Figure 10.3.6).

Figure 10.3.6 Cross-section through the Aleutian subduction zone with a depiction of the increasing depth of earthquakes “inshore” from the trench. [Image Description]

In the 1950s, scientists from the Scripps Oceanographic Institute in California persuaded the U.S. Coast Guard to include magnetometer readings on one of their expeditions to study ocean floor topography. The first comprehensive magnetic data set was compiled in 1958 for an area off the coast of B.C. and Washington State. This survey revealed a bewildering pattern of low and high magnetic intensity in sea-floor rocks (Figure 10.3.7). When the data were first plotted on a map in 1961, nobody understood them — not even the scientists who collected them. Although the patterns made even less sense than the stripes on a zebra, many thousands of kilometres of magnetic surveys were conducted over the next several years.

Figure 10.3.7 Pattern of sea-floor magnetism off of the west coast of British Columbia and Washington.

The wealth of new data from the oceans began to significantly influence geological thinking in the 1960s. In 1960, Harold Hess, a widely respected geologist from Princeton University, advanced a theory with many of the elements that we now accept as plate tectonics . He maintained some uncertainty about his proposal however, and in order to deflect criticism from mainstream geologists, he labelled it geopoetry. In fact, until 1962, Hess didn’t even put his ideas in writing—except internally to the U.S. Navy (which funded his research)—but presented them mostly in lectures and seminars. Hess proposed that new sea floor was generated from mantle material at the ocean ridges, and that old sea floor was dragged down at the ocean trenches and re-incorporated into the mantle. He suggested that the process was driven by mantle convection currents, rising at the ridges and descending at the trenches (Figure 10.3.8). He also suggested that the less-dense continental crust did not descend with oceanic crust into trenches, but that colliding land masses were thrust up to form mountains. Hess’s theory formed the basis for our ideas on sea-floor spreading and continental drift , but it did not deal with the concept that the crust is made up of specific plates . Although the Hess model was not roundly criticized, it was not widely accepted (especially in the U.S.), partly because it was not well supported by hard evidence.

Figure 10.3.8 A representation of Harold Hess’s model for sea-floor spreading and subduction.

Collection of magnetic data from the oceans continued in the early 1960s, but still nobody could explain the origin of the zebra-like patterns. Most assumed that they were related to variations in the composition of the rocks—such as variations in the amount of magnetite—as this is a common explanation for magnetic variations in rocks of the continental crust. The first real understanding of the significance of the striped anomalies was the interpretation by Fred Vine, a Cambridge graduate student. Vine was examining magnetic data from the Indian Ocean and, like others before, he noted the symmetry of the magnetic patterns with respect to the oceanic ridge.

At the same time, other researchers, led by groups in California and New Zealand, were studying the phenomenon of reversals in Earth’s magnetic field. They were trying to determine when such reversals had taken place over the past several million years by analyzing the magnetic characteristics of hundreds of samples from basaltic flows. As discussed in Chapter 9, it is evident that Earth’s magnetic field becomes weakened periodically and then virtually non-existent, before becoming re-established with the reverse polarity. During periods of reversed polarity, a compass would point south instead of north.

The time scale of magnetic reversals is irregular. For example, the present “normal” event, known as the Bruhnes magnetic chron, has persisted for about 780,000 years. This was preceded by a 190,000-year reversed event a 50,000-year normal event known as Jaramillo and then a 700,000-year reversed event (see Figure 9.3.3).

In a paper published in September 1963, Vine and his PhD supervisor Drummond Matthews proposed that the patterns associated with ridges were related to the magnetic reversals, and that oceanic crust created from cooling basalt during a normal event would have polarity aligned with the present magnetic field, and thus would produce a positive anomaly (a black stripe on the sea-floor magnetic map), whereas oceanic crust created during a reversed event would have polarity opposite to the present field and thus would produce a negative magnetic anomaly (a white stripe). The same idea had been put forward a few months earlier by Lawrence Morley, of the Geological Survey of Canada however, his papers submitted earlier in 1963 to Nature and The Journal of Geophysical Research were rejected. Many people refer to the idea as the Vine-Matthews-Morley (VMM) hypothesis.

Vine, Matthews, and Morley were the first to show this type of correspondence between the relative widths of the stripes and the periods of the magnetic reversals. The VMM hypothesis was confirmed within a few years when magnetic data were compiled from spreading ridges around the world. It was shown that the same general magnetic patterns were present straddling each ridge, although the widths of the anomalies varied according to the spreading rates characteristic of the different ridges. It was also shown that the patterns corresponded with the chronology of Earth’s magnetic field reversals. This global consistency provided strong support for the VMM hypothesis and led to rejection of the other explanations for the magnetic anomalies.

