Advantages of relative dating in geology

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.

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.

One of the advantages of dendrochronology is that, providing reliable reference records are available, the technique can be used to date events to the nearest year. About 5 Ma of this record is shown in Figure 8.19, where the black bands represent periods of normal magnetism (“normal” meaning similar to the current magnetic field) and the white bands represent periods of reversed magnetism.

[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.

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922 – 923, 30 October 1997.] Changes in Earth’s magnetic field can also be used to date events in geologic history.

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.

This process typically involves measuring and calculating what remains of the chemical and/or physical properties naturally present in everything from rocks to human remains, and the most widely accepted methods of absolute dating rely on measuring the radioactive decay of naturally occurring elements and isotopes within an object or artifact.

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.

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