Age dating rocks
So the Cretaceous sandstone bed in first figure was deposited during the middle part of the Cretaceous, not at the beginning or end.The discovery of new data may cause the numbers deﬁning the boundaries of periods to change, which is why the term numerical age is preferred to absolute age.Why don’t we ﬁnd rocks with ages between 4.03 and 4.57 Ga in the Earth’s crust?Geologists have come up with several ideas to explain the lack of extremely old rocks.Geologists consider 4.57-Ga meteorites to be fragments of planetesimals like those from which the Earth ﬁrst formed.Thus, these dates are close to the age of the Earth’s birth, for models of the Earth’s formation assume that all objects in the Solar System developed at roughly the same time from the same nebula.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.
The number 4.57 billion is so staggeringly large that we can’t begin to comprehend it.
We must answer this question if we want to add numerical ages to the geologic column.
Geologists obtain dates for sedimentary rocks by studying cross-cutting relationships between sedimentary rocks and datable igneous or metamorphic rocks.
The route to a solution didn't appear until 1896, when Henri Becquerel announced the discovery of radioactivity.
Geologists immediately realized that the Earth’s interior was producing heat from the decay of radioactive material.