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When magma rises to the Earth's surface at a mid-ocean ridge, it flows out onto both sides of the ridge, gradually cooled by the seawater. Like tiny compass needles, the magnetic minerals in the hot magma are at first free to align themselves with the Earth's magnetic field when the magma settles into the tectonic plate, but once the lava cools below the Curie point, their orientation becomes fixed. When readings of the strength of the magnetic field are taken along sections of the ocean floor near such ridges, segments where it is anomalously high alternate with segments where it is anomalously low. Anomalously high readings occur because the magnetometer is picking up both the reading from today's magnetic field and that from the minerals in the rock that are aligned with it, adding to the total strength of the field, while anomalously low readings occur when the magnetic minerals are aligned against the Earth's magnetic field, diminishing the total strength. The rocks that yield these anomalously low readings therefore must have formed at a time when the Earth's magnetic field was reversed—oriented in such a way that the north magnetic pole was roughly where today's south magnetic pole is, and vice versa. These magnetic reversals, in which the direction of the field is flipped, are believed to occur when small, complex fluctuations of magnetic fields in the Earth's outer liquid core interfere with the Earth's main dipolar magnetic field to the point where they overwhelm it, causing it to reverse. The length of time between magnetic reversals is not always the same, but is on the order of 200,000 to 1,000,000 years; the last magnetic reversal was about 750,000 years ago. Because the pattern of positive and negative readings is more or less symmetrical about the axis of the mid-ocean ridge and remains the same throughout the length of the ridge, geophysicists have been able to construct a calendar of the Earth's magnetic record dating back to as far as 150–200 million years ago. It is not known when the next magnetic reversal will be, or how long the process will take, though it will certainly have a significant impact on the artificial and biological navigational systems of humans and animals.