mersenneforum.org  

Go Back   mersenneforum.org > Extra Stuff > Science & Technology

Reply
 
Thread Tools
Old 2012-12-26, 07:35   #23
xilman
Bamboozled!
 
xilman's Avatar
 
"𒉺𒌌𒇷𒆷𒀭"
May 2003
Down not across

278C16 Posts
Default

Quote:
Originally Posted by ewmayer View Post
One of the reasons I've long had a dual interest in antipodean geography and plate tectonics is the riddle of the Deccan traps flood basalts, which are - within error bars - contemporaneous with the K-T mass extinction event now attributed to the Chicxulub impact event. Before the latter crater was discovered in the 90s deep under sediments in the Gulf of Mexico, the Deccan Traps were often cited as a possible cause of the K-T extinction. Since then the consensus has shifted to the impact event, but the near-contemporaneity still bears noticing. Back in grad school I met some folks doing many-particle simulations of large asteroid-earth impact events, and one thing that immediately struck me (no pun intended) about the results of their simulations was that besides the impact site, there was a very large similar-magnitude crustal displacement at the antipode, due to geometric wave recoalescence. So when studying impact events, one should always look both at (or for) the impact site and its antipode. That's why I would be interested to see an accurate reconstruction of earth's surface 65 Myr ago, to see where the antipode of the Chicxulub impact site would have been.

Perusing Wikipedia just now, I see there is in fact a lively scientific debate going on about just such scenario in the context of the earlier Permian-Triassic "Great Dying", the largest mass extinction event in earth's history. Unlike the K-T event there is no known impact crater clearly associated with the P-T extinction, but there is the fact of the contemporaneous - again within error bars - Siberian traps flood basalts, one of the largest known such volcanic events in earth's history. The lack of any obvious impactor event (but the note about the purported ice-buried huge Antarctic crater in the quoted text below) led to quasi-general consensus that a large mantle plume "just erupted" in Siberia over a huge area. I see now that scientists are pursuing a possible link of the kind I speculate on above:

Again, both better time constraints and accurate tectonic models are needed here, but the idea is quite tantalizing.
Would the geographical distribution of KT-era ejecta give an idea of where the antipodes would have been?

Apart from the length of the day, the relevant terrestial criteria such as gravitational mass, radius, atmospheric density and temperature would all have been very similar to their present values. It's possible to model (and measure to some extent) the mass per unit area and particle size distribution as a function of distance from an impact site. The impact was certainly big enough for ejecta to be distributed globally.
xilman is offline   Reply With Quote
Old 2012-12-26, 12:31   #24
Brian-E
 
Brian-E's Avatar
 
"Brian"
Jul 2007
The Netherlands

26×3×17 Posts
Default

Ernst: I appreciate your comments and analogies with waves on ponds and bells. Yes, clearly shock waves travelling round near the surface of the Earth would be the ones to consider as possible causes of volcanic features at the antipodal point, not waves which pass through the interior of the Earth which was my mindset when I wrote that reaction.

Paul: yes, I believe the Alvarez iridium layer is distributed globally but will still presumably show varying concentrations of iridium at different points on Earth which could indeed be used to estimate where the antipodal point of the Chicxulub impact was at the time.

Of course we had a notable impact in the solar system in 1994. While huge, gaseous Jupiter is obviously no model for Earth, it would still be very nice if someone could spot any feature at the antipodal point of the impacts of the pieces of comet on that planet on pictures from the months following that event. It would at least provide some sort of confirming evidence that a reaction at the antipodal point of an impact is to be expected.
Brian-E is offline   Reply With Quote
Old 2012-12-26, 20:59   #25
ewmayer
2ω=0
 
ewmayer's Avatar
 
Sep 2002
República de California

9,791 Posts
Default

Quote:
Originally Posted by xilman View Post
Would the geographical distribution of KT-era ejecta give an idea of where the antipodes would have been?

