LinkBack URL
About LinkBacksI was reading article , and it said "Fifty million years ago, the Eurasian and Indian continents collided".
How is it possible that we know it happened so long ago?
February 16th, 2010, 03:41 AM
I was reading article , and it said "Fifty million years ago, the Eurasian and Indian continents collided".
How is it possible that we know it happened so long ago?
A number of different branches of geology all give evidence:
Geochronological data dating metamorphic rocks and fossils of animals and plants from one continent not appearing on the other till after this time are a couple such.
ask yourself the question : how does a forensic scientist figure out how a crime was committed that happened several months ago with no witnesses to use as evidence ?
now extend this several million-fold - the evidence is there, it's just a matter of knowing how to interpret it
And Like forensics it is a bit of a guessing game. The results are based on a number of assumptions and the quality of the result is highly dependent on the quality of the assumptions. It is an inexact science, necessarily because much of the evidence is generally lost or destroyed.
be that as it may, the available evidence is sufficient to state with a fair degree of confidence that india did indeed crash into asia around 50 million years ago, thereby forming the himalayas
there's still a vast difference between sparse evidence and no evidence at all
And the following is an example (from several hundred, if not several thousand examples) of how that 'sparse' evidence is more than sufficient to build a detailed and convincing case.
Bin Zhu, et al "Age of Initiation of the India‐Asia Collision in the East‐Central Himalaya" The Journal of Geology, 2005, volume 113, p. 265–285
Abstract
We document the stratigraphy and provenance of the lower Tertiary terrigenous sections in the Zhepure Shan region of the Tethyan Himalaya, southern Tibet, using petrographic and geochemical whole‐rock and single‐grain techniques. The Cretaceous–early Tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former Indian passive margin near the western end of the Zhepure Shan are conformably overlain by lower Tertiary clastic rocks. Sandstones in the Jidula Formation (Paleocene) mostly contain monocrystalline quartz grains of cratonic origin. In contrast, significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the Youxia (Early Eocene) and Shenkeza (post–Early Eocene) formations. Major, trace, and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the Jidula Formation consisted primarily of quartzose basement rocks, probably of Indian continental origin, whereas the sediments of the Youxia Formation were mainly derived from the uplifted Gangdese arc‐trench system associated with the obduction of the Asian subduction complex. The compositions of Cr‐rich spinels in the Youxia and Shenkeza sandstones resemble those from fore‐arc peridotites and were most likely derived from the arc and ophiolite rocks along the developing Yarlung‐Zangbo suture to the north. No spinels have been observed in the Jidula sandstones. Therefore, the early Tertiary detrital clastics in the Zhepure Shan record a marked change in provenance and sediment character and specifically at the time of deposition of the Youxia Formation, which contains a zone P‐8 foram assemblage. This change indicates that the onset of India‐Asia collision and the first development of the foreland basin immediately south of the India‐Asia suture zone occurred at Ma in the both the western (Zanskar) and eastern (this study) Tethyan Himalaya.
So what specific fault do you find with this cypress? Are you quibbling about a million years here, or there, or do you doubt that plate tectonics even occurs? Just what is your position in this matter?
NO fault. Just answering the OP's question as accurately as possible. Uncertainty is a component of science. Best to be ever aware.
umm ... how about "the actual number is missing" ...Originally Posted by Ophiolite
pierreSimons, you might do well to ignore the anti-science interjections of our resident creationist-who-claims-not-to-be-one. His purpose here seems to be to one to obfuscate with deception and speciosity.
The fact is there is a preponderance of evidence that converges on the fact that the Indian and Eurasian continents have not only found themselves in collision but have done so at around 50 million years ago.
There isn't simply a single line of shaky evidence that points to this fact, but a preponderance of evidence that is both corroborating and complimenting. Not the least of which are fossil strata that can be dated with potassium-argon and argon-argon methods; paleomagnetic strata which are observed both undersea and in terrestrial environs; and measurements of plate motion in modern, real time. These, of course, are hardly the only data, there are many more, but they are nothing to take lightly. And they are hardly the sort of data that can be characterized as "a bit of a guessing game" and "an inexact science." Those who do so are either ignorant beyond the pale or have an agenda to undermine scientific achievements to those who are asking good questions such as yourself.