In 1963, J. Tuzo Wilson of the University of Toronto proposed the idea of a mantle plume or hot spot —a place where hot mantle material rises in a stationary and semi-permanent plume, and affects the overlying crust. He based this hypothesis partly on the distribution of the Hawaiian and Emperor Seamount island chains in the Pacific Ocean (Figure 10.3.9). The volcanic rock making up these islands gets progressively younger toward the southeast, culminating with the island of Hawaii itself, which consists of rock that is almost all younger than 1 Ma. Wilson suggested that a stationary plume of hot upwelling mantle material is the source of the Hawaiian volcanism, and that the ocean crust of the Pacific Plate is moving toward the northwest over this hot spot. Near the Midway Islands, the chain takes a pronounced change in direction, from northwest-southeast for the Hawaiian Islands and to nearly north-south for the Emperor Seamounts. This change is widely ascribed to a change in direction of the Pacific Plate moving over the stationary mantle plume, but a more plausible explanation is that the Hawaiian mantle plume has not actually been stationary throughout its history, and in fact moved at least 2,000 km south over the period between 81 and 45 Ma. [2]

Figure 10.3.9 The ages of the Hawaiian Islands and the Emperor Seamounts in relation to the location of the Hawaiian mantle plume.

Exercise 10.2 Volcanoes and the Rate of Plate Motion

The Hawaiian and Emperor volcanoes shown in Figure 10.3.9 are listed in the table below along with their ages and their distances from the centre of the mantle plume under Hawaii (the Big Island).

Ages of Hawaiian and Emperor volcanoes and their distances from the centre of the mantle plume. Calculate their rate of movement in centimetres per year.
Island Age Distance Rate
Hawaii 0 Ma 0 km
Necker 10.3 Ma 1,058 km 10.2 cm/y
Midway 27.7 Ma 2,432 km
Koko 48.1 Ma 3,758 km
Suiko 64.7 Ma 4,860 km

Plot the data on the graph provided here, and use the numbers in the table to estimate the rates of plate motion for the Pacific Plate in cm/year. (The first two are plotted for you.)

There is evidence of many such mantle plumes around the world (Figure 10.3.10). Most are within the ocean basins—including places like Hawaii, Iceland, and the Galapagos Islands—but some are under continents. One example is the Yellowstone hot spot in the west-central United States, and another is the one responsible for the Anahim Volcanic Belt in central British Columbia. It is evident that mantle plumes are very long-lived phenomena, lasting for at least tens of millions of years, possibly for hundreds of millions of years in some cases.

Figure 10.3.10 Mantle plume locations. Selected Mantle plumes: 1: Azores, 3: Bowie, 5: Cobb, 8: Eifel, 10: Galapagos, 12: Hawaii, 14: Iceland, 17: Cameroon, 18: Canary, 19: Cape Verde, 35: Samoa, 38: Tahiti, 42: Tristan, 44: Yellowstone, 45: Anahim

Although oceanic spreading ridges appear to be curved features on Earth’s surface, in fact the ridges are composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge (Figure 10.3.11). In a paper published in 1965, Tuzo Wilson termed these features transform faults . He described the nature of the motion along them, and showed why there are earthquakes only on the section of a transform fault between two adjacent ridge segments. The San Andreas Fault in California is a very long transform fault that links the southern end of the Juan de Fuca spreading ridge to the East Pacific Rise spreading ridges situated in the Gulf of California (see Figure 10.4.9). The Queen Charlotte Fault, which extends north from the northern end of the Juan de Fuca spreading ridge (near the northern end of Vancouver Island) toward Alaska, is also a transform fault.

Figure 10.3.11 A part of the mid-Atlantic ridge near the equator. The double white lines are spreading ridges. The solid white lines are fracture zones. As shown by the yellow arrows, the relative motion of the plates on either side of the fracture zones can be similar (arrows pointing the same direction) or opposite (arrows pointing opposite directions). Transform faults (red lines) are in between the ridge segments, where the yellow arrows point in opposite directions.

In the same 1965 paper, Wilson introduced the idea that the crust can be divided into a series of rigid plates, and thus he is responsible for the term plate tectonics .

Exercise 10.3 Paper transform fault model

Figure 10.3.12

Tuzo Wilson used a paper model, a little bit like the one shown here, to explain transform faults to his colleagues. To use this model either print this page or download the image above and print that, then cut around the outside, and then slice along the line A-B (the fracture zone) with a sharp knife. Fold down the top half where shown, and then pinch together in the middle. Do the same with the bottom half.

Figure 10.3.13

When you’re done, you should have something like the example shown on Figure 10.3.13, with two folds of paper extending underneath. Find someone else to pinch those folds with two fingers just below each ridge crest, and then gently pull apart where shown. As you do, the oceanic crust will emerge from the middle, and you’ll see that the parts of the fracture zone between the ridge crests will be moving in opposite directions (this is the transform fault) while the parts of the fracture zone outside of the ridge crests will be moving in the same direction. You’ll also see that the oceanic crust is being magnetized as it forms at the ridge. The magnetic patterns shown are accurate, and represent the last 2.5 Ma of geological time.

