Download Integrated Science Geologic Time Notes Section 1: Geologic Time

Integrated Science
Geologic Time Notes
Section 1: Geologic Time
To help in the analysis of Earth’s rocks, geologists have divided the history of Earth into time units based
upon the fossils contained in the rocks.
1. Geologic Time Scale – a record of Earth’s history from its origin 4.6 billion years ago to the present.
• Eon – the longest time unit in the scale, and is measured in billions of years
→ The eons are: Archean (4,600 to 2,500 millions of years before present (M.Y.B.P.), the
Proterozoic (2,500 to 540 M.Y.B.P), and the Phanerozoic (540 M.Y.B.P. to present)
• Era – the next longest time unit, measured in hundreds of millions of years
→ Within the Phanerozoic Eon there are three eras: Paleozoic, Mesozoic, and Cenozoic
→ Eras are defined by the differences in life-forms found in rocks
→ The Paleozoic Era was dominated by a variety of marine plants and animals
♦ At the end of the Paleozoic Era (245 M.Y.B.P.) 90 % of all marine species became extinct
→ The Mesozoic Era was dominated by the development of amphibians and reptiles
♦ Dinosaurs were the dominant land creature
♦ Mammals also began to evolve during this era
♦ By the end of the Mesozoic Era (66 M.Y.B.P.) the dinosaurs were extinct
• Periods – time units measured in tens of millions of to hundreds of millions of years
→ Periods are defined by the life-forms that were abundant or became extinct during the time in
which specific rocks were deposited
→ Some periods were named for the geographic region in which the rocks of that age were first
observed, studied, and described
→ There are seven periods in the Paleozoic Era, three in the Mesozoic Era, and three in the
Cenozoic Era
• Epochs – time units measured in millions of years to tens of millions of years
→ The fossil record of the Cenozoic Era is relatively complete, and the rocks and fossils are
easily accessed and studied
→ The Paleogene Period is divided into three epochs, the Neogene into two, and the
Quaternary into two epochs
→ Different groups of fossils have been used to distinguish the various epochs
• Regardless of how a geologic time unit is defined, each unit contains specific characteristics that
set it apart fro the rest of geologic history
Section 2: Age Dating of Rocks
Geologists believe that Earth is 4.6 billion years old. How did they determine the age of the Earth? How
can geologists determine the age of fossils and the rocks in which they are found? Scientists use two
methods to determine the age of the Earth and the rocks and fossils present in the Earth.
1. Relative Age Dating of Rocks
• Relative age dating places the ages of rocks and the events that formed them in order, but
without exact dates. This is done by comparing one event or rock layer to another.
• Relative age dating is based the principle of uniformitarianism which states that the processes
occurring today have been occurring since Earth formed. Only the rate, intensity, and scale with
which they occur have changed
• There are geologic principles that guide the process of relative age dating
→ The principle of original horizontality – states that sedimentary rocks are deposited in
horizontal or nearly horizontal layers
♦
→
→
While the actual age of the rock
layers is not known, geologists
assume that the oldest layers are
on the bottom and each layer
going toward the top is younger
The principle of superposition – states
that in an undisturbed rock sequence,
the oldest rock is on the bottom and
each successive layer is younger than
the layer beneath
The principle of cross-cutting
relationships – states the intrusion or
fault is younger than the rock it cuts
across
♦ The Black Hills is an igneous
intrusive, however, most of the
rock through which it intruded has
been eroded away
•
Relative age also can be determined
where an overlying rock layer contains
particles of rock material from the layer
beneath it. These particles are called
Using the principle of superposition, geologists
inclusions
have determined the relative ages of these rock
•
It is very difficult to find an undisturbed
layers.
sequence of rock layers. The Earth is
constantly changing as a result of
processes such as weathering, erosion, earthquakes, and volcanism. Where these processes
have removed a portion of the rock record, and a new layer of rock is then deposited there is a
gap in the rock record. This gap is called an unconformity.
→ Disconformity – when horizontal layers of rock overlie horizontal layers of rock
→ Nonconformity – the contact point between nonsedimentary and sedimentary rock
→ Angular unconformity – the contact between uplifted and tilted rock layers and the
horizontal rock layers that were deposited at a later time
A discontinuity forms when a sedimentary rock layer is deposited on top of an eroded
sedimentary rock layer. A nonconformity forms when a sedimentary rock layer is deposited
on top of an eroded metamorphic rock layer.
2. Absolute Age Dating of Rocks
• Absolute age dating allows scientists to determine the actual age of a rock, fossil, or other object.
• Scientists have developed a method for dating very old objects using the decay rate of
radioactive isotopes.
