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Chapter 11: Geologic Time And The Rock Record

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Title: Chapter 11: Geologic Time And The Rock Record


1
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

3
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.

4
Figure 11.1
5
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.

6
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.

7
Figure 11.2
8
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.

9
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.

10
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.

11
Figure 11.3
12
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.

13
Figure 11.4
14
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.

15
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.

16
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.

17
Figure 11.6
18
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.

19
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.

20
Figure 11.7
21
Figure 11.9
22
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.

23
Figure 11.10
24
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.

25
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).

26
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.

27
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

28
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.

29
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.

30
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.

31
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.

32
Figure 11.11
33
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.

34
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.

35
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.

36
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.

37
Figure 11.12
38
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.

39
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.

40
Figure 11.13
41
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.

42
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.

43
Figure 11.14
44
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.

45
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.

46
Figure 11.15
47
Figure B01
48
Figure B02
49
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.

50
Figure 11.16
51
Figure 11.17
52
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.

53
Figure 11.18
54
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.

55
Figure 11.19
56
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.
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