There are other versions of this model available here: Paper Models of Transform Faults. [3]

Image Descriptions

Figure 10.3.2 image description: At 500 Ma, rocks in Europe had upward-pointing magnetic orientations. At 400 Ma, the magnetic orientation leveled. From 300 Ma to the present, rocks in Europe shown an increasingly downward-pointing magnetic orientation. [Return to Figure 10.3.2]

Figure 10.3.6 image description: A cross section of the trench formed at the Aleutian subduction zone as the Pacific plate subducts under the North American plate in the middle of the Pacific Ocean. The farther away an earthquake is from this trench (on the North America plate side), the deeper it is. [Return to Figure 10.3.6]

Media Attributions

  • Figures 10.3.1, 10.3.2, 10.3.3, 10.3.5, 10.3.6, 10.3.8, 10.3.11, 10.3.12, 10.3.13: © Steven Earle. CC BY.
  • Figure 10.3.4: “Elevation” by NOAA. Adapted by Steven Earle. Public domain.
  • Figure 10.3.7: “Juan de Fuca Ridge” by USGS. Adapted by Steven Earle. Public domain. Based on Raff, A. and Mason, R., 1961, Magnetic survey off the west coast of North America, 40˚ N to 52˚ N latitude, Geol. Soc. America Bulletin, V. 72, p. 267-270.
  • Figure 10.3.9: “Hawaii Hotspot” by National Geophysical Data Center. Adapted by Steven Earle. Public domain.
  • Figure 10.3.10: “Hotspots” by Ingo Wölbern. Public domain.
  1. Ted Irving later set up a paleomagnetic lab at the Geological Survey of Canada in Sidney, B.C., and did a great deal of important work on understanding the geology of western North America. &crarr
  2. J. A. Tarduno et al., 2003, The Emperor Seamounts: Southward Motion of the Hawaiian Hotspot Plume in Earth’s Mantle, Science 301 (5636): 1064–1069. &crarr
  3. For more information see: Earle, S., 2004, A simple paper model of a transform fault at a spreading ridge, J. Geosc. Educ. V. 52, p. 391-2. &crarr

magnetism of a body of rock that formed at the time the rock formed and is consistent with the magnetic field orientation that existed at that time and place

past variations in the intensity and polarity of the Earth’s magnetic field

a path of varying magnetic pole positions defined by paleomagnetic data (in fact it is now understood that the continents have wandered, not the poles, so a more appropriate terms is “apparent polar wandering path”)

measurement of the properties of sediments based on detection of sounds generated at surface and reflected from layers beneath the surface

The concept that the Earth’s crust and upper mantle (lithosphere) is divided into a number of plates that move independently on the surface and interact with each other at their boundaries.

the formation of new oceanic crust by volcanism at a divergent plate boundary

the concept that tectonic plates can move across the surface of the Earth

a region of the lithosphere that is considered to be moving across the surface of the Earth as a single unit

a plume of hot rock (not magma) that rises through the mantle (either from the base or from part way up) and reaches the surface where it spreads out and also leads to hot-spot volcanism

the surface area of volcanism and high heat flow above a mantle plume

a boundary between two plates that are moving horizontally with respect to each other


Discovery of new geologic process calls for changes to plate tectonic cycle

Elements of a newly discovered process in plate tectonics include a mass (rock slab weight), a pulley (trench), a dashpot (microcontinent), and a string (oceanic plate) that connects these elements to each other. In the initial state, the microcontinent drifts towards the subduction zone (Figure a). The microcontinent then extends during its journey to the subduction trench owing to the tensional force applied by the pull of the rock slab pull across the subduction zone (Figure b). Finally, the microcontinent accretes to the overriding plate and resists subduction due to its low density, causing the down-going slab to break off (Figure c). Credit: Erkan Gün/University of Toronto

Geoscientists at the University of Toronto (U of T) and Istanbul Technical University have discovered a new process in plate tectonics which shows that tremendous damage occurs to areas of Earth’s crust long before it should be geologically altered by known plate-boundary processes, highlighting the need to amend current understandings of the planet’s tectonic cycle.

Plate tectonics, an accepted theory for over 60 years that explains the geologic processes occurring below the surface of Earth, holds that its outer shell is fragmented into continent-sized blocks of solid rock, called “plates,” that slide over Earth’s mantle, the rocky inner layer above the planet’s core. As the plates drift around and collide with each other over million-years-long periods, they produce everything from volcanoes and earthquakes to mountain ranges and deep ocean trenches, at the boundaries where the plates collide.

Now, using supercomputer modelling, the researchers show that the plates on which Earth’s oceans sit are being torn apart by massive tectonic forces even as they drift about the globe. The findings are reported in a study published this week in Nature Geoscience.

The thinking up to now focused only on the geological deformation of these drifting plates at their boundaries after they had reached a subduction zone, such as the Marianas Trench in the Pacific Ocean where the massive Pacific plate dives beneath the smaller Philippine plate and is recycled into Earth’s mantle.

The new research shows much earlier damage to the drifting plate further away from the boundaries of two colliding plates, focused around zones of microcontinents — continental crustal fragments that have broken off from main continental masses to form distinct islands often several hundred kilometers from their place of origin.

