Laboratory Manual for Introductory Geology 4e

LEARNING
OBJECTIVES
■■
Determine the relative ages of
rocks and geologic processes
and use these data to interpret
complex geologic histories
■■
Learn how numerical ages
of rocks are calculated and
apply these methods to dating
geologic materials and events
■■
Understand how geologists
piece together Earth history
from evidence found in widely
separated locations
MATERIALS
NEEDED
■■
Pen, pencils, calculator
12.1 Introduction
You’ve learned to identify minerals, use mineralogy and texture to interpret the
origin of rocks, deduce what types of deformation have affected a given area, and
recognize evidence of tectonic events. With these skills, you can construct a threedimensional
picture of the Earth, using topography and surface map patterns to
infer underground relationships.
This chapter adds the fourth dimension: time. Geologists ask two different questions
about the ages of rocks and geologic processes: (1) Is a rock or process older
or younger than another? (its relative age) and (2) Exactly how many years old is a
rock or process? (its numerical age). In this chapter, we’ll look first at how relative
ages are determined, then at methods for calculating numerical age, and finally
combine them to decipher geologic histories of varying complexity.
12.2 Physical Criteria for Determining
Relative Age
Common sense is the most important resource for determining relative ages. Most
of the reasoning used in relative dating is intuitive, and the basic principles were
used for hundreds of years before we could measure numerical ages. Geologists use
two types of methods to determine relative age: physical methods based on features
in rocks and relationships among them, and biological methods that use fossils. We
focus first on the physical methods and return to fossils later, starting with the simplest
situations and building toward more complicated histories.
12.2.1 The Principles of Original Horizontality
and Superposition
The principle of original horizontality states that most sedimentary rocks are deposited
in horizontal beds (there are exceptions, such as inclined sedimentation in
alluvial fans, dunes, and deltas). But sedimentary rocks are not the only ones that
form in horizontal layers. Lava flows and volcanic ash falls also form layers that are
typically horizontal (FIG. 12.1). It is easy to determine the relative ages in a sequence
of rocks by superposition, as long as the layers are still in their original horizontal position.
EXERCISE 12.1
Name:
Course:
Relative Ages of Horizontal Rock: The Principle of Superposition
Section:
Date:
Using common sense, label the oldest and youngest layers in each of the three photographs in Figure 12.1. Explain your
reasoning.
Congratulations! You have just repeated a discovery by Niels Stensen (also known as Nicolaus Steno) almost 400 years
ago that is one of the foundations for relative dating. This principle of superposition states that in a sequence of strata
that is still in its original horizontal position, the youngest beds will be at the
and the oldest will be at the
.
300 CHAPTER 12 INTERPRETING GEOLOGIC HISTORY