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Chapter 4: Plate Tectonics
As we learned in Chapter 1, the basic idea of plate tectonics is simplicity itself: Earth’s outer, brittle shell is divided into several large, rocky slabs (lithospheric plates) that slide around atop a more mobile interior (the asthenosphere).
Plate boundaries are sites of geologic action. Most earthquakes and volcanoes occur at plate boundaries. Most mountain building also occurs at plate boundaries.
You should review these plate tectonic animations before proceeding.
Here’s a worldwide plate boundary map:
Now, let’s zoom in to examine selected plate boundaries in more detail:
Lithospheric plate boundaries. (courtesy of Wikipedia; http://en.wikipedia.org/wiki/File:Plates_tect2_en.svg)
In the image below, the Pacific plate is subducting northwestward, beneath Alaska (North American plate). The Aleutian trench marks the sea floor location of the plate boundary. Inboard from the trench we find a chain of volcanic islands—a volcanic arc:
Northern Pacific Ocean (image courtesy of Google Earth).
In the image below, the Mid-Atlantic Ridge—actually a series of short ridge segments offset by transform plate boundaries (red arrows)--is clearly visible along the boundary between the South American and African plates.
Southern Atlantic Ocean (image courtesy of Google Earth).
African Plate
S. Am. Plate
Mid-Atlantic Ridge N
N
35 0 mi approx
N
Pacific Plate (moving NW)
Aleutian Trench (subduction zone)
Aleutian Islands (volcanic arc) N. American Plate
300 mi approx
N
oceanic crust like a snowplow plows through snow, or a drifting boat pushes through water, as shown in Figure 4.7 in your text (hence, the phrase ―continental drift‖).
More recent work has established that the crust and uppermost mantle comprise a single unit (the lithosphere) that moves atop a weaker, more plastic region within the mantle (asthenosphere) underneath.
Wegener also mistakenly thought that the outward directed forces associated with a spinning Earth (which Wegener called the ―pole fleeing force‖) and the Moon’s gravitation caused the continents to drift. This too, was completely wrong.
And yet, Wegener cited compelling evidence that the continents had moved, including matching coastlines (click on 2.1) , matching rock units, matching mountain ranges, and matching fossils across vast ocean basins. In addition, he explained polar wander curves and ancient climates in terms of continental drift as well. The problem was that much of his evidence could also be understood in other ways, so most folks weren’t convinced that Wegner had made a good case for mobile continents. Unfortunately for our hero, he never lived to see his idea of mobile continents widely accepted .
Things really began to change in the years following World War II, when our understanding of the ocean floor dramatically improved. In addition, researchers began to realize that Earth’s magnetic field hadn’t always been oriented as it is today, but that it had occasionally flip-flopped in the past, completely reversing itself (more on this later).
One of geology’s great heroes is Harry Hess. Make sure you understand Hess’ version of seafloor spreading. Hess proposed that new seafloor oozes out of the mid-ocean ridges, solidifies, and moves in opposite directions away from the mid-ocean ridge, like a conveyor belt, eventually disappearing in what Hess called the ―jaw crusher of the Earth’s mantle.‖
It’s important to keep in mind that Hess’ idea of seafloor spreading was very different than Wegener’s notion of continental drift. According to Hess, the continents and ocean floors moved together in lockstep, rather than mobile continents moving through stationary sea floor, as Wegener thought. Hess was such a brilliant researcher and a scientific superstar that he was eventually put in charge of the Apollo space program…
Check out the following animated links to make sure you get a visual-spatial understanding of how seafloor spreading works, both in map view (click on 2.5) and cross section (click on 2.7).
As your book points out, modern plate tectonic theory represents a combination of seafloor spreading and continental drift (continental drift + seafloor spreading = plate tectonics)
In the mid-1950s and 1960s, surveys of seafloor magnetism revealed something amazing!
As shown in Figure 4.14 in your text, researchers detected magnetic stripes parallel and symmetrical to the mid-ocean ridge.
Such stripes alternated between positive magnetic anomalies--seafloor regions with unusually strong magnetism, and negative magnetic anomalies--regions with weaker than expected magnetism.
If one colors the positive magnetic stripes dark and the negative stripes white, they resemble a zebra.
For this reason, researchers dubbed these stripes ―zebra stripes.‖ Keep in mind that they don’t really look this way; coloring them black and white just makes them stand out.
Here’s an image of the zebra stripe pattern:
Formation of zebra pattern (courtesy of USGS; http://pubs.usgs.gov/gip/dynamic/developing.html)
How can the ―zebra‖ pattern seen on the sea floor be explained? Simple! In terms of flip- flopping magnetic poles in combination with seafloor spreading.
Watch the following link to get a sense of how the ―zebra pattern‖ originates via seafloor spreading: Zebra Pattern—Figure 4.14.
The zebra pattern was first explained by Canadian geologist, Lawrence Morley, who tried unsuccessfully to get his ideas published in 1963.
