Prologue: Yesterday’s diary (URL http://www.dailykos.com/...) came and went quickly due to so much other important news coverage. Before reading the conclusion to this primer and spiel I recommend reading the first diary. Afterward, today's continuation will recap some of that spiel, while adding more to the pile of learning afforded this moving subject (pun intended). Let’s call this a Geology 201 course extension.
More Details About Plate Tectonics: Continents not only grow and shrink over time, but also tend to meld their real estates from time to time. Actually, cycles in millions of years confirms such a phenomenon. Thus the changing planetary landscapes almost since the beginning of our Earth’s time, which includes adding new material by a process called seafloor spreading (at mid-oceanic ridge points, instigating faulting).
When Pangea initially fractured into two large segments, the lesser sized landmass, Gondwana, had amassed its real estate by merging with smaller land segments. This exotic-sounding name derives from a region of central northern India, tracing its roots from the Sanskrit “Gonhavana,” meaning forest of gond.
When Pangea initially fragmented into two distinct halves comprised of Gondwana and its larger counterpart, Laurasia, the stage was set for further fragmentation, and of course more continental pieces that continued to separate from the whole. Laurasia's downsizing fragments included most of the landmasses that make up today’s continents of the Northern Hemisphere. Chiefly, Laurentia (the name give to the North American craton), Baltica (now called Europe), Greenland, Siberia, Kazakhstania (i.e., a small continental region in the interior of Asia), a part of northwest Africa, and the North China and East China cratons. Laurentia and Eurasia combined is Laurasia.
What geologists know about both initial and larger estates is that Gondwana was once on or just below the equator. Its estate included present day’s South America, Africa, India, Australia/Antarctica, New Guinea and New Zealand, bits of Asia, Madagascar; also Arabia and the India subcontinent was included in the package. Laurasia comprised all the other landmasses and both fragments were, for a time, separated by the Tethys Sea.
Plate Migrations And Favored Geologic Clues: How any of these plate migrations ends up with a fantastic story, as credible and empirical, with a plus and minus factor measured in hundreds of millions of years, cannot be pinned down given more precise dates for such world-changing events. However, there are rough and reliable estimates, where ‘roughly’ in this sense may mean anywhere from 10 to 20 million years one way or the other. Otherwise, there are five essential clues that verify what has happened throughout time and around the world by way of the old standby in math and philosophy (but also useful to earth sciences: Q.E.D., meaning quod erat demonstrandum (Latin for “which had to be demonstrated” before accepting any theory or principle).
• GEOGRAPHICAL evidence demonstrates howThe upshot of this information is this: if you have ever wondered why people find the fossil remains of tropical plants and animals in places that today are temperate or even cold; if you have ever wondered what causes the oceans and seas to move inland across large portions of a continental land mass, then withdraw. . .you have your answer. And if you ever scale the heights of mountains, like Mt. Everest, and could dig deep beneath the ice and snow and find sea life remains, then you know mountain-building activity was at work, that is, where a former landmass was much lower, but eventually raised to impressive heights.
the coastline of some continents would nearly interlock if rearranged like perfect pieces of a vast jigsaw puzzle that really isn’t as complicated as one might think.
For instance, look how easily South America and Africa 'right side' up fit together.
• GEOLOGICAL evidence demonstrates how old mountain zones of matching ages appear as “belts” crossing southern continents, that is, if these were joined together in a precise way.
• CLIMATIC evidence demonstrates how glacial deposits and rocks scratched by stones and harder minerals in moving ice show that thick ice once covered huge tracts of southern continents some 300 million years ago (hereafter, “mya”), which further suggests these places were located in polar regions.
• PALEOMAGNETIC evidence demonstrates alignments of magnetized particles in old rocks show that southern continents all lay near the South Pole some 300 mya.
• BIOLOGICAL evidence demonstrates identical fossil land plants and land animals crop up in southern continents now widely separated by the sea.
Fossil correlation, geography, and mountain chain disruption puts Humpty Dumpty, our fractured planet, back together. This is the corollary and corroboration geologists use to prove their empirical science.
Supercontinents Through Time: Pangea-like masses do not necessarily indicate how all the continents were once glued together. Rather, portions of greater size are also considered. Consider contemporary super sized continents given the following examples:
• Afro-Eurasia (~ 5 mya present-day supercontinent)As for so-called historical supercontinents, these are the known landmasses that had formed over time (Ga means billions of years ago):
• Americas (~ 15 mya present-day supercontinent)
• Eurasia (~ 60 mya present-day supercontinent)
• Gondwana (~600 — ~30 mya)Some of this information is technical, yet relates in depth details entailing this subject matter, and will (in part) be more fully explained below.