“Our work discovers that a completely different part of the plate is being pulled apart because of the subduction process, and at a remarkably early phase of the tectonic cycle,” said Erkan Gün, a PhD candidate in the Department of Earth Sciences in the Faculty of Arts & Science at U of T and lead author of the study.

The researchers term the mechanism a “subduction pulley” where the weight of the subducting portion that dives beneath another tectonic plate, pulls on the drifting ocean plate and tears apart the weak microcontinent sections in an early phase of potentially significant damage.

“The damage occurs long before the microcontinent fragment reaches its fate to be consumed in a subduction zone at the boundaries of the colliding plates,” said Russell Pysklywec, professor and chair of the Department of Earth Sciences at U of T, and a coauthor of the study. He says another way to look at it is to think of the drifting ocean plate as an airport baggage conveyor, and the microcontinents are like pieces of luggage travelling on the conveyor.

“The conveyor system itself is actually tearing apart the luggage as it travels around the carousel, before the luggage even reaches its owner.”

The researchers arrived at the results following a mysterious observation of major extension of rocks in alpine regions in Italy and Turkey. These observations suggested that the tectonic plates that brought the rocks to their current location were already highly damaged prior to the collisional and mountain-building events that normally cause deformation.

“We devised and conducted computational Earth models to investigate a process to account for the observations,” said Gün. “It turned out that the temperature and pressure rock histories that we measured with the virtual Earth models match closely with the enigmatic rock evolution observed in Italy and Turkey.”

According to the researchers, the findings refine some of the fundamental aspects of plate tectonics and call for a revised understanding of this fundamental theory in geoscience.

“Normally we assume — and teach — that the ocean plate conveyor is too strong to be damaged as it drifts around the globe, but we prove otherwise,” said Pysklywec.

The findings build on the legacy of J. Tuzo Wilson, also a U of T scientist, and a renowned figure in geosciences who pioneered the idea of plate tectonics in the 1960s.

The research was made possible with support from SciNet and Compute Canada, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Scientific and Technological Research Council of Turkey.

Reference:
Erkan Gün, Russell N. Pysklywec, Oğuz H. Göğüş, Gültekin Topuz. Pre-collisional extension of microcontinental terranes by a subduction pulley. Nature Geoscience, 2021 DOI: 10.1038/s41561-021-00746-9

Note: The above post is reprinted from materials provided by University of Toronto. Original written by Sean Bettam.


10: Plate Tectonics - Geosciences

It has been often repeated in this text and elsewhere that convection of the mantle is critical to plate tectonics, and while this is almost certainly so, there is still some debate about the actual forces that make the plates move. One side in the argument holds that the plates are only moved by the traction caused by mantle convection. The other side holds that traction plays only a minor role and that two other forces, ridge-push and slab-pull, are more important (Figure 10.28). Some argue that the real answer lies somewhere in between.

Figure 10.28 Models for plate motion mechanisms [SE]

Kearey and Vine (1996) [1] have listed some compelling arguments in favour of the ridge-push/slab-pull model, as follows: (a) plates that are attached to subducting slabs (e.g., Pacific, Australian, and Nazca Plates) move the fastest, and plates that are not (e.g., North American, South American, Eurasian, and African Plates) move significantly slower (b) in order for the traction model to apply, the mantle would have to be moving about five times faster than the plates are moving (because the coupling between the partially liquid asthenosphere and the plates is not strong), and such high rates of convection are not supported by geophysical models and (c) although large plates have potential for much higher convection traction, plate velocity is not related to plate area.

In the ridge-push/slab-pull model, which is the one that has been adopted by most geologists working on plate-tectonic problems, the lithosphere is the upper surface of the convection cells, as is illustrated in Figure 10.29.

Figure 10.29 The ridge-push/slab-pull model for plate motion, in which the lithosphere is the upper surface of the convective systems. [SE]

Although ridge-push/slab-pull is the favoured mechanism for plate motion, it’s important not to underestimate the role of mantle convection. Without convection, there would be no ridges to push from because upward convection brings hot buoyant rock to surface. Furthermore, many plates, including our own North American Plate, move along nicely — albeit slowly — without any slab-pull happening.


77 11.2 Earthquakes and Plate Tectonics

The distribution of earthquakes across the globe is shown in Figure 11.7. It is relatively easy to see the relationships between earthquakes and the plate boundaries. Along divergent boundaries like the mid-Atlantic ridge and the East Pacific Rise, earthquakes are common, but restricted to a narrow zone close to the ridge, and consistently at less than 30 km depth. Shallow earthquakes are also common along transform faults, such as the San Andreas Fault. Along subduction zones, as we saw in Chapter 10, earthquakes are very abundant, and they are increasingly deep on the landward side of the subduction zone.

Figure 11.7 General distribution of global earthquakes of magnitude 4 and greater from 2004 to 2011, colour coded by depth (red: 0-33 km, orange 33-70 km, green: 70-300 km, blue: 300-700 km) [from Dale Sawyer, Rice University, http://plateboundary.rice.edu ,used with permission]

Earthquakes are also relatively common at a few intraplate locations. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. Examples of intraplate earthquake regions include the Great Rift Valley area of Africa, the Tibet region of China, and the Lake Baikal area of Russia.

