Chapter 17 Unraveling Geologic History

404 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY
TERMS TO KNOW
Scientists use the ratio of carbon-12 to carbon-14 to establish
with great precision when the plant died.
Radiocarbon dates have been verified by checking them
with dates obtained from counting tree rings. The oldest trees
alive today are more than 5000 years old. Radiocarbon dating
has been especially useful in work on prehistoric human
habitations and events during or since the last ice age.
OTHER ISOTOPES The remaining three isotopes in Table 17-1
have longer half-life periods. This means that they are more
useful for dating older rocks, from 100,000 years old and back
to the origin of Earth. These three radioisotopes are found in
selected minerals. Each can be measured with great precision.
The age of an igneous rock is generally the age of its
minerals. In some places, these isotopes have been used to
find the age of layers of volcanic ash. Therefore, ash beds are
often useful horizons of absolute time that can be found over
a broad geographic area.
absolute age intrusion radioisotope
decay product isotopes relative age
decay-product ratio original horizontality superposition
geologist outcrop unconformity
half-life period radioactive uniformitarianism
inclusion
CHAPTER REVIEW QUESTIONS
1. Which geological events destroy a part of the rock record?
(1) uplift and erosion over a long period of time
(2) continuous deposition of sediments in a deep ocean basin
(3) folding of rock layers deep under Earth’s surface
(4) a long period of volcanic activity