Chapter 11: Geologic Time And The Rock Record

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Chapter 11 Geologic Time And The Rock Record
2
Introduction

• The concept that most geologic processes happen
  very slowly was proposed by James Hutton
  (1726-1797).
• Geologists sort Earths history into a sequence
  of events.
• Position in that sequence identifies relative
  age.
• Numerical age can be determined through analysis
  of the products of radioactive decay

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Reading The Record Of Layered Rocks

• Layered sedimentary or volcanic rocks contain
  important clues about past environments at and
  near Earths surface.
• Their sequence and relative ages provide the
  basis for reconstructing much of Earths history.
• The study of strata is called stratigraphy.

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Figure 11.1
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The Laws of Stratigraphy

• Most sediment is laid down in the sea, generally
  in relatively shallow waters, or by streams on
  the land.
• Each new layer is laid down horizontally over
  older ones.
• The law of original horizontality states that
  water-laid sediments are deposited in strata that
  are horizontal or nearly horizontal.

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Stratification, Superposition, and the Relative
Ages of Strata (1)

• The principle of stratigraphic superposition
  states that any sequence of sedimentary strata
  was deposited from bottom to top.
• Charles Lyell and other geologists of the
  nineteenth century speculated that it might be
  possible to determine numerical ages by using
  stratigraphic record.

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Figure 11.2
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Stratification, Superposition, and the Relative
Ages of Strata (2)

• Two assumptions must be correct for the method to
  work
• It must be assumed that the rate of sedimentation
  was constant throughout the time of sediment
  accumulation.
• It must be assumed that all strata exhibit
  conformity, meaning they have been deposited
  layer after layer without interruption.

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Stratification, Superposition, and the Relative
Ages of Strata (3)

• The first assumption is false because it can be
  observed today that sedimentation rates vary
  widely from place to place and time to time.
• The second and even more important assumption is
  false because sedimentation can be disrupted
  periodically by major environmental changes, such
  as sea level changes and tectonic activity that
  lead to intervals of erosion or non deposition.

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Kinds of Unconformities (1)

• An unconformity is a substantial break or gap in
  a stratigraphic sequence.
• Three important kinds of unconformities are found
  in sedimentary rocks
• Angular unconformity.
• The older strata were deformed and then cut off
  by erosion before the younger layers were
  deposited across them.

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Figure 11.3
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Kinds of Unconformities (2)

• Disconformity.
• It is an irregular surface of erosion between
  parallel strata.
• A disconformity implies a cessation of
  sedimentation and erosion, but not tilting.
• It is often hard to recognize, because the strata
  above and below are parallel.
• Nonconformity.
• Strata overlie igneous or metamorphic rock.

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Figure 11.4
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The Significance of Unconformities

• The many unconformities exposed in rocks of
  Earths crust are evidence that former seafloors
  were uplifted by tectonic forces and exposed to
  erosion.
• Preservation of a surface of erosion occurs when
  later tectonic forces depress the surface.
• The surface, in turn, becomes a site of
  deposition of sediment.

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Stratigraphic Classification (1)

• A rock-stratigraphic unit is any distinctive
  stratum that differs from the strata above and
  below.
• The basis of rock stratigraphy is the formation.
• A formation is a collection of similar strata
  that are sufficiently different from adjacent
  groups of strata so that on the basis of physical
  properties they constitute a distinctive,
  recognizable unit that can be used for geologic
  mapping over a wide area.

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Stratigraphic Classification (2)

• Each of the boundaries of a time-stratigraphic
  unit, upper and lower, is uniformly the same age.
• The primary time-stratigraphic unit is a system,
  which is chosen to represent a time interval
  sufficiently great so that such units can be used
  all over the world.

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Figure 11.6
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Stratigraphic Classification (3)

• The primary unit of geologic time is a geologic
  period, which is the time during which a geologic
  system accumulated.
• Correlation is the determination of equivalence
  in time-stratigraphic or rock-stratigraphic units
  of the succession of strata found in two or more
  different places.

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How Correlation Is Accomplished

• Correlation involves two main tasks
• Determining the relative ages of units exposed
  within a local area being studied (identifying
  the same formation wherever it crops out).
• Establishing the ages of the local rock units
  relative to a standard scale of geologic time.
• Distinctive fossils (index fossils)are especially
  useful for this purpose. If a distinctive index
  fossil is recognizable at an outcrop, a rapid and
  reliable means of correlation is available.

