Answer the following question given this paragraph: 

Some rocks contain little compasses too! As lava cools, tiny iron-rich crystals line up with Earth’s magnetic field. Anywhere lavas have cooled, these magnetite crystals point to the magnetic poles. The little magnets point to where the north pole was when the lava cooled. Scientists can use this to figure out where the continents were at that time. This evidence clearly shows that the continents have moved.


Q: As lava cools, tiny iron-rich crystals line up with earth’s what?


A:
magnetic field