At Mesquite Mountain, the plateau ages for the hornblende indicate closure at around 80 Ma, the initial phase of thrusting and uplift.
Biotites and potassium feldspar have 0% Ar intercept ages around 20-30 Ma, indicative of early Miocene cooling during extension and unroofing in the Colorado River Extensional Corridor.
In addition, faunal succession and the use of "key" diagnostic fossils were used to correlate lithologic units over wide geographic areas.
Although lithologic units could be placed within a known sequence of geologic periods of roughly similar age, absolute ages, expressed in units of years, could not be assigned.
Ar dating is a major method that researchers have used to understand the structural evolution of the Maria Fold and Thrust Belt.
Argon-argon dating works because potassium-40 decays to argon-40 with a known decay constant. This led to the formerly-popular potassium-argon dating method.
Ar step-heating results from Mesquite Mountain and the Northern Granite Wash Mountains.
Knapp and Heizler (1990) took samples from the migmatitic Mesquite Gneiss in the Mesquite Mountain locality and from both the hanging wall and foot wall of multiple structures in the Northern Granite Wash Mountains.
Second, the sample is irradiated along with a standard of a known age. A major advantage of the argon-argon method is that the sample can be heated incrementally.
The first two dacite lava domes built within the crater (late June and early August 1980) were destroyed by subsequent explosive eruptions (July 22 and October 17).
He knew the nitrogen in air could be reacted with oxygen to form, ultimately, nitrous acid.
This process, known as "step heating", provides additional information on the age of the sample.
Reheating events and diffusion of argon from the boundaries of the grain can result in lower Ar date.
Sometimes, when a large amount of argon has been lost, this is not possible.