Author Question: A rock that is 90 million years old comes from the ________ Period. A) Cretaceous B) Mesozoic ... (Read 50 times)

oliviahorn72 · **on:** Jul 16, 2018

A rock that is 90 million years old comes from the ________ Period.

A) Cretaceous
B) Mesozoic
C) Phanerozoic
D) Jurassic

Question 2

How do geologists recognize gaps in the rock record?

What will be an ideal response?

Eunice618 · **Reply #1 on:** Jul 16, 2018

Answer to Question 1

Answer: A

Answer to Question 2

Answer: Unconformities are former land or seafloor surfaces between rock layers that represent intervals not recorded in the local rock record. Angular unconformities separate sedimentary or volcanic rocks that are inclined at different angles. When an angular unconformity is present, the time required for tilting the lower rocks and eroding them to produce a new surface of deposition for the upper rocks is not represented in the geologic record. Disconformities separate sedimentary or volcanic rocks that are inclined at the same angle but are separated by an irregular erosion surface. This surface indicates a pause in deposition or erosion of the older rocks. Nonconformities separate sedimentary or volcanic rocks from underlying plutonic-igneous or metamorphic rocks. When a nonconformity is present, the time required to erode the materials that originally covered the plutonic and metamorphic rocks before the onset of the more recent sediment accumulation is not represented in the geologic record.