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Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks


Fine-grained rocks A rock in which most of the mineral grains