Geology Reference
In-Depth Information
detection can be improved through training designed to
let the conscious mind play a supervisory role. Over the
years we've made efforts to deconstruct the meteorite rec-
ognition process, and we now recognize two “trainable”
factors: the visual clues provided by the meteorite itself,
and development of an internal catalog of local terres-
trial lithologies.
Improving the latter for ANSMET field party members
is fairly simple and follows the old maxim “The best geol-
ogist is the one who has seen the most rocks” (attributed
originally to H.H. Read; see
Young
[2003]). The first few
days of ANSMET fieldwork are routinely dedicated to
looking at lots of rocks during searches at sites rich in
local lithologies. Typically, the search site will be a
moraine where previous work has suggested not only a
thorough representation of local lithologies but also the
likely presence of a few “example” meteorites (Figure 2.3).
During such searches field party members are strongly
encouraged to consciously examine every rock that
catches their eye and bring any rock they are curious
about to the attention of the team as a whole and the vet-
erans in particular for identification. False positives are
par for the course early on and accepted as a crucial part
of the training. Anecdotal evidence suggests that this
early exposure to a very complex lithological environ-
ment quickly trains the brain; it is not unusual for an indi-
vidual's meteorite finds to increase at a nearly exponential
rate during this training period. It sometimes leads to a
phenomenon we affectionately call a feeding frenzy,
where the team's rapidly increasing power to recognize
meteorites overwhelms leadership's attempts at managing
systematic progress during the search. There are worse
problems to have given our goals.
When meteorites are encountered during these early
searches, focus shifts to the other trainable factor (recog-
nition of the features of Antarctic meteorite finds). Most
field party members have some prior experience with
meteorites in hand sample. During training in McMurdo,
they are asked to familiarize themselves with hundreds of
images of previous finds. Meteorites in the wild can look
very different than those images, due to lighting and
background conditions, and even experienced veterans
benefit from a refresher course on the features that distin-
guish Antarctic meteorites.
The most distinctive feature of meteorites and the one
that most often distinguishes them from terrestrial rocks
is fusion crust. On their way to the ground, meteorites
develop a thin shell of melt as 10-20 km/s of velocity is
converted into thermal energy within the Earth's
atmosphere. The resulting layer of melt, once chilled to a
glass, is called fusion crust. With notable exceptions,
fusion crust is distinct from a meteorite's interior and
much darker than the weathering rind common on native
Antarctic rocks. It often shows flow lines and fluid fea-
tures characteristic of a semi-liquid state and is rarely
more than a few mm in thickness. Fusion crusts can range
from a matte black, polygonally fractured surface remi-
niscent of a charcoal briquette to a smooth glassy black
resembling furnace slag. Fusion crust is almost always
black but can vary in color depending on the minerals
being melted; gray, green, and even yellowish fusion crust
has been noted on some unusual specimens (Figure 2.3).
Only a very small percentage of Antarctic meteorites
show no fusion crust whatsoever, usually due to physical
weathering.
In the absence of visible fusion crust, other clues can
help one recognize meteorites. Meteorites are often well
rounded and equant in comparison to their terrestrial
neighbors; their fiery plunge through the atmosphere
tends to take off any sharp corners, and structural con-
trols on their shape (such as bedding, jointing, etc.) are
virtually absent in meteorites and common in terrestrial
rock. Meteorites often are different in size than the local
rocks, particularly in settings where aeolian sorting has
occurred; they can be either larger than the wind-sorted
rocks around them simply because they were delivered
there by different means, or smaller because their higher
density and rounded shape sorts them differently. The
density of meteorites, and their ability to absorb solar
energy when fusion crusted, can also lead to them sitting
differently at the ice surface (often slightly sunken in).
Because most meteorites contain native metal that oxi-
dizes very easily, they can show significant spots of rust
when weathered; this highly localized distribution of rust
is quite distinct from the broader coloring associated
with terrestrial oxidation of FeO in oxides and silicates
(Figure 2.3). The presence of native metal is also readily
detected by examination with a hand magnet, a test used
by ANSMET field party members when other clues sug-
gesting a meteoritic origin are not convincing enough.
Chondrules can also be very diagnostic when exposed.
Finally, most meteorite lithologies are distinct from most
terrestrial lithologies, so any rock that just “looks differ-
ent” has potential, whether or not you're a trained geolo-
gist. During ANSMET fieldwork we strive to recover any
rock suspected of having fusion crust or that just seems
exceptionally out of place, accepting some level of false
positives and trusting the curatorial process that follows
to weed these out.
In summary, ANSMET meteorite searches are an
economic compromise. Maximizing recoveries for any
given season means balancing currently available logis-
tical access to a site with our understanding of local
meteorite density, a site's propensity for foul weather,
recent snow cover, the density of local terrestrial rock
coverage, and even the expertise of a given year's field
team. Our visual searches are prey to all the failings of
the flesh, as well as the quirks of wind, snow cover, terres-
