The age of a rock is determined through the analysis of isotopes—minute amounts of radioactive elements in the rocks. The most common methods for dating rocks include U/Pb: analyzing for the amount of lead left behind by the steady decay of radioactive uranium, or K/Ar dating: measuring the amount of argon remaining from the steady decay of radioactive potassium.
With advances in technology, new ways of determining the ages of rocks, or even their cooling and exposure histories are available. They include the two techniques used by Matthew Morriss in his study of Hells Canyon: uranium + thorium /Helium dating, and cosmogenic radionuclide dating.
The uranium plus thorium/helium technique is used to determine the rate of uplift of a region. The measurement can tell how long ago a mineral grain (usually the uranium-rich mineral, apatite) has cooled to a temperature below 60 degrees Celsius (140 F). This temperature corresponds to burial beneath less than a half-mile of crustal rock. Both uranium and thorium decay to (among other things) the gas, helium. At temperatures above 140 degrees F, the helium can escape from the mineral as it forms. But below 140 F. the helium is trapped in the mineral. So, measure the amount of helium and bingo, you can figure out how long the mineral has been cooler than 140F and hence buried less than a half-mile beneath the surface.
Cosmogenic radionuclides are elements produced when cosmic rays bombard the surface of a mineral grain. The best-known system if the change of oxygen (O16) to a form of the element beryllium Be10. Cosmic rays also interact with silicon (Si28) to produce a form of aluminum. (Al26) Both oxygen and silicon are abundant in rocks, especially as the mineral quartz. When a rock is on the surface, exposed to cosmic rays, both oxygen and silicon react, producing very small quantities of beryllium and aluminum—at different rates. But as soon as the rock is covered by more than six feet of sediment (or washed into a cave) production stops and the ration of Be10 to Al26 can be used to determine the time since its burial.