How is the magnetic field direction preserved in the rocks of the sea floor?

Question

What kind of rocks must they be?

Answer 1

Magnetic field direction is preserved mostly in the mineral magnetite that is present in most rocks. Nevertheless, in order to preserve the magnetic field, magnetite must align or crystallize following the earth magnetic field of the time. There are two basic ways to do this.
1) sedimentary rocks,
2) in igneous rocks.

1) In sedimentary rocks you have to look for fine grained sedimentary rock, a fine grained sandstone or finer. The typical sand found at the beach will not work, that is too coarse.

2) Igneous rocks: Crystallization of magnetite in lavas would be very useful. Crystallization of magnetite in plutonic rocks will not work very well.

The finer the grain of magnetite in your rock, the better it will retain the paleomagnetic data you are looking for. If you have coarse magnetite it'll form different magnetic domains and it gets complicated.

You also have to consider and be aware of later hydrothermal events that can re-magnetize your rocks (by precipitating new magnetite, or by oxidizing magnetite to hematite or maghemite).

Other magnetic mineral to consider is pyrrhotite which is not as common. Maghemite is more common (but a lot less common than magnetite, fortunately) and is easy to re-magnetize, becoming a nightmare for paleomagnetists.

Answer 2

i think what you are referring to is the event where the direction of the earth's magnetic field reverses after some million years. it had happened a lot of times in the geological past. these rocks are mostly magnetite, capable of aligning their atoms with respect to the direction of the earth's magnetic field at the time of their formation and cooling

Answer 3

The seabed rocks and many mountains have revealed many facts of earth's magnetism..
for one, (and most spectacular) that it keeps changing its poles.
Well, the rocks are mainly of igneous rocks of lava origin .. In liquid form they are aligned by earth's field which solidifies and makes a layer of it in the bed (or in mountain)..
The rocks aree collected and are worked to find their age and the magnetic alignment they're having...
research Shows earth will reverse its polarity in next 100 or so years now..
(as it is weakening now at the poles)..

Answer 4

as far as I know the magnetism in rocks is formed when they cool from being lava. as they cool the atoms line up to face north and south in turn determining the field for that individual rock.