• Laurasia (~ 300 — ~60 mya)
• Pangaea (~300 — ~180 mya)
• Euramerica (~300 mya)
• Pannotia (~600 — ~540 mya)
• Rodinia (~1.1 Ga— ~750 mya)
• Columbia, also called Nuna (~1.8 — 1.5 Ga ago)
• Nena (~1.8 Ga)
• Kenorland (~2.7 Ga ago)
Note: Neoarchean sanukitoid cratons and new continental crust formed Kenorland. Protracted tectonic magna plume rifting occurred 2.48 to 2.45 Ga and this contributed to the Paleo-proterozoic glacial events in 2.45 to 2.22 Ga. Final breakup occurred ~2.1 Ga.
• Ur (~3 Ga ago)
Note: Ur is classified as the earliest known landmass. However, Ur was probably the largest, perhaps even the only continent three billion years ago. While probably not a supercontinent, one can argue that Ur was a supercontinent for its time, even if it was smaller than Australia is today. Still, an older rock formation now located in Greenland dates back from Hadean times.
• Komatii Formation (3.475 Ga)
• Vaalbara (~3.6 Ga. Evidence is the Yilgarn Craton, Western Australia and the world-wide Archean greenstone belts that were subsequently spread out across Gondwana and Laurasia)
• Yilgarn (4.3 - 4.3 Ga
Note: Zircon crystals from the Jack Hills of the Narryer Gneiss Terrane, Yilgarn craton, Western Australia and also 186 miles south point to a continental crust formation between this timeframe. Evidence is the high Oxygen-18 values of 8.5 and micro-inclusions of SiO2 (silicon dioxide) in these zircon crystals consistent with growth from a granitic source supracrustal material, low-temperature interactions and a liquid ocean.
Pangea, Revisited: I think one of the most tangible demonstrations I can think of that demonstrates the power and magnitude of shifting tectonic plates is to imagine yourself standing on a continental plate, being familiar with the surroundings, then falling asleep. Millions of years later you wake up and realize the environment is completely different. You only have a vague memory of the before and a sobering account of the after. Had you been around, say, the late Permian Period (about 230 or 240 mya), and woke up to today’s world you would probably think you were living on another planet. The ride of your life was indeed a slow migration, where the plate you were on had steadily drifted across many and varying latitudes. It follows along the way environments also changed, in this example, starting from below the equator.
Let’s take this example and turn it into another time machine excursion. For example, you chose what would one day be called the North American plate. First, you positioned yourself in one locale, Pangea. The image of its final concretion is not what you might expect, that is, a mass of welded continental mass comprised in a neat circle and revealing neat pie-shaped segments of continents. Instead, Pangea’s arrangement was a somewhat askew north-south arrangement, with a larger segment protruded from the upper mass and oriented toward the west. From top to bottom, the names, though here translated to modern and major continental landmasses, are as follows:
Eurasia in the upper right (the largest segment), North America, Africa, with South America fitted nicely into its western edge, India (just below Africa, and to the west), then Antarctica and Australia, which sort of shoulders India just above the two.Once the segment you’re standing on separated from the larger mass it was off and on its own. This North American segment also began its migration starting from the far southern latitudes. One slow inch at a time it migrated north-northeast. It was a steady ride all the way. Oh, there were bumps along the way, and one in particular, a collision with another plate, pivoted the clockwise motion of your plate north-northwest. That’s also what continental and oceanic crustal plates do from time to time: bump into one another. Sometimes the merging affects a ricocheting counter direction, and sometimes it’s more serious, such as often happens when plates subduct––diving over or on top of one another.
Let’s take another example to bring this point home. At the Grand Canyon the latitude is 36º and let’s say this is where you finally ended your migration while standing on the plate. For now, at least. What you see in front of you is a great and gaping chasm. That geologic facade is also a literal treasure trove of many environments and latitudes now frozen in time. BUT. . .had the North American plate stayed where it was it is obvious the original environmental template would also be the same. Obviously, this is not the case. Instead, the chasm is another way of verifying how this particular plate you rode all this way had indeed crossed many environmental climatic patterns. For instance, subtropical, tropical, and temperate zones. In time, the continuing migration will drift farther north into more frigid zones. From the historical perspective, the geologic record certainly reveals the transition thus far.
