CHAPTER 8: GEOLOGIC TIME
SUMMARY
This chapter begins the discussion of geologic time by acknowledging that our personal
conceptions of time are firmly grounded in time intervals measured in hours, days, and years.
Yet Earth is very old (~4.6 billion), and many slow-acting events have occurred that have shaped
it into the world we observe today. Students are reminded that Earth’s environment changed
from that of a molten mass, to having solid continents, to an environment that included an
atmosphere, and eventually, oceans. Some of those events were unlike changes we observe today
and caused catastrophic changes in the face of Earth. Other events were not unlike what we
observe today and are uniformitarian (or similar) with events we observe today. Regardless of
the type of process involved, geologists use the principles of relative time, correlation, and
numerical time to figure out what happened and when. That evidence indicates that primitive life
is at least 3.8 billion years old and developed in a hostile environment compared to today. In the
past 542 million years, life flourished, but occasionally underwent mass extinctions. Some of
those extinctions may have been catastrophic events. Many types of fossils are preserved in the
rock record and those changes marked significant modification in the type of life that dominated
earlier episodes of Earth history. Humans now have the most dramatic environmental impact of
all species on Earth. That may change some day in the future as our species fights to survive as
have so many in the past.
8.2 The History of (Relative) Time.
The study of relative time involves placing events in the order in which they occurred. The
principles include:
a) Original Horizontality: sedimentary rocks are deposited in nearly horizontal layers (and if
tilted, they must have undergone deformation after their formation).
b) Superposition: in a series of sedimentary rocks (that are not deformed so much that the
sequence is overturned) the rocks at the bottom of the stack are the oldest and the rocks at the top
are the youngest.
c) Cross-cutting relationships: older rocks may be cut by younger rocks or other geologic
features (such as faults, joints).
d) Inclusions: younger rock units sometimes incorporate pieces of older rocks. It can be used to
identify older pieces of rock surrounded by younger igneous rocks (such as a magma intrusion
that incorporates surrounding rocks).
see pages 213-214 and figures on these pages for more on the above principles.
James Hutton's geologic principles helped to invent our modern concept of geologic time (that
the Earth is very old and Earth's processes take a very long time to produce any significant
change in the shape of the Earth's surface). These principles are known as
Uniformitarianism: the same slow-acting geologic processes that operate today must have
operated in the past ("The present is the key to the past"); he noted that all land should be worn
flat unless some process acts to renew the landscape by forming new mountains (so there are
processes of construction, such as mountain building during plate tectonics and processes of
destruction such as weathering and erosion).
Another of Hutton's principles is Unconformities: when rock layers are worn down by erosion
(while they are exposed at Earth's surface) and then later buried beneath new horizontal layers of
sediments they form a feature called an unconformity (located at the physical boundary between
the two sections of rocks)...since erosion removed rocks layers, it is equivalent to removing
geologic time...so an unconformity surface represents a gap in geologic time. Please see pages
218-219 "Geological History of the Grand Canyon" and follow the block diagrams from #1 to #8
to get a better understanding of how unconformities form (and to understand other principles of
relative time dating techniques).
The above diagram depicts (using block diagrams) how to apply relative age dating techniques
to rock units. At (a.) three sedimentary rock units are shown, the oldest bed on the bottom (A)
and the youngest bed on top (C) (we know this by the principle of superposition). After
deposition of these units they were tilted and eroded to a relatively flat surface (so the
environment changed from depositional...such as under water; to erosional...exposed to
processes of weathering and erosion at the Earth's surface). Then at (b.) a renewed period of
deposition placed (D) on top of the tilted units. At (c.) all of the units are now tilted (remember,
the principle of horizontality states that these rock units are originally deposited horizontally).
Also note that the physical boundary between (D) and the other tilted units represents an
unconformity (gap in geologic time). Keep in mind the processes described above can take
millions of years to produce these features.
The above should help you with checkpoint 8.2 and 8.3. Checkpoint 8.3 also shows
examples of cross-cutting relationships and inclusions which will help you interpret the
sequence of events.