Earthquakes at Divergent and Transform Boundaries

Figure 11.8 provides a closer look at magnitude (M) 4 and larger earthquakes in an area of divergent boundaries in the mid-Atlantic region near the equator. Here, as we saw in Chapter 10, the segments of the mid-Atlantic ridge are offset by some long transform faults. Most of the earthquakes are located along the transform faults, rather than along the spreading segments, although there are clusters of earthquakes at some of the ridge-transform boundaries. Some earthquakes do occur on spreading ridges, but they tend to be small and infrequent because of the relatively high rock temperatures in the areas where spreading is taking place.

Figure 11.8 Distribution of earthquakes of M4 and greater in the area of the mid-Atlantic ridge near the equator from 1990 to 1996. All are at a depth of 0 to 33 km [SE after Dale Sawyer, Rice University, http://plateboundary.rice.edu]

Earthquakes at Convergent Boundaries

The distribution and depths of earthquakes in the Caribbean and Central America area are shown in Figure 11.9. In this region, the Cocos Plate is subducting beneath the North America and Caribbean Plates (ocean-continent convergence), and the South and North America Plates are subducting beneath the Caribbean Plate (ocean-ocean convergence). In both cases, the earthquakes get deeper with distance from the trench. In Figure 11.9, the South America Plate is shown as being subducted beneath the Caribbean Plate in the area north of Colombia, but since there is almost no earthquake activity along this zone, it is questionable whether subduction is actually taking place.

Figure 11.9 Distribution of earthquakes of M4 and greater in the Central America region from 1990 to 1996 (red: 0-33 km, orange: 33-70 km, green: 70-300 km, blue: 300-700 km) (Spreading ridges are heavy lines, subduction zones are toothed lines, and transform faults are light lines.) [SE after Dale Sawyer, Rice University, http://plateboundary.rice.edu]

There are also various divergent and transform boundaries in the area shown in Figure 11.9, and as we’ve seen in the mid-Atlantic area, most of these earthquakes occur along the transform faults.

The distribution of earthquakes with depth in the Kuril Islands of Russia in the northwest Pacific is shown in Figure 11.10. This is an ocean-ocean convergent boundary. The small red and yellow dots show background seismicity over a number of years, while the larger white dots are individual shocks associated with a M6.9 earthquake in April 2009. The relatively large earthquake took place on the upper part of the plate boundary between 60 km and 140 km inland from the trench. As we saw for the Cascadia subduction zone, this is where large subduction earthquakes are expected to occur.

In fact, all of the very large earthquakes — M9 or higher — take place at subduction boundaries because there is the potential for a greater width of rupture zone on a gently dipping boundary than on a steep transform boundary. The largest earthquakes on transform boundaries are in the order of M8.

Figure 11.10 Distribution of earthquakes in the area of the Kuril Islands, Russia (just north of Japan) (White dots represent the April 2009 M6.9 earthquake. Red and yellow dots are from background seismicity over several years prior to 2009.) [SE after Gavin Hayes, from data at http://earthquake.usgs.gov/earthquakes/eqarchives/subduction_zone/us2009fdak/szgc/ku6_trench.pdf]

The background seismicity at this convergent boundary, and on other similar ones, is predominantly near the upper side of the subducting plate. The frequency of earthquakes is greatest near the surface and especially around the area where large subduction quakes happen, but it extends to at least 400 km depth. There is also significant seismic activity in the overriding North America Plate, again most commonly near the region of large quakes, but also extending for a few hundred kilometres away from the plate boundary.

The distribution of earthquakes in the area of the India-Eurasia plate boundary is shown in Figure 11.11. This is a continent-continent convergent boundary, and it is generally assumed that although the India Plate continues to move north toward the Asia Plate, there is no actual subduction taking place. There are transform faults on either side of the India Plate in this area.

Figure 11.11 Distribution of earthquakes in the area where the India Plate is converging with the Asia Plate (data from 1990 to 1996, red: 0-33 km, orange: 33-70 km, green: 70-300 km). (Spreading ridges are heavy lines, subduction zones are toothed lines, and transform faults are light lines. The double line along the northern edge of the India Plate indicates convergence, but not subduction. Plate motions are shown in mm/y.) [SE after Dale Sawyer, Rice University, http://plateboundary.rice.edu]

The entire northern India and southern Asia region is very seismically active. Earthquakes are common in northern India, Nepal, Bhutan, Bangladesh and adjacent parts of China, and throughout Pakistan and Afghanistan. Many of the earthquakes are related to the transform faults on either side of the India Plate, and most of the others are related to the significant tectonic squeezing caused by the continued convergence of the India and Asia Plates. That squeezing has caused the Asia Plate to be thrust over top of the India Plate, building the Himalayas and the Tibet Plateau to enormous heights. Most of the earthquakes of Figure 11.11 are related to the thrust faults shown in Figure 11.12 (and to hundreds of other similar ones that cannot be shown at this scale). The southernmost thrust fault in Figure 11.12 is equivalent to the Main Boundary Fault in Figure 11.11.