Back To The Man Who Really Tore The Planet Apart (in a manner of speaking): Amazing stuff, Wegener’s Continental Drifty theory, wouldn’t you say? I have often imagined being present in his time when he took his case to the German Geological Society, which was on January 6, 1912. Confident about his theories, his notions, perhaps his intuitions, he must have sensed he was on a major geologic breaking notion that would utterly change the world as scientists knew it. True, other 19th Century scientists also up with ideas how there was once a massive continent surrounded by a single world ocean. They envisioned this super landmass had amassed its size by merging with smaller land segments over millions of years. Then came the steady and great breakup of the landmass, first two fractured segments, then others, and in time all others, as continental fragments showing up on today’s global scene.
As Wegener explained these ideas to his colleagues, he likely mentioned enlightened others who had similar notions, particularly those scientists who conducted their studies, say, from 1848 to 1895. For evidence, it was thought at the time how volcanic activity due to thermal expansion was the initial catalyst that broke continents apart, therefore new continents drifted away from each other. Thus new seas or oceans had formed from the primal and single body of water, the Tethys Ocean. The impetus behind the theory actually pointed to continental drift, which suggested continents were dragged toward the equator by some sort of increased lunar gravity forming the Himalayas and Alps and other dramatic mountain ranges.
Wegener agreed with his far-seeing predecessors, but felt their theories were not fully developed. His was, however. And that is why he brought his findings to the German Geological Society. He was also the first scientist to officially use the phrase continental drift (in 1912 and 1915). The popular and graphic description he described to his peers was therefore suggesting how the present continents not only had formed a single landmass that eventually fractured (drifted apart), but likened the audacious idea to icebergs of low density (granitic) floating on a sea of much denser basalt. That part of his argument surely must have gotten Alfred scant looks of disbelief. After all, at the time it was thought the planet was a solid crust with a liquid core was one thing. But iceberg-like continents riding around the planet’s surface was quite outlandish, even silly.
Still, Wegener pressed on with his robust arguments and hoped to convince the scientists there was a paradigm in the making that would change everything given how geologists (especially this discipline) thought about the world. He explained to these men before him how all continental masses, as former and individual estates, had merely drifted apart in the primal past (sometime after the close of the Paleozoic Era). The only thing he could not substantiate was the precise physical processes that caused continents to drift in the first place. His ideas (again, in part based on previous scientific notions submitted by his counterparts) suggested the movement might have been caused by the centrifugal pseudo force of the planet’s rotation. Perhaps astronomical precession was responsible. These ideas, however, were flatly rejected by the geoscientific community, at large, and the same verdict or opinion was given to his continental drifty theory.
Still, Wegener was not one to be easily dismissed given his bold ideas that carried many scientific comrades (from the past and present) far into the future. I can only imagine he didn’t leave that symposium feeling awful for not convincing the others. Rather, he sensed his findings would one day be vindicated by stronger proof. Then the last laugh would be on his closed-minded peers who matter of fact dismissed his notions as something akin to, perhaps, quackery.
Dylan Was Right: The Times Are A-Changing: Eventually, sound evidence mounted in favor of Wegener’s theory and scientists in later years were not prone to dismissing such radical ideas. For instance, how similar plant and animal fossils show up in abundance on different continental shores. Thus there is striking correlation that some of the continents were really joined in the past. Somehow North and South America, like Australia, India and Antarctica (all latter day named continents) may have joined their respective borders, that is, this or that continent was glued together before the great breakup began. (You will recall the five essential clues earlier mentioned that confirm such activity. Namely, geographical, geological, climatic, paleomagnetic and biological.) Using, as a likely example, South America and Africa, their respective profiles reveals a former matched-pair, as it were, and not something in the way of two different shapes that came by way of mere suggestion or coincidence. Fossils of ancient creatures are also replicated on each continent, yet both are obviously separated by a great expanse of water. Moreover, with Wegener’s continental drift theory such a contemporary arrangement of the world’s landmasses will continue to change by processes such as slab pull and ridge-push and seafloor-spreading.