Apart from the length of the day, the relevant terrestial criteria such as gravitational mass, radius, atmospheric density and temperature would all have been very similar to their present values. It's possible to model (and measure to some extent) the mass per unit area and particle size distribution as a function of distance from an impact site. The impact was certainly big enough for ejecta to be distributed globally.
I expect the Iridium, being derived from shattering/vaporization of the impactor, would be closely correlated with the primary impact site itself, but that (and most all of the surrounding area for quite a large distance) is buried under deep later accumulations of sediment. What I've read and seen on TV about the Ir anomaly indicates that the distribution is global, with no obvious "ground zero" elevations of concentration, at least not ones easily discernible from surface stratigraphy. OTOH, heavier ejecta appear to show useful biases (underlines mine):
Quote:
The evidence for the Alvarez impact theory is supported by chondritic meteorites and asteroids which have an iridium concentration of ~455 parts per billion,[6] much higher than ~0.3 parts per billion typical of the Earth's crust.[4] Chromium isotopic anomalies found in Cretaceous–Tertiary boundary sediments are similar to those of an asteroid or a comet composed of carbonaceous chondrites. Shocked quartz granules and tektite glass spherules, indicative of an impact event, are also common in the K–T boundary, especially in deposits from around the Caribbean. All of these constituents are embedded in a layer of clay, which the Alvarez team interpreted as the debris spread all over the world by the impact.[4]

Using estimates of the total amount of iridium in the K–Pg layer, and assuming that the asteroid contained the normal percentage of iridium found in chondrites, the Alvarez team went on to calculate the size of the asteroid. The answer was about 10 km (6.2 mi) in diameter, about the size of Manhattan.[4] Such a large impact would have had approximately the energy of 100 trillion tons of TNT, or about 2 million times greater than the most powerful thermonuclear bomb ever tested.
The other possible phenomenon that occurs to me w.r.to the antipode is a kind of "small entry wound, large exit wound" effect analogous to that which often happens with bullet wounds to the skull. In this case, however, the "exit wound" would be caused not by the impactor and the associated pressure waves traveling through the interior, but due to the surface waves refocusing, as described above. But that might explain the crater-at-impact-site / huge-crustal-disruption-at-antipode effect, if those dual phenomena are in fact correlated.

Re. the Deccan Traps volcanism, Wikipedia has this to say:
Quote:
Before 2000, arguments that the Deccan Traps flood basalts caused the extinction were usually linked to the view that the extinction was gradual, as the flood basalt events were thought to have started around 68 Ma and lasted for over 2 million years. The most recent evidence shows that the traps erupted over 800,000 years spanning the K–T boundary, and therefore may be responsible for the extinction and the delayed biotic recovery thereafter.[90]

The Deccan Traps could have caused extinction through several mechanisms, including the release of dust and sulfuric aerosols into the air, which might have blocked sunlight and thereby reduced photosynthesis in plants. In addition, Deccan Trap volcanism might have resulted in carbon dioxide emissions that increased the greenhouse effect when the dust and aerosols cleared from the atmosphere.[91]

In the years when the Deccan Traps hypothesis was linked to a slower extinction, Luis Alvarez (who died in 1988) replied that paleontologists were being misled by sparse data. While his assertion was not initially well-received, later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions that produced the Indian Deccan Traps, and these may have contributed to the extinctions.[92]
Now for some back-of-the-envelope-style computation regarding "what were the respective positions of the modern-day Caribbean and India 65 Myr ago? Here are some data on the Indian plate, which is by far the faster mover in this tale:
Quote:
140 million years ago the Indian Plate formed part of the supercontinent Gondwana together with modern Africa, Australia, Antarctica, and South America. Gondwana broke up as these continents drifted apart with different velocities,[7] a process which led to the opening of the Indian Ocean.[8]

In the late Cretaceous about 90 million years ago, subsequent to the splitting off from Gondwana of conjoined Madagascar and India, the Indian Plate split from Madagascar. It began moving north, at about 20 centimetres (7.9 in) per year,[7] and is believed to have begun colliding with Asia between 55 and 50 million years ago,[9] in the Eocene epoch of the Cenozoic, although this is contested, with some authors suggesting it was much later at around 35 million years ago.[10] If the collision occurred between 55 and 50 Ma, the Indian Plate would have covered a distance of 3,000 to 2,000 kilometres (1,900 to 1,200 mi), moving faster than any other known plate.
...
The Indian Plate is currently moving north-east at 5 centimetres (2.0 in) per year, while the Eurasian Plate is moving north at only 2 centimetres (0.79 in) per year. This is causing the Eurasian Plate to deform, and the India Plate to compress at a rate of 4 millimetres (0.16 in) per year.
So the plate was moving very fast until the collision, slower since. Crudely, let's say an average of 10 cm/year for the past 65 Myr, that means ~6500 km of northward motion. Deccan traps are currently at between 17°–24°N, 73°–74°E, each degree of latitude ~= 110 km (km/degree for longitude is latitude-dependent, but still ~100 km/degree for the range of latitudes we're interested in). So 6500 km represents ~60 degrees of northward motion, meaning K-T-era latitude of ~40° S, longitude perhaps on the order of 10° further westward, say ~60° E. (Approximating the surrounding plates as latitudinally fixed, which is suspect, but OK for order-of-magnitude estimation such as this).

