Advantages of relative dating in geology
Figure 8.21 The pattern of magnetism within the area of the Juan de Fuca Plate, off the west coast of North America.
Chemists have established the rate at which radioactive decay occurs for particular chemical isotopes and isotope sequences, so researchers now can estimate age as precisely as possible by examining how much radioactive material is left in a discovery.
Evidence from a major tsunami that struck Japan on January 27, 1700, narrowed the timing of the earthquake to sometime in the evening of January 26, 1700. Chapter 9 has a discussion of Earth’s magnetic field, including where and how it is generated and why its polarity changes periodically.
(For more information, see https://ca/earle/1700-quake/) Figure 8.18 Sites in Washington where dead trees are present in coastal flats. Figure 8.20 Depiction of the formation of magnetized oceanic crust at a spreading ridge.
At all of the locations studied, the trees were found to have died either in the year 1699, or very shortly thereafter (Figure 8.18). As it cools, tiny crystals of magnetite that form within the magma become aligned with the existing magnetic field and then remain that way after all of the rock has hardened, as shown in Figure 8.20.
On the basis of these results, it was concluded that a major earthquake took place in this region sometime between the end of growing season in 1699 and the beginning of the growing season in 1700. Crust that is forming today is being magnetized in a “normal” sense, but crust that formed 780,000 to 900,000 years ago, in the interval between the Brunhes and Jaramillo normal periods, was magnetized in the “reversed” sense.