Question

explaining how to date sedimentary layers
how would a geologist use absolute dating to determine the age of sedimentary layers?

• by dating the age of several radioactive elements in a sedimentary rock layer
• by dating the age of intrusions and extrusions near a sedimentary rock layer
• by comparing the relative ages of several sedimentary rock layers
• by identifying fossils in nearby intrusion and extrusions and determining their age

Explanation:

To determine the age of sedimentary rock layers using absolute dating, geologists use the principle that sedimentary rocks often contain intrusions (like igneous rock formed within) or extrusions (igneous rock formed on the surface) nearby. By dating these intrusions and extrusions (which can be dated with absolute methods like radiometric dating), and knowing the relative timing of the sedimentary layer to these igneous features (sedimentary layers are older than intrusions that cut through them, and younger than extrusions that cover them, or vice versa depending on context), they can infer the age of the sedimentary layer. Let's analyze each option:

• Option 1: Dating radioactive elements in sedimentary rock is not reliable for absolute dating of sedimentary rocks because sedimentary rocks are composed of particles from other rocks, so the radioactive elements in them are not in a closed system for dating (they are detrital).
• Option 2: Dating intrusions and extrusions near the sedimentary layer is a valid method. Intrusions (igneous rock that intruded into the sedimentary layers) and extrusions (igneous rock that erupted onto the surface and then sedimentary layers might be deposited on top or below) can be dated with absolute methods (like radiometric dating). Then, using the relative relationship between the sedimentary layer and these igneous features (e.g., a sedimentary layer below an intrusion is older than the intrusion; a sedimentary layer above an extrusion is younger than the extrusion), the age of the sedimentary layer can be constrained.
• Option 3: Comparing relative ages of sedimentary layers is relative dating, not absolute dating.
• Option 4: Identifying fossils and dating intrusions/extrusions is a mix, but the key for absolute dating of the sedimentary layer itself (indirectly) is dating the associated igneous features (intrusions/extrusions), not just identifying fossils (fossils are used in relative dating via biostratigraphy).

So the correct option is the one about dating intrusions and extrusions near the sedimentary rock layer.

Answer:

by dating the age of intrusions and extrusions near a sedimentary rock layer