Chicxulub crater is currently centered at 21°24′N 89°31′W, so we're only in the very rough ballpark relative to the current antipode. I need to do some more digging on the relative plate motions to see if I can refine these crude estimates somewhat.

Last fiddled with by ewmayer on 2012-12-26 at 21:02
ewmayer is offline   Reply With Quote
Old 2012-12-26, 23:34   #26
science_man_88
 
science_man_88's Avatar
 
"Forget I exist"
Jul 2009
Dumbassville

20B116 Posts
Default

Quote:
Originally Posted by ewmayer View Post
I expect the Iridium, being derived from shattering/vaporization of the impactor, would be closely correlated with the primary impact site itself, but that (and most all of the surrounding area for quite a large distance) is buried under deep later accumulations of sediment. What I've read and seen on TV about the Ir anomaly indicates that the distribution is global, with no obvious "ground zero" elevations of concentration, at least not ones easily discernible from surface stratigraphy. OTOH, heavier ejecta appear to show useful biases (underlines mine):


The other possible phenomenon that occurs to me w.r.to the antipode is a kind of "small entry wound, large exit wound" effect analogous to that which often happens with bullet wounds to the skull. In this case, however, the "exit wound" would be caused not by the impactor and the associated pressure waves traveling through the interior, but due to the surface waves refocusing, as described above. But that might explain the crater-at-impact-site / huge-crustal-disruption-at-antipode effect, if those dual phenomena are in fact correlated.

Re. the Deccan Traps volcanism, Wikipedia has this to say:


Now for some back-of-the-envelope-style computation regarding "what were the respective positions of the modern-day Caribbean and India 65 Myr ago? Here are some data on the Indian plate, which is by far the faster mover in this tale:

So the plate was moving very fast until the collision, slower since. Crudely, let's say an average of 10 cm/year for the past 65 Myr, that means ~6500 km of northward motion. Deccan traps are currently at between 17°–24°N, 73°–74°E, each degree of latitude ~= 110 km (km/degree for longitude is latitude-dependent, but still ~100 km/degree for the range of latitudes we're interested in). So 6500 km represents ~60 degrees of northward motion, meaning K-T-era latitude of ~40° S, longitude perhaps on the order of 10° further westward, say ~60° E. (Approximating the surrounding plates as latitudinally fixed, which is suspect, but OK for order-of-magnitude estimation such as this).

Chicxulub crater is currently centered at 21°24′N 89°31′W, so we're only in the very rough ballpark relative to the current antipode. I need to do some more digging on the relative plate motions to see if I can refine these crude estimates somewhat.
I took a rough approach (your estimate is involved) using:

1)http://www.antipodemap.com/ ( I zoomed in to find the city on google maps first)

2)http://www.freemaptools.com/radius-around-point.htm ( took a radius of 6500 km)

and:

3) https://maps.google.ca/

it appears to me to roughly estimate the Himalayan mountains or at least intersect them.

Last fiddled with by science_man_88 on 2012-12-26 at 23:44
science_man_88 is offline   Reply With Quote
Old 2012-12-27, 23:54   #27
ewmayer
2ω=0
 
ewmayer's Avatar
 
Sep 2002
República de California

9,791 Posts
Default

Regarding the relative positions of the K-T impact site and the Deccan traps, the key plate motions are the rapid NNE motion of the Indian plate, and the more-gradual but quite steady spreading apart of the continents now on opposite sides of the Atlantic ocean. Both of these would tend to move the 2 points of interest apart over time, meaning that 65 Myr ago the sites were significantly closer than they are today. Since even today they are closer than 180 degrees longitudinally across the Atlantic, Africa and the Indian Oceans, they would have been much closer than longitudinally antipodal at the time of the K-T event. Thus, while the relative N/S latitudinal motions allow for the possibility of latitudinal antipodality, "we have a longitude problem". I found some nice tectonic-reconstruction links on this Wikipage, including a nifty 10 Myr-interval animation by the Geodynamics group at the Geological Survey of Norway, "Global plate reconstructions with velocity fields from 150 Ma to present in 10 Ma increments". That confirms the longitude problem: At 65 Ma, the present-day Caribbean and India were much closer than 180 degrees of longitude.

