#Geologic Time Table
10 messages in this thread
How are they determining the age of the ocean basin rock?
That's out of my league and it has been a while, but I expect such methods as
K/Ar ratios and similar age dating techniques.
Well I don't even know what that dateing system is. All the matter on earth is
the same age.
Frank Reed did instruct me as to how uranium can change its apparent age in
respect to the ratio of lead to uranium.
The original age of the formation of the uranium cannot be determined though.
I was wondering how basalt and granites can be dated.
You might say all matter on earth is the same age (as the molecules or
molecular parts), but the crystals, grains, etc. that make up rock are of
different ages, i.e. when a lava flow is formed it takes on a certain ration of
minerals, and thus elements and many of these are unstable and break down at
certain rates (such as Uranium breaking down to Lead and all those nasty
radiocative things, and many other elements do the same thing). Because of
this, it is possible to measure the ratios of isotopes of U, Pb to each other,
Potassium to Argon, etc. to determine the ages of rocks. Thus, the igneous
rocks can all be dated because they formed by the each mineral crystallizing
out of a melt of some sort and have a determined ratio of one element to
another at the time of formation. Measure the amount of change and you can
determine the age.
I remember a Russian team a while back that went out onto the flows of a
volcano with a definite known age.
They use the normal dating process and were able to get just about any age of
rock from some side of the slope.
Isn't the dating of half lives of radioactive materials based on assuming that
there is an infinite amount of the radioactive parent at the formation of the
structure being dated? If some of the child material like lead in the case of
uranium is mixed in the ore at the formation then wouldn't we get a much older
date in that area?
Hi, Kamron–
If lead and uranium are both present when a rock is formed, the lead will
contain very little of the isotope lead 206, and by checking the amounts of the
other isotopes of lead, one can tell how much lead 206 must have been in the
original lead. Any excess lead 206 over that amount must be from uranium.
Also, they will often be in different minerals – if I find any lead 206 free of
other lead isotopes, I know that all of it came from uranium decay.
Once in a while, any dating method may give wild results. Something very
unusual, like a hydrothermal solution removing lead or adding uranium, perhaps?
But when many many samples all give dates that place certain strata at certain
dates, and that date fits the pattern from many more samples from other strata,
one can be confident it is valid. Oddball readings from a few weird pieces may
indicate unknown factors or invalid techniques, but too much of a small-scale
local surprise to help validate your theories when confronted with the
large-scale consistency of huge numbers of tests.
–Doug
Both the isotope ratios and the radio active isotope decay ratio methods are
completely accepted by the scientific community. Most problems and errors are
understood and recognized.
Generally the "accuracy" of these age dating methods are proven by other
methods of dating rocks.
Lanny
>> Both the isotope ratios and the radio active isotope decay ratio methods
are completely accepted by the scientific community. Most problems and errors
are understood and recognized.<<
Still is crude dating when you think of the even minor leaching that water
could do or heat could melt out the child of the radio active parent.
>>Generally the "accuracy" of these age dating methods are proven by other
methods of dating rocks. <<
What are the other ways that dating is done?
Hi, Kamron–
The geologists had worked out a basic time table before radioactivity was
discovered. They had sufficient confidence in these estimates that they argued
vigorously against Earth/Sun creation dates figured by the physicists near the
turn of the century, led by Lord Kelvin. Assuming gravitational contraction
was the major source of heat, the physicists dated both Earth and Sun as about
100 million years old.
But as radioactivity was studied, it was appreciated as a heat souce that kept
Earth warm more than contraction could, and as indicating an energy source for
the Sun beyond contraction also. Radioactive isotope dating eventually
confirmed the broad outlines of the geologists' time tables, giving them
greater precision, but within the uncertainty limits of the old estimates.
I am not sure of all the methods used, but I am sure observed sedimentation
rates had a big play in those old datings.
–Doug
>>> Both the isotope ratios and the radio active isotope decay ratio methods
are completely accepted by the scientific community. Most problems and errors
are understood and recognized.<<
Still is crude dating when you think of the even minor leaching that water
could do or heat could melt out the child of the radio active parent.>>
For most radiometric dating methods, there are techniques for verifying the
usefulness of the sample. One of the best methods, and the one I happen to
know something about is Argon-Argon dating, which is a method of Potasium Argon
dating . Using Argon-Argon in combination with single crystal methods, very
accurate and reliable dates can be obtained from the proper sample, and samples
can be tested to see if they are "good" independantly of the suspected age of
the sample.
>>Generally the "accuracy" of these age dating methods are proven by other
methods of dating rocks. <<
What are the other ways that dating is done? <<
Several ways. One is to compare the dates with known or reconstructed
biostratigraphy. Another is to check to see that the dates are "in order" (on
top of each other in the correct age-sequence). A recent (well, ten years ago
now) correlation that was very powerful in this regard is the Tulu Bor tuff in
East Africa. This is one of the most widespread and extensive tuffs in the
world (at least for the last 50 or so million years), and it fell over much of
East Africa and the Red Sea and beyond. The tuff shows up in all sorts of
geological strata, and is always found in a place that a tuff of that age
"makes sense" in terms of local stratigraphy. Best of all, it is found in the
Indian Ocean and Red Sea ocean cores, layered in among all those nice annual
varves, in the right position!
Cheers,
GTL
– Greg Laden, Written 06-Sep-1994 @ 21:21:19
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