When volcanic lava solidifies, it becomes uniformly magnetized in the direction in which the Earth' s magnetic field points. There are significant differences in the direction of magnetization among solidified lava flows from different volcanoes that erupted at different times over the past several million years. Ttherefore, it must be that the direction of the Earth' s magnetic field has changed over time. Since lava flows differing by thousands of years in age often have very similar directions of magnetization, the change in the direction of the Earth' s magnetic field must take place very gradually over hundreds of thousands of years. The argument that the direction of the Earth's magnetic field has changed over time requires the assumption that ['not all solidified lava has changed the direction of its magnetization unpredictably', "only lava can be used to measure the direction of the Earth's magnetic field as it existed in the distant past", 'as lava flows down the side of a volcano, it picks up magnetized rocks', 'a single volcano can produce lava of differing consistencies during different eruptions']