The movement of a tectonic plate away from a mid-ocean ridge can be measured by the orientation of the field.

geomagnetic field.

Explanation:

The earth magnetic field can be likened to a bar magnet with a North and South pole. It originates from the movement of molten iron and nickel in the outer core.

At every point in time, the magnetic field orientation can change. We must note that the magnetic north lies close to that of geographic north axis of the earth.

Around the mid-oceanic ridges, new materials are brought to the surface. These new materials can contain magnetic minerals such as haematite which are capable of being magnetized as a result of the prevalent magnetic field of the earth. As different materials are brought up, the magnetic minerals preserves the details of the magnetic field at that point in time. Using this information can know the orientation of the magnetic field at any point in time if the magnetic history chart of the earth is constructed. Oceanic ridges becomes older as they move away from the divergent margins.

xvx sorry didnt mean to answer this