Figure 11.12 Schematic diagram of the India-Asia convergent boundary, showing examples of the types of faults along which earthquakes are focussed. The devastating Nepal earthquake of May 2015 took place along one of these thrust faults. [SE after D. Vouichard, from a United Nations University document at: http://archive.unu.edu/unupress/unupbooks/80a02e/80A02E05.htm]

There is a very significant concentration of both shallow and deep (greater than 70 km) earthquakes in the northwestern part of Figure 11.11. This is northern Afghanistan, and at depths of more than 70 km, many of these earthquakes are within the mantle as opposed to the crust. It is interpreted that these deep earthquakes are caused by northwestward subduction of part of the India Plate beneath the Asia Plate in this area.

Exercises

Exercise 11.1 Earthquakes in British Columbia

This map shows the incidence and magnitude of earthquakes in British Columbia over a one-month period in March and April 2015.

1. What is the likely origin of the earthquakes between the Juan de Fuca (JDF) and Explorer Plates?

2. The string of small earthquakes adjacent to Haida Gwaii (H.G.) coincides closely with the rupture surface of the 2012 M7.7 earthquake in that area. How might these earthquakes be related to that one?

3. Most of the earthquakes around Vancouver Island (V.I.) are relatively shallow. What is their likely origin?

4. Some of the earthquakes in B.C. are interpreted as being caused by natural gas extraction (including fracking). Which of the earthquakes here could fall into this category?


NEW HIGH P-T EXPERIMENTS

We undertook new high P-T experiments at 825–1000 °C and 1.6–2.2 GPa on a primitive and depleted (relatively high MgO and low light rare earth elements [LREEs], Th, and U) anhydrous sample from the Ontong Java oceanic plateau (OJP) (see the Methods section of the GSA Data Repository 1 , and Tables DR1 and DR2 therein). All of the previous starting compositions reported in the literature are significantly different from our OJP sample in at least several major elements (Table DR1).

Evidence for Eoarchean subduction compelled us to explore a subduction environment from which to generate ETTG. A shallow subducting slab is converted to an amphibolite with ∼2–3 wt% water (Peacock, 1993), and therefore, a similar amount of water was added to the anhydrous OJP material to form partial melts in equilibrium with an amphibolite containing plagioclase and/or garnet depending on the P-T conditions. Above ∼900 °C, the OJP sample undergoes partial melting to generate tonalite liquids (Fig. 1A Table DR3) and our experiments replicate melt-generating processes that occurred at the top of a subducting Eoarchean slab. Lower crustal sections (<3–4 km depth) would be essentially anhydrous (Foley et al., 2002 Moyen and Martin, 2012 Tang et al., 2016), and therefore, our results do not represent intracrustal melting mechanisms deep within Eoarchean oceanic crust.

With the exception of K2O, our tonalite melts plot within the major element liquid lines of descent for ETTG (Hoffmann et al., 2011 Nutman et al., 2009), and Figures 1B and 1C show this using TiO2 and MgO as examples (see Table DR4 for a full major element comparison). Previous experimental melts are highly variable but generally have a poor fit with regards to either TiO2 or MgO (or other major elements). Our K2O values are below those for ETTG (previous experimental liquids are again highly variable), but K2O, unlike other major elements, is easily mobilized in subducted slab-derived aqueous fluids, and so ETTG may have gained K2O from fluids derived by dehydration of subducted crust as well as from slab melts. Accordingly, we use the methodology of Kogiso et al. (1997) to mix our tonalites with a theoretical K2O-enriched aqueous slab fluid that increases the K2O content such that all of our experimental major element compositions now plot with ETTG (Fig. 1D Table DR4). Using a primitive oceanic plateau starting composition with higher K2O concentrations to increase the K2O abundances in our melts is not practical because primitive oceanic plateau lavas have very low K2O (average of ∼0.1 wt% from the OJP and Caribbean, similar to our starting material) (Fitton and Godard, 2004 Hastie et al., 2016). Nevertheless, future experiments using more differentiated oceanic plateau material may be able to generate melts with higher K2O without requiring the addition of a slab fluid.

Figure 2A shows that the trace element concentrations of our tonalite liquids also have compositions nearly identical to that of ETTG (Table DR5). Importantly, the range of heavy REE (HREE) concentrations is replicated, from high-HREE contents with residual plagioclase to progressively lower HREE concentrations as residual garnet increases in modal abundance. Additionally, the liquids have low Eoarchean-like Sr contents ranging from 133 to 474 ppm, with melts in equilibrium with residual plagioclase having lower values (Fig. 2A). Residual amphibole and titanomagnetite also generate a characteristic negative Ti anomaly. Data from previous experimental liquids derived from Hadean greenstone (Adam et al., 2012) and back-arc starting materials (Rapp et al., 1999) largely overlap the ETTG data, but several elements plot outside the ETTG field (e.g., Sr), and the melts generally do not replicate the overall ETTG pattern as well as our OJP melts—particularly the negative Ti anomaly (even with residual rutile) (Fig. 2B).