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Figure 11.7
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Figure 11.9
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The Geologic Column and the Geologic Time Scale

• In the nineteenth century, geologists began to
  assemble a geologic column, which is a composite
  column containing, in chronological order, the
  succession of known strata, fitted together on
  the basis of their fossils or other evidence of
  relative age.
• The corresponding column of time is the geologic
  time scale.

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Figure 11.10
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Eons

• An eon is the largest interval into which
  geologic time is divided.
• There are four eons.
• The Hadean Eon is the oldest
• Some of the samples brought back from the moon
  were formed during the Hadean Eon.
• The Archean Eon follows the Hadean.
• Archean rocks, which contain primitive
  microscopic life forms are the oldest rocks we
  know of on the Earth.
• The Proterozoic Eon follows the Archean.
• The Phanerozoic Eon is the most recent of the
  four eons.

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Eras (1)

• Each of the eons is subdivided into shorter time
  units called eras.
• The Phanerozoic Eon is divided into the
• Paleozoic (old life).
• Mesozoic (middle life).
• Cenozoic (recent life).

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Eras (2)

• In the Paleozoic Era, early land plants appeared,
  expanded and evolved. Developing animal life
  included marine invertebrates, fishes,
  amphibians,and reptiles.
• The Mesozoic Era saw the rise of the dinosaurs,
  which became the dominant vertebrates on land.
  Mammals first appeared during the Mesozoic Era as
  did flowering plants.
• Mammals dominated the Cenozoic Era. Grasses
  evolved during the Cenozoic Era, and became an
  important food for grazing mammals.

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Periods

• The Eras of the Phanerozoic Eon are divided into
  periods.
• The periods are defined on the basis of the
  fossils contained in the equivalent rocks.
• The two Periods are the Quaternary Period and the
  Tertiary Period

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Epochs

• Periods are further subdivided into epochs on the
  basis of the fossil record.
• The Tertiary Period is divided into these epochs
• Paleocene.
• Eocene.
• Oligocene.
• The Quaternary Period is divided into these
  epochs
• Holocene.
• Pleistocene.

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Early Attempts to Measure Geologic Time
Numerically (1)

• Early attempts to measure geologic time
  numerically were inaccurate.
• Edmund Halley suggested, in 1715, that sea salt
  might be used to date the ocean.
• John Joly finally made the necessary measurements
  and calculations in 1889. His determination of
  the oceans age, 90 million years, was not
  correct.
• Salts are added both by erosion and by submarine
  volcanism, but salts are also removed by
  solution.

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Early Attempts to Measure Geologic Time
Numerically (2)

• Lord Kelvin, a physicist, attempted to calculate
  the time Earth has been a solid body.
• By measuring the thermal properties of rock and
  estimating the present temperature of Earths
  interior, he calculated the time for the Earth to
  cool to its present state.
• His estimate of 100 million years is incorrect.
• The Earths interior is cooling so slowly that it
  has a nearly constant temperature over periods as
  long as hundreds of millions of years.

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Radioactivity (1)

• In 1896, the discovery of radioactivity provided
  the needed method to measure the age of the Earth
  accurately.
• Different kinds of atoms of an element that
  contain different numbers of neutrons are called
  isotopes.
• Most Isotopes of the chemical elements found in
  Earth are generally stable and not subject to
  change.

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Figure 11.11
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Radioactivity (2)

• A few isotopes, such as 14C, are radioactive.
• Radioactivity arises because of instability
  within an atomic nucleus.
• If the ratio of the number of neutrons (n) to the
  number of protons (p) is too high or too low, the
  atomic nucleus of a radioactive isotope will
  transform spontaneously to a nucleus of a more
  stable isotope of a different chemical element.

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Radioactivity (3)

• The process is called radioactive decay.
• An atomic nucleus undergoing radioactive decay is
  said to be the parent.
• The product arising form radioactive decay is
  called a daughter.

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Kinds of Radioactive Decay (1)

• Radioactive decay can happen in five ways
• 1. Beta decay emission of an electron from the
  nucleus.
• 2. Positron emission emission of a particle with
  the same mass as an electron but with a positive
  charge.
• 3. Electron capture by capture into the nucleus
  of one of the orbital electrons, a process that
  decreases the number of protons in the nucleus by
  one.