→ A radioactive isotope is an unstable form of an atom that, over time, decays into a stable form
of a different atom
♦ Example: Uranium 238, which is radioactive, decays into Lead 206, which is very stable
→ The original radioactive isotope is called the parent, and the new, stable element is called the
daughter
→ The radioactive isotopes decay by emitting (giving up) protons and neutrons at a constant
rate
♦ As the number of parent atoms decreases the number of daughter atoms increase by the
same amount
→ As the number of protons and neutrons change with each nuclear emission, the element is
converted to a different element
→ Radioactive decay – the process in which a radioactive isotope emits protons and neutrons
at a constant rate until a stable nuclear structure is achieved
♦ Because it often takes a very long time for an entire amount of a radioactive isotope to
decay, geologists use the length of time it takes for one-half of the original amount to
decay. This amount of time is known as the half-live of the isotope
♦ Half-life – period of time it takes for a radioactive isotope to decay to one-half of its
original amount
♦ Different radioactive isotopes have different half-lives
Half-lives of Selected Radioactive Isotopes
•
Radioactive Isotope
(parent)
Rubidium-87
Approximate
Half-live
48.6 billion years
Decay Product
(daughter)
Strontium-87
Thorium-232
14.0 billion years
Lead-208
Potassium-40
1.3 billion years
Argon-40
Uranium-238
4.5 billion years
Lead-206
Uranium-235
0.7 billion years
Lead-207
Carbon-14
5,730 years
Nitrogen-14
Some igneous and metamorphic rocks contain radioactive isotopes that allow geologists to
determine the absolute age of the rocks
→ Radiometric dating – the process whereby scientists attempt to determine the ratio of parent
nuclei to daughter nuclei for the purpose of determining the absolute age of the object
♦ By finding this ratio the scientists know how many half-lives have occurred since the
radioactive decay began
♦ Example: The radioactive decay of Carbon-14 to Nitrogen-14
Time 1
Percent Parent
Element
100.0
Percent Daughter
Element
0.0
Elapsed
Years
0
Number of
Half-lives
0
Time 2
50.0
50.0
5,730
1
Time 3
25.0
75.0
11,560
2
Time 4
12.5
87.5
17,290
3
→
The isotope used for dating depends on the estimated age of the object to be dated
♦ Example: If Uranium-238 (half-life = 4.5 billion years) is used to date an object estimated
to be only a few thousand years old, the ratio of parent atoms to daughter atoms will be
too large to be useful.
♦ Likewise if Carbon-14 (half-life = 5,730 years) is used to date a rock estimated to be over
a billion years old, the parent/daughter ratio will to low to be useful
3. Other Ways to Determine Age
• Determining the relative of absolute age of an object or event is not limited to the use of rocks or
chemical elements.
→ Naturally occurring materials, such as trees, lake-bottom sediment, and volcanic ash can also
be used to help geologists determine the age of an object or events, such as a forest fire, a
drought, a flood, or a volcanic eruption
• Tree Rings
→ The age of trees can be determined by counting the number of annual tree rings in a cross
section of the tree
→ During the spring months a tree experiences its greatest growth, during the winter months its
growth is less. A pair of spring and winter growth rings represents an annual tree ring
• Seasonal Climate Changes
→ About 11,000 years ago, glaciers covered the northern part of the United States
→ During the summer the glaciers would partially melt, and the water would carry sediment
downstream and deposit the sediment in large lakes
→ Summer sediment deposits are generally light-colored and relatively thick compared to the
thinner, dark-colored sediments of winter
→ Deposits from different glacial lakes can be compared to determine ages from about 15,000
to 20,000 years ago
• Distinctive Sediment Layers
→ When a rock layer is formed by an instantaneous or short lived event, geologists can
determine the date of the event by radiometric dating
→ The layer then becomes a time marker called a key bed, which can be used to correlate rock
layers across wide areas
♦ Key bed – sediment layer that serves as a time marker in the rock record and results
from volcanic ash or meteorite-impact debris that spread out and covered large areas of
the Earth
♦ Example: at the end of the Cretaceous Period a large meteor impact threw large amounts
of dirt and stuff into the air and resulted in an overall lowering of the planet’s temperature
and caused the mass extinction of the dinosaurs. This dust later settled and formed a
sediment layer that can be found in many parts of the world today.
