Magnetic Rocks

Most of the rocks that we find on the Earth's surface have some iron atoms among their constituents. Rocks such as magnetite (three atoms of iron joined to four atoms of oxygen—Fe3C>4) contain so much iron that they are noticeably attracted to a magnet placed near their surface. On a microscopic scale, such iron-rich rock materials show tiny separated magnetic domains in which

Magnetic Rocks
FIGURE 2.7 ► The satellite magnetometer is located at the end of a long boom to avoid the noisy magnetic fields from satellite electrical systems.

all the atoms with magnetic properties are aligned in a single direction. When most of these domains show a similar directional alignment, the rock is said to be magnetized (Figure 2.8). If such a magnetized rock is suspended with a thread near the middle of its long axis, the rock will align itself north-south as a compass needle does. Scientists studying the structure of magnetic rocks have been able to create new materials in which the field domain and boundary regions (Figure 2.8) have been modified to maximize and concentrate the magnetization. Such materials find use in everything from refrigerator magnets to temporary fasteners to industrial magnets for production-line service in manufacturing.

For each magnetic material at temperatures above a specific high level called the Curie Temperature (about 500 to 800° Centigrade or 932 to 1472° Fahrenheit), the microscopic magnetic domains become randomly oriented due to the heat so that the rock material loses its magnetization. Paleomag-

Magnetic Poles Reversal Lava Flows
FIGURE 2.8 ► Areas enclosed by curved lines indicate the microscopic magnetic domains in a rock before and after magnetization. Arrows show the dipole field alignment within the domains.

neticians study the ways that rocks become naturally magnetized and what such rocks reveal about the paleo years of Earth formation.

Pieces of iron and those rocks that contain a considerable amount of iron atoms (called ferrous atoms) can be artificially magnetized several ways:

1. By heating them and then letting them cool to below the Curie Temperature in a magnetic field,

2. By placing them adjacent to an extremely strong magnetic field (early sailing ships always carried a strong loadstone for the occasional re-magnetization of the ship's compass needle), and

3. By sudden jarring so that the magnetic domains realign with the Earth's strong local natural field—while holding the long axis of the material along the direction shown by a compass. Jarring is probably what accidentally magnetized the iron clock weights of my grandfather clock (see Section 1.3.2, p. 14). Try magnetizing an iron file by aligning its long axis with the Earth's main field direction and then sharply hit the end of the file with a hammer.

Rocks about the Earth are often found to be naturally magnetized. Although geophysicists, who study these rocks, continue to discover new ways that this remanent (leftover) magnetization occurs in nature, let us pause in our tour to look at how most natural rock magnetization arises.

Hot lava (magma from deep within the Earth) is at temperatures higher than the Curie Temperature and therefore composed of many randomly oriented magnetic domains. As this liquid rock material cools into igneous

FIGURE 2.9 ► Remanent magnetism of igneous rock results from the cooling of hot volcanic lava, which preserves a record of the local main field at the time the magma hardens.

(formed-in-heat) rock in the Earth's main field, many of the magnetic domains align themselves with that local field (Figure 2.9). The rock thus formed is said to have a remanent magnetism indicative of the Earth's field at the time of the cooling—which may have been many thousands of years ago. Using either special radioactive dating techniques or historical information on the volcanic eruption to identify the age of the cooled magma, the paleomagneti-cian measures the rock sample to establish the ancient paleofield direction.

Fine rock dust is dissolved in the water of streams and lakes. Such dust often has the remains of magnetic domains that were jointly oriented in their earlier rock formation. While moving with the water, the overall alignment of the many particles is, at first, scrambled by the water currents. The rock dust eventually settles to the bottom and is gradually compacted to form sandstone and mudstone. The particles have time to align their magnetic domains with the Earth's local magnetic direction of that formation period for the sedimentary (formed by settling) rock. Often many layers of these rocks are subsequently exposed by natural land uplift (Figure 2.10) or road cuts. When

Magnetic Rock

FIGURE 2.10 ► Iron is responsible for the red color of these spires at Bryce Canyon, Utah. The pictured formations were deposited in lakes that existed about 70 million years ago. Sedimentary layers at the base of these formations were created about 150 million years ago by a shallow sea. Laboratory measurements of magnetic fields from mud-, silt-, and sandstone rock samples can reveal the direction of the Earth's field that existed at the time of each layer formation.

FIGURE 2.10 ► Iron is responsible for the red color of these spires at Bryce Canyon, Utah. The pictured formations were deposited in lakes that existed about 70 million years ago. Sedimentary layers at the base of these formations were created about 150 million years ago by a shallow sea. Laboratory measurements of magnetic fields from mud-, silt-, and sandstone rock samples can reveal the direction of the Earth's field that existed at the time of each layer formation.

the paleomagnetician measures the remanent field of a vertical series of such rock samples, he or she can determine the Earth's ancient field direction corresponding to the time that each sedimentary rock layer was formed. Scientists have also identified metamorphic (form-changed) rocks in which gradual physical and chemical changes over time have altered their rock structure and composition along with their remanent magnetization.

Magnetic rock materials also exist in the clay used for bricks and pottery. At high baking kiln temperatures, the Curie Temperature level of ferrous clay is exceeded. Upon cooling and hardening, the randomly oriented magnetic domains in the clay become magnetized along the Earth's local field direction. Because the bricks are fired horizontally and the molded clay pots are usually fired in an upright or upside-down position, archaeologic relics can reveal the geomagnetic main field dip angle (angle of the field from the horizontal plane) at the time of pot firing (Figure 2.11). Studies of this type are called

FIGURE 2.11 ► A Mayan pot from Mexico in which the local magnetic field was preserved during the original firing of the clay.

archaeomagnetism because of the importance to those specialists interested in ancient man-made (archaeological) structures.

2.1.3 Prehistoric Fields and Continental Drift

Often when scientists can date rock specimens from other evidence, the magnetization of rock samples from layers at the sample site (formed over a long period of time) can reveal the history of the Earth's changing field direction. Because the main geomagnetic field has a dipole-like field pattern, the field makes a unique angle (dip) with the Earth's surface at each latitude on our globe. That angle determines the north or south magnetic latitudes and the apparent magnetic polar locations at the time of magnetization (recall Figure 1.5). After allowing for continental drift, the field direction evidence shows a continuous westward movement of the magnetic poles (with respect to the Earth's north and south geographic spin-axis poles) over millions of years. In addition, the geomagnetic main field has, on many occasions, completely reversed its direction. Now, armed with rather accurate charts of the ancient field behavior, rocks that cannot be dated in the laboratory by radio-

Polarity

(black = same as present; white ■ reversed from present)

Was this article helpful?

+1 0

Post a comment