Chapter Twenty One

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Chapter Twenty One Sections 1 and 2, and pg
752-754
Geology literally means "study of the
Earth." Physical geology examines the
materials and processes of the Earth. Historical
geology examines the origin and evolution of our
planet through time.
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• Geology is an evolving science - the theory of
  plate tectonics was just accepted in the 1960's.
• Plate tectonics is the unifying theory in
  geology.
• Although geologists treat it as a law - plate
  tectonics is still and will likely remain a
  theory

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Geology is an extremely controversial science -
the theory of evolution (paleontology) is central
to geology. Geology seeks to understand the
origin of our planet and our place in the
Universe - answers to these questions are also
posed outside of the realm of science.
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History of Early Geology
Catastrophism (James Ussher, mid 1600s) - He
interpreted the Bible to determine that the Earth
was created at 4004 B.C. This was generally
accepted by both the scientific and religious
communities. Subsequent workers then developed
the notion of catastrophism, which held that the
the Earths landforms were formed over very short
periods of time. Uniformitarianism (James
Hutton, late 1700s) - He proposed that the same
processes that are at work today were at work in
the past. Summarized by The present is the key
to the past. Hutton, not constrained by the
notion of a very young planet, recognized that
time is the critical element to the formation of
common geologic structures. Uniformitarianism is
a basic foundation of modern geology.
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BLAMMO!
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Although catastrophism was abandoned, there is
certainly evidence that sudden events do occur.
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Uniformitarianism

• Uniformitarianism the physical, chemical and
  biological laws that operate today have also
  operated in the geologic past
• Put forth by James Hutton in the late 1700s
• To understand the past, we must first understand
  present day processes and their results

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Geologic Time
Relative Dating Putting geologic events into
proper order (oldest to youngest), but without
absolute ages. We use a number of principles and
laws to do this Law of Original Horzontality -
Sedimentary units and lava flows are deposited
horizontally. Law of Superposition - the layer
below is older than the layer above. Principle of
fossil succession - life forms succeed one
another in a definite and determinable order and
therefor a time period can be determined by its
fossils. Law of Cross-cutting Relationships - A
rock is younger than any rock across which it
cuts.
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Rock Record

• Rocks record geological events and changing life
  forms of the past
• Much of the rock record has been removed by
  erosion
• By unraveling the clues in the rock record you
  can learn about the conditions of the past

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Relative Dating

• Identifying what rock layers formed 1st, 2nd,
  3rd, etc
• Tells us the sequence events occurred in
• Does not tell us how long ago the events occurred

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Original Horizontality

• layers of sediments are generally deposited in a
  horizontal position
• -its got layers

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Angular Unconformity-still layered, just not
horizontally
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2. Law of Superposition

• In an undeformed sequence of sedimentary rocks,
  each bed is older than the one above it, and
  younger than the one below it
• Deeper older

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3. Cross-cutting Relationships

• When a fault cuts through a rock or magma
  intrudes other rocks and crystallizes, we can
  assume the intrusion or fault is younger than the
  rocks affected
• If its cutting through it, it must be younger
  than what it is cutting through

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What can you tell about A-G?
D
G
C
B
F
A
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Or a more fun one
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Correlation of Rock Layers

• Correlation is the matching of rocks of similar
  ages
• Done to develop a geologic time scale that can be
  applied to the entire Earth
• This can create a more complete view of the
  geologic record of an area

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Geologic Time (using index fossils to date
things)

• The concept of geologic time is new (staggering)
  to many nongeologists.
• The current estimate is that the Earth is
  4,600,000,000 (4.6 billion) years old.
• As humans we have a hard time understanding the
  amount of time required for geologic events.
• We have a good idea of how long a century is.
  One thousand centuries is only 100,000 years.
  That huge amount of time is only 0.002 of the
  age of the Earth!
• An appreciation for the magnitude of geologic
  time is important because many processes are very
  gradual.

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• Geologic time is divided into different types of
  units.
• Note that each Eon, Era or Period represents a
  different amount of time. For example, the
  Cambrian period encompasses 65 million years
  whereas the Silurian period is only 30 million
  years old.
• The change in periods is related to the changing
  character of life on Earth and other changes in
  environment.
• The beginning of the Phanerozoic represents the
  explosion of life.
• The time before the Phanerozoic is commonly
  referred to as the PreCambrian and represents
  over 4 billion years of time. The Phanerozoic
  eon (abundant life) represents only the last 13
  of Earth time.

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So how do we put actual dates on things?
Absolute (Radiometric) Dating Using radioactive
decay of elements to determine the absolute age
of rocks. This is done using igneous and
metamorphic rocks.
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Radiometric Dating

• A method to determine the age of a rock formation
  by measuring the ratios of certain radioactive
  isotopes
• Radioactive isotopes decay at a constant rate
  (measured in Half-lives)
• Half-life is how long it takes for half of a
  radioactive isotope to decay into a stable
  daughter product (stable isotope)

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Radiometric Dating
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Radiometric Dating cont.

• Usually only performed on igneous rocks
• The minerals in igneous rocks all formed around
  the same time
• Sedimentary and Metamorphic rocks are formed from
  preexisting rocks, so the date would be from when
  the minerals were formed, not when the rock layer
  formed
• How do we do it?