It is definitely worth our time and effort on a science forum to entertain discussions such as this from passers-by who are genuinely curious about how science is done and how it is that we come to conclusions like a 50 Ma date on the initial collision of the Indian and Eurasian plates. And I think there is much more to be said on the topics of paleomagnetic data and dating as well as radiometric dating techniques like K-Ar. And I'm sure geologists like Ophiolite would welcome opportunities to discuss stratigraphy of the fossil-bearing members that are in collision and doing many wonderful and fascinating things like folding and uplifting.
But I'm not going to permit anti-science creationists like cypress to have their opportunity to pollute the minds of the curious and the interested. Your comments, cypress, were not only unwelcome, they were patently false, misleading and betray your true agenda once and for all. You claim to not be a creationist in several posts where you've hinted at and all but admitted favoring "intelligent" design (a creationist bit of pseudoscience). But you repeatedly demonstrate a desire to present an anti-science and pseudoscience agenda in very subtle, but clear ways.
If you want to make such posts, you'll need to confine yourself to the pseudoscience forum. From now on, such posts will be summarily deleted without warning or moved to pseudoscience.
Any response to my previous two paragraphs will need to find their way to the Site Feedback subforum or my PM box. Metadiscussion (that discussion which is about discussion) be deleted from this thread as off-topic.
I think what happened is that when you copy/paste from the journal, the figure is a very small image file or a different font, thus it doesn't paste very well and you might not notice it. But the actual numbers in the journal is "50.6 +/- 0.2 Ma."umm ... how about "the actual number is missing" ...
To get the "+/-" effect where the plus sign is over the minus, the editors of the online version of the journal use a tiny image file for the number.
The authors describe in their dating methods several convergent lines of data. They look at the sharp change of sedimentary composition that occurred between the Zhepure Shan and Youxia formations, arriving at an estimated date of 50.8 to 50.4 Ma; they examine the stratigraphic data of the Zanskar region, arriving at a date of 50.6 Ma; they examine the Tso Marari area, arriving at a date of 55 +/- 6 Ma.
In addition, the authors falsified a previously hypothesized date of 64 Ma by showing the absence of ultrastable heavy minerals like Zr and Cr in Late Cretaceous and Paleocene sediments.
They also discuss convergence velocity and other data. The paper is a fascinating read.
Here's an image from the paper itself. Lots of ophiolites in the strata there!
weird - when ALT-0177 does the trick just as well
but when Ophiolite led with his chin like that, I just couldn't resist ... :PI think what happened is that when you copy/paste from the journal, the figure is a very small image file or a different font, thus it doesn't paste very well and you might not notice it. But the actual numbers in the journal is "50.6 +/- 0.2 Ma."umm ... how about "the actual number is missing" ...
To get the "+/-" effect where the plus sign is over the minus, the editors of the online version of the journal use a tiny image file for the number...
........
there will always be questions about the numerical accuracy of a report, especially in situations when the number changes significantly (eg, when I started studying Earth Sciences, the ~64 Ma was the number in the (older) reference books) -
but, at any given time, the number is the best available from the evidence and the methods applied - the trend over time is that the methods and accuracy are improving ...
hand-waving about undiscovered evidence, method assumptions, margins of error, rounding off, etc, are about as meaningful as equating the common definition of theory with the scientific definition - it doesn't provide a better or more accurate result; when new evidence is discovered, and a better result is shown to be such, it is adopted and the previous archived without regret ...
The reason for this was likely the K-T boundary. It was clear enough that the collision started after the Cretaceous, but there still wasn't enough data to pinpoint the time range further. Now, they're within a few million years based on better stratigraphic study and mineral analysis. I think that's reasonable given that the process of colliding continents is hardly an overnight event.when I started studying Earth Sciences, the ~64 Ma was the number in the (older) reference books
BTW, I agree on the easy dig on Ophiolite's post. :-)
I just wanted to be sure I cleared the air on why and how in case there's someone genuinely looking for information.
come on guys, you don't actually think I read anything I post do you. :? I'm only here for the beer.
Hey, we have beer and geology here?! I thought I had to wait for GSA for that!
from memory - which isn't as trustworthy as it used to be - I think that's right ...
good ... now that's done, bring on the beer!It was clear enough that the collision started after the Cretaceous, but there still wasn't enough data to pinpoint the time range further. Now, they're within a few million years based on better stratigraphic study and mineral analysis. I think that's reasonable given that the process of colliding continents is hardly an overnight event.
BTW, I agree on the easy dig on Ophiolite's post. :-)
I just wanted to be sure I cleared the air on why and how in case there's someone genuinely looking for information.