So now 2 things come to mind:

1. Why does the earlier-mentioned article by Von Frese et al. so confidently mention the alleged antipodality of the much-older Siberian Traps and the (purported) Wilkes Land crater in the modern-day Antarctic? (By way of answering my own question, It may be that the supercontinent which contained both sites at that time (250 Ma) makes them much easier to fix relative to each other than the various sites on the multiple relatively moving continents 65 Myr ago).

2. Given the relative rarity in the past billion years of extinction-level bolide impacts and mega-volcanic-flood events, it still seems highly improbable to me that the K-T impact and the Deccan Traps volcanism just happened to overlap in a very narrow window of geologic time defined by the error bars associated with the latter volcanism. So perhaps the idea of antipodality as being crucial, despite its "sexiness", is in fact relatively unimportant. Earth's lithosphere is ~100 km thick, so the idea that large disturbance waves from a K-T-style impact will just slosh around the globe, relatively harmlessly except close to the primary impact and secondary recoalescence point antipodal to it, quite likely is simply wrong. Rather, what if such an event fractures more or less the whole earth surface? Not necessarily in the sense of large-scale relative displacements (except near the impact site and antipode), but simply in terms of wide-scale weakening of the entire crust. Then, anywhere on earth where you had a mantle plume or hotspot which had previously been overlain by a slab of more-or-less solid plate acting as a "pressure cooker lid", now suddenly the lid gets shattered, leading to a spate of massive wide-scale volcanism. Once that has spent most of its pent-up fury - which might well take thousands of years for a really big hotspot - things quiet down again, the shattered crust slowly re-knits and "heals", and things get back to normal.

Last fiddled with by ewmayer on 2012-12-28 at 00:04 Reason: correct lat <--> long prefix mix-up
ewmayer is offline   Reply With Quote
Old 2012-12-28, 11:18   #28
Brian-E
 
Brian-E's Avatar
 
"Brian"
Jul 2007
The Netherlands

1100110000002 Posts
Default

Perhaps shock waves travelling in rock close to the surface of the earth move at different speeds depending on differing conditions? The most obvious difference that comes to mind is that between the earth's crust in areas which are covered by land and the basalt under oceans. This might lead to the waves coalescing at a point elsewhere than the antipodal point if the original suggestion of coalescing shock waves is correct after all.
Brian-E is offline   Reply With Quote
Old 2012-12-29, 22:21   #29
ewmayer
2ω=0
 
ewmayer's Avatar
 
Sep 2002
República de California

9,791 Posts
Default

Perhaps, but based on the plate-tectonic reconstruction of the K-T impact, that would have needed the shocks to perform a 90-degree right-turn in order to coalesce under India.
ewmayer is offline   Reply With Quote
Old 2013-02-22, 11:46   #30
jasong
 
jasong's Avatar
 
"Jason Goatcher"
Mar 2005

66618 Posts
Default

Quote:
Originally Posted by ewmayer View Post
I think I actually figured out what the Fed's "exit strategy" is:

"Keep digging, through the center of the earth if necessary, in hopes of eventually emerging on the other side."

The question is: where is "the other side" at which the eventual exit will occur? Hint: It's not in China. (Though our Chinese friends are strongly encouraged to resume robust buying of US debt in order to secure a piece of this great value proposition). No, using the publicly available geographical coordinates of the Fed-quarters building, (38.892778,-77.045833) puts the geographical antipode at (-51.107222,102.954167), which is in the southern ocean roughly midway between the SW corner of Australia and the nearest point of Antarctica.

But fear not - I'm sure Bernanke et al can cite an academic economic model which indicates that by the time their "keep digging ever faster and deeper" policy achieves a "breakthrough", there will be fertile "growthiness supporting" land as far as the eye can see at that location.
I'm 99.9% certain I'll get flamed for this, but...

The Federal Reserve Bank ISN'T Federal, has no reserves and isn't a bank. Technically, American money is still gold and silver. Haven't you ever wondered about the terms Federal Reserve NOTE and dollar BILL? These are both terms used when someone owes someone else money.

Try this fun thing: Google high resolution pictures of American money where the bills are with 10k or more. Compare the writing on the bill on the web to a bill in your pocket. You will discover that the bill on the web has 4 extra words,"payment of" and "in gold."

