How do unconformities make it difficult to determine ages in rock layers?

Answer 1

Sedimentary rock is composed of the weathered remains of a variety of rocks of all ages. It would therefore only be possible to date specific components of a sample, none of which would equate to the age of the sedimentary rock as a unit. Sedimentary rock is dated by its association in the geologic column with igneous and metamorphic rock which can be dated by radioactive dating techniques.

Answer 2

Sedimentary rock can only rarely be dated directly by radiometric means. A sedimentary rock may include particles that contain radioactive isotopes, but the rock's age cannot be accurately determined because the grains making up the rock are not the same age as the rock in which they occur. Rather, the sediments have been weathered from rocks of diverse ages.

Because of geologic events such as plate tectonics and volcanism, suitable material for radiometric dating, such as volcanic tuff, solidified lava, and igneous intrusions have been found as layers on, in, and cut through layers of sedimentary rock. Intrusions are always younger than the rock body they penetrate, meaning that the sedimentary rock in which intrusions are found will be older than a radiometrically dated sample of the intrusion. Lava flows and volcanic ash which form layers in rock will be younger than the rock below and older than the rock above. The solidified intrusions and lava flows can be dated with radiometric techniques.

The radiometric dating of suitable rocks (i.e. igneous or metamorphic) in proximity to their sedimentary counterparts, therefore allows the sedimentary rock to be dated as well.

How_do_you_determine_the_age_of_a_sedimentary_rock_or_a_fossil_contained_within_it

Answer 3

It is difficult to determine the absolute age of a sedimentary rock because sedimentary rocks are made up of diverse particles that are all different ages. As a result radioactive dating can only determine the age of the particles, not the whole rock.

Answer 4

Because most sedimentary rocks are composed of sediment which derived from many sources any dating process would find many different results. For example, a zircon from a sedimentary rock may have been formed in an igneous rock many hundreds of millions of years before. Testing the zircon would give the date of the igneous rock, not the sedimentary.

It is much safer to find the nearest contemporary igneous rock in the sequence and date that, even a thin layer of volcanic ash will do.

Answer 5

Because the rock formation is easily weathered away by water and wind. This causes parts and peices to be carried away, where they can reform into newer formations.

Answer 6

Unconformities represent missing layers which could have eroded or were not deposited.

Answer 7

because they are different