Geologists agree that these volcanoes erupted only thousands of years ago, spilling lava into an already eroded Grand Canyon, even temporarily damming the Colorado River.Rocks from these lava flows have been dated by the same rubidium-strontium isochron method used to date the Cardenas Basalt, giving an "age" of 1.34 billion years.
It is based on the fact that potassium-40 (an isotope or "variety" of the element potassium) spontaneously "decays", becoming argon-40 (an isotope of the element argon).
To date rocks or other objects, scientists typically use radiometric dating.
In short, the ratio of radioactive and stable isotopes in the sample are determined and the measured rate at which the isotopes decay is used as an indicator of the age of the sample.
During 17 so-called dome-building eruptions, from October 18, 1980 to October 26, 1986, thick pasty lava oozed out of the volcanic vent much like toothpaste from a tube.
Dacite lava is too thick to flow very far, so it simply piled up around the vent forming the mountain-like dome, which now sits as a plug over the volcanic orifice.
Isotopic Fractionation is a physical separation of isotopes and a non-radioactive source of isotope ratios.