Advantages of relative dating in geology
Most experts concur that while superior to relative dating practices, even absolute dating cannot truly be "absolute"; therefore, site and artifact dating--even when absolute methodologies are used--can only be estimated within a few hundred years or so.
Arguably the most significant archaeological and paleontological breakthrough of the 20th century, the discovery of carbon-14 dating makes it possible for these scientists to learn the age of artifacts containing organic matter--including all flora and fauna--that has been dead for several centuries to as many as 50,000 years.
[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.
For example, the Juan de Fuca Plate, which lies off of the west coast of B.
The longest such records can take us back over 25 ka, to the height of the last glaciation. By studying magnetism in volcanic rocks that have been dated isotopically, geologists have been able to delineate the chronology of magnetic field reversals going back for some 250 Ma.
(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.
By carefully matching the sea-floor magnetic stripes with the known magnetic chronology, we can determine the age at any point on the plate.
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.
Dendrochronology has been used to date the last major subduction zone earthquake on the coast of B. When large earthquakes strike in this setting, there is a tendency for some coastal areas to subside by one or two metres. These periods of consistent magnetic polarity are given names to make them easier to reference.
Seawater then rushes in, flooding coastal flats and killing trees and other vegetation within a few months. The current normal magnetic field, known as Brunhes, has lasted for the past 780,000 years.