Advantages of relative dating in geology
There are numerous other techniques for dating geological materials, but we will examine just two of them here: tree-ring dating (i.e., dendrochronology) and dating based on the record of reversals of Earth’s magnetic field. The magnetic field makes compasses point toward the North Pole, but, as we’ll see in Chapter 10, this hasn’t always been the case.Dendrochronology can be applied to dating very young geological materials based on reference records of tree-ring growth going back many millennia. At various times in the past, Earth’s magnetic field has reversed itself completely, and during those times a compass would have pointed to the South Pole.If we find a rock with normal magnetism, we can’t know which normal magnetic interval it represents, unless we have some other information.Using Figure 8.19 for reference, determine the age of a rock with normal magnetism that has been found to be between 1.5 and 2.0 Ma based on fossil evidence.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.[SE] Magnetic chronology can be used as a dating technique because we can measure the magnetic field of rocks using a magnetometer in a lab, or of entire regions by towing a magnetometer behind a ship or an airplane.
(Note that in this diagram, sea-floor magnetism is only shown for the Juan de Fuca Plate, although similar patterns exist on the Pacific Plate.) [SE] places significant limits on the applicability of magnetic dating. Fw-300 .qstn-title #ya-trending-questions-show-more, #ya-related-questions-show-more #ya-trending-questions-more, #ya-related-questions-more /* DMROS */ .Less than a century ago, when archaeologists, geologists and other scientists sought to determine the age of artifacts, fossils, buildings or geological events, they turned to "relative" methods to approximate a date sequence they hoped was meaningful.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.