Again, we do not know, so we need to make an assumption. It is only after we have made these assumptions that we can calculate an ‘age’ for the rock.
In most situations we don’t know since we didn’t measure it, so we need to make an assumption—a guess.This is one of the rare instances in which, to the question, ‘Were you there? ’ The dating method Dr Austin used at Mount St Helens was the potassium-argon method, which is widely used in geological circles.It is based on the fact that potassium-40 (an isotope or ‘variety’ of the element potassium) spontaneously ‘decays’ into argon-40 (an isotope of the element argon).2 This process proceeds very slowly at a known rate, having a half-life for potassium-40 of 1.3 billion years.1 In other words, 1.0 g of potassium-40 would, in 1.3 billion years, theoretically decay to the point that only 0.5 g was left.Today, resistant volcanic flows cap mesas such as Grand Mesa, White River Plateau, Raton Mesa, and those near Basalt, Colorado.In the southwestern part of the state, ash-flow tuffs cover thousands of square miles.Smaller plutons take a variety of shapes, each with its own name, such as stocks, plugs, dikes and sills.Magma that reaches the surface forms a variety of volcanic landforms and deposits.It is made of dacite, a fine-grained volcanic rock that contains a sprinkling of larger, visible crystals, like chopped fruit in a cake.Actually, the present lava dome at Mount St Helens is the third dome to form since the 1980 eruption, the previous two having been blasted away by the subsequent eruptions.However, this generalized map still gives a good feel for the distribution and ages of the intrusions.Radioisotope dating conveys an aura of reliability both to the general public and professional scientists. In most people’s minds it is the best ‘proof’ for millions of years of Earth history. In August of 1993, with geologist Dr Steven Austin and others from the Institute for Creation Research, I climbed into the crater of Mount St Helens to view the lava dome.