Field book of common rocks and minerals

by Frederic Brewster Loomis

Meteorites

Meteorites can hardly be called common, but there is always a chance of finding one, and their interest is so great, that none should escape because unrecognized.

Meteorites are visitors to the earth from space, and they bring to us knowledge of the composition of planets and solar systems, other than our own. It is of interest to note, that while they have brought to us some combinations of elements which do not occur in the earth, still they have not brought any element with which we were not already familiar. They are popularly known as “falling” or “shooting stars,” though of course they are not stars, but only small masses of matter which are entirely invisible until they come inside our atmosphere.

In space there are many small (compared with the size of the earth) chunks of matter, each pursuing its solitary way around the sun, or wandering through space along paths entirely unrelated to the sun. From time to time one of these passes near enough to the earth, so as to be influenced by its attraction, and then comes rushing toward it at tremendous speed, 20 to 30 miles per second. As soon as it comes into the atmosphere, even the very attenuated atmosphere, a couple of hundred miles above the surface, friction heats the surface of the meteor until it glows, and by that light we see the so-called shooting star, often with a trail of luminous matter streaming out behind. Of course in using this term “shooting star,” we understand the meteor is no star, for they are bodies as big as our sun, shining at distances billions of miles away.

As the meteor rushes through the atmosphere it may all burn up, no large fragment reaching the earth’s surface. The luminous matter streaming out behind is material which has melted and dripped off the main mass. As this oxidizes and cools, that part which did not become gaseous will finally fall to the earth as fine dust. When however a meteor actually falls to the earth, its surface is still hot, though probably there has not been time enough for much heat to be transmitted to the interior. At any rate they do not show any alteration due to this cause. On landing and sometimes before they land meteors break into two or more pieces. When found the surface always shows the effects of the heat generated by the friction of passing through the air, the surface being smoothed, and covered with stream lines and melted out pits and hollows, and the outer surface consisting of a thin crust, making an appearance, which once seen, can hardly be mistaken.

There are two types of meteorites, those made wholly or largely of iron with some nickel, and appearing like great chunks of iron, and those which are stony and resemble a granite boulder. In collections the first sort, i.e. iron meteorites, are most abundantly represented, because most easily recognized when found. They consist of masses of iron and nickel with small amounts of other elements, ranging in size from the Cape York meteorite, which fell in northern Greenland in 1894 and was later brought by Peary to the American Museum, and weighs some 36 tons, down to small grains as small as a grain of wheat. The largest one which has fallen in the United States was the Willamette meteorite weighing some 15 tons, and falling 19 miles south of Portland, Oregon. These and all iron meteorites have the iron in crystalline form which is readily seen if the meteorite is cut, and the surface thus made polished, then etched with acid, which is put on and quickly washed off. Every meteorite has its particular pattern, as illustrated on [Plate 72], and by these patterns can be identified. Meteorites have a high value and are eagerly sought by certain large institutions and collectors. Since the crystalline structure is so characteristic of each fall, when a new meteorite is found, it is usually cut in two, and one part retained by the finder or some institution; while the other part is cut into small pieces, an inch or two on a side and a quarter of an inch thick, but each large enough to show the characteristic pattern. These are distributed largely by sale to other collectors. Thus a great meteorite collection consists of a few large meteorites and a great many small portions of other meteorites.

The second type of meteorite is the stony meteorite. Where meteorites have been located as they fell and recovered, the majority of them were of this type, so that probably more than half of the meteorites which fall are of the stony type. However when the stony meteorite is exposed to weathering it takes only a very short time before the surface is eroded off and then such a meteorite looks like any other boulder and probably most of them fail to be recognized, and so have been lost. Because they have so much greater variety, they are in many ways of greater interest than the iron type.

It is desirable that every one have his eye out for meteorites, and when found it is desirable that the fact should be reported to some one of the great institutions which collect them, such as the National Museum in Washington, or the American Museum in New York. Each one should be on record even if it is desired to keep it in a private collection.