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Chapter 1
The Science of Historical
The Earth formed about 4.6 billion years
Homo sapiens appeared on Earth between
about 300,000 and 150,000 years ago.
Humans ask questions about their surroundings.
How did the Earth form?
Why do earthquakes occur?
What lies beneath the land and below the ocean floor?
Curiosity leads to exploration.
Why Study Earth History?
The Earth has changed through time.
Understanding past geologic events
will help us predict future geologic
Past geologic events include:
Volcanic eruptions.
Continents flooded by inland seas.
Drifting and colliding continents.
Glaciers have covered large parts of continents.
Meteorite and asteroid impacts.
Changes in chemistry of oceans and atmosphere.
Changes to life on Earth through time - sometimes
slow, sometimes swift and deadly.
Geology is the study of the Earth.
Two major branches of geology:
• Physical Geology - deals with Earth
materials and processes
• Historical Geology - deals with origin and
changes of Earth and life through time and
What do Geologists Do?
• Study the structure of mountain ranges
• Attempt to predict geologic hazards like
earthquakes and volcanic eruptions.
• Identify minerals in meteorites to learn how
Earth formed.
• Study rivers, floods, glaciers, and underground
• Look at results of past events and work
backward in time to discover causes of those
• Search for fossil fuels and mineral resources.
Scientific Method in Geology
Science operates through the use of the Scientific
The scientific method is a method for finding
answers to questions and solutions to problems.
Scientists work like detectives to gather data, to
try to figure out what happened.
The data may be obtained through observations
and/or experiments, which can be repeated and
verified by others.
Summary of Scientific Method
1. A question is formulated.
2. Observations (collect data)
3. Develop multiple working hypotheses (ideas to explain
the observations)
4. Test the hypotheses by experimenting and either accept,
reject, or modify the hypothesis.
The simplest explanation is best.
5. When a hypothesis has considerable experimental or
observational support, it is accepted and others are
rejected, and it may become a theory.
6. A theory ultimately may become a scientific law.
What is a Theory?
A hypothesis that survives repeated challenges,
and is supported by a large body of evidence,
may be elevated to the status of a theory.
A theory is not just an wild idea or a guess.
Theories have survived close examination, and
can be accepted with confidence.
A theory has a very high probability of being
Examples of theories include the theory of relativity, plate
tectonics theory, evolutionary theory, and atomic theory.
Major Themes in Earth History
1. Deep time
2. Plate tectonics
3. Evolution of life
Deep Time
• Recognition of immensity of geologic time
is geology's most important contribution to
human knowledge.
• The science that deals with determining
the ages of rocks is called geochronology.
Methods of Dating Rocks
• Absolute age - The actual age.
Quantifying the age of the rock or mineral
in years.
• Relative age - Determining which rocks
are older and which are younger.
Absolute Age
The discovery of radioactivity in 1896 gave us the
tools to find the absolute age of a rock.
Radiometric dating involves analysis of the
breakdown of unstable radioactive elements in
Radioactive elements decay by releasing
subatomic particles from their nuclei. Through
this process, the unstable radioactive element is
converted to a stable "daughter" element.
Example: Uranium-235 decays to form lead-207.
Radioactive Decay
Many radioactive elements can be used as
geologic clocks. Each radioactive element
decays at its own nearly constant rate.
The rate of decay can be measured.
Once this rate is known, geologists can
determine the length of time over which
decay has been occurring by measuring
the amount of radioactive parent element
and the amount of stable daughter
Half Life
Each radioactive element has its own
unique half-life.
A half-life is the time it takes for half of the
parent radioactive element to decay to a
daughter product.
Example: Uranium-235 has a half-life of about
704 million years.
Uranium-235 decays to form lead-207. Uranium235 has a half-life of about 704 million years.
1. After 704 million years, only half (50%) of the
uranium atoms in the mineral remain. (The rest
have decayed to lead-207.)
2. After another 704 million years, only half of
that amount (or 25%) of the uranium atoms
3. So, a rock with 25% uranium-235 and 75%
lead-207 must be 1,408 million years old (or
1.408 billion years old).
Using radiometric dating, some rocks found
in Canada's Northwest Territories have
been dated at 4.04 billion years old.
Relative age
• Determining which rocks are older and which
are younger. “Rock unit A is older than rock unit
• The geologic time scale was developed through
relative dating.
• Relative age determinations provide a
framework or geologic time scale in which to
place events of the geologic past.
• Using radiometric dating, actual dates in years
have been determined for the geologic time
Major Themes in Earth History
1. Deep time
2. Plate tectonics
3. Evolution of life
Plate Tectonics
The theory of plate tectonics has
revolutionized the understanding of
geology. Plate tectonics explains many
large scale patterns in the Earth's
geological record.
It is a "great unifying theory" in geology.
Plate Tectonics
The Earth's surface or lithosphere is divided into
plates (about 7 large plates and 20 smaller ones).
Plate Tectonics
The lithosphere is about 100 km thick and
consists of the rigid, brittle crust and
uppermost mantle.
Rigid lithospheric plates rest (or "float") on
the asthenosphere, the easily deformed,
or partially molten part of mantle below the
The plates are moving, but their rates and
directions of movement vary.
Plate Movements
Plate movement is due to convectional flow
(circular movement of the asthenosphere
due to hot material rising and cooler
material sinking).
The plates only move a few millimeters per
year, about the rate at which your
fingernails grow.
Types of plate boundaries:
• Divergent - where plates move apart from one
• Convergent - where plates move toward one
• Transform - where two plates slide past one
Major Themes in Earth History
1. Deep time
2. Plate tectonics
3. Evolution of life
Evolution of Life
In biology, evolution is the
"great unifying theory" for understanding
the history of life.
Evolution of Life
As a result of evolution, plants and animals living
today are different from their ancestors. They
differ in appearance, genetic characteristics,
body chemistry, and in the way they function.
These differences appear to be a response to
changes in the environment and competition for
Fossils record the changes in organisms over time.
Natural Selection
Charles Darwin and Alfred Wallace were the
first scientists to assemble a large body of
convincing observational evidence in
support of evolution.
They proposed a mechanism for evolution
which Darwin called natural selection.
Natural selection is based on the following
• Any given species produces more offspring than
can survive to maturity.
• Variations exist among the offspring.
• Offspring must compete with one another for
food and habitat.
• Offspring with the most favorable characteristics
are more likely to survive to reproduce.
• Beneficial traits are passed on to the next
Lines of evidence for evolution
cited by Darwin
• Fossils provide direct evidence for changes in
life in rocks of different ages.
• Certain organs or structures are present in a
variety of species, but they are modified to
function differently (homologous structures).
• Modern organisms contain vestigial organs that
appear to have little or no use. These structures
had a useful function in ancestral species.
• Animals that are very different, had similarlooking embryos.
Other lines of evidence for
evolution come from the fields of:
• Genetics (DNA molecule)
• Biochemistry (Biochemistry of closelyrelated organism is similar, but very
different from more distantly related
• Molecular biology (sequences of amino
acids in proteins)
Organic Evolution
These discoveries indicate that plants and
animals of each geologic era arose from
earlier species by the process we call
"organic evolution".
Organic evolution refers to changes that
have occurred in organisms with the
passage of time.