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Old 2012-12-16, 21:44   #12
Brian-E
 
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Quote:
Originally Posted by science_man_88 View Post
how much of the land has an antipodal point that's on land ?
Nice question. When all the land was concentrated in one huge continent, the answer would have been very little if any. If we now assume that the three tenths of the world's surface which is land is uniformly distributed around the globe, the answer would be three tenths of the land having an antipodal point on land. But of course the distribution of land is still concentrated on one "side" with the Pacific Ocean on the other. So it will be much less than three tenths.
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Old 2012-12-16, 21:50   #13
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Quote:
Originally Posted by Brian-E View Post
Nice question. When all the land was concentrated in one huge continent, the answer would have been very little if any. If we now assume that the three tenths of the world's surface which is land is uniformly distributed around the globe, the answer would be three tenths of the land having an antipodal point on land. But of course the distribution of land is still concentrated on one "side" with the Pacific Ocean on the other. So it will be much less than three tenths.
well by playing around with that page a bit I found south east Asia seems to be in south America.

if we make the white around the sphere on Wikipedia see through can we estimate the percentage that has orange in the non see through area by scanning the photo ?

edit 2:
Quote:
All together, less than 4% of land is antipodal to land
well I don't look obviously.

Last fiddled with by science_man_88 on 2012-12-16 at 22:27
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Old 2012-12-17, 18:08   #14
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Quote:
Originally Posted by Brian-E View Post
Our progression around the globe due to plate tectonics would need to be measured relative to something else of course. What would change sufficiently slowly compared to continental drift to make a suitable frame of reference?
I doubt anything would be so stable as to provide a sufficient "fixed reference frame" on those kinds of time scales, but the land masses themselves should serve. You'd simply have to pick a point on a stable piece of crust (one that won't get subducted into the mantle while you wait :), watch things slide around, and track what it and its antipode are doing.
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Old 2012-12-20, 09:27   #15
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Originally Posted by Uncwilly View Post
That is a fascinating tool. I found it interesting that there is no place in Mexico or the lower 48 U.S. states that has an antipode point on land. It appears that extreme northern Alaska has an antipode point in extreme southern Antarctica. Hawaii also has an antipode point in the southern part of Africa.

I was always told as a kid that if you dug through the earth from the middle U.S., you would hit China. Now I see that it is in the southern Indian Ocean. My childhood image has been destroyed!

Last fiddled with by gd_barnes on 2012-12-20 at 09:33
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Old 2012-12-21, 04:24   #16
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A bit of Montana antipodes to Foch Island (TAAF).
(-; And both have population of ~25 ;-)

Fascinating read:
Quote:
The German auxiliary cruiser Atlantis called at Kerguelen during December 1940. During their stay the crew performed maintenance and replenished their water supplies. This ship's first fatality of the war occurred when a sailor, Bernhard Herrmann, fell while painting the funnel. He is buried in what is sometimes referred to as "the most southerly German war grave" of World War II.
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Old 2012-12-21, 05:50   #17
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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."
Let daredevils, the Fed and Verne chart a path through the earth; celestial beacons rock: Three wise men [xkcd what-if]
Quote:
If the three wise men had a hovercar that could move at highway speed over land and water (it’s in the gnostic gospels somewhere) and decided to follow Venus, they’d take a particularly weird path:

At one point, they wind up near the North Pole in October. There, the Sun and Venus spend months near the horizon, rising and setting, nudging the Magi into a month-long spiral around the pole, a chaotic strange Magi attractor around the North Pole which some argue provides the theological foundation for the story of Santa Claus[citation needed].
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Old 2012-12-24, 18:14   #18
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Quote:
Originally Posted by gd_barnes View Post
It appears that extreme northern Alaska has an antipode point in extreme southern Antarctica.
I.e., the South Pole? :-) I think you meant northern Antarctica (near its coast).

