磁极产生稳定磁场。
当你降低温度时，或者在某些情况下对材料施加压力或施加磁场，这个系统会试图变成一种更稳定的其他状态。
When you lower the temperature, or in some cases stress the material or impose a magnetic field, the system tries to reach a different state that is more stable.
研究者测量了一些在南非卡普瓦尔（Kaapvaal）克拉通（craton，稳定地块）被发现的岩石的磁场强度，卡普瓦尔克拉通是公认为可以追溯到30多亿年前的一块地质区域。
The researchers measured the magnetic strength of certain rocks found in the Kaapvaal craton of South Africa, a geologic region known to date back more than 3 billion years.
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