Re: How do we know how much radioactive isotope is in fresh volcanic rock?

The question you ask is one of the most important in the science of 
geochronology, or the study of the age of rocks and the earth.  As you have 
realized, unless we know the original isotopic composition of a rock, we 
can't figure out its age.  First, for some background....

First, the most commonly used geologic dating techniques are 
potassium-argon, uranium-lead, rubidium-strontium, and samarium-neodymium. 
 In each case, the "parent" isotope is what's called primordial; it was 
manufactured in a supernova explosion before the earth formed and 
transported to the pre-solar nebula by being blown through space.  By 
studying meteorites that formed at this time, we can come up with good 
estimates of the chemical and isotopic composition of the solar nebula and 
make some guesses about the composition of the bulk earth.  Ever since that 
time, these have been decaying away.  That means that radiation levels on 
the early earth were higher than they are now, too.

To date a rock, you need to have something that has remained "closed" to 
both parent and progeny isotopes since the rock formed.  This means that 
the rock or mineral has not lost any of these atoms (or not many), making 
it possible for us to count the number of atoms of both parent and progeny 
nuclide in a mass spectrometer (we don't use geiger counters for this).  By 
counting the number of, say, K-40 and Ar-40 atoms in a piece of feldspar, 
we come up with the atomic ratio and, using some math, can figure out the 
age of that rock.  So, in this case, we don't need to know how much 
potassium was in the rock to begin with, we just need to know how much has 
decayed to Ar.

You can find out more about isotopic dating and the assumptions behind it 
in the following books:

The Age of the Earth, G. Brent Dalrymple, Stanford University Press, 1991 
(an excellent book)

Principles of Isotope Geology, Gunter Faure, MacMillan Press, 1990 (written 
by my graduate school advisor and the classic college-level textbook on the 
subject)

Radiogenic Isotope Geology, Allan Dickin, Cambridge University Press, 1995 
(the most recent and best addition to the college textbook collection)