Page 236 - Laboratory Manual in Physical Geology

ACTIVITY Introduction
If you could dig a hole deep into Earth’s crust, you
8.1 Geologic Inquiry for would encounter the geologic record , layers of rock
Relative Age Dating stacked one atop the other like pages in a book. As
each new layer of sediment or rock forms today, it
THINK How can you tell relative age relationships covers the older layers of the geologic record beneath
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About It among the parts of geologic cross it and becomes the youngest layer of the geologic
sections exposed in outcrops? record. Thus, rock layers form a sequence from oldest
at the bottom to youngest at the top. They also have

OBJECTIVE Identify features of geologic cross different colors, textures, chemical compositions, and
sections exposed in outcrops, infer their relative ages, fossils (any evidence of ancient life) depending on
and suggest rules for relative age dating. the environmental conditions under which they were
formed. Geologists have studied sequences of rock layers
PROCEDURES wherever they are exposed in mines, quarries, river
1. Before you begin , do not look up defi nitions and beds, road cuts, wells, and mountain sides throughout
information. Use your current knowledge, and the world. They have also correlated the layers (traced
complete the worksheet with your current level of them from one place to another) across regions and
ability. Also, this is what you will need to do the continents. Thus, the geologic record of rock layers is
activity: essentially a stack of stone pages in a giant natural book
____ Black or blue pen of Earth’s history. And like the pages in any old book,
____ Activity 8.1 Worksheet (p. 217 ) and pencil the rock layers have been folded, fractured (cracked),
torn (faulted), and even removed by geologic events.
2. Complete the worksheet in a way that makes Geologists tell time based on relative and absolute
sense to you.
dating techniques. Relative age dating is the process
3. After you complete the worksheet , be prepared of determining when something formed or happened
to discuss your observations, interpretations, and in relation to other events. For example, if you have
inferences with others. a younger brother and an older sister, then you could
describe your relative age by saying that you are younger
than your sister and older than your brother. Absolute
age dating is the process of determining when something
formed or happened in exact units of time such as days,
ACTIVITY months, or years. Using the example above, you could

describe your absolute age just by saying how old you
8.2 Determining Sequence of are in years.
Events in Geologic Cross Geologists “read” and infer Earth’s history from
rocky outcrops and geologic cross sections by observing
Sections rock layers, recognizing geologic structures, and evaluat-
ing age relationships among the layers and structures.
THINK How can geologic cross sections be The so-called geologic time scale is a chart of named
About It interpreted to establish the relative intervals of the geologic record and their ages in both
ages of rock units, contacts, and other relative and absolute time. It has taken thousands of
geologic features? geoscientists, from all parts of the world, more than a
century to construct the present form of the geologic
OBJECTIVE Apply principles of relative age dating time scale.

to analyze and interpret sequences of events in
geologic cross sections.
Relative Age Dating Based on
PROCEDURES Physical Relationships
1. Before you begin , read the Introduction
A geologist’s initial challenge in the field is to subdivide the
and Relative Age Dating Based on Physical
local sequence of sediments and bodies of rock into map-
Relationships. Also, this is what you will need :
pable units that can be correlated from one site to the next.
____ dark (black or blue) pen Subdivision is based on color, texture, rock type, or other
____ Activity 8.2 Worksheet (p. 219 ) and pencil physical features of the rocks, and the mappable units are
2. Then follow your instructor’s directions for called formations . Formations can be subdivided into
completing the worksheets. members , or even individual strata. Surfaces between any of
these kinds of units are contacts .

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