年龄< p > < a href = " http://en.wikipedia.org/wiki/Earth%27s_magnetic_field " >的地球磁场的< / >可以一样古老的存在能产生磁场的物理条件。因为我们相信的地磁场来源于液体外核,你的问题可能是重申:地球是什么时候形成液体核心? How the Earth's core formed is still not well understood and the area of research is referred to as planetary differentiation.