Fossils and Chronology: The most straightforward method of correlating sedimentary rocks is
to compare their fossils. Fossils can be the key to predicting the sequence of layers in a given
location and to match outcrops of similar rocks between different locations. The best fossils to
use for correlation are called index fossils: species that existed for relatively short periods of
geologic time and are found over large geographic areas. Paleontologists have used index fossils
to correlate rocks around the world!
Checkpoint 8.6: Outcrops of rock are examined in four different locations in a state. The rock
types and the fossils they contain are illustrated in the following diagram. Which fossil would be
the best choice to use as an index fossil for these rocks? Which fossil is least characteristic of a
specific set of geological conditions?
a) Fossil 1
b) Fossil 2
c) Fossil 3
8.3: Geologic Time
Read on this! Geologic Time and Mass Extinctions. The geologic time scale is like a calendar
of Earth's history. Earth's past can be divided into three big chunks of time known as Eons.
Today we are in the Phanerozoic Eon, which is further subdivided into three Eras: the Cenozoic
(new life), Mesozoic (middle life) and Paleozoic (ancient life). Note the boundaries between the
Cenozoic/Mesozoic Eras is marked by a mass extinction (the K-T event that ended the reign of
the dinosaurs about 66 million years ago) and the Mesozoic/Paleozoic Eras is marked by an even
greater mass extinction (the P-T event that killed off 96% of marine species and 70% of land
species about 251 million years ago). These Eras are further sudivided into Periods (see Table
8.1). Finally, note that the beginning of the Paleozoic Era (the Cambrian Period) is marked by an
explosion of organisms with hard skeletons about 542 million years ago. Prior to this, fossils are
rare in the geologic record.
8.4: Numerical Time
Atoms of the same element with a different number of neutrons are called isotopes (the mass
number is the combined number of protons and neutrons in the nucleus). Unstable isotopes can
breakdown by radioactive decay and change to a new element accompanied by the release of
energy. The unstable original isotope is termed the "parent" isotope and the product of decay is
termed the "daughter" atom. Radioactive decay of specific isotopes occurs at a constant rate
regardless of the physical or chemical conditions. By keeping track of the relative amounts of
isotopes in individual minerals we can calculate an age for the rock containing the mineral.
To calculate the age we use half-lives. The half-life of an isotope is the time taken for half of the
parent isotopes to convert to daughter atoms. As time passes the amount of parent isotope
decreases and the quanitity of daughter atom increases. After one half-life, 50% of the parent
remains and the other 50% of the original atoms are converted to daughter atoms. After
two half-lives, the number of parent isotopes is halved again (25%) and the number of daughter
atoms increases by an equivalent amount (to 75%). The relative proportion (ratio) of parent
isotopes and daughter atoms can be used to determine how many half-lives have passed, and the
time for these half-lives to occur is know for certain isotopes (see Table 8.3).
Checkpoint 8.15: The half-life of a radioactive isotope is 500 million years. Scientists testing a
rock sample discover that the sample contains three times as many daughter atoms as parent
isotopes. What is the age of the rock?
a) 500 million years
b) 1,500 million years (1.5 billion)
c) 1,000 million years (1 billion)
d) 2,500 million years (2.5 billion)
When answering the above question is it helpfull to refer to Figure 8.21 on page 230. Once you
figure out how many half-lives have passed, multiply the time for the half-life to occur by the
number of half-lives that have passed.
LEARNING OBJECTIVES
1. Students will explain concepts related to geologic time.
2. Students will explain rules that are used to determine the sequence of geologic events: the
principles of superposition, cross-cutting relationships, and original horizontality.
3. Students will apply the principles to determine the order of geologic events.
4. Students will recognize the importance of index fossils and identify the best index fossil in a
sequence of rocks and fossils.
5. Students will describe characteristics of the eons of the geologic time scale.
6. Students will determine how the age of a rock is related to the relative proportions of parent
and daughter isotopes.
7. Students will distinguish between high- and low-magnitude events and list examples of each.