Lesson 3a: Relative Time

Question 1

Compare relative dates vs. absolute dates

Answer 1

Relative dates: place rocks in a sequence of formation that allows us to determine if something is older or younger than something else
Absolute dates: number of years since an event occurred thus we can say how old something is

Question 2

How do geologists determine relative dates?

Answer 2

Stratigraphy Principles

Question 3

Definition and example of Superposition

Answer 3

in an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.

Example: The Grand Canyon, Arizona

Question 4

Definition of Cross-cutting relationships

Answer 4

younger features cut across older features.

Question 5

Definition and example of Inclusions

Answer 5

fragments of one rock unit that are enclosed within another rock unit

Example: fruit slices in jello

Question 6

Definition of fossil successions

Answer 6

Layers with the same fossils are the same age.

Question 7

Define unconformities

Answer 7

A break in the rock record produced by nondesposition or erosion of rock units.

Example: Shepard Point, Utah

Question 8

What are the three types of unconformities?

Answer 8

1. Angular unconformity - tilted rocks are overlain by flat-lying rocks
2. Disconformity - sedimentary strata on either side are parallel
3. Nonconformity - sedimentary strata overlay metamorphic or igneous rocks