Pat, Sterling, others, moving or resting sand is not conserving meteorites but is one of the major threats to the preservation of meteorites. Most if not all meteorite aggregation fields in the Sahara have one thing in common: they are free of sand. In fact they are shielded against the sandflow by ridges or mountain chains, e.g. the Djebel al Aswad is bordering the east of the Dar Al Ghani.
These natural barriers that prevent the sand from entering certain areas are commonly situated in the east or northeast of the aggregation field. And the winds on the African continent tend to blow from the northeast during the whole year since at least ten thousand years. The surface horizons of the meteorite concentration areas such as the HAH and the DAG are mostly composed of pebbles and top soil and are free of quartz sand. Terrestrial lifetimes of meteorites decrease as soon as quartz sand is involved due to the quick mechanical abrasion. It is true that meteorites found in aggregation areas in the Sahara were sedimented and protected by soils from wheathering before the recent wind erosion uncovered them. This process started about 12 - 10,000 years ago and continues until present. But these sediments that are being eroded today were not composed of quartz sand. There is a good paper on the mechanisms responsible for meteorite concentration in the dense aggregation areas by Schlueter et al. : J. Schlüter et al. (2002) The Dar al Gani meteorite field, Meteoritics & Planetary Science 37, 1079-1093. I fully second Sterling's post on this subject but take the liberty to add, that desertification in the Sahara may not be such an irreversible process as suggested. A single acacia growing on a hammadah or on an erg-surface accumulates soil in a radius of 25 - 40 meters. This soil layer can reach up to 1 m height during the lifetime of the plant. A climate change involving a noticeable growth of humidity, triggerd by a change of e.g. ocean currents, would soon lead to the spreading of Savannah vegetation into the Saharan voids. Cheers Svend www.meteorite-recon.com ----- Original Message ----- From: "Pat Brown" <[EMAIL PROTECTED]> To: "Jason Utas" <[EMAIL PROTECTED]>; "Meteorite-list" <meteorite-list@meteoritecentral.com> Sent: Friday, January 11, 2008 7:41 AM Subject: Re: [meteorite-list] intriguing Question > Hi Jasoon, Peter and the List, > > Some thoughts on why the Sahara is so productive: > > Another significant factor is the climate over the > past several thousand years. The sahara and parts of > the American southwest have been dry for a long time, > this increases the terrestrial recognizability time of > any meteorites that are there. The sahara has another > interesting and significant factor having to do with > the sand dunes; slowly moving dunes provide a soft > landing and a protective environment as meteorites are > covered by a slow moving dune. Later (sometimes much > later) meteorites are uncovered in great shape with > significant terrestrial age. > > Another factor is economic; most of us in the US that > hunt meteorites do so on weekends and vacation time. > Our motivation is to get out in nature and do a bit of > 'big science'. Many of the hunters in the sahara are > subsistance hunting, many are truely professional full > time hunters. I get in 25 days of meteorite hunting > tops per year, the locals in the sahara get in well > above 100 days per year in many cases. They have in > many cases spent their lives in the desert. I have > only spent a total of 10 or 12 weeks in the areas > where I hunt. They are better at seeing the odd rock. > > A few American hunters that are retired and in good > health likely pass the 100 days per year in the field > hunting mark. I hope to be one of them in 12 or 13 > years:) > > Pat > --- Jason Utas <[EMAIL PROTECTED]> wrote: > >> Hola Peter, All, >> I know individual aspect of your questions have been >> addressed, but >> I'd like to as well... >> >> >I know the Sahara desert is about a galgillion >> square miles. >> Then there are the deserts in Calif., South America, >> the Antarctic continent >> and God only knows where else. Why don't I see any >> meteorites from the >> Gobi desert, or maybe the Mongolia desert. >> >> Well, yeah, it's big. But the main reason that >> there are more >> meteorites found in the Sahara than elsewhere is >> because there are >> more people looking. There are just as many martian >> and lunar >> meteorites elsewhere, but they tend to be harder to >> recognize due to >> harder hunting terrain (similar rocks, etc) - and >> because there are >> probably twenty or so meteorite hunters in the >> southwest who get out >> into the wilderness with some frequency - probably a >> few more, since >> Franconia became popular. Compared to the undefined >> number of hunters >> who have been scouring the Sahara for the past >> decade, well, it's just >> not much of a comparison. >> Same goes for South America, though Antarctica has >> seen a good deal of >> thorough hunting, as results would suggest. >> >> >And then there is little dinky Roosevelt Co, NM at >> just 2,455 sq >> miles and it has a >> staggering >> 109 meteorites, which comes to one for every 22.5 sq >> miles. What gives? >> >> This is due to, as has been said, the hunting of >> Skip Wilson, who has >> spent years in the area, hunting blowout after >> blowout with remarkable >> success. A good bit of hunting land paired with his >> diligence has >> turned up pretty spectacular results... >> That said, the density of meteorites that actually >> exist on the land >> should be, at the very least, several per square >> mile; yes, many have >> been found, but there are still countless more >> waiting to be >> discovered. >> >> >They are of a wide variety of classifications, so >> it can't be turning >> every piece in >> for classification. I can't speak for anyone else, >> but I find this >> very puzzling. >> >> I don't see why he couldn't be turning every stone >> in for >> classification; data gathered suggest that smaller >> falls would be more >> common than larger ones, and this would mean that >> the majority of >> falls would consist of small, individual stones. >> >> I also don't know how many of his finds have been >> paired with one >> another, but I have the feeling that if he has been >> concentrating on >> individual areas (as opposed to moving on whenever >> he finds a >> meteorite), it would stand to reason that he has >> found at least a few >> paired meteorites, whether or not they are listed as >> such in the >> catalogue. >> >> Regards, >> Jason Hi, All, > The [S]ahara and parts of the American southwest > have been dry for a long time... The American SW, yes. The Sahara, no. It's desertification began in the eastern Sahara about 14,000 years ago and worked its way slowly west. Cyrene on the Egyptian-Libyan border was a flourishing Greek city-state 2700 to 2200 years ago, then dried up and blew away. Western North Africa was the "breadbasket" of the Roman Empire and was still producing agricultural surpluses for Rome as late as 400 CE. Agriculture was practiced more than 200 miles further inland from the North African shore than is possible today. During the most recent glaciation, the Sahara was a bountiful land with a substantial Neolithic population. Some 8000 to 10,000 years ago, most of the interior Sahara was grassy plains with scattered forests, lakes and rivers. The present progressive desertification of the Sahara is irreversible because of runaway sand generation The Sahara will be a desert now for millions of years to come, most likely. Sterling K. Webb --------------------------------------------------------------------------- ----- Original Message ----- From: "Pat Brown" <[EMAIL PROTECTED]> To: "Jason Utas" <[EMAIL PROTECTED]>; "Meteorite-list" <meteorite-list@meteoritecentral.com> Sent: Friday, January 11, 2008 12:41 AM Subject: Re: [meteorite-list] intriguing Question Hi Jasoon, Peter and the List, Some thoughts on why the Sahara is so productive: Another significant factor is the climate over the past several thousand years. The sahara and parts of the American southwest have been dry for a long time, this increases the terrestrial recognizability time of any meteorites that are there. The sahara has another interesting and significant factor having to do with the sand dunes; slowly moving dunes provide a soft landing and a protective environment as meteorites are covered by a slow moving dune. Later (sometimes much later) meteorites are uncovered in great shape with significant terrestrial age. Another factor is economic; most of us in the US that hunt meteorites do so on weekends and vacation time. Our motivation is to get out in nature and do a bit of 'big science'. Many of the hunters in the sahara are subsistance hunting, many are truely professional full time hunters. I get in 25 days of meteorite hunting tops per year, the locals in the sahara get in well above 100 days per year in many