Sir Harold Jeffreys in Britain, in particular, pointed out that the deeper layers were not nearly fluid enough and would strongly resist the proposed motion. After Wegener died on an arctic expedition, in 1931, only a handful of loyal supporters continued to promote his ideas. More evidence was needed, and it came from the Earth's magnetism . After molten lava emerges from a volcano, it solidifies to a rock. Its magnetization is in the direction of the local magnetic force at the time when it cools down.
Instruments can measure the magnetization of basalt.
Therefore, if a volcano has produced many lava flows over a past period, scientists can analyze the magnetizations of the various flows and from them get an idea on how the direction of the local Earth's field varied in the past. Surprisingly, this procedure suggested that times existed when the magnetization had the opposite direction from today's. All sorts of explanation were proposed, but in the end the only one which passed all tests was that in the distant past, indeed, the magnetic polarity of the Earth was sometimes reversed. Click here to bring up four of those questions , with their answers. In the 1950s electronic magnetometers were developed.