All kinds of people think they have found a meteorite, and
eventually they, a friend or relative bring it to a museum or
university for authentication, often after the treasured specimen
has sat on a mantelpiece or beside the barn for a year or a decade!
This month let's examine a few examples of the real thing...
1. Left: On the left is an ordinary chondrite,
displaying a black fusion crust and a pale
interior with traces of terrestrial rust-staining around
mm-size metallic flecks of nickel-iron alloy. On the
right is a small piece of iron meteorite showing the
dimpling typical of many irons.
2. Right: The Kitchener chondrite, a small (200-g) 1998 fall
in southern Ontario.
Note the large chondrule near the centre, visible in
outline beneath the intact fusion crust.
3. Left: One view of the Toronto iron meteorite, a "find" recognized
in 1997 and, for lack of precise information on its recovery, named for
the city in which its meteoritic origin was finally verified.
4. Right: A second view of Toronto, a 2.7-kg (6-pound) iron.
"Rock of the Month # 3, posted September 2001" --- four images,
a "special" presentation to
mark the annual meeting of
MIAC, the Meteorites & Impacts Advisory Committee
to the Canadian Space Agency. Photographs by Karyn Gorra,
Department of Geology, University of Toronto.
Meteorites have caught the attention of the public in the past decade,
in part because of well-publicized episodes such as the 1992 Peekskill
fall in upstate New York, which struck a car as it landed, and
bright "fireball" meteors which resulted in recovery of freshly fallen
meteorite "showers" such as St-Robert (Quebec, 1994) and Tagish Lake
(northern British Columbia, 2000). There is also the question of scarcity and
value: trade prices of most meteorites have risen sharply in the 1990s,
despite the fact that most aesthetically-beautiful specimens are
priced beyond the means of humble collectors. An alternative
reason for the increased interest resides in the exotic origins of these
"space rocks", distant in space and time from our own
perspective, on the one hospitable body in the Solar System.
Although no one set of features will define all meteorites,
these examples illustrate ordinary chondrites and irons, the
broad families of meteorites which a lucky finder is most
likely to encounter. In brief, some of the most useful pointers
are as follows:
As employed for railway use, slag fragments are
typically 3-8 cm in size, and few are larger
than 12 cm in maximum dimension. Study
these photographs of typical slag samples:
- 1. A thin black fusion crust, like a thin glassy glaze, covers
many meteorites, and is most prominent on many of the stony
types, where it forms a sharp contrast to the pale interior
typical of common chondrites (photo 1 - left side: photo 2).
The fusion crust may show fine lineations indicative of
flow of liquid glass along the face of the meteorite
tumbling in the atmosphere, rather like rain running down a car's
- 2. The interior of the most common stony meteorites often
contains small (1 mm or less) flecks of silvery metal and
bronzey sulphide, often stained by rust on weathered samples.
- 3. Broken interiors of many of the most abundant
class of meteorites, the chondrites, contain pale rounded
structures which are mostly less than 3 mm across, but exceptionally
larger than 5 mm.
These "chondrules" (photo 2) are spherical aggregates
of silicate minerals, thought to have formed in the solar
nebula shortly before the origin of the Earth and the
other planets (meteorites are old!).
- 4. Iron meteorites are intensely magnetic, while most but not
all stony meteorites are somewhat magnetic due to the
disseminated blebs of metal. Iron meteorites are very dense,
and may weigh three times as much as similar-sized pebbles
of common Earth rocks, such as limestone or granite.
- 5. The surfaces of many iron meteorites are variably indented
(photo 1 - right side: photos 3, 4) with rounded depressions.
These rather resemble the marks of a thumb or finger in putty.
- 6. It is worth noting the absence of other features, such as
abundant bubbles, often concentrated along particular planes,
and coarse, ropy flow textures, more like a lava flow than the
delicate features noted in (1) above. Both these textural features
are typical of blast-furnace slag, which often has a reddish surface tinge,
and is commonly found in abundance as
ballast along railway lines.
Left: a large sample, 15x10x3 cm, courtesy of
John Rucklidge (University of Toronto).
sample 1896, smelter slag from Sudbury, Ontario.
Anyone who thinks that they may have found a meteorite is encouraged to
take their find to a museum or university department with
expertise in geology, astronomy or related branches natural history,
proportion of "found" samples that turn out to be meteorites is
usually small, less than one in a hundred, the
"treasure" is sometimes notable for other reasons, which
probably made the sample sufficiently unusual to attract the finder's eye
and awaken a sense of curiosity. As a caveat, novice finders of
potential meteorites should study the cautionary tale of
of Washington University in St. Louis.
Here are some modern books which offer distinct,
informative introductions to meteorites and related topics,
including tektites and impact events.
GRIBBIN,JR and GRIBBIN,M (1996) Fire on Earth: Doomsday,
Dinosaurs, and Humankind. St. Martin's Press, New York, 264pp.
HUTCHISON,R and GRAHAM,A (1994) Meteorites. Natural History
Museum, London, 2nd impression, 61pp.
Meteorites and their Parent Bodies.
Cambridge University Press, 2nd edition, 310pp.
NORTON,OR (1994) Rocks from Space: Meteorites and Meteorite
Hunters. Mountain Press Publishing Co., 449pp.
NORTON,OR and CHITWOOD,LA (2008) Field Guide to Meteors and Meteorites.
Springer-Verlag London Limited, 287pp.
ZANDA,B and ROTARU,M (editors) (2001)
Meteorites, their Impact on Science and History.
Cambridge University Press, 128pp.
Graham Wilson, for September 2001, last amended 21 November 2010.
Interested? Please see THIS (updated 2-page review, as 494 kb pdf file)
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