Section 3: Fossils
1. Fossils – the evidence of once living plants or animals
• Provide evidence of the past existence of a wide variety of life-forms, most of which are extinct
• Also provide evidence that populations have undergone change through time in response to
changes in their environment
→ Evolution – adaptations of life-forms due to changing environmental conditions
• Fossils can also provide information about past environmental conditions and can be used to
correlate rock layers from are to another
2. Three concepts are important in the study and use of fossils:
• Fossils represent the remains of once-living organisms
• Most fossils are the remains of extinct organisms; that is, they belong to species that are no
longer living anywhere on Earth
• The kinds of fossils found in rocks of different ages differ because life on Earth has changed
through time
•
The three concepts are summarized in the Law of Fossil Succession: The kinds of animals and
plants found as fossils change through time
♦ If we begin at the present and examine older and older layers of rock, we will come to a
level where no fossils of humans are present.
♦ If we continue backwards in time, we will successively come to levels where no fossils of
flowering plants are present, no birds, no mammals, no reptiles, no four-footed
vertebrates, no land plants, no fishes, no shells, and no animals.
The Law of Fossil Succession: The kinds of animals and plants found as fossils change through time
3. Evolution
• How do scientists explain the changes in life forms, which are obvious in the record of fossils in
rocks?
→ Early explanations were built around the idea of successive natural disasters or catastrophes
that periodically destroyed life
♦ After each catastrophe, life began anew
→ In the mid-nineteenth century, both Charles Darwin and Alfred Wallace proposed that older
species of life give rise to younger ones
♦ According to Darwin, this change or evolution is caused by four processes: variation,
over-reproduction, competition, and survival of those best adapted to the environment in
which they live
♦ Darwin's theory accounts for all of the diversity of life, both living and fossil.
♦ His explanation gave scientific meaning to the observed succession of once-living
species seen as fossils in the record of Earth's history preserved in the rocks.
♦ Darwin's theory of evolution has been refined and modified continuously as new
information has accumulated
♦ All of the new information has supported Darwin's basic concept--that living beings have
changed through time and older species are ancestors of younger ones
4. Types of fossils
• Original preservation – describes a fossil with soft and hard parts that have not undergone any
change since the organism’s death
→ These fossils are very uncommon because their preservation requires extraordinary
circumstances such as freezing, drying out, or oxygen-free environments
♦ In Alaska the woody parts of plants are imbedded in the permafrost from 10,000 year old
bogs
♦ Soft parts of mammoths and saber-toothed tigers are preserved in the La Brea Tar Pits in
California
♦
•
Soft parts are also preserved when plants or animals have been dried out and their
remains mummified. Most mummies are found in dry caves or deserts
Altered hard parts – fossils whose organic material has been removed and whose hard parts
have been changed by recrystallization or mineral replacement
→ Permineralization – process in which pore spaces in a fossil are filled with mineral
substances
♦ Example: When volcanic ash spreads over a large area, entire forests may become
buried. Over a long period of time, quarts and other minerals in the ash combine with
groundwater and slowly fill in the spaces between the cell walls of the trees and silicify
them. This fossilized tree is called petrified wood.
The process of permineralization
→
•
•
Changes in temperature and pressure may also result in changes in shell or bone material
♦ Shells or exoskeletons (from coral) can be affected by a process called recrystallization.
The exterior of the shell remains the same, but shell microstructure is destroyed in the
process
Index fossil – remains of plants or animals that can be used by geologists to correlate rock
layers over large geographical areas or to date a particular rock layer
→ An index fossil is easily recognized, abundant, and distributed over a large geographic area
→ An index fossil must also have lived during a short period
Some fossils do not contain any shell or bone material. They may be molds or casts.
→ Mold – fossil that is formed when a shelled organism decays in sedimentary rock and is
weathered away leaving a hollowed impression
→ Cast – fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes
filled with minerals or sediment
The process of cast formation
•
Indirect evidence of plant and animal life are called trace fossils
Examples of trace fossils include worm trails, dinosaur tracks, gastroliths, and corprolites
→
♦
♦
Gastroliths – smooth, rounded rocks dinosaurs had in their stomachs to help them
digest and grind their food.
Coprolites – the remains of solid waste materials of animals
5. Why study fossils
• The study of fossils allows scientists to interpret and describe Earth’s history
• Fossils from different geologic time periods describe how organisms have changed throughout
time
• Fossils also show evidence of ancient environmental conditions
• They also may help scientists find patterns and cycles that can be used to predict future
phenomena, such as climate changes
• The study of fossils allows geologists to locate energy resources
6. How fossils are studied
• The study of fossils and the rocks that contain them occurs both out of doors and in the laboratory
• The field work can take place anywhere in the world
• In the laboratory, rock saws, dental drills, pneumatic chisels, inorganic and organic acids, and
other mechanical and chemical procedures may be used to prepare samples for study
♦ Preparation may take days, weeks, or months--large dinosaurs may take years to
prepare.
♦ Once the fossils are freed from the rock, they can be studied and interpreted
♦ In addition, the rock itself provides much useful information about the environment in
which it and the fossils were formed