A German-English Alliance Long Before The Great World Wars: Mentioned in yesterday’s diary was Arthur Holmes, an English geologist. He something similar in mind given Wegener’s notion when he took a closer look at plate junctions beneath the sea. Holmes suggested convection currents deep within the mantle act as the driving force for whatever is on top (of the lithosphere). By the mid-1950s a variable magnetic field direction in rocks dated from differing ages and added collaboration and evidence to what both scientists came up with decades earlier, and both acting on independent information and observation. The expansion of the global crust thus accounted for the magnetic field reversals and seafloor spreading because of the new rock upwelling evidence. That was precisely the geophysical force behind the plate tectonic movement.
By the 1960s, seismic imaging techniques were underway. Namely, the intense studies of the deep ocean floor mapping. The entire subject matter was also traced back to Wegener’s impassioned views and were ready to separate fact from fantasy. All told, this new evidence confirmed what Wegener had conceived and believed in to the day he died. His idea, combined with all the others who worked to either prove or disprove his theory, revolutionized the earth sciences. Finally, there was an empirical way to explain a diverse range of geological phenomena. Paleogeography and paleobiology were in full swing, as scientific disciplines that enriched the earth sciences.
Later discoveries of mid-ocean ridges, whose summits were higher than most continental mountain peaks, and the deep-ocean trenches that lay more than 30,000 feet beneath the surface, is what sparked a revolution in geology during the 1960s. Here it should also be mentioned, as a courtesy, Maria Tharp, a geologist and oceanographic cartographer, was a pioneer in this field. She helped produce the first real map of the ocean floor, along with the discovery of a series of mid-ocean ridges that circled the globe. Incidentally, her name seldom comes up in the usual publicized material of seafloor-spreading, though some of us think hers should be at the top of the list of discoverers.
Wegener, himself, did not live long enough to see his fantastic hypothesis finally embraced as credible. By the 1950s, and into the 1960s, it finally came to light the scientific community was about to vindicate the forward thinking German. The old Continental Drift theory was reinstated, revamped, and researched with more open-minded science. The emerging theory under the new name, Plate Tectonics, was vogue. The upshot is how large-scale motions of the Earth’s lithosphere were indeed plausible. After the concepts of seafloor spreading were refined plate tectonics, its theory, was embraced and seasoned with empiricism. Hence, substantiated. As it turned out, seafloor spreading at mid-ocean ridges came to Wegener’s rescue. Subsequently, new oceanic crust is continually formed through underwater volcanic activity, which gradually vectors away from the ridge.
A Global Beltway For Plates: To get a better grasp of what scientists were excited about given their new discoveries in the 1950s and 60s, imagine the lithosphere broken up into tectonic plates. As you already this, this band represents the elastic upper mantle, a sort of beltway for continents and oceanic masses to wander about the globe. There are seven or eight major plates, the number depending on how each plate is define, and numerous minor plates. Because plates above and below the water are mobile, it sets up periodic scenarios where plates come in contact with one another. Where plates meet, is also where their relative movement determines what type of boundary (meeting) happens. In this case, there are three types: convergent, divergent or transform (also briefly mentioned in yesterday’s diary, but here more fully explained).
What happens when plates merge is what you already sense are typical results, that is, depending on what type of boundary occurs. Earthquakes, volcanic activity and mountain-building are the big three events, and oceanic trench formations also result along plate boundaries.
The calculated movement of plates, at least those that are on the loose as it were, is minimal, say, measured anywhere from zero to as much as a few inches annually. True, the movement seems almost negligible, but over millions of years the distance mobile plates travel is quite impressive. The geography of the planet also changes considerably. For the sake of learning another relevant term, think in terms of push-pull tectonic plate activity.
Let’s go a bit farther into this fascinating discussion, because from the perspective seen through a window of time encompassing hundreds of millions of years the restive planet always calls the shots. Thus one day you might might wake up and think it’s going to be another day of routine, only to find out the plate you’re living on has come in contact with another plate, either continental or oceanic, and all hell is about to break out. . .literally.
As you know, tectonic plates are either oceanic lithosphere material or the thicker continental lithosphere types. Each is therefore topped by its own brand of crust. Continental plates, which are mainly composed of granite, are heavier than oceanic plates, which are most basaltic (lava-based). What carries plates along their respective convergent boundaries is a geophysical process called subduction. This activity is what transports plates into the mantle itself. As a result, the material that’s lost in the exchange is nonetheless roughly balanced by the formation of new oceanic crust formed along divergent margins. Hence, the above mentioned seafloor spreading phenomenon. What this means is the total surface of the planet always remains about the same. Scientists have a phrase for this, too: the conveyor belt principle. Earlier scientists, however, claimed the net result of plate migrations created a gradual shrinking of the planet, a mode of contraction or else it was a gradual expansion. This theory still has its share of supporters.
