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klk
Trad climber
cali
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Sep 16, 2014 - 02:16pm PT
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anyone here attending agu?
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 16, 2014 - 09:42pm PT
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I'm likely to go to AGU.
I'd be posting new material to this thread, have a bunch of it stacked up, but I'm at other meetings this week without much spare time.
Had breakfast with the fellow who named the Transantarctic Mountains. Many of us on ST have named routes or walls, but it's something else to name a 3,500 kilometer mountain range.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 17, 2014 - 08:58pm PT
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Interesting models; the question is are they real? For instance, note rhat in all of those images the "spreading continent" (green) has a leading edge that curls under at contact with the subducting plate, as of course it should be given these assumptions. But where real continents override subducting plates as in the eastern Pacific,the leading edge of the overriding plate (submarine) is typically *not* crumpled as this model requires. In some locations that leading edge has deep undisturbed sediments, incompatible with images of the overriding plate literally riding over another plate but better explained if the overriding plate is sliding into a gap as the subducting plate sinks.
The trick is to find theories that fit all of the observations, this one may fit just a few. I heard an explanation today that links the Sierra to Gulf of Mexico oil.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 21, 2014 - 07:52am PT
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So let's go back one last time to Hawaii. The Emperor/Hawaii chain is a series of progressively younger volcanic islands or seamounts, the inspiration for the "hotspot" concept of narrow plumes from a fixed deep mantle location, erupting at the surface as plates move overhead. This model has proven to be wrong in every respect, and yet appears in most textbooks, inspires much research, and became the ideal type for other, less clearly defined "hotspots" claimed all over the world.
From Wikipedia,
The Hawaii hotspot is a volcanic hotspot located near the namesake Hawaiian Islands, in the northern Pacific Ocean. One of the most well-known and heavily studied hotspots in the world,[1][2] the Hawaii plume is responsible for the creation of the Hawaiian – Emperor seamount chain, an over 5,800 kilometres (3,600 mi) long chain of volcanoes, four of which are active, two of which are dormant, and more than 123 of which are extinct, many having since been ground beneath the waves by erosion as seamounts and atolls. The chain extends from south of the island of Hawaiʻi to the edge of the Aleutian Trench, near the eastern edge of Russia. While most volcanoes are created by geological activity at tectonic plate boundaries, the Hawaii hotspot is located far from nearby plate boundaries; the classic hotspot theory, first proposed in 1963 by John Tuzo Wilson, proposes that a single, fixed mantle plume builds volcanoes that then, cut off from their source by the movement of the Pacific Plate, become increasingly inactive and eventually erode below sea level over millions of years. According to this theory, the nearly 60° bend separating the Emperor and Hawaiian segments of the chain was caused by a sudden shift in the movement of the Pacific Plate. In 2003, fresh investigations of this irregularity led to the proposal of a mobile hotspot theory, suggesting that hotspots are mobile, not fixed, and that the 47 million year old bend was caused by a shift in the hotspot's motion rather than the plate's.
The idea that the hotspot pipe, and not just the plates, might be moving around (blowing in "mantle winds"?) was a bandaid correction applied later to work around the first and most obvious disproof, but there have been many other reality failures as well. For example, a deep source implies that the lava erupted along the chain should be compositionally similar, but even the adjacent Mauna Kea and Mauna Loa rocks are compositionally distinct. A bandaid for that one involves multiple hotspot pipes from distant mantle origins that angle sideways and happen to erupt at almost the same place. But a simpler explanation is just that they each have shallow origins, where composition is already known to be distinct.
And note that the cartoon still shows one straight fixed narrow pipe from the deep mantle. And Wikipedia's caption for this cartoon says,
A diagram demonstrating the migration of the Earth's crust over the hotspot
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 21, 2014 - 08:39am PT
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Foulger and Anderson have a more detailed, point-by-point takedown of evidence regarding the Hawaii/hotspot paradigm. Worth reading in its entirety for the 'ologists here or others interested in seeing how strong the evidence against a theory can be, and yet that theory still retains dominance in the field.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 21, 2014 - 09:09am PT
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So if it's not the plumologists' hotspot, what is the Emperor/Hawaii chain? Its origin in the triple plate junction marking the west end of the Aleutian Trench, instead of being just a plumological coincidence, becomes the centerpiece for a non-plumological understanding, as Ian Norton works it out.
Beneath the insulating lid of the upper crust, rocks of the lower crust are quite hot. An eruption from this region does not require added heat from below, but simply the release of pressure from above. In simple terms, the Emperor/Hawaii chain most plausibly represents not a deep-rooted hotspot either fixed or madly blowing in mantle winds. Instead it appears to be a series of shallow eruptions tracking a pull-apart weakness or crack, propagating from a triple junction where the subducting Pacific plate is pulled in two different directions.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 21, 2014 - 10:18am PT
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Well, we already know for sure that the Pacific Plate did not change direction to create the Emperor/Hawaii elbow, even though textbooks still say it did -- and authorities still cite this as the defining example for hotspots worldwide. The plate's realitive motions are well constrained by geology on all sides, as in the overview map below. The Emperor/Hawaii 60-degree change in direction happened 45 million years ago, contemporary with the orange bands dated paleomagnetically at the spreading ridges in the map below. The Pacific Plate obviously did not shift direction at that time.
A propagating weakness (imagine tearing a sheet of paper by pulling it apart?) might change direction more easily, and the character of the triple junction also has changed over time. Norton focuses mainly on how the crack started (85 Ma), just speculating briefly on why it changed direction (45 Ma) -- I gather that event is not well explained.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 05:36am PT
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It'll get stranger when we leave Earth, and *really* heterodox at the last planet on this tour. I'll still try to keep it real, though, and in some respects the evidence there is more visible -- we can all be armchair planetologists, if folks stay curious.
