Download Chapter 12 Geologic Time

Chapter 12 Geologic Time
12.1 Discovering Earth’s History
 Rocks Record Earth History
o Rocks ______________ geological events and changing life forms of the past.
o We have learned that Earth is much ______________ than anyone had previously imagined and that
its surface and interior have been changed by the same ______________ processes that continue
______________.
 A Brief History of Geology
o ____________________ means that the forces and processes that we observe today have been at
work for a very long time.
 The ideas behind uniformitarianism originated with the work of Scottish geologist
______________.
 Relative Dating—Key Principles
o ______________ dating tells us the ______________ in which events occurred, not how long ago
they occurred.
 Law of Superposition
o The law of ______________ states that in an ______________ sequence of sedimentary rocks, each
bed is ______________ than the one ______________ it and younger than the one below it.
 Principle of Original Horizontality
o The principle of original ______________ means that layers of sediment are generally
______________ in a horizontal position.
 Principle of Cross-Cutting Relationships
o The principle of ______________ relationships states that when a fault cuts ______________ rock
layers, or when ______________ intrudes other rocks and crystallizes, we can assume that the fault
or intrusion is ______________ than the rocks affected.
 Inclusions
o ______________ are rocks contained within other ______________.
o Rocks containing inclusions are ______________ than the inclusions they contain.
 Unconformities
o An ______________ represents a long period during which deposition ______________, erosion
removed previously formed rocks, and then ______________ resumed.
o An ______________ unconformity indicates that during the ______________ in deposition, a period
of deformation (folding or tilting) and erosion occurred.
o A ______________ is when the erosional surface separates ______________ metamorphic or
intrusive igneous rocks from younger sedimentary rocks.
o A ______________ is when two sedimentary rock layers are separated by an ______________
surface.
 Correlation of Rock Layers
o ______________ is establishing the equivalence of rocks of ______________ age in different areas.
 By correlating the rocks from one ______________ to ______________, it is possible create
a more complete ______________ of the geological history of a region.
12.2 Fossils: Evidence of Past Life
 Fossil Formation
o ______________ are the remains or traces of prehistoric life. They are important components of
sediment and sedimentary rocks.
o The ______________ of fossil that is formed is determined by the ______________ under which an
organism died and how it was buried.
o ______________ Remains
 Some remains of organisms—such as teeth, bones, and shells—may ______have been
altered, or may have changed hardly at all over time.
o ______________ Remains
 The remains of an organism are likely to be ______________ over time.
 Fossils often become ______________ or turned to ______________.
 ______________ and ______________ are another common type of fossil.
 ______________ is particularly effective in preserving leaves and delicate animals. It occurs
when an organism is buried under fine ______________.
o ______________ Evidence
 ______________ fossils are indirect evidence of prehistoric life such as ______________
o Conditions Favoring Preservation
 Two conditions are important for preservation: ______________ and the possession of
______________ parts.
 Fossils and Correlation
o The principle of fossil ______________ states that fossil organisms ______________ one another in
a definite and determinable order. Therefore, any time period can be recognized by its
______________ content.
o ______________ fossils are widespread geographically, are ______________ to a short span of
geologic time, and occur in ______________ numbers.
 Fossil Formation
o Interpreting Environments
o Fossils can also be used to ______________ and describe ______________ environments.
12.3 Dating with Radioactivity
 Basic ______________ Structures
o Orbiting the nucleus are ______________, which are ______________ electrical charges.
o ______________ number is the number of ______________ in the atom’s nucleus.
o ______________ number is the number of ______________ plus the number of ______________ in
an atom’s nucleus.
 Radioactivity
o ______________ is the spontaneous ______________ of certain unstable atomic nuclei.
 Half-Life
o A ______________ is the amount of time necessary for ______________ of the nuclei in a sample to
______________ to a stable isotope.
 Radiometric Dating
o Each ______________ isotope has been decaying at a ______________ rate since the formation of
the rocks in which it occurs.
o Radiometric dating is the procedure of ______________ the ______________ ages of rocks and
minerals that contain radioactive isotopes.
o As a radioactive isotope ______________, atoms of the ______________ product are formed and
accumulate.
o An ______________ radiometric date can be obtained only if the mineral remained in a
______________ system during the entire period since its ______________.
 Dating with Carbon-14
o Radiocarbon dating is the method for determining age by comparing the amount of ______________
to the amount of ______________ in a sample.
o When an organism dies, the amount of carbon-14 it contains gradually ______________ as it decays.
By comparing the ______________ of carbon-14 to carbon-12 in a sample, ______________ dates
can be determined.
 Importance of Radiometric Dating
o Radiometric dating has supported the ideas of ______________, ______________, and others who
inferred that geologic time must be ______________.
12.4 The Geologic Time Scale
 Structure of the Time Scale
o Based on their interpretations of the rock record, geologists have divided Earth’s 4.56-billion-year
history into ______________ that represent specific amounts of ______________. Taken together,
these time spans make up the ______________.
o ______________ represent the greatest expanses of time. Eons are divided into ______________.
Each era is subdivided into ______________. Finally, periods are divided into smaller units called
______________.
o There are ______________ eras within the ______________ eon: the ______________, which
means “ancient life,” the ______________, which means “middle life,” and the ______________,
which means “recent life.”
o Each ______________ within an era is characterized by somewhat less profound ______________
in life forms as compared with the changes that occur during an ______________.
o The periods of the ______________ era are divided into still smaller units called epochs, during
which even less profound changes in life forms occur.
 Precambrian Time
o During ______________ time, there were ______________ life forms. These life forms are more
______________ to identify and the rocks have been ______________ often.
 Difficulties With the Geologic Time Scale
o A ______________ rock may contain particles that contain radioactive isotopes, but these particles
are ______________ the same age as the rock in which they occur.
o The age of a particular mineral in a ______________ rock does not necessarily represent the time
when the rock was ______________ formed. Instead, the date may indicate when the rock was
______________________.