The Earth's magnetic field is generated mainly by the movement of molten metal in its:

The Earth's magnetic field is produced mainly by the movement of molten, electrically conducting metal in the liquid outer core. This process is called the geodynamo.

Layers of the Earth's interior:

• Crust: the thin, solid outer layer.
• Mantle: a thick layer of hot, mostly solid rock that can flow very slowly.
• Outer core: a liquid layer of molten metal, about 2,200 km thick.
• Inner core: a solid metal ball at the centre, with a radius of about 1,220 km.

How the geodynamo works:

• Heat escaping from the core, and the cooling and slow growth of the solid inner core, cause the molten metal in the outer core to move in large convection currents.
• The Earth's rotation twists these currents into spiral patterns, because of the Coriolis effect.
• Moving electrically conducting liquid generates electric currents, and these currents produce a magnetic field.
• The magnetic field in turn affects the flow, so the process is self-sustaining, like a dynamo.

Evidence that the outer core is liquid:

Seismic S-waves (secondary waves) cannot travel through liquids.

These waves do not pass through the outer core, creating a "shadow zone" on the opposite side of the Earth after an earthquake. This shows that the outer core is liquid.

Importance of the magnetic field:

• It forms the magnetosphere, which shields the Earth from the solar wind and cosmic radiation.
• It allows compasses to work and helps many animals, such as birds and turtles, to navigate.

Changes in the magnetic field:

The magnetic poles slowly move. The north magnetic pole has been shifting from Canada towards Siberia.

The field has reversed many times in the Earth's history; the last full reversal happened about 7,80,000 years ago.

Why not the other options?

• ✗The crust and upper mantle are mostly solid rock and do not produce the main magnetic field.
• ✗The inner core is solid; it influences the field but does not generate it by flowing.