By all means do argue when things sound wrong. Skepticism is warranted (even better if it looks both directions). There should be skepticism at Anderson's AGU session this fall. What's the province of his quote above?
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 08:09am PT
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Chemical plumes in the mantle
DON L. ANDERSON
Dude, that's 1975!
:-)
I recently discovered and began reading the first volume (top left) of Henry Frankel's epic history of the continental drift controversy.
Some of the names and tales are familiar to me from childhood, when these topics were dinner-table conversation at our house. Anyway Frankel does an excellent, exhaustively researched job of it, tracing the development of ideas from Wegener through plate tectonics. And also the fierce resistance those new ideas met, including efforts at delay and suppression, as they challenged the life's work of earlier geologists. Frankel likens the drift/tectonics revolution to the Darwinian revolution in biology.
One connection between the continental drift controversy and the newer stories I'm telling here is that some of the same scientists are involved. Another is that the drift-era pattern of orthodox resistance is being repeated now against top-down (instead of hotspot/plume bottom-up) concepts in tectonics. But more on that later.
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BASE104
Social climber
An Oil Field
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Sep 22, 2014 - 08:28am PT
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I saw this in a news story when on vacation. It is very exciting, considering that plate tectonics seemed so complete.
I have a lot of reading to do now...
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 08:30am PT
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Glad you showed up here, Base, hope it will be a fun ride.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 08:44am PT
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Are plate movements driven mainly by cooling and sinking from above? Or by hot mantle rising from below? Does buoyant material come mainly from the deep mantle, or from shallower sources -- above the 660 km discontinuity, or even from the lower crust? Do deep-mantle hotspots or plumes exist at all? Those seem to be major points of division.
In cartoon vision, do we picture the North American plate being pushed from thousands of miles away by mid-Atlantic spreading so that it overrides the Pacific? Or is the Pacific edge cooling and sinking, pulling the N America plate west and more distantly causing spread in mid-Atlantic?
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 11:11am PT
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But I can't believe there are tensile stresses adjacent to subduction zones. You have proof of otherwise?
You can't believe there are, or are not? Seems obvious there are many kinds of stress associated with these zones.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 11:24am PT
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I'm happy to keep discussing Earth but it could also be interesting to branch outwards, "To Infinity and Beyond." Or at least, to the terrestrial planets.
These are the rocky worlds of Mercury, Venus, Earth/Moon and Mars/Phobos. The image shows them approximately to scale. One takeaway is that Venus is just a bit smaller than Earth, whereas Mars is tiny. All those sci-fi films we’ve seen depicting Earthlike gravity on Mars: Not True.
The terrestrial planets have some relevance to orthodoxy and revolution in plate tectonics.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 11:39am PT
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The Moon’s surface is basically saturated with impact craters, meaning you could not create a new crater without obliterating older ones. Dating of rock samples returned by the Apollo missions suggests that most of the bombardment happened early in lunar history, between about 4.5 and 3.8 billion years ago. Similar or larger bombardment must have happened on Earth, but the evidence has been recycled by tectonics. Lunar dating provides our best constraint on the bombardment phase of planetary accretion.
Close to the Sun, Mercury evolved differently but its surface too is saturated with craters, probably reflecting a similar history of ancient (~3.8 Ga and before) bombardment.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 11:46am PT
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Unlike the Moon or Mercury, Mars has an atmosphere and once lots of water. The surface shows much evidence of water erosion from vanished sources, and ongoing reshaping by wind. Where a less weathered or sedimented surface shows through, it too appears saturated by ancient impacts. When Mariner 4 returned the first closeup photos of Mars (1965), scientists were disappointed to see it looked so much like the Moon.
Rocky planetoids we have visited seem to be variations on this ancient impact-dominated theme. Jupiter’s moon Callisto:
Saturn’s moon Mimas:
But Earth is different. Because we have tectonics.
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TwistedCrank
climber
Released into general population, Idaho
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Sep 22, 2014 - 12:07pm PT
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Hot the phone, the ink's not dry.
I'm trying to wrap what's left of my head around this.
I'm a Neogene continental stratigrapher - or at least I was one until I found out there's more money to be made in the private sector.
Am I reading this thread correctly? (it's taken a few days)
Are hot spot's dead? It that correct?
(As a sidebar - and if this is true - I'm actually more interested in how scientific method can be corrupted into groupthink science like this. Or is that a philosophic titwillow that's beyond the grasp of my grey matter.)
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 12:18pm PT
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Are hot spot's dead? It that correct?
Some geologists think so, and I'm channeling their alternative vision. By far the dominant view is still that hotspots do exist, although predictions from this theory have failed one after another. Alternative simpler explanations have been proposed instead -- see Hawaii discussion above, for example; or Yellowstone, Iceland etc. papers on the mantleplumes.org website.
(As a sidebar - and if this is true - I'm actually more interested in how scientific method can be corrupted into groupthink science like this. Or is that a philosophic titwillow that's beyond the grasp of my grey matter.)
That's what I was suggesting by introducing Frankel's Continental Drift Controversy books upthread. And may get back to, if this thread lasts long enough. I could eventually cite a new publication (currently in review) that uses the term "groupthink," after defining it for geologists.
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Chiloe
Trad climber
Lee, NH
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Topic Author's Reply - Sep 22, 2014 - 12:57pm PT
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What about Venus? That gets complicated. A thick atmosphere hides surface details from telescope or satellite eyes. In visible light we only see clouds.
So the Magellan spacecraft orbiting Venus from 1990 to 1994 mapped the surface by radar, looking through the clouds. The result (false color below) looks different from the obviously crater-saturated surfaces of other rocky planets (except Earth). What accounts for that difference?
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