Marine Geology Phần 3 docx

MAGNETIC SURVEYS

Geologists looking for a decisive test for seafloor spreading stumbled upon

magnetic reversals on the ocean floor. Recognition of the reversal of the geo￾magnetic field began in the early 1950s. In 1963, the British geologists Fred

Vine and Drummond Mathews thought that magnetic reversal would be a

decisive test for seafloor spreading. Experiments using sensitive magnetic

recording instruments called magnetometers towed behind ships over the

midocean ridges (Fig. 40) revealed magnetic patterns locked in the volcanic

rocks on the seafloor.These patterns alternated from north to south and were

mirror images of each other on both sides of the ridge crest. The magnetic

fields captured in the rocks also showed the past position of the magnetic poles

as well as their polarities.

As the iron-rich basalts of the midocean ridges cool, the magnetic fields

of their iron molecules line up in the direction of Earth’s magnetic field at the

time of their deposition. As the ocean floor spreads out on both sides of the

ridge, the basalts solidify.They establish a record of the geomagnetic field at

each successive reversal, somewhat like a magnetic tape recording of the his￾Figure 40 A crew

member lowers a

magnetometer over the

stern of the oceanographic

research ship USNS

Hayes.

(Photo courtesy U.S. Navy)

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Marine Geology

tory of the geomagnetic field. Normal polarities in the rocks are reinforced by

the present magnetic field, while reversed polarities are weakened by it.This

process produced parallel bands of magnetic rocks of varying width and mag￾nitude on both sides of the ridge crest (Fig. 41). Here at last was clinching

proof for seafloor spreading. In order for the magnetic stripes to form in such

a manner, the ocean floor had to be pulling apart.

Two or three times every million years, Earth’s geomagnetic field

reverses polarity, with the north and south magnetic poles switching places.

Over the last 4 million years, the field reversed 11 times. Over the last 170 mil￾lion years, Earth’s magnetic field has reversed 300 times. No reversals occurred

during long stretches of the Permian and Cretaceous periods. Furthermore, a

sudden polar shift of 10 to 15 degrees occurred between 100 million and 70

million years ago.

Since about 90 million years ago, reversals have steadily become more fre￾quent, and the polar wandering has decreased to only about 5 degrees.The last

time the geomagnetic field reversed was about 780,000 years ago, and Earth

appears to be well overdo for another one.The magnetic field in existence 2,000

years ago was considerably stronger than it is today. Earth’s magnetic field seems

to have weakened over the past 150 years, amounting to a loss of about 1 per￾cent per decade.If the present rate of decay continues,the field could reach zero

and go into another reversal within the next 1,000 years or so.

Figure 41 Magnetic

stripes on the ocean floor

are mirror images of each

other and indicate that the

ocean crust is spreading

apart.

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Marine Exploration

Midocean ridge

The magnetic stripes also provided a means of dating practically the

entire ocean floor.This is because the magnetic reversals occur randomly and

any set of patterns is unique in geologic history (Table 6).The rate of seafloor

spreading was calculated by determining the age of the magnetic stripes by

dating drill cores taken from the midocean ridge and measuring the distance

from their points of origin at the ridge crest. During the past 100 million

years, the rate of seafloor spreading has changed little. Periods of increased

acceleration were accompanied by an increase in volcanic activity. During the

past 10 to 20 million years, a progressive acceleration has occurred, reaching a

peak about 2 million years ago.

The spreading rates on the East-Pacific Rise are upward of 6 inches per

year, which results in less topographical relief on the ocean floor.The active

tectonic zone of a fast-spreading ridge is usually quite narrow, generally less

that 4 miles wide. In the Atlantic, the rates are much slower, only about 1 inch

per year. This allows taller ridges to form. Calculating the rate of seafloor

spreading for the Atlantic indicates that it began to open around 170 million

years ago—a time span remarkably concurrent with the estimated date for the

breakup of the continents.

SATELLITE MAPPING

In 1978, the radar satellite Seasat (Fig. 42) precisely measured the distance to

the ocean surface over most of the globe. Buried structures beneath the ocean

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Marine Geology

TABLE 6 COMPARISON OF MAGNETIC REVERSALS WITH

OTHER PHENOMENA (DATES IN MILLIONS OF YEARS)

Magnetic Unusual Meteorite Sea Level Mass

Reversal Cold Activity Drops Extinctions

0.7 0.7 0.7

1.9 1.9 1.9

2.0 2.0

10 11

40 37–20 37

70 70–60 65

130 132–125 137

160 165–140 173