Age dating rocks
And if a datable volcanic ash buried the strata, then the strata must be older than the ash.
rocks and sedimentary rocks or for layers of datable volcanic rocks inter-bedded with sedimentary rocks.
They argued that physical processes that shape the Earth and form its rocks, as well as the process of natural selection that yields the diversity of species, all take a very long time.
Geologists and physicists continued to debate the age issue for many years.
(Recall that “Ga” means “billion years ago.”) Individual clastic grains of the mineral zircon have yielded dates of up to 4.4 Ga, indicating that rock as old as 4.4 Ga did once exist.
On this scale, the oldest rocks preserved on Earth date from early February, and the ﬁrst bacteria appear in the ocean on February 21.
For example, if we ﬁnd a sequence of sedimentary strata deposited unconformably on a datable granite, the strata must be younger than the granite (figure above).
If a datable basalt dike cuts the strata, the strata must be older than the dike.
One idea comes from calculations deﬁning how the temperature of our planet has changed over time.
These calculations indicate that during the ﬁrst half-billion years of its existence, the Earth might have been so hot that rocks in the crust remained above the closure temperature for minerals, and isotopic clocks could not start “ticking.” Another idea comes from studies of cratering events on other moons and planets.