Click here to view the "Meteorite Article Archive". Desert meteorites in situ Part 2 Breakup through weathering   Remains of a ~1 kg chondrite. The meteorite's fragmentation is caused by surface weathering. Fragments are distributed over several meters. (meteorite under classification) Breakup through weathering and horizontal movement Meteorites exposed on desert surfaces for a very long time or during more arid phases often break up along pre-existent cracks due to chemical and mechanical weathering. This is particularly true for finds located in sandy areas. The fragments of these meteorites are sometimes found distributed over several meters with the smaller fragments often but not always transported further away than the larger fragments. At first sight there is no obvious reason for this strange phenomenon, particularly if the find location is situated on an even surface with no hydraulic gradient. Visualizing the soil erosion process in the vertical dimension provides the answer. While breaking apart on a deflation surface over the millennia, soil is constantly removed under the meteorite and its fragments. This results in a vertical downward movement of the meteorite and its fragments. Variations in the soil composition, roots and camel grass tussocks affect the macro-local surface resilience towards erosion and deflect the downwards movement of meteorite fragments over the time.   Larger meteorite fragments located on a slight gravel slope. Note the lower fragment (550 g) which has already moved 40 cm downwards and which is in the process of further fragmentation. (meteorite under classification) Thus during the erosion and deflation process temporarily local gradients, small channels and grooves can occur which further contribute to a horizontal movement of surface rocks - particularly during the periods with frequent rainfalls. Because the final surface, in many cases a perfectly even gravel pavement or sand plane, no longer shows any evidence of these processes, the migrating meteorites are often the only telltale signs that these surfaces in fact transform over the millennia. Corrasion Another interesting feature visible on meteorites found on desert surfaces is corrasion (not to be confused with "corrosion"). The term describes the wearing away of the meteorite's surface by natural sandblasting. Usually corrasion is found only on the windward surface of the meteorite and is mostly limited to an area not exceeding 3-4 centimeters from ground. The limitation to the windward side is an indication for a steady wind direction over the complete period of the meteorite's surface exposure.   1/200sec exposure image showing wind borne sand abrading a meteorite. Most of the activity takes place in the space up to three centimters from the surface (Rub al-Chali, meteorite under classification, specimen 6 cm longest dimension) In the Sahara and in most parts of the Rub al Khali only the the east- or northeastern surfaces of meteorite's display the effects of corrasion. In case a meteorite is exposed for a long time and/or composed of a relatively soft lithology then the windborne sand can literally carve tunnels and deep regmaglypt like cavities into the rock.   Ink black meteorite weighing 590 g on a bright gravel surface. (meteorite under classification, scale cube is 1 cm) Detectability One would think that a fusion crusted or desert patina coated meteorite is easy to spot on a bright gravel surface over a great distance. Unfortunately this isn't the case. The advantage of a good background contrast is lost as soon as the sun is not in the ideal six o'clock position of the viewer. Rocks, pebbles and roots in the nearer surrounding cast shadows as black as the meteorite in their middle causing a plethora of potential targets that all have to be ckecked. A meteorite the size of a golf ball will blur with the surrounding in broad midday light if more than a couple of paces away from the viewer. In fact we have spotted no meteorite this size any further than from a distance of five or six meters. On anything smaller you have to step on in order to find it, even in ideal light conditions. In situ photos showing an ink black stone on a plain white surface often give a wrong impression in this regard. The striking contrast is often put into perspective if one steps back and pictures the meteorite in its wider surrounding. Most desert surfaces show a wide variety of look-alikes and pseudo-meteorites littering one's search area.   The same meteorite as in the previous image, this time photographed from a distance of seven meters. Numerous black spots can be seen in the surrounding, all are terrestrial rocks. A meteorite will look no different to these dark rocks from a certain distance. Not unless the keen prospector picked up a couple of hundred of these and intimately internalized their appearances from a thousand angles during every altitude of the sun and on every surface background imaginable. Only then he might notice the insignificant difference in an outline or the slight shade of tint that separates the genuine meteorite from its terrestrial Doppelganger. Return to part 1, Shielding from weathering Part 1 Shielding from weathering Part 2 Breakup through weathering Return to the "Meteorite Article Archive".   © 2001-2010 NigerMeteoriteRecon