Volcanic rock is also known as igneous rock
These are rocks that were once very hot and which cool down to become very hard and resistant to erosion. They begin below the earth’s crust, and cool on reaching air. The dating method used to set dates is the single main foundation of all linear time claimed to have elapsed on earth ‘since its claimed formation’ over 4 billion years ago.
Yet it is a method that has serious problems that can make it completely unreliable, and quite unfit for such an important use as confirming these long ages. To start with, there are no rocks older than a claimed age of 3 billion years or so, found in Greenland for example. In fact the method, and the linear formation of earth itself, relies on several key assumptions invented over 350 years ago.