[greenwolf]

It was only 75,000 years ago that Toba exploded and only about 2,000 humans worldwide survived.

That's sort of hard to buy off on. As I understand it you've got a couple of dating systems including dendrochronology and radiocarbon dating which will date things from the present back to about 30,000 years ago and then you've got decay type dating systems which will let you fix approximate dates in millions of years, but I'm not really aware of anything which will fix dates in the 50,000 - 100,000 year old range.

[blam]

The main methods used to assigned to pre-historical volcanic deposits are:

dendrochronology
Carbon-14
radioisotopes (radiocarbon and isotope dating)
magnetostratigraphy