Where are meteorites found?
Nearly all meteorites are found in deserts. (Yes, Antarctica is a desert because the annual precipitation rate is very low.) Deserts are places that accumulate meteorites over thousands of years and then nothing much happens to the meteorite. Also, meteorites are easier to find in deserts than in places with topography, vegetation, and other rocks.
Nearly 42,000 meteorites have been found in Antarctica by government funded expeditions, mainly by the U.S. and Japan. Nearly all of these have been found since 1976. Over 14,000 meteorites have been found in the Sahara Desert of northern Africa, most since 1995, mainly by nomads and private collectors. About 4200 have been found on the Arabian Peninsula, mostly in the Sultanate of Oman, a desert country about the size of New Mexico.
The geographic distribution of meteorites is considerably different if calculated by mass instead of by number (Fig. 4). The field for Antarctica shrinks considerably. In Antarctica, all meteorites in a search area are collected regardless of size. Thus, the average mass of meteorites from Antarctica is 55 g (about 2 ounces), compared with 248 g (about a pound) for meteorites from the Sahara where little ones are easier to miss. (The smallest 90% of meteorites from Antarctica average only 17 g in mass.) Some of the other slices of Fig. 4 are much larger than in Fig. 1 because of massive iron meteorites from China (Aletai, 74 tons), Namibia (Hoba, 60 tons, andGibeon, 26 tons), Greenland (Cape York, 58 tons), Argentina (Campo del Cielo, 50 tons), U.S. (Canyon Diablo, 30 tons), Mexico (Chupaderos, 24 tons, andBacubirito, 22 tons), Australia (Mundrabilla, 24 tons), and Russia (Sikhote-Alin, 23 tons. Together, these 10 meteorites (391.5 metric tons) account for 56% of the mass of all known meteorites.
Finds and falls
Only a small fraction of collected meteoriteshave been observed to fall, 1.8% for the whole world. “Observed” usually means that a meteor was seen or heard and the stone or stones causing the effects was recovered shortly thereafter, usually within a few days. Such meteorites are called “falls” by meteoriticists. The other 98% are called “finds” – someone found the meteorite but the meteor was not observed.
Stony and iron meteorites
If only falls are considered, most meteorites are stony meteorites. Only a few percent are iron meteorites, also known as “irons.” Mesosideritesandpallasitesare rare types of meteorites that contain subequal volumes of both metal and stony material.
In populated places like North America, however, people find (Fig. 9) a greater fraction of irons than the fraction of irons among falls (Fig. 7) because irons tend to be more massive and are more likely to catch peoples’ attention. Many have been found by farmers plowing a field.
Types of stony meteorites
Most stony meteorites (93.1%) are chondrites, and most chondrites (93.9%) are ordinary chondrites. Put another way, 87% of stony meteorites are ordinary chondrites.
Chondrites contain iron-nickel metal, which is what makes themattract a magnet. Most other stony meteorite types contain little metal. The rare achondrites (6.9%) resemble Earth rocks more closely than do other meteorite types. It usually requires chemical or mineral analysis to determine if a rock is an achondrite. In the absence of a fusion crust, most of us cannot tell the difference just “by looking.”
The picture is a bit different when measured by mass. The aubrite slice is much larger here than in Fig. 11, because among the 93 enstatite achondrites (aubrites), one is huge, Norton County (1.1 metric ton). Similarly, the enstatite chondrite field is bigger in the mass diagram because of huge Al Haggounia 001 (3 metric tons).
Antarctica vs. northern Africa
As noted above, in Antarctica icefields are searched systematically and all meteorites are collected. I suspect that in northern Africa, most meteorites that are found are actually collected. On the basis of Fig. 13, however, I suspect that many of the ordinary chondrites are not classified and, consequently, are not listed in theMeteoritical Bulletin Database. Unclassified NWA chondrites are available for sale on the internet for less than a dollar per gram.
I gleaned all data presented here from theMeteoritical Bulletin DatabaseofThe Meteoritical Society. Thanks, Jeff.
Some Meteorite Information
