Magnetism Help (page 2)
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.
In 1930, Motonori Matsuyama, a Japanese mathematician and physicist, began studying magnetism in rocks. He took a closer look at the reason some rocks pointed in one direction, while others pointed in another. Matsuyama studied magnetic anomalies and proposed the idea that they were the result of magnetic reversal .
When geologists took samples of lava flows in Hawaii and other places, they found that some lava samples contained grains with reversed polarity. This meant that thousands of years ago the northern magnetic pole was located where the southern magnetic pole is today and vice versa.
Polarity reversal is when the North Pole location and the South Pole location switch places.
The dating of lavas is possible through the use of radiometric methods using 40 K/ 40 Ar measurements. By using both radiometric dating and magnetic polarity measurements on ancient extruded lava layers, geologists were able to record the average between magnetic reversals. They found that, on average, the magnetic poles flip approximately every 200,000 years. By geological time the flip was overnight, but they actually happened over a gradual period of between 300 and 1000 years.
A magnetic switch begins with the reversal of an area of magma flow deep in the core of the Earth. As the switch area grows larger and more polar, the countercurrent works its way upward and begins to affect the magnetic currents in the crust and atmosphere. When this happens, areas of the Earth’s outer magnetic field begin to weaken. The countercurrent below cancels out the charges above.
Magnetic field strength or field intensity is the force applied to a magnetic pole at any point.
Weakened patches in a magnetic field are called anomalies . A magnetic anomaly may be high or low, subcircular, ridge-like, valley-like, or oval when studying a magnetic topographical map. The range of values of magnetic intensity over an anomaly or an area is called the magnetic relief .
The South Atlantic Anomaly is one of these weakened patches. In this area, the magnetic field is 30% weaker than other areas around the planet and it is growing. Geologists studying magnetic reversal over the past 10 years have used supercomputer programs, along with thousands of lava samples and the compass readings from British Naval officers’ journal notes from the past 300 years, to study magnetism. The result is an excellent prediction method of magnetic reversal.
These studies have revealed that the Earth is long overdue for a magnetic reversal. The last major reversal happened over 700,000 years ago. Knowing this, geologists now think that the South Atlantic Anomaly is the beginning of a magnetic switch. It will not happen in our lifetime, but probably sometime in the next 1000 years if the model holds true.
Magnetic polarity can be minor or major. The tectonic and environmental effects of a magnetic reversal are not known. Scientists are just starting to study and understand the implications of a planet-wide magnetic reversal.
Times of mostly normal polarity, like what we have today, or times of mostly reversed polarity, are called magnetic epochs or chrons . The Matsuyama Epoch , a major polar reversal around 0.5–2.5 million years ago, is named after Motonori Matsuyama.
As lavas from many magnetic epochs pile on top of each other, they build up layers with opposite magnetic polarities. Figure 6-8 shows how these reversed lava layers might look if you were to take a cross-sectional slice.
Fig. 6-8. Different lava layers contain igneous rock magnetized in reversed magnetic fields.
Igneous rock provides geologists with many clues to the wild and crazy actions of ancient and recent magmas as they blasted or slowly forced their way to the Earth’s surface in different magnetic fields. Studying these clues will help us better understand magma’s tricks and the Earth’s future.
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