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Kinds of Radioactive Decay (2)

• 4. Alpha decay emission from the nucleus of a
  heavy atomic particle consisting of two neutrons
  and two protons called an a (alpha) particle.
• 5. Gamma ray emission emission of ? rays (gamma
  rays), which are very short-wavelength,
  high-energy electromagnetic rays.
• Gamma rays have no mass, so gamma ray emission
  does not affect either the atomic number or the
  mass number of an isotope.

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Figure 11.12
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Rates of Decay and the Half-Lives of Isotopes (1)

• The rate at which radioactive decay occurs varies
  among isotopes.
• Decay rates are unaffected by changes in the
  chemical and physical environment.
• The decay rate of a given isotope is the same in
  the mantle or in a sedimentary rock.
• In radioactive decay, the proportionfraction or
  percentageof parent atoms that decay during each
  unit of time is always the same.

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Rates of Decay and the Half-Lives of Isotopes (2)

• The rate of radioactive decay is measured in
  terms of half-life, the amount of time needed for
  the number of parent atoms to be reduced by one
  half.
• At the end of each unit of time (half-life), the
  number of parent atoms has decreased by exactly
  one-half.

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Figure 11.13
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Using Radioactivity to Measure Time

• Radioactivity in a mineral is like a clock.
• The length of time this clock has been ticking is
  the minerals radiometric age.
• Many natural radioactive isotopes can be used for
  radiometric dating, but six predominate in
  geologic studies
• Two radioactive isotopes of uranium plus
  radioactive isotopes of thorium, potassium,
  rubidium and carbon are used.
• In practice, an isotope can be used for dating
  samples that are no older than about six
  half-lives of the isotope.

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Radiocarbon Dating (1)

• 14C is especially useful for dating geologically
  young samples.
• The half-life of radiocarbon is short5730
  yearsby comparison with the half-lives of most
  isotopes used for radiometric dating.
• Radiocarbon is continuously created in the
  atmosphere through bombardment of 14C by neutrons
  created by cosmic radiation.

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Figure 11.14
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Radiocarbon Dating (2)

• Though some variations have been identified, the
  proportion of 14C is nearly constant throughout
  the atmosphere and biosphere.
• Living organisms have the same proportion of 14C
  In their bodies as exists in their environment.
• No carbon is added after death, so by measuring
  the radioactivity remaining in an organic sample,
  we can calculate how many half-lives ago the
  organism died.

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Radiometric Dating and the Geologic Column

• Through various methods of radiometric dating,
  geologists have determined the dates of
  solidification of many bodies of igneous rock.
• Moon dust brought back by astronauts, is 4.55
  billion years old.
• The Earth was formed approximately 4.55 billion
  years ago.

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Figure 11.15
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Figure B01
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Figure B02
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Magnetic Polarity Time Scale (1)

• Certain rocks become permanent magnets as a
  result of the way they form.
• Magnetite and certain other iron-bearing minerals
  can become permanently magnetized.
• Above a certain temperature (called the Curie
  point), the thermal agitation of atoms is such
  that permanent magnetism is impossible.
• Below that temperature, however, the magnetic
  fields of adjacent iron atoms reinforce each
  other.

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Figure 11.16
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Figure 11.17
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Magnetic Polarity Time Scale (2)

• As solidified lava cools, the temperature will
  drop below 580oC, the Curie point for magnetite.
• When the temperature drops below the Curie point,
  all the magnetite grains in the rock become tiny
  permanent magnets with the same polarity as
  Earths field.
• All lava formed at the same time records the same
  magnetic polarity information.

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Figure 11.18
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Magnetic Polarity Time Scale (3)

• The Earths polarity has shifted in the past. A
  period in which polarity remains stable is called
  a magnetic chron.
• The four most recent chrons have been named for
  scientists who made great contributions to
  studies of magnetism. The four chrons below
  occurred during the last 4.5 million years. From
  the most recent to the oldest
• Brunhes.
• Matuyama.
• Gauss.
• Gilbert.

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Figure 11.19
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Primordial Gasses

• Studies of volcanic gases provide other clues to
  the age of the Earth.
• Three gases, 40Ar (daughter of 40K), 3He, and
  36Ar (both primordial gases trapped in Earth from
  the solar nebula), are being released, but they
  are not being recycled.
• Because they accumulate in the atmosphere, their
  growing proportion can be used to estimate the
  age of the Earth.