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.
This dating method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.
Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.
C., Washington, and Oregon, is being and has been formed along the Juan de Fuca spreading ridge (Figure 8.21).
We can see, for example, that the oldest part of the Juan de Fuca Plate that has not subducted (off the coast of Oregon) is just over 8 million years old, while the part that is subducting underneath Vancouver Island is between 0 and about 6 million years old.
The potassium-argon dating method has been used to measure a wide variety of ages.
The potassium-argon age of some meteorites is as old as 4,500,000,000 years, and volcanic rocks as young as 20,000 years old have been measured by this method.
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