An Explanation How Plates Migrate Across The Lithosphere: Because it has a higher strength and lower density, that is, compared to the substratum called the asthenosphere, you can think of the lithosphere in a mechanical sense, that is, relatively and much cooler and more rigid compared to the hotter asthenosphere, which also flows more easily. Consequently, the lithosphere loses its heat by conduction whereas the asthenosphere transfers its heat by convection. The crux of plate tectonics essentially and therefore comes down to this point: the lithosphere exists as separate and distinct tectonic plates riding on the visco-elastic solid asthenosphere (which is more fluid).
Clarifying this layer, the lithosphere, by contrast to the underlying asthenosphere, is highly viscous. It’s also weaker. Compare its average depth, say, somewhere between 62 and 125 miles below the surface, which may extend as far as 425 (or more) miles, to the average 60 or so mile thickness of the lithosphere. It’s also the lateral density, its variations in the mantle, that results in convection currents. These moving currents are thought to be driven by a combination of factors: the motion of seafloor from the spreading ridge and a process known as basal drag (friction), denoting downward suction at the subduction zones (gravity). However, this notion is far from complete in view of other factors that likely are also involved.
For instance, some scientists think the real explanation can be traced to different forces generated by the rotation of the Earth, as well as tidal forces created by the sun and the moon. These ad hoc ideas, however, which are more complex and scientists continue debating the matter, ad infinitum.
Plate Boundaries, An Expanded Explanation: Given various plate boundaries, sometimes when plates meet it’s not so catastrophic, while at other times you certainly don’t want to be present when the collision happens. Here’s an explanation of what happens in each of the previously mentioned boundary collision episodes.
Transform boundaries occur where plates slide, and sometimes it’s more accurate to say, grind, against each other. This event happens along transform faults. Of course, some folks claim this type of collision is San Andreas fault (small pun), but many places around the world, and not just Southern California, exhibit transform boundary contact.
A divergent boundary contact occurs where two plates merely slide apart from each other. For example, the Mid-Atlantic ridge is an active zone of such rifting. So is Africa’s East African Rift. Compared to transform boundaries, divergent contacts are less threatening to those of us far above such events.
The third type, known as convergent boundary contact, are utterly destructive. These occur where two plates slide toward each other, sort of like bumper cars that just can’t seem to get out of each other’s way. Bam! Such events usually form either a subduction zone, that is, if one of the plates moves directly below the other, or a continental collision of precipitous magnitude. Deep marine trenches are typically associated with the subduction zones. The potent side effect, at least one of them, resulting from convergent boundaries releases super heated water causing the mantle to melt. Thus extreme volcanism on an epic scale.
There are also plate boundary zones to consider, which occur where the effects of such interactions (collisions) are somewhat vague. Still, these contacts tend to occur along a broad belt and may indicate different types of movements in different episodes. Again, such boundary contacts are not well defined. So far, that is.
The Whim And The Ways Of Mother Nature: Plate tectonics is, for some of us, a fascinating subject. For all of us, however, what happens in such movements really does matter. After all, old Mother Nature is constantly rearranging her floor plan, both above the water and below. The North American continental plate, for instance, is what it is because of the migration from below the equator. When it eventually came in contact with the eastern edge of the Pacific (oceanic) plate, big events soon followed. All the mountains, volcanoes and earthquakes happened because of plate tectonics and one kind of a collision or another. Consider, too, how the Farallon micro oceanic plate subducted beneath the larger North American plate, and that event caused the entirety of a new born province to literally rise from a planed geography turned a fantastic topography. Hence, the great uplifting of the Colorado Plateau.
So, every time you gaze at an inspiring mountain range or volcano you know these landmarks are directly related to moving plates. Earthquakes, even tsunamis, happen for this same reason.
Closing Notions, Applause And Lingering Arguments: What we thus far comprehend about plate tectonics, and not so much a theory as a reality, is that such events are traced to an essential kinematic phenomenon. In short, each plate moves, and has moved, relative to other plates. How and when they move remains debatable. Thus scientists are keen to discover the engine behind the movement; the ultimate geodynamic mechanism that defines the exercise of such mobile transitioning around the globe. It’s likely thought the movement happens due to the aforementioned relative density of oceanic lithosphere, its belt, and the relative weakness of the asthenosphere. It follows heat dissipation (from the mantle itself) may be the primal source of energy that drives the plates by means of convection currents. The oceanic lithosphere is also quite dense, and this factor incites a sinking effect in known (or unknown) subduction zones. It is thought (or presumed) this energy motor is the potent source that generates plate movement.
