Paleomagnetism

Many rocks contain small amounts of magnetic and paramagnetic minerals that acquire a weak remnant magnetism at the time of crystallization of the rock. Thus igneous rocks preserve evidence of the direction of the Earth's magnetic field at the time they were formed. In some cases sedimentary rocks also preserve remnant magnetism. Studies of the remnant magnetism in rocks are known as paleomagnetism and were pioneered in the early 1950s by Blackett (1956) and his colleagues.

This paleomagnetic work demonstrated inconsistencies between the remnant magnetic field orientations found in old rocks and the magnetic field in which the rocks are found today. In some cases, corrections had to be made for the effects of the local tilting and rotation that the rocks had undergone since their formation. Even so, having taken into account these and other possible effects, discrepancies remained. Several explanations for these discrepancies were given: (1) variations in the Earth's magnetic field, (2) movement of the entire outer shell of the Earth relative to the axis of rotation (i.e., polar wander), and (3) continental drift.

The systematic variations in the remnant magnetism strongly favored the third explanation. When the remnant magnetic vectors for a series of rocks with different ages from the same locality were considered together, the orientation had a regular and progressive change with age, with the most recent rocks showing the closest alignment with the present field. This is shown graphically by plotting a series of "virtual magnetic poles." For each rock in the time series, a "pole position" is derived from its magnetic inclination and declination. If sufficient points are plotted, they form a curved line terminating near the present pole for the youngest rocks; when rocks in North America and Europe are compared, the opening of the North Atlantic is clearly illustrated as shown in Figure 1.12. In the late 1950s these studies were taken by their proponents as definitive evidence supporting continental drift

Figure 1.12. Polar wander paths based on observations from North America and Europe. Points on the path are identified by age in millions of years (C = Cambrian 540-510; S-D = Silurian/Devonian 440-290; Cu = Upper Carboniferous 325-290; P = Permian 290-245; Tr, l, u = Triassic, lower, upper 225-190; K = Cretaceous 135-65). The shapes of the paths are approximately the same until the Triassic when the continents began to separate.

Figure 1.12. Polar wander paths based on observations from North America and Europe. Points on the path are identified by age in millions of years (C = Cambrian 540-510; S-D = Silurian/Devonian 440-290; Cu = Upper Carboniferous 325-290; P = Permian 290-245; Tr, l, u = Triassic, lower, upper 225-190; K = Cretaceous 135-65). The shapes of the paths are approximately the same until the Triassic when the continents began to separate.

(Runcorn, 1956, 1962a). The opponents of continental drift argued that the results could be due to variations in the structure of the Earth's magnetic field.

There was another important result of the paleomagnetic studies. Although consistent and progressive changes in the magnetic inclination and declination were observed for each continent, the polarity of the remnant magnetic field was highly variable and in some cases agreed with the polarity of the present field and in others was reversed (Cox et al., 1963, 1964). The recognition that virtually all rocks with reversed polarity had formed within specific time intervals, regardless of latitude or continent, led to the conclusion that the reversed magnetic polarities were the result of aperiodic changes in the polarity of the Earth's magnetic field. These reversals were to play a key role in quantifying the seafloor spreading hypothesis, discussed in the next section.

Was this article helpful?

0 0

Post a comment