Final answer:
The direction of Earth's magnetic field when a rock cools below its blocking temperature is recorded through the permanent alignment of magnetic minerals in the rock. This allows geologists to study the rock's paleomagnetic properties to determine its age and the history of Earth's magnetic field changes.
Step-by-step explanation:
The direction of Earth's magnetic field when a rock reaches its blocking temperature is recorded through the orientation of magnetic minerals in the rock. As rocks form, particularly igneous rocks, magnetic minerals within them align with the Earth's magnetic field at the time of their crystallization. When these minerals cool below what is known as the Curie temperature or blocking temperature, they lock in the direction and intensity of the magnetic field at that specific time. This process is known as thermoremanent magnetization.
Using these principles, geologists can study the paleomagnetic properties of ancient rocks to understand past directions of the Earth's magnetic field. Additionally, similar processes occur in sedimentary rocks, where sediments can become aligned with the magnetic field while they are being deposited. The information gathered from these rocks can be matched with the known historical record of Earth's magnetic field reversals and changes, which can then be used to estimate the age of the sediments, a method known as magnetostratigraphy.
This paleomagnetic data provides crucial insights into the historical changes in the Earth's magnetic field and helps geologists correlate and date geological formations, in a way somewhat analogous to using a compass to map out the magnetic field lines around a magnet, a method that reveals the orientation of magnetic fields in a given space.