Earth Materials as Time Keepers

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Earth Materials as Time Keepers

• Smith Pun, Chapter 7

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Why study the ages of rocks?

• 7.1 How do you determine the order of events?
• 7.2 How are geologic events placed in relative
  order?
• 7.3 How do geologists determine the relative ages
  of rocks in widely separated places?
• 7.4 How was the geologic time scale constructed?
• 7.5 How do you recognize gaps in the rock record?
• 7.6 How have scientists determined the age of
  Earth?
• 7.7 How is the absolute age of a rock determined?
  
• 7.8 --skip
• 7.9 How do you reconstruct geologic history with
  rocks?

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Why study the ages of rocks?

• The keys to explaining process are
• The sequence of events
• The time required for each step
• Geologists use rocks to tell time in two ways
• In the field, they can look at a landscape and
  decipher the order of events that produced it
• To actually know when an event occurred, or to
  know when a rock formed, requires laboratory
  analysis

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Why study the ages of rocks?

• Objectives In this chapter you will learn
• and apply the principles for placing geologic
  events in order from oldest to youngest.
• to understand how geologists measure the ages of
  rocks.

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Why study the ages of rocks?
This field sketch shows observations of a
landscape. How do we ascertain the order in which
the rocks were placed there? By first determining
the rocks relative ages.
Fig 7.1
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7.1 How do you determine the order of events?

• Relative age the ordering of objects or
  features from oldest to youngest. Things that
  happened first, then next, and last
• Absolute age establishing the date of an event
  (in years before the present time).

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7.2 How are geologic events placed in relative
order?

• By the application of a set of geological rules
• First is the Principle of Superposition This
  states that when rocks were placed, newer ones
  were placed or formed on top of the older ones

So here at the Grand Canyon the old rocks are
on the bottom of the pile, and the new ones are
on top.
Fig. 7.2
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7.2 How are geologic events placed in relative
order?

• Principle of Original Horizontality sedimentary
  layers are (more or less) horizontal when they
  form.

Flat layers that are no longer horizontal
Fig 7.4
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7.2 How are geologic events placed in relative
order?

• Principle of Cross-Cutting Relationships
  geologic features that cut across rock must be
  younger than the rock they cut through.

Fig 7.4
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7.2 How are geologic events placed in relative
order?

• Principle of Inclusions This states that
  objects enclosed in rock must be older than the
  time of rock formation

Fig. 7.5
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7.3 How do geologists determine the relative ages
of rocks in widely separated places?

• Principle of Lateral Continuity layers are
  continuous until encountering an obstruction

Here at the Grand Canyon, we can see that layers
continue for large expanses, but are broken
(cross-cut) by the canyon itself.
Fig 7.6
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7.3 How do geologists determine the relative ages
of rocks in widely separated places?
Fig 7.7
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7.3 How do geologists determine the relative ages
of rocks in widely separated places?

• Principle of Faunal Succession William Smith
  noted that limestones bounding coal layers
  contained different fossils.
• Furthermore, he noted that these layers and
  fossils were found in the same succession in
  other mines
• Therefore, the principle states
• Fossils of different organisms first appear at
  different times.
• Fossils of related organisms change in the same
  fashion in progressively younger rocks every
  place they occur.
• Fossil species disappear from the rock record
  everywhere when they become extinct and do not
  reappear in younger rocks.

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7.3 How do geologists determine the relative ages
of rocks in widely separated places?
Correlation is the process of matching up rocks
found in different places.
Fig 7.8
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Fig 7.9 The Geologic Time Scale
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7.4 How was the geologic time scale constructed?
Fig 7.10b
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7.5 How do you recognize gaps in the rock record?

• Unconformities a gap in the rock record when
  erosion occurred rather than deposition.

Fig 7.11
An angular unconformity is the meeting of two
layers that are inclined at different angles to
one another.
angular unconformity
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7.5 How do you recognize gaps in the rock record?
Fig 7.12
A disconformity is a gap between two sedimentary
layers.
disconformity
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7.5 How do you recognize gaps in the rock record?
A nonconformity is where sedimentary or volcanic
rocks accumulate atop igneous or metamorphic
rocks.
Fig 7.13
nonconformity
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7.6 How have scientists determined the age of
Earth?

• Radioactivity in the 1890s a new science aided
  these attempts.
• It also provided a measure for the absolute ages
  of rocks, and the 4.5-billion-year age of Earth.

Fig 7.16
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7.6 How have scientists determined the age of
Earth?

• Radioactivity
• Isotopes atoms of the same element with the
  same number of protons, but a different number of
  neutrons
• Radioactive decay a change in the number of
  protons, neutrons, or both that transforms an
  unstable isotope towards a stable one

Fig 7.16
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7.7 How is the absolute age of a rock determined?

• Measure the isotopic abundance
• An unstable parent isotope decays towards a
  stable daughter isotope.
• Assuming each daughter comes from a parent
  isotope, and that we know the average time it
  takes to convert, we can use the ratio of the two
  to calculate the age of the rock they are in.
• Half-life the amount of time it takes for ½ of
  the parent to turn into the daughter isotope.

Fig 7.17
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Validating the method
Fig 7.19a
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7.7 How is the absolute age of a rock determined?
Combining absolute and relative age dating
Fig 7.21
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7.7 How is the absolute age of a rock determined?
Combining absolute and relative age dating
Fig 7.22
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Age of Earth

• The oldest rock found so far is gneiss resulting
  from metamorphism of a tonalite intrusion in
  northwestern Canada is 4.030 billion years old
• Sandstone in Australia contains zircon sand
  grains as old as 4.4 billion years - oldest
  mineral
• Various estimates of Earths age range from 4.4
  to 4.56 billion years, so 4.50 0.06 billion
  years old

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7.9 How do you reconstruct geologic history with
rocks?
By describing the rock layers in an exposed
section, along with a knowledge of the basic
geologic principles and relative and absolute
dating, one can form narrative of the
construction of a geologic structure. For
example, let us revisit the sea cliff in Fig 7.1.
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7.9 How do you reconstruct geologic history with
rocks?
Fig 7.1
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7.9 How do you reconstruct geologic history with
rocks?
Fig 7.24
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7.9 How do you reconstruct geologic history with
rocks?
Fig 7.24
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7.9 How do you reconstruct geologic history with
rocks?

• Both relative and absolute dating methods, which
  reveal the order and age of events, allow
  geologists to determine the geologic history of
  an area.
• Combined with knowledge of rock-forming processes
  and the origins of other geologic features,
  relative and absolute dating methods permit
  narrative descriptions of geologic history.

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Approximately how old is Earth?

• About 6000 years old
• About 1.5 million years old
• About 65 million years old
• About 4.5 billion years old
• About 13 billion years old

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Which is a correct interpretation that uses the
law of superposition?

• The sandstone is older than the dike
• The dike is older than the sandstone
• The sandstone is older than the limestone
• The limestone is older than the sandstone

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What kind of unconformity is indicated by the red
line in the photograph ?