To reconstruct a volcano's history,
accurate ages must be determined for layers of ash and other deposits
formed by past eruptions. The two most commonly-used methods for
determining age are radiocarbon dating and tree-ring dating. Radio-Carbon
dating is used to determine the age of once living objects less than
30,000 years old.
When a plant or animal is alive,
its concentration of radioactive carbon is equal to the concentration of
radioactive carbon in the atmosphere.
But when an organism dies, the
amount of carbon-14 in its tissues decreases in measurable amounts. Since
the amount of carbon-14 in its tissues can be measured precisely and the
rate of decay is known accurately, the year it died can easily be
calculated.
With tree-ring dating, geologists
count the annual growth rings of stumps and trees rooted on top of recent
volcanic deposits. Then, they can figure out how old the deposits are.
These ages are approximate. They
do not account for the period of time required for trees to establish and
grow in new deposits.
Depending on the tree species, the
climate, and the type of deposit, tree establishment might take 1 to 20
years.
A more precise method of dating
volcanic deposits of recent age is to identify unusual growth patterns in
the annual rings of trees that were growing at the time of an
eruption.
Falling pumice can turn a stand of
giant Douglas fir trees into a ghost forest by bombarding their
crowns.
Because pumice is so rough and
abrasive, it can damage needles and strip them from limbs and
branches.
Trees lucky enough to survive a
shower of pumice may lose so many needles that they may grow very little
for several years following an eruption.
Under normal conditions, trees grow
a ring of new wood every year.
But when a tree is stressed because
of drought or volcanic eruption, it will add very little new wood during
the next few years. A sequence of very narrow rings marks these stressful
years.
Another method of tree-ring dating
can reveal the age of volcanic deposits in which dead trees are buried,
but no living trees are rooted.
By matching ring patterns of the
dead trees with ring patterns of living trees, an approximate date for
the eruption that buried the tree is established. This method of matching
ring patterns between trees, called cross-dating, is used extensively for
dating archaeological ruins.
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Mount St. Helens & Other
Volcanoes
Geological History
Before the Eruption
Mount St. Helens
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