Induced Magnetism in Volcanic Rocks

Abstract

AN interesting note by G. Folgheraiter, on the magnetism induced in volcanic rocks by the earth's magnetic field, appears in the Atti della Reale Accademia dei Lincei (vol. iv. part 5, March 3, 1895). The author has performed a number of experiments on volcanic rocks, in order to determine the amount of induced magnetism left when, after heating to such a temperature that they entirely lose their permanent magnetism, they are either allowed to cool slowly or are suddenly cooled, in each case under the influence of the earth's field. From such observations he hopes to be able to deduce some conclusions as to the conditions under which the rocks experimented on, which were originally permanently magnetised, became magnetised. The rocks are cut into small parallelopepedons weighing about 50 grams, and such that the length is about two or three times the depth or breadth, care being always taken to cut the rock so that the axes of these pieces were vertical when the rock was in its place in the earth. The intensity of magnetisation was in every case measured by the method of deflection; a freely suspended magnetic needle being deflected by the sample, which was placed with its length east and west. After measuring the intensity of magnetisation of the samples, they were heated to redness, and then either allowed to cool slowly, or are rapidly quenched with their axes vertical. Their magnetic moment was determined, first immediately they were cool, and then after they had stood under the influence of the earth's field for three months. The specimens of basalt examined may be divided into two groups: in the first may be placed those specimens which were originally only slightly magnetised, and in this case, after heating to redness, the magnetisation is always increased, but to a very different degree in the different specimens. The second group includes those basalts which were originally strongly magnetised, and in this case after heating the magnetisation was considerably reduced. In both groups the magnetisation underwent no change during three months, and sudden cooling gave the same results as slow cooling. Experiments have also been made on tuff and peperino. The results obtained with the first of these rocks are similar to those obtained with the first group of basalts. Peperino, however, differs in that, before being heated, its coercive force seems almost nil, the bar becoming only temporarily magnetised. After heating, the character of the rock is altered, as it can now become permanently magnetised and behaves just like the tuff. From this the author concludes that peperino has been formed at a low temperature, probably by the action of water on cinders, &c.