Vocabulary Review
Summary of Key Ideas
29.1 Fossils, evidence of earlier life preserved in
rock, are the basis for the geologic time scale and are
an important part of the rock record. Fossils may occur
as original remains, replaced remains, molds and casts,
trace fossils, and carbonaceous films.
29.2 Relative dating is used to place past events
in order without determining actual dates during
which events occurred. Relative dating is also used
to determine relative ages of specimens such as
rocks or fossils. Relative dating is based on the
principle of superposition and the principle of
cross-cutting relationships. A gap in relative time
may be the result of an unconformity in the rock
record.
29.3 Absolute time identifies actual dates of
events or actual ages of specimens. Scientists
use radioactive-dating methods to measure
absolute time. Radiometric dating relies on
radioactive decay and half-lives of elements
such as carbon, uranium, rubidium, and
potassium.
Write the term from the key vocabulary list that best
completes the sentence.
1. The time it takes for half the radioactive atoms
in a sample to decay to a stable product is
?
the
.
2. Indirect evidence of earlier life preserved in rock
?
is a(n)
.
3. Layers of rock are called
?
.
4. The matching of rock layers from one area to
?
another is
.
5. Scientists correlate layers of rock over long
?
distances by comparing the
found
embedded in the rocks.
Concept Review
6. Describe three types of fossils and how they are
formed.
7. Are the trees in Petrified Forest National Park
fossils? Why or why not?
8. Compare and contrast relative dating and
absolute dating.
9. Describe an angular unconformity, a
disconformity, and a nonconformity. What does
each indicate about past events?
10. Describe how radiocarbon dating can be used to
determine when a plant or animal died.
KEY VOCABULARY
absolute time (p. 656)
parent isotope (p. 656)
cast (p. 648)
radioactive decay (p. 656)
correlation (p. 652)
radiometric dating (p. 658)
daughter isotope (p. 656)
relative dating (p. 650)
half-life (p. 657)
strata (p. 650)
index fossils (p. 653)
trace fossil (p. 649)
key bed (p. 654)
unconformity (p. 651)
mold (p. 648)
varve (p. 656)
11. Graphic Organizer Copy and complete the
concept map below.
?
forms
daughter
nuclei
paleontology (p. 648)
by
?
662
Unit 8 Earth’s History
?
? capture