I was thinking that pierreSimons deserved a more complete answer to his question than "based on the evidence the body of science has estimated the date at about 50 mya". He should know that the estimates are based on assumptions and the quality of those assumptions impact the quality of the result. This is especially true of historical sciences. In my field, the quality of the assumptions make or break the success or failure of the systems we design and construct. There is no fault in mentioning this reality. It is being complete.pierreSimons, you might do well to ignore the anti-science interjections of our resident creationist-who-claims-not-to-be-one. His purpose here seems to be to one to obfuscate with deception and speciosity.
The fact is there is a preponderance of evidence that converges on the fact that the Indian and Eurasian continents have not only found themselves in collision but have done so at around 50 million years ago.
Three key categories in these uncertainties are the assumptions that go into radio isotope rock dating, general dating of geologic strata and assumptions about the rate and consistency of the rate of tectonic movement. I am not suggesting or implying these assumptions are wrong, I am simply informing the OP that there are a number of assumptions and the quality of these impact the results. The actual time can be off in either direction, but as I implied in my response to Ophiolite, I accept the 50 mya estimate.
So, as per usual, nothing specific from Cypress... merely continued efforts to sow FUD (fear uncertainty doubt) without any backing or clarity.
Uh-uh. You're not off the hook that easily. You were trying to pull the creationist wool over the eyes of someone you hoped to be a fence-sitter or gullible. The assumptions you refer to aren't that vague or inaccurate and methods of dating such as argon-argon, potassium-argon, paleomagnetic, etc are based on physics and the isotopes involved in those like the first two have decay rates that are both constant and known to a fine degree.
This is a prime example of a creationist pushing his own superstitious agenda albeit in a manner by which you hope to appear rational.
Sorry, it doesn't fly.
Is science free of uncertainty and doubt? If not, then what is your issue? I believe I was quite clear that I don't take issue with the estimates or process. I do believe it the OP asked why the estimate of 50 mya and I believe that any answer must include a disclosure that conclusions from historical sciences are uncertain due to quality of assumptions. I also note that my response prompted exactly that discussion along with the predictable complaints from the bleachers.
Your "disclosure," like most of your contributions to this forum, contained nothing related to a meaningful discussion of science.I do believe it the OP asked why the estimate of 50 mya and I believe that any answer must include a disclosure that conclusions from historical sciences are uncertain due to quality of assumptions. I also note that my response prompted exactly that discussion along with the predictable complaints from the bleachers.
"Like forensics it is a bit of a guessing game. The results are based on a number of assumptions and the quality of the result is highly dependent on the quality of the assumptions. It is an inexact science, necessarily because much of the evidence is generally lost or destroyed."
It was a broad hand wave with no purpose other than to cast doubt on the methods used by science. It also lacked any detail or indicator that you have any actual understanding of those methods. Nor did it reveal that the published science, for those who bother to actually read and understand their content, are replete in identifying the applicability of the methods, statistical uncertainty, error bars and any other limitations.
In essence you added nothing to the discussion.
That's your opinion. Like I say, it seems to have instigated a reasonable discussion of the uncertainties and assumptions. It seems to have served its purpose well.Your "disclosure," like most of your contributions to this forum, contained nothing related to a meaningful discussion of science.I do believe it the OP asked why the estimate of 50 mya and I believe that any answer must include a disclosure that conclusions from historical sciences are uncertain due to quality of assumptions. I also note that my response prompted exactly that discussion along with the predictable complaints from the bleachers.
the estimates are based on evidence, the interpretation of which involves assumptions.
Your tendency to use imprecise wording is one reason your posts are not always well-received.
February 28th, 2010, 02:33 PM
As you might guess, it depends on what's being studied. For tectonism, the geochemical approach is very powerful. For example, one possibility is to U-Pb date a suite of zircon crystals, and then compare it with a suite of Rb-Sr dates from the mineral biotite. Biotite has a lower "closure temperature" than zircon, and thus provides information about local heat flow conditions which is a function of tectonic character. This, in conjuction with an understanding of tectonics, can determine very precise spatial and temporal constraints on a tectonic setting. Accurancy is also good, and can be bolstered by relative dating techniques.
Cypress, you accept the 50 million year figure. I accept that properly I should say 50 million years +/- X million years. The assumptions determine the magnitude of X. This is fundamental. It is one of the great strengths of science. You seem to imply it is a weakness.
| « hyperspectral sensing application in tropical areas | global warming threads » |
| Bookmarks |
Bookmarks |