UC San Diego professor, Dr. Naomi Oreskes has edited a wonderful book about the plate tectonic revolution, entitled Plate Tectonics : an Insider’s history of the Modern Theory of the Earth (Oreskes 2001). Morley’s entire paper is reproduced in her book.
Morley argued that as molten rock rises upward at the ridge and cools, the upwelling rock:
…must become magnetized in the direction of the Earth’s field prevailing at the time. If this portion of the rock moves upward and then horizontally to make room for new upwelling material and if, in the meantime, the Earth’s field has reversed…it stands to reason that a linear magnetic anomaly pattern of the type observed would result.
One of the reviewers for the journal that rejected Morley’s paper wrote that his idea, ―is an interesting one – I suppose – but it seems most appropriate over martinis, say, than in the Journal of Geophysical Research‖ (Oreskes 2001).
Eventually, Morley got his paper published, but not before two other researchers, Vine and Matthews, got their very similar explanation of the zebra pattern published first.
Mid-ocean Ridge
Normal magnetic polarity
Reversed magnetic polarity
Mid-ocean Ridge
Sure enough, this is exactly what we find, and it’s just what Harry Hess and colleagues predicted we’d find once we had the technology to measure sea floor sediment thickness.
Also, plate tectonic theory nicely explains why the oldest continental rocks (typically billions of years old) are much, much older than the oldest seafloor rocks (only about 180-200 million years old). Simply put, continental rocks don’t subduct because they’re too buoyant, whereas oceanic rocks do subduct and so are eventually recycled back into the mantle.
Figure 4.18B highlights an interesting property of transform plate boundaries. They tend to occur in close association with divergent boundaries, but only between mid-ocean ridge segments.
Notice in Figure 4.18B that it’s only in the region between two spreading ridges that the two plates move in opposite directions.
Here’s a link that will demonstrate this: Transform Faulting (click on 2.8).
Here’s a picture (from the Ch 1 study guide), below. The large image on top and the lower left- hand image are in map view (viewed from above), whereas the lower right-hand image is in cross section (vertical slice).
Transform plate boundary.
N
A B
MOR
segment
MOR
Segment
Cross Section (view west along line X-X’)
A
(away)
B
(toward)
X X’
Map View
Plate A
Plate B
Many pieces of evidence support plate tectonic theory. Here's a long (but by no means complete) list:
- the approximate puzzle-like fit of the continental coastlines, particularly eastern South America and western Africa. This suggests the two continents were originally joined and later split apart, migrating their separate ways;
- the better fit of the continental shelves--the submerged edges of the continents--which supports the idea that now-separated continents were once joined;
- the alignment of coastal features, such as rock types, fossils, and mountain ranges, across continental boundaries, which provides yet more evidence that many continents now separated by thousands of kilometers of open ocean were once joined;
- seafloor spreading--the lateral, conveyor-like movement of two oceanic plates in opposite directions away from a mid-ocean ridge. This style of plate movement has been confirmed by the trend of increasing seafloor age and sediment thickness away from the mid-ocean ridges—both observations that are explicable only in the context of plate divergence;
- the symmetrical alignment of "zebra stripes"--strips of strong and weak magnetism on the seafloor-- across the world's mid-ocean ridges. This observation also confirms seafloor spreading, and can only be explained as seafloor spreading occurs in the presence of Earth's occasionally reversing magnetic field. Such reversals, together with seafloor spreading, tend to "fossilize" regions of normal and reversed magnetism on either side of the mid ocean ridges. When the sea floor diverges at the mid-ocean ridge, these fossilized magnetic strips split in two, with part of the strip moving one way, and part of the strip moving in the opposite direction. This creates the apparent magnetic symmetry across the mid-ocean ridges.
- the alignment of earthquake epicenters along many of the world's active plate boundaries-- suggesting that the plates really do move;
- the presence of volcanic arcs near active subduction zone plate boundaries, which suggests that subducting plates really do partially melt to create magma, as predicted by plate tectonic theory;
- the presence of dipping zones of earthquakes beneath subduction zones, which provides independent evidence of the subduction process. As a plate subducts, it cracks up, creating earthquakes that trace out the subducting plate in 3 dimensions;
- the large difference in age between continental and oceanic crust. Continental crust is, on average, billions of years old, particularly in plate interiors, whereas oceanic crust is relatively young--only as old as about 180-200 million years. Plate tectonic theory explains why: continental crust is generally too buoyant to subduct and so, once formed, it's more likely to be preserved into geologic old age. In contrast, oceanic crust easily subducts and is thus destroyed;
- the occurrence of mountain belts--zones of active compression and uplift-- at active convergent plate boundaries, thus explaining how and where mountain belts tend to form;
- the occurrence of ―mismatched‖ climatic indicators in ancient rocks, such as coal deposits (indicative of low- to mid-latitude, humid and temperate climates) in high-latitude places like Antarctica, and extensive continental glacial deposits (indicative of cool climates) in warm places like India. Given that climatic zones don’t change much throughout geologic time, it must be that the continents move through different climate zones during their plate tectonic march.