The US is owned by the owners of the Federal Reserve Bank. The debt will NEVER be re-paid because it's more profitable for the owners of our "money" to continually string us along. It'd be like if you paid for everything with a credit card, paid the credit card debts with exactly the same credit card, and handed 100% of your belongings to the credit card company for them to profit from as they see fit.
jasong is offline   Reply With Quote
Old 2013-02-22, 19:04   #31
ewmayer
2ω=0
 
ewmayer's Avatar
 
Sep 2002
República de California

9,791 Posts
Default

Quote:
Originally Posted by jasong View Post
I'm 99.9% certain I'll get flamed for this, but...
Not by me ... but we already have a dedicated series of threads in Soapbox for discussing such things.
ewmayer is offline   Reply With Quote
Old 2013-11-14, 23:59   #32
ewmayer
2ω=0
 
ewmayer's Avatar
 
Sep 2002
República de California

9,791 Posts
Default

A multiple-impactor variant [think comet Shoemaker-Levy getting pulled into multiple chunks by the planet's tidal forces before eventually impacting Jupiter] of the K-Pg [formerly K-T] impact hypothesis appears to be gaining some traction in the scientific community - a separate impactor hitting at the then-location of India and causing the Deccan flood basalt eruption would obviate the need for the Chicxulub impactor to perform magic tricks.

The Wiki entry on the largest of the purported co-impactor sites, the (disputed) Shiva Crater notes that
Quote:
At the time of the Cretaceous–Paleogene extinction, India was located over the Réunion hotspot of the Indian Ocean. Hot material rising from the mantle flooded portions of India with a vast amount of lava, creating a plateau known as the Deccan Traps. It has been hypothesized that either the crater or the Deccan Traps associated with the area are the reason for the high level of oil and natural gas reserves in the region.[3]
...
Unlike typical known extraterrestrial impact structures, Shiva is teardrop shaped, roughly 600 × 400 km (370 × 250 mi). It is also unusually rectangular. Chatterjee argues that the low angle of an impact combined with boundary fault lines and unstable rock led to this unusual formation.[2] Other researchers have noted that rock faults and impacts could modify the crater shape.[6] The crater also is reported to contain larger than average amounts of alkaline melt rocks, shocked quartz, and iron oxide laced with iridium. These types of rocks and features suggest an impact origin. The age of the crater is inferred from the Deccan traps, which contain relatively high amounts of iridium (an element extremely rare in the Earth's crust but more common in asteroids).[2][7]
The Ir enrichment, if true, strikes me as very telling. As with the original impact hypothesis, skeptical scientists can easily come up with reasons to explain away just about every other piece of evidence - heck, there are still papers appearing claiming Chicxulub is not in fact contemporaneous {within error bars] with the global K-Pg mass extinction event - but as with the said boundary layer, the Ir enrichment is a smoking gun impossible to explain away by non-impactor means.
ewmayer is offline   Reply With Quote
Old 2013-11-15, 11:15   #33
Brian-E
 
Brian-E's Avatar
 
"Brian"
Jul 2007
The Netherlands

26×3×17 Posts
Default

Quote:
Originally Posted by ewmayer View Post
[...] [think comet Shoemaker-Levy getting pulled into multiple chunks by the planet's tidal forces before eventually impacting Jupiter] [...]
...with the difference, I'm guessing, that it would almost certainly have been the sun's gravitational field, not the planet's as in the case of the recent Jupiter implact, which pulled the eventually impacting object into multiple chunks? Presumably it would have been a comet which has just broken up on passing perihelion which then impacted Earth on the way back.

Fascinating. Thanks.
Brian-E is offline   Reply With Quote
Reply

Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
Some arithmetic... science_man_88 science_man_88 11 2014-07-30 22:53
modular arithmetic science_man_88 Miscellaneous Math 42 2011-07-26 02:02
Simple Arithmetic! mfgoode Puzzles 82 2007-05-02 08:13
Easy Arithmetic davar55 Puzzles 6 2007-03-20 17:47
Check my arithmetic R.D. Silverman Factoring 3 2006-06-05 23:49

All times are UTC. The time now is 18:09.

Mon Oct 26 18:09:30 UTC 2020 up 46 days, 15:20, 0 users, load averages: 2.25, 2.02, 1.98

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.

This forum has received and complied with 0 (zero) government requests for information.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation.
A copy of the license is included in the FAQ.