This Day In History
These methods have been used to
provide independent lower limits for the age of the Earth. Also, by
radiometrically dating these items we can cross-check the accuracy of
radiometric dating. This has shown that radiometric dating is in general
Tree Rings (dendrochronology)
a ring for each year of growth. Counting these rings gives an age for
the tree. This has been done for the the bristlecone pine trees, which grow very slowly and live up to 6,000
years. These trees grow in a very dry region near the California-Nevada border. Dead trees in this dry climate take many thousands of years to decay.
The tree ring patterns of living and dead trees can be compared and then
linked together into a continuous history or tree rings.
method the continuous rings have been dated back to 11,800 years ago.
A continuous count of
ice core layers exists back as far as 160,000 years. Volcanic
eruptions and other major geologic events can be used to correlated between ice
cores. Historical eruptions, such as Vesuvius can used to verify the accuracy
daily growth bands These can be used to tell how many days there were in a
year when the coral was growing. Modern corals have about 365 day-bands
per year. By measuring the rate at which Earth's rotation is slowing, it
can be determined how many days were in a year in times past. This method
predicts that 400 million years ago each year was about 400 days. Corals
that grew in formations radiometrically dated as 400 million years old, show
about 400 day-bands per year.
Varves are variations in sedimentary layers deposited underwater
caused by the change in the seasons. These are useful in deep lakes where
the bottom layer is not disturbed. Varves have been measured going back to about 35,000 years.
Green River Shale
Lake Suigetsu Algae
- Tiny one-celled algae bloom each year in Lake Suigetsu, Japan.
When they die, they sink to the bottom of the lake creating a thin, white
layer. This layer is then covered with the dark clay sediments that
settle to the bottom the rest of the year. This forms annual
alternating light and dark layers. These number up to 45,000 layers
representing 45,000 years. This dates have been verified against
carbon-14 dating, giving the same age. The fact that carbon-14 dating
and algae layering produce the same age is good evidence that these methods
are accurate, since the factors that affect one don't affect the other.
Discussion of various radiometric and non-radiometric dating methods