cases. They have in many cases spent their lives in the desert. I have only spent a total of 10 or 12 weeks in the areas where I hunt. They are better at seeing the odd rock. A few American hunters that are retired and in good health likely pass the 100 days per year in the field hunting mark. I hope to be one of them in 12 or 13 years:) Pat --- Jason Utas <[EMAIL PROTECTED]> wrote: > Hola Peter, All, > I know individual aspect of your questions have been > addressed, but > I'd like to as well... > > >I know the Sahara desert is about a galgillion > square miles. > Then there are the deserts in Calif., South America, > the Antarctic continent > and God only knows where else. Why don't I see any > meteorites from the > Gobi desert, or maybe the Mongolia desert. > > Well, yeah, it's big. But the main reason that > there are more > meteorites found in the Sahara than elsewhere is > because there are > more people looking. There are just as many martian > and lunar > meteorites elsewhere, but they tend to be harder to > recognize due to > harder hunting terrain (similar rocks, etc) - and > because there are > probably twenty or so meteorite hunters in the > southwest who get out > into the wilderness with some frequency - probably a > few more, since > Franconia became popular. Compared to the undefined > number of hunters > who have been scouring the Sahara for the past > decade, well, it's just > not much of a comparison. > Same goes for South America, though Antarctica has > seen a good deal of > thorough hunting, as results would suggest. > > >And then there is little dinky Roosevelt Co, NM at > just 2,455 sq > miles and it has a > staggering > 109 meteorites, which comes to one for every 22.5 sq > miles. What gives? > > This is due to, as has been said, the hunting of > Skip Wilson, who has > spent years in the area, hunting blowout after > blowout with remarkable > success. A good bit of hunting land paired with his > diligence has > turned up pretty spectacular results... > That said, the density of meteorites that actually > exist on the land > should be, at the very least, several per square > mile; yes, many have > been found, but there are still countless more > waiting to be > discovered. > > >They are of a wide variety of classifications, so > it can't be turning > every piece in > for classification. I can't speak for anyone else, > but I find this > very puzzling. > > I don't see why he couldn't be turning every stone > in for > classification; data gathered suggest that smaller > falls would be more > common than larger ones, and this would mean that > the majority of > falls would consist of small, individual stones. > > I also don't know how many of his finds have been > paired with one > another, but I have the feeling that if he has been > concentrating on > individual areas (as opposed to moving on whenever > he finds a > meteorite), it would stand to reason that he has > found at least a few > paired meteorites, whether or not they are listed as > such in the > catalogue. > > Regards, > Jason > > On Jan 9, 2008 7:01 PM, Peter A Shugar > <[EMAIL PROTECTED]> wrote: > > Hello, > > I'm the newbie, so please explain this to me. This > is an intriguing question. > > I can't figure it out. I know the Sahara desert is > about a galgillion square miles. > > Then there are the deserts in Calif., South > America, the Antarctic continent > > and God only knows where else. Why don't I see any > meteorites from the > > Gobi desert, or maybe the Mongolia desert. > > And then there is little dinky Roosevelt Co, NM at > just 2,455 sq miles and it has a > > staggering > > 109 meteorites, which comes to one for every 22.5 > sq miles. What gives? > > They are of a wide variety of classifications, so > it can't be turning every piece in > > for classification. I can't speak for anyone else, > but I find this very puzzling. > > Any thoughts, List? > > Pete > > > > ______________________________________________ > > http://www.meteoritecentral.com > > Meteorite-list mailing list > > Meteorite-list@meteoritecentral.com > > > http://six.pairlist.net/mailman/listinfo/meteorite-list > > > ______________________________________________ > http://www.meteoritecentral.com > Meteorite-list mailing list > Meteorite-list@meteoritecentral.com > http://six.pairlist.net/mailman/listinfo/meteorite-list > ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list -- www.niger-meteorite-recon.de ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list
