To unravel the story in the rocks, we rely on the Principle of Uniformitarianism which states that "the present is the key to the past". In other words, the processes we see creating and deforming rocks today were operating millions of years ago in much the same way. Today for example, lava erupting from volanic craters and fissures in Hawaii cools to form basalt, a dark volcanic rock (the photo at left from the Hawaiian Volcanic Observatory). From the Principle of Uniformitarianism we can assume that the basalt shown at right in northern Arizona was the result of similar volcanic activity- even though no one was around to see it.
Thus basalt and other volcanic (also called extrusive) rocks, are evidence of volcanic activity in the past. [LeftImage courtesy of the NPS]