Moreover, as new crust forms at mid-ocean ridges, which creates less dense oceanic lithosphere material compared to the underlying asthenosphere, over time the material becomes more dense and conductively cools and thickens. Scientists who advocate this finding claim it’s the greater density of the former (older) lithosphere relative to the asthenosphere that sinks into the deeper mantle at these subduction zones. Consequently, here is the real driving force for plates. It’s also the weakness of the asthenosphere that incites tectonic plate movement toward the subduction zones.
The bottom line? Possibly, it’s subduction that is the strongest driving force behind plate movement. The conundrum, however, is the fact the North American plate is moving along the grid, as it were, but where is the subduction now? A shrug from a scientist’s shoulders tells us where this idea stands, for the moment.
Meanwhile, what is being scrutinized by the scientific community at large is the search for the driving force behind the plates. Is it one mechanism (i.e., geodynamic) or some other kinematic pattern waiting to be discovered? For the present, the focus remains on sheer mantle dynamics, gravity related, and mostly secondary forces, and possibly the Earth’s rotation.
Sometimes Argumentative Debates Turn Out A Positive Outcome: The lively debates since Wegener’s time before his fellow German scientists, chiefly among the “drifters” or “mobilsts” (the proponents of Continent Drift Theory), and the “fixists” (opponents of same), have taken us to today’s accepted science of how and why plate tectonics works. With stronger evidence coming from paleomagnetism (i.e., the fact that rocks of different ages show a variable magnetic field direction), and how the magnetic north and south poles tend to reverse through time, thus correlating to the importance of paleo-tectonic studies, there is certitude these days, whereas before incertitude, along with conjectures and refutations were common from the 1920s through the 1940s. When, finally, the North Pole’s location was proven to have shifted its position throughout time, these new sciences felt vindicated. There was also an alternative explanation that showed how the continents, themselves, had moved (either shifted or rotated) relative to the North Pole. Still, continental drift was finally accepted as nominal science. We, of course, have been on the move ever since, that is, the plates, just as it was almost from the beginning of the Earth’s creation. We have the science of paleogeography and paleobiology to thank for gathering continuing evidence. If only scientists can predict where and when the next major catastrophic event will happen, people living in different parts of the world, so forewarned, can simply move elsewhere. Some say the Japanese Government may indeed be considering such advice.
Finis, Sort Of: Folks, let’s give a big hand to Alfred Wegener for his proposals based on the thoughts and minds of similar scientists in his time and before. When he first announced his idea that plates move about the world, most people thought the man, mad; at least, simple-minded and far-fetched with his notions. Today, however, we see all too clearly the planet constantly changes its facial and oceanic appearance, by moving plates here and there. Indeed, the very concept of Pangea, as a major supercontinent, was not the first time the planet joined its landmasses together. There were other similar events long before this, the most famous, supercontinent formed. When it came apart early into the Mesozoic Era, the stasis of the planet, that is, nothing major happened, as planetary events, was about to experience a literal shakeup. The world thus tore its floor plan apart and everything has been in motion since that time. Millions of years from now the pieces will come back together. Wouldn’t you like to be around to see the new look of an old planet that never tires of its dynamism?
And so, DKos community, we come to the end of this special two-part series. There will be other scenic places to tour and supplemental topics to read and think about, so stay tuned for a continuation in this series.
As always, your thoughtful commentaries are welcomed.
Please Note: Apart from the usual Wikipedia source material (which serves a decent and generalized background for this diary's narrative), I also recommend the following sites for those desiring to know even more about plate tectonics. In my diaries I also tend to shy away from direct quotes, and therefore do not site source material other than site suggestive background that has added to my research over many years. Still, I find sharing some of the material, as URL's, noteworthy. For an even greater reference list (pertaining to my own composition's source material (from wench these diaries originate) feel free to contact me and I will gladly share such reference material.
YouTube Video Documentary:FYI: For a list of all diaries posted to date, please see the growing inventory by clicking on my profile or by dialing in this URL: http://www.dailykos.com/...
This Dynamic Earth (USGS):
Plate Tectonic Theory: Plate Boundaries and Interplate Relationships
Voyage to the Deep (Plate Tectonics):
Plate Tectonics: The Mechanism
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