Rock Magnetism in Earth's Crust and Mantle

The magnetic properties of rocks vary depending on their mineral composition and the physical conditions (temperature, pressure) they are subjected to. Understanding these differences is key to interpreting Earth's structure.

Upper Crust Rocks

Rocks found in the Earth's upper crust commonly contain significant amounts of iron-bearing minerals, such as magnetite (Fe₃O₄). At the temperatures and pressures typical of the crust, these minerals often exhibit strong magnetic properties. Specifically, they are generally ferromagnetic, meaning they can be strongly magnetized and are attracted to magnets.

Lower Mantle Rocks

In contrast, the conditions within the Earth's lower mantle are drastically different, characterized by extremely high temperatures and pressures. While iron is present in lower mantle minerals, these intense conditions tend to suppress strong magnetic ordering. Consequently, rocks in the lower mantle typically display paramagnetic behavior. Paramagnetic materials are only weakly attracted to magnetic fields and lose their magnetism when the field is removed.

Conclusion

Based on these distinct conditions and mineral behaviors:

• Upper crust rocks are predominantly ferromagnetic.
• Lower mantle rocks are predominantly paramagnetic.

Therefore, the correct description is that rocks in the upper crust and lower mantle are ferromagnetic and paramagnetic, respectively.