Our tonalites have a variably small negative Nb anomaly (MORB-normalized [mn] La/Nbmn ratios of 0.7–2.3) compared with ETTG (La/Nbmn ratios of 1.3–11.5). However, the La/Nbmn ratios in our melts can be increased if we mix them with the same slab-derived fluid that we used to increase the K2O (Fig. 1D). We assume that only Th, U, Sr, and the LREEs are mobile in a slab-derived aqueous fluid (Kogiso et al., 1997) (Table DR6). A 96% tonalite and 4% slab fluid mixture generates a higher La/Nbmn ratio of 1.4–3.5 that brackets about half of the ETTG samples while still retaining ETTG-like concentrations for the other elements (Fig. 2C). Oceanic plateau starting material with higher TiO2 concentrations may stabilize rutile as a residual phase instead of titanomagnetite here, and this could lead to higher La/Nbmn in subsequent melts. Primitive oceanic plateau samples commonly have low TiO2 abundances similar to that in the starting material in our experiments (Fitton and Godard, 2004 Hastie et al., 2016) however, more differentiated oceanic plateau material does have commonly higher TiO2 and potentially could stabilize rutile. Again, future experiments using more differentiated oceanic plateau material are required to explore this possibility. Nonetheless, assuming that Eoarchean oceanic crust is similar to primitive oceanic plateau basalts, our tonalite melt and slab fluid mixtures represent the simplest model to explain ETTG major and trace element compositions.


Do impacts impact global tectonics?

Plate tectonics is the framework through which we understand the large-scale Phanerozoic history of Earth. The question of when and how plate tectonics began remains the subject of debate, in no small part because through subduction, plate tectonics destroys much of the evidence of its earlier activity. Estimates of the onset of plate tectonics vary from the Hadean (Hopkins et al., 2008), to the Archean (Brown, 2006), to the Neoproterozoic (Stern 2005, 2008). There is no rock record from the Hadean, and only a limited rock record from the Archean. Thus, it is unlikely that we will determine whether any deformation recorded during this time period was part of a globally connected plate boundary system or a regional, transient event.

Spherule beds are preserved within Archean age rocks in the Barberton greenstone belt, South Africa (Lowe and Byerly, 1986 Lowe et al., 1989) and the Pilbara craton, Australia (Glikson et al., 2016). Archean spherule beds formed from the distal ejecta of large bolide impacts. These beds contain important clues regarding their formation—the thickness of the beds can be used to estimate the size of the impactor (Johnson and Melosh, 2012). Lowe et al. (2014) described four additional spherule beds and placed their formation at the same time as the first major episode of orogeny and crustal deformation in the Barberton greenstone belt (3.26–3.23 Ga). Lowe et al. further suggested that these impacts may have been the trigger that initiated the modern plate tectonic regime. A new contribution by O’Neill et al. (2020, page 174 in this issue) uses the characteristics of these recently described spherule beds to constrain the size and velocity of the impactors that formed them, extending the Archean impact record. They then use the Archean impact record as input to geodynamic models to test Lowe et al.’s hypothesis that these impacts could have initiated a modern style of plate tectonics.

Lowe et al. (2014) were not the first to postulate that the Archean greenstone belts record plate tectonic activity. There are multiple lines of evidence that plate tectonics may have been operating in the Archean, including apparent polar wander curves (O’Neill et al., 2007), felsic volcanism consistent with melting of a water-rich source, and isotopic systematics similar to modern-day arcs (Hugh Smithies et al., 2018 O’Neill et al., 2018). The absence of clearly identified fold-and-thrust belts, tectonic mélanges, or ophiolites in the Archaean rock record casts doubts on the subduction interpretation (e.g., Stern, 2005 Moyen and van Hunen, 2012).

Geodynamic calculations have become an important hypothesis-testing tool when combined with the Precambrian geological record (c.f., van Hunen and Moyen, 2012: O’Neill et al., 2018 Stern and Gerya, 2018). These models are based on basic laws of physics, including the conservation of mass and energy, as well as Newton’s second law, sometimes misleadingly described as the conservation of momentum. To reduce the number of variables and create a set of equations that can be solved, a set of constitutive equations are required. For mantle convection, stress and strain rate are related through the viscosity. The viscosity of silicate minerals depends on temperature, pressure, composition, stress, grain size, water content, and history (c.f., King, 2016). Our understanding of viscosity is limited, in part due to the challenge of measuring the viscosity of silicate minerals at high pressures and temperatures, and the reality that the strain-rates achievable in such laboratory measurements must be extrapolated by orders of magnitude to mantle conditions. Geodynamic calculations are built upon solid physical principles the calculations are limited in so far as our understanding of mantle viscosity is limited, and the appropriateness of the initial and boundary condition choices.

