Fossil dating method
Rocks of this kind in the ancient record may very well have resulted from rapid uplift and continent collision.
Without absolute ages, investigators could only determine which fossil organisms lived at the same time and the relative order of their appearance in the correlated sedimentary rock record.Unlike ages derived from fossils, which occur only in sedimentary rocks, absolute ages are obtained from minerals that grow as liquid rock bodies cool at or below the surface.When rocks are subjected to high temperatures and pressures in collide, certain datable minerals grow and even regrow to record the timing of such geologic events.These units, called in their molten form, constitute major crustal additions.By contrast, crustal destruction occurs at the margins of two colliding continents, as, for example, where the subcontinent of India is moving north over Asia.Some method of correlating rock units must be found.
In the ideal case, the geologist will discover a single rock unit with a unique collection of easily observed attributes called a marker horizon that can be found at widely spaced localities.
For example, the presence of recycled bricks at an archaeological site indicates the sequence in which the structures were built.
Similarly, in geology, if distinctive granitic pebbles can be found in the sediment beside a similar granitic body, it can be inferred that the granite, after cooling, had been uplifted and eroded and therefore was not injected into the adjacent rock sequence.
Although with clever detective work many complex time sequences or relative ages can be deduced, the ability to show that objects at two separated sites were formed at the same time requires additional information.
A coin, vessel, or other common artifact could link two archaeological sites, but the possibility of recycling would have to be considered.
Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.