How do rocks record the changes in the Earth's magnetic field?

Answer 1

When new seafloor is created by upwelling magma at the mid-ocean ridges, the magnetic minerals orient themselves to the magnetic orientation of Earth at the time of their solidification in the rock matrix. Thus, the history of magnetic reversals are recorded in the rock.

Answer 2

The rocks that record the changes in Earth's magnetic field are igneous. The easiest place to observe the record is in the igneous rock of the ocean floor. At the mid-ocean ridges, new crust is being created by the infilling of gaps between diverging oceanic plates. The infilling material is magma, which then solidifies into rock. Magnetic minerals in the magma are aligned with the magnetic poles, much like the needle of a compass. When the magma solidifies, the magnetic minerals are locked in place, recording the direction of the Earth's magnetic field at the time the rock is formed. Radioactive elements in the igneous rock can then be dated by isotopic decay rates to determine their age, and therefore the ages of the magnetic field shifts.

Answer 3

Deep-ocean trenches

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No. That's where the rock may be, but the real answer is igneous rock, particularly that making up new oceanic crust at constructive plate boundaries.

Answer 4

Any magnetic ore, stuff that contains iron.