(The following article was written by Ron Oriti and is from the November 1965 issue of the "Griffith Observer", reprinted with the authors permission - ed.)
DRY LAKES AND METEORITES
Dry lakes or playas, as they are sometimes called, are found only in the desert regions of the world, and specifically, those regions which are characterized by internal drainage, i.e., regions in which desert streams and waterways die out before reaching the sea. Dry lakes are themselves the drainage basins of the desert, and into them drain the streams and rivulets, which result from occasional cloudbursts and flashfloods. Quite remarkable in appearance, they are extremely flat places, barren of vegetation, often barren of rocks, and sometimes extending for many square miles in area.
Dry lakes are composed of silt and clay sediments, with varying amounts of salt. When the amount of salt is low, the dry lake is nothing more than an enormous mud flat, but when the percentage of salt is high, our mud flat becomes an alkali flat or salina. The so-called Bonneville Salt Flats in Utah is an example of a salina which is part mud flat in places. These places are so hard and flat that they provide a fine race course for high speed land vehicles. Rogers Dry Lake (formerly Muroc Dry Lake) in the Mojave Desert covers 100 square miles of area, and is used by the Air Force as a landing field and runway for experimental aircraft. Not all dry lakes, however, have such hard surfaces. In some cases they are so soft that a jeep will bog down in them.
When the water from a transient desert stream finally reaches a dry lake and spreads out over its surface, it moves so slowly that it can carry, in suspension, only the finest particles of clay and silt. These particles as well as dissolved salts are deposited when the water evaporates. According to R. O. Stone (Geology Department, University of Southern California) the rate of sedimentation of many of the playas in the Mojave Desert of California is less than one foot per thousand years, and may well be just a few inches per thousand years. Of course, the rate of sedimentation has not been constant, and was probably much higher about one million years ago when the playas were just forming. Even lower than this rate of sedimentation, however, is the rate of erosion, the principal cause of which is the wind.
Now what has all of this got to do with meteorites, you might ask, and the answer, of course, is that some meteorites have been found on dry lakes and for good reason, we think. Most of the meteorites which fall are of the stony variety, and almost invariably break up in the atmosphere and produce a multiple fall or shower. The speed of the individual specimens is often so reduced that they fall like ordinary rocks dropped from high places, and no penetration of the ground results. Such meteorites are the ones likely to be found on dry lakes. Now it is not that meteorites fall more frequently onto dry lakes than elsewhere. They have been found, and will doubtless continue to be found on them, only because dry lakes are more easily searched than other regions, and also because any meteorites on them will probably be better preserved. Some of the reasons for this are the following:
1. Dry lakes often have so little rock material on them that even small pebbles are conspicuous; this is especially so when the sun is low and shadows are long. With binoculars it is easy to see rocks up to a distance of about half a mile.
2. Their surfaces are usually hard enough to support jeeps or autos so that many square miles can be explored with relative speed and comfort.
3. Because of the low rate of sedimentation and lack of erosion, meteorites are not so apt to be buried or broken, and in the usually dry environment weathering proceeds slowly.
It is sometimes difficult to explain why, here and there, over the surface of a dry lake one can sometimes find isolated terrestrial rocks. Any flow of water, or strong wind, sufficient to transport such rocks would also carry lots of smaller debris, but such detritus may not be found. Some of these rocks are found to be scoriaceous lava. These, undoubtedly, landed there after being ejected by some nearby volcano. It is an interesting diversion to search dry lakes for isolated rocks, and who knows. if one is lucky he may even find one which has had a cosmic origin.
Dry lakes sometimes become wet lakes during the rainy season and appear as impressive bodies of water. The appearance, however, is misleading, for such water is generally not more than an inch or two deep, and will probably evaporate in a couple of weeks. During such times meteorites will undergo their worst weathering. The surface of a clay dry lake is generally so impermeable to water that only the upper few inches get wet, and at a depth of about one foot, or so, the clay is dry.
Large meteorites, which have a tendency to retain some of their original cosmic velocity, may bury themselves in a dry lake to depths exceeding one foot. These meteorites will remain in an excellent state of preservation, perhaps for thousands of years. With the right kinds of equipment it may be possible to locate such sub-surface specimens. Ordinary metal detectors, however, are not adequate for this job. Such detectors cover too little area, and they are not very sensitive to the stony meteorites. We are presently investigating the feasibility of applying geophysical techniques to this problem.
*Lucerne Dry Lake pictured above is just one of more than one-hundred dry lakes in California. Nine small meteorites have been found on its surface*
In southern California alone there are more than one hundred dry lakes which exceed one square mile in area. To my knowledge, meteorites have been discovered on three of them, namely, Rogers, Rosalind, and recently Lucerne Dry Lake. One meteorite is named Dale Dry Lake, but it was actually found a few miles away from the playa. The author and his colleagues are currently engaged in a survey of playas for surface meteorites, and wish to extend to all an invitation to join us in the hunt. You may send any suspected meteorites, or samples thereof, to the author for free analysis.
-Ronald A. Oriti