Several modes of surface behavior are recognized in geodynamic models. In stagnant-lid convection, the lithosphere is immobile with surface heat flow limited by conduction. In mobile-lid convection, the lithosphere is part of the convecting system, cooling as it advects along the surface, and sinking back into the warmer mantle. All other factors being equal, a stagnant-lid planet will have a hotter mantle than a mobile-lid planet. The transition from stagnant-lid to mobile-lid tectonics in geodynamic modeling has enriched our understanding of plate tectonics on Earth. As the mantle becomes hotter than at present day, many models show that plate-like behavior becomes episodic, with alternating periods of mobile-lid and stagnant-lid behavior (c.f., van Hunen and Moyen, 2012). An implication of these models is that evidence for plate tectonics may appear and disappear in the geological record, and subduction may repeatedly fail (O’Neill et al., 2018).

The primary force driving plate tectonics is the negative buoyancy in subducted slabs (Forsyth and Uyeda, 1975). It is unclear what additional processes could produce the large forces necessary to initiate subduction on a pre–plate tectonic planet. To identify and test candidate processes, researchers have modeled the arrival of large plumes under the lithosphere (Gerya et al., 2015), magmatic weakening and volcanic loading (Moore and Webb, 2013 Nakagawa and Tackley, 2014), and bolide impacts (O’Neill et al., 2017). In many cases, subduction is transient, with the mantle reverting to a stagnant-lid state after a relatively brief time interval.

In a previous study, O’Neill et al. (2017) examined the effect of bolide impacts on a Hadean Earth, finding that the thermal anomalies produced by extremely large impacting bolides (>∼700 km in diameter) induce mantle upwellings that are capable of driving transient subduction events. These transient events terminate because the hotter Hadean mantle had a lower viscosity than the present-day mantle, reducing the coupling between the mantle and lithosphere (O’Neill et al., 2007), and the higher temperatures weakened the core of the subducting slab, resulting in necking and breakoff that reduces the slab pull force on the plate (van Hunen and Moyen, 2012).

The present O’Neill et al. study (2020) addresses whether the estimated size and frequency of Mesoarchean impacts could have initiated subduction, and whether these events could have developed into a globally connected plate boundary network that continued without interruption to the present. There are several significant differences between the Hadean and Archean environments that will directly impact the geodynamic modeling. The Archean mantle should be cooler than the Hadean mantle (c.f., Christensen, 1985) and there should be smaller, less-frequent bolide impacts in the Archean when compared to the Hadean (Bottke et al. 2012). Thus, it is not possible to rescale the results of O’Neill et al.’s (2017) Hadean Earth study to address impact-induced subduction in the Archean.

The two studies by O’Neill et al. (2017, 2020) are among the first studies to model the role of bolide impacts on planet-wide tectonic behavior applied to Earth. Both studies concluded that, if initiated, subduction would be a transient event, lasting only tens of millions of years, and leaving open the question of when plate tectonics (as the globally connected plate boundary network that we recognize today) took hold. The O’Neill et al. (2017, 2020) results are at odds with the results of Foley et al. (2014). The reason for the discrepancy may be that O’Neill et al. used a yield-stress formulation while Foley et al. (2014) used a formulation where the plate boundaries are weakened by grain-size reduction (Bercovici and Ricard, 2014).

Foley (2018) suggested that the difference between the two formulations is related to how they respond to changes in lithospheric stress. In yield-stress formulations, when the lithospheric stress decreases, the stress level at the boundaries may no longer exceed the yield stress, resulting in lithospheric stagnation. In contrast, when lithosphere stress drops with increasing mantle temperature or heat production rate, the deformational work, which drives grain-size reduction, increases. Thus, in grain-size reduction formulations, the ability to form weak plate boundaries is not impeded by early Earth thermal conditions. Both formulations are based on sound physical principles. Yet it is not clear how each method applies to the complex environment of subduction zones. Our understanding of subduction initiation is limited by our understanding of the process by which a stagnant lithosphere begins to deform.

Some may wonder whether appealing to a bolide impact as a trigger for subduction initiation is truly necessary. It is clear from the other inner Solar System bodies (Mercury, Venus, Mars, and the Moon) that bolide impacts were a significant process in the inner Solar System during the Hadean and Archean (Bottke et al., 2012). Thus, bolide impacts played a significant role in the Hadean and Archean on Earth. Geodynamicists assumed that because Earth’s mantle is convecting well above the critical Rayleigh number, over the course of Earth history the initial state of the mantle will be long forgotten. Weller and Lenardic (2012) showed that models with identical parameters starting from different initial conditions may end up in different states (e.g., stagnant-lid versus mobile-lid convection). When and whether a planet exhibits plate tectonics are likely functions of the initial state and history of the planet.

When plate tectonics began has a fundamental control on the thermal evolution of Earth, because planets lose heat more effectively with a mobile lid than a stagnant lid. Thus, the earlier plate tectonics began, the further Earth has cooled from its initial state. The hypothesis that plate tectonics is necessary to create the kind of stable surface environment necessary for a habitable planet is of interest to the exoplanet community (Kasting and Catling, 2003).


Watch the video: Session 10 Plate Tectonics and Continental Drift Live Session (September 2021).