Certain minerals, the most important of which is magnetite , can become permanently magnetized. This comes about because the orbiting electrons around a nucleus form an electric current and create a very small magnetic field. Figure 6-5 shows how orbiting electrons set up an electric field. A magnetic field is the space through which the force or influence of a magnet is applied.
Fig. 6-5. Electrons orbiting a nucleus create a magnetic field.
Above a temperature called the Curie point or temperature , the thermal excitement of atoms does not allow them to become permanently magnetized. They are too busy zipping around to slow down and allow magnetism to take place.
The temperature above which all permanent magnetism is destroyed is called the Curie point or temperature.
Curie ’ s law , named after Pierre Curie, who with his wife, Marie, received Nobel prizes in chemistry for their work with radioactive elements. Curie’s law describes the ability of an element to be magnetized as inversely proportional to the absolute temperature. In other words, the hotter it gets, the more the atoms get excited and the less likely magnetism is to occur.
The Curie point for magnetite is 500°C. Any temperatures higher than that cause atoms to get excited and vibrate wildly in no particular direction. This random dancing around causes the atoms’ electrical currents to cancel each other out instead of lining up and forming a stable field.
When the temperature is less than 500°C, small “islands” of electric current in a solid stabilize and reinforce each other. When an external magnetic field is nearby, all the magnetic islands in a solid, parallel to the magnetic field, become larger and expand, taking over the neighboring, nonparallel islands. In no time, parallel islands of current form a “continent” of electric current and a permanent magnet is created.
This is true for cooling lava. All the minerals crystallize at temperatures above 700°C, a lot higher than the Curie points of any of the magnetic lava minerals. As crystallized lava slowly cools, its temperature drops below 500°C, the Curie point for magnetite. When this happens, all the magnetite grains in the rock turn into tiny permanent magnets. They are affected by the much greater magnetic field of the Earth.
Geologists have discovered from core samples of ancient lava and modern-day lava flows that the magnetic poles of the magnetite grains in the lava sample have the same magnetic inclination as the Earth’s magnetic field.
Figure 6-6 shows how the magnetic field “islands” look above and below 500°C.
Fig. 6-6. Atoms in tiny “islands” cause the formation of more magnetic islands.
The magnetic poles of the magnetite grains in the lava will align in the same direction as the Earth’s magnetic field. When lava samples are collected, they have unique magnetic polarities depending on the time and the magnetic field of the Earth that was in place when they were formed. The magnetic signature of a lava’s formation will stay the same as long as the lava exists. The signature will be the same as when the lava’s cooling temperature passed 500°C.
Figure 6-7 shows the difference that an external magnetic field makes when the magnetite grains are below the 500°C Curie point.
This study of the magnetism of lava crystals allowed geologists to understand the changes that took place during the development and cooling of the Earth.
When the cooling of lava results in the creation of permanent magnetism, it is called thermoremanent magnetism.
No magnet is permanently magnetic. Over time, it loses magnetism. This is called the magnetic relaxation time . Permanent magnets have very long relaxation times.
Relaxation of magnets is affected by many things including the following:
- mineral composition,
- grain size,
- neighboring minerals, and
- the strength of the original magnetization.
The time it takes for a magnet to lose its magnetic ability is called the relaxation time.
In order for geologists to determine relaxation times and magnetization ages of rock samples, a few of the magnetic grains (perhaps those that were somewhat weaker to start with), must have already relaxed beyond the age of the first magnetization. Measuring the relaxation times of rock samples in the laboratory is performed as a temperature function.
Many igneous rock samples have relaxation times much greater than the magnetization age. These samples, collected from ancient, exposed lava flows around the world, are used to figure out where the magnetic poles were located thousands of years ago.
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