Last fiddled with by cheesehead on 2012-12-24 at 18:16
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Old 2012-12-25, 20:35   #19
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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:
Quote:
The Siberian Traps (Russian: Сибирские траппы, Sibirskije trappy) form a large region of volcanic rock, known as a large igneous province, in the Russian region of Siberia. The massive eruptive event which formed the traps, one of the largest known volcanic events of the last 500 million years of Earth's geological history, continued for a million years and spanned the Permian–Triassic boundary, about 251 million to 250 million years ago.
...
Vast volumes of basaltic lava paved over a large expanse of primeval Siberia in a flood basalt event. Today the area covered is about 2 million km2—roughly equal to western Europe in land area—and estimates of the original coverage are as high as 7 million km2. The original volume of lava is estimated to range from 1 million to 4 million km3.

The area covered lies between 50° and 75° north latitude and 60° to 120° east longitude.

Origin

The source of the Siberian Traps basalt has variously been attributed to a mantle plume which impacted the base of the earth's crust and erupted through the Siberian Craton, or to processes related to plate tectonics.[1] Another possible cause may be the impact that formed the Wilkes Land crater, which may have been contemporaneous and would have been antipodal to the Traps.[2] This controversial scientific debate is ongoing.[3]
Again, both better time constraints and accurate tectonic models are needed here, but the idea is quite tantalizing.

Last fiddled with by ewmayer on 2012-12-25 at 23:14
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Old 2012-12-25, 22:55   #20
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Tantalising indeed. Without any knowledge whatsoever of how shock waves from a large impact would behave, I wonder why you would particularly expect the antipodal point to show such effects. Two factors which I would naively expect to affect the position on the earth's surface of such a reaction are (1) the angle of impact of the asteroid or comet which would not generally be directed towards the earth's centre, and (2) the differing structures of material within the earth, especially the boundary between mantle and core, which might deflect shock waves.
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Old 2012-12-26, 02:13   #21
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Quote:
Originally Posted by Brian-E View Post
Tantalising indeed. Without any knowledge whatsoever of how shock waves from a large impact would behave, I wonder why you would particularly expect the antipodal point to show such effects. Two factors which I would naively expect to affect the position on the earth's surface of such a reaction are (1) the angle of impact of the asteroid or comet which would not generally be directed towards the earth's centre, and (2) the differing structures of material within the earth, especially the boundary between mantle and core, which might deflect shock waves.
Good points - while the impact angle will definitely greatly influence the details of the initial "splash", I was thinking mainly in terms of deep-energy deposition in the larger object being struck, which is strongly affected by the shape and composition of the latter. Think of striking a bell with a hammer: If one hits it a steeply angled blow that certainly affects how loud the resulting sound is, but the energy deposited is almost entirely from the normal component of the impact.

And while there are appreciable asymmetries even deep within the earth (e.g. mantle plume structure, descending slabs of subducted crust, etc), perhaps for a very large impact these and the seeming solidity of the crust are dominated by large-amplitude surface waves spreading out like ripples on a pond. Notice that when you skip a stone off water, the ripples spread out symmetrically from the impact points, even though the striking occurs at an extremely shallow angle. In our case, the "pond" has a spherical surface, so the ripples recoalesce at the antipode.

Lots of interesting physics to think about ... shortly after making the above post I e-mailed Ralph von Frese, the lead author of ref. [2] on the Siberian Traps (the 2009 paper, "GRACE gravity evidence for an impact basin in Wilkes Land, Antarctica") to ask for a reprint of that paper, which should make for very interesting reading material.
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Old 2012-12-26, 04:35   #22
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Thanks for those links. I just had a fascinating excursion through a half dozen Wiki articles. The idea of antipodal reactions to massive impacts seems to make sense. I think I've even seen it animated in a cable series on solar system and planetary formation. However, that might have been the hypothetical "glancing blow" of a Mars-sized impactor which may have resulted in the moon. But I think it was more likely an illustration of the Late Bombardment period which showed the possible results of planetary shock waves.
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