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Radiation Types and Half Life a brief foray into
nuclear chemistry

• Radiation is use for many purposes
• Unless it is in large amounts, it is not usually
  harmful
• Many objects naturally give off radiation
• We can use this tendency to help us put dates to
  rocks and fossils

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Types of Radiation

• Alpha (?) a positively charged helium isotope
  - we usually ignore the charge because it
  involves electrons, not protons and neutrons
• Beta (ß) an electron
• Gamma (?) pure energy called a ray rather than
  a particle

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Other Nuclear Particles

• Neutron
• Positron a positive electron
• Proton usually referred to as hydrogen-1
• Any other elemental isotope

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Balancing Nuclear Reactions

• In the reactants (starting materials on the
  left side of an equation) and products (final
  products on the right side of an equation)
• Atomic numbers must balance
• and
• Mass numbers must balance
• Use a particle or isotope to fill in the missing
  protons and neutrons

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Nuclear Reactions

• Alpha emission

Note that mass number (A) goes down by 4 and
atomic number (Z) goes down by 2.
Nucleons (nuclear particles protons and
neutrons) are rearranged but conserved
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Nuclear Reactions

• Beta emission

Note that mass number (A) is unchanged and atomic
number (Z) goes up by 1.
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Other Types of Nuclear Reactions You Dont need
to Know

• Positron (01b) a positive electron

Electron capture the capture of an electron
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Learning Check

• What radioactive isotope is produced in the
  following bombardment of boron?
• 10B 4He ? 1n
• 5 2
  0

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Write Nuclear Equations!

• Write the nuclear equation for the beta emitter
  Co-60.

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Transuranium Elements

• Elements beyond 92 (transuranium) made starting
  with an g reaction
• 23892U 10n ---gt 23992U g
• 23992U ---gt 23993Np 0-1b
• 23993Np ---gt 23994Pu 0-1b

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Half-Life

• HALF-LIFE is the time that it takes for 1/2 a
  sample to decompose.
• The rate of a nuclear transformation depends only
  on the reactant concentration.

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Half-Life
Decay of 20.0 mg of 15O. What remains after 3
half-lives? After 5 half-lives?
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Kinetics of Radioactive Decay

• For each duration (half-life), one half of the
  substance decomposes.
• For example Ra-234 has a half-life of 3.6
  daysIf you start with 50 grams of Ra-234

After 3.6 days gt 25 grams After 7.2 days gt 12.5
grams After 10.8 days gt 6.25 grams
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Learning Check!

• The half life of I-123 is 13 hr. How much of a
  64 mg sample of I-123 is left after 39 hours?

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Radiocarbon Dating

• Radioactive C-14 is formed in the upper
  atmosphere by nuclear reactions initiated by
  neutrons in cosmic radiation
• 14N 1on ---gt 14C 1H
• The C-14 is oxidized to CO2, which circulates
  through the biosphere.
• When a plant dies, the C-14 is not replenished.
• But the C-14 continues to decay with t1/2 5730
  years.
• Activity of a sample can be used to date the
  sample.

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So now, we could use relative dating and absolute
dating techniques to figure out the ages of all
the layers seen below
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• The Earth is composed of several integrated parts
  (spheres) that interact with one another
• atmosphere
• hydrosphere
• solid earth (lithosphere)
• biosphere
• (cryosphere)

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The Earth System Youll see more of this later
Hydrosphere the global ocean is the most
prominent feature of our (blue) planet. The
oceans cover 71 of our planet and represent 97
of all the water on our planet. For now,
though, we care more about Atmosphere the
swirling clouds of the atmosphere represent the
very thin blanket of air that covers our planet.
It is not only the air we breathe, but protects
us from harmful radiation from the sun.
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A bit more about Earths Atmosphere

• The creation of the Earth through collisions
  initially left the surface of the planet molten
• Earth slowly cooled and formed a solid crust
• The gasses that were originally dissolved in the
  molten rocks were released
• Earths first atmosphere resembled the gasses
  from volcanic eruptions.
• The first atmosphere
• was composed of Water
• vapor, carbon dioxide,
• nitrogen and several
• trace gasses. No oxygen!

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And then what happened?

• As the planet continued to cool the water vapor
  in the atmosphere fell as rain
• Rain evaporated before or upon reaching the
  surface
• Evaporation increased the rate of cooling
• Eventually water accumulated in the low areas
  creating oceans and seas
• Most of the Carbon dioxide was dissolved in the
  oceans
• The remaining atmosphere was Nitrogen rich

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Continued Atmospheric Evolution

• Eventually primitive organisms started to use
  photosynthesis to generate energy
• This produced a byproduct of oxygen gas
• Initially the oxygen gas would attach to metals
  (iron oxide, aluminum oxide etc)
• Eventually the oxygen collected in the atmosphere
  (2.5 billion years ago)

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Or Graphically.
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Modern Composition

• 79 N2 Nitrogen gas
• 20 O2 Oxygen gas
• .93 Argon an inert (Noble) gas
• .039 Carbon Dioxide (CO2)
• Although it only makes a small percent it has
  large impacts globally
• .031 other gasses