The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- During the seventeenth and eighteenth centuries, catastrophism
influenced the formulation of explanations about Earth. Catastrophism states that Earth's
landscapes have been developed primarily by great catastrophes. By contrast,
uniformitarianism, one of the fundamental principles of modern geology
advanced by James Hutton in the late 1700s, states that the physical,
chemical, and biological laws that operate today have also operated in the geologic past.
The idea is often summarized as "The present is the key to the past." Hutton argued that
processes that appear to be slow-acting could, over long spans of time, produce effects that
were just as great as those resulting from sudden catastrophic events.
- The two types of dates used by geologists to interpret Earth history are (1)
relative dates, which put events in their proper sequence of
formation, and (2) numerical dates, which pinpoint the time in
years when an event took place.
- Relative dates can be established using the law of superposition, principle of
original horizontality, principle of cross-cutting relationships, inclusions, and
- Correlation, the matching up of two or more geologic phenomena in
different areas, is used to develop a geologic time scale that applies to the entire Earth.
- Fossils are the remains or traces of prehistoric life. The special
conditions that favor preservation are rapid burial and the possession of
hard parts such as shells, bones, or teeth.
- Fossils are used to correlate sedimentary rocks from different regions
by using the rocks' distinctive fossil content and applying the principle of fossil
succession. It states that fossil organisms succeed one another in a definite and
determinable order, and therefore any time period can be recognized by its fossil content.
- Each atom has a nucleus containing protons (positively charged
particles) and neutrons (neutral particles). Orbiting the nucleus are
negatively charged electrons. The atomic number of an atom is
the number of protons in the nucleus. The mass number is the number of protons
plus the number of neutrons in an atom's nucleus. Isotopes are variants of the
same atom, but with a different number of neutrons and hence a different mass number.
- Radioactivity is the spontaneous breaking apart (decay) of certain
unstable atomic nuclei. Three common types of radioactive decay are (1) emission of alpha
particles from the nucleus, (2) emission of beta particles (electrons) from the nucleus, and
(3) capture of electrons by the nucleus.
- An unstable radioactive isotope, called the parent, will
decay and form stable daughter products. The length of time for half of the
nuclei of a radioactive isotope to decay is called the half-life of the
isotope. If the half-life of the isotope is known and the parent/daugher ratio can be
measured, the age of a sample can be calculated.
- The geologic time scale divides Earth's history into units of varying
magnitude. It is commonly presented in chart form, with the oldest time and event at the
bottom and the youngest at the top. The principal subdivisions of the geologic time scale,
called eons, include the Hadean, Archean, Proterozoic (together,
these three eons are commonly referred to as the Precambrian), and, beginning
about 540 million years ago, the Phanerozoic. The Phanerozoic (meaning "visible life") eon
is divided into the following eras: Paleozoic ("ancient life"),
Mesozoic ("middle life"), and Cenozoic ("recent life").
- A significant problem in assigning numerical dates to units of time is that
not all rocks can be dated radiometrically. A sedimentary rock may contain
particles of many ages that have been weathered from different rocks that formed at various
times. One way geologists assign numerical dates to sedimentary rocks is to relate them to
datable igneous masses, such as dikes and volcanic ash beds.