In addition to volcano caused cold weather and shadow, there are also layers of volcanic ash and lava. Those layers can be tested for age and things under or above them are older or younger. Where there are multiple layers separated by centuries, one can find and date old trees approximately and see the between eruption conditions recorded.
... there are also layers of [datable] volcanic ash and lava. ...
One of the greatest advancements in the 'scientific research process' during my lifetime, has been the cross-matching of multiple and independent dating sources. Use of this dendrochronology to cross-check with volcanic ejecta and ice-cores and human records and fossil records and geo-magnetism ...
This, by itself, has been an amazing tool for measuring our past, both recent and deep. Yes, it is susceptible to interpretation errors and corrections deriving from improved techniques but a real advance as a tool!