One important modification to plate tectonic theory is the so-called deep mantle plume hypothesis. Think of a deep mantle plume as a cylindrical column of hot rock that rises from a great depth to express itself at Earth’s surface as a ―hot spot‖—a region of intense volcanic activity. In contrast to the mobile plates, mantle plumes are thought to be roughly stationary. So when a mobile plate like the Pacific plate moves over a stationary mantle plume in the central Pacific ocean, magma occasionally finds its way to the surface to create a chain of island volcanoes—the Hawaiian Islands. Just as we’d expect, the Hawaiian Islands become progressively older to the northwest, because the Pacific Plate is also moving to the northwest.
Here’s a link to an animation of a deep-mantle plume that gives rise to a Hot Spot (click on 2.10)
I won’t go into much detail about deep mantle plumes. In part, because I’m not convinced they really exist in the large numbers implied by Figure 4.32 in your book. I’m certainly no plume expert, but I’ve been strongly influenced by the views of famous plume skeptics like Don Anderson at Caltech, Warren Hamilton at Colorado School of Mines, and Gillian Anderson at the U.S. Geological Survey, among others.
The basic problem with at least some purported deep mantle plumes is that they’re not deep, not stationary, and not hot! That is, heat flow around many such plumes doesn’t appear to be any greater than in non-plume areas. Nor are plumes always stationary. Some plumes appear to ―drift‖ within a ―mantle wind‖ at velocities approaching plate velocities. Bottom line, if deep mantle plumes aren’t hot and aren’t spots, how can they be associated with hot spots?
Slab pull (gravity sinking) and trench suction (secondary mantle flow) mechanisms.
Sea Surface Volcanic Arc
Trench
“Slab Pull” Gravitational sinking of subducting plate due to increased density.
“Trench Suction” Secondary asthenospheric flow assoc. w/slab pull draws upper, non-subducting plate toward trench.
Many researchers wonder whether something else is happening in Earth’s interior below some hotspots. There’s more and more evidence mounting that many regions initially thought to be underlain by deep mantle plumes are actually underlain by shallower magma sources. In addition, widespread melting in Earth’s interior can also be accomplished by changing the composition of the rocks rather than by heating. Furthermore, there’s good evidence that as a plate breaks up and cracks, this process actually triggers melting within the underlying mantle.
Some regions of widespread, point-source volcanism may simply be underlain by rocks with lower melting temperatures than the surrounding rocks. Finally, although the Hawaiian island chain shows a nice age progression in accordance with plume theory (e.g., older islands having already moved off the stationary plume underneath), some other island chains don’t show this age progression.
Right now, there’s a raging debate going on among researchers regarding the extent to which (or even if) deep-mantle plumes exist…Let’s listen in ;-)
(Announcer steps into a Las Vegas wrestling ring, at the center of a large, jam-packed, smoke-filled auditorium…a microphone descends from above…the announcer clears her throat…)
“Good evening, ladies and gentlemen…and I use those terms loosely (crowd boos…). I’m Sandy Beach, your announcer…Welcome to the Great Plume Debate! (crowd goes wild…). Please, please, settle down!
In one corner, weighing in on the side of shaky scientific consensus, we have the reigning champions, those mighty plume advocates…The Plumes of Doom! (crowd chants, Doom! Doom! Doom!)…
In the other corner, we have tonight’s challengers--those pesky, nasty plume skeptics (crowd hisses and boos!) who are determined to throw cold water on a nice, tidy, idea that’s been around for decades...(crowd starts chanting, “No Anti-Plumers! No Anti-Plumers! We love plumes! We love plumes!)…Ladies and gentlemen, I give you the No Ploomer Boomers! (crowd hisses and boos)…
Quiet! Quiet! Let’s get this match underway!
Tonight’s rules are…Number 1—all arguments for either side must be published in peer-reviewed scientific journals...Reader’s Digest doesn’t count! (crowd boos!).
Number 2—each side may occasionally misrepresent the arguments of the other side, but only to a point.
And Number 3—This match is to the death…that is, the death of the losers’ scientific prestige…Young researchers, take note…if you’re on the losing side, you may not get tenure, and then you won’t even be able to get a job as a dishwasher at Burger King, let alone a community college instructor! To the winners go the spoils—glory, honor, tenure, speaking engagements, and lots of grant money!
And finally…a disclaimer: Any resemblance of tonight’s event to actual persons, scientific debates, fast- food chains, or educational institutions, whether intentional or unintentional, implied or unimplied, real or fictional, funny or not, is unintended…P.S. Our lawyers made me say this…
Let the mayhem begin!” (crowd cheers!)…
Truth be told, this match will probably continue for years if not decades into the future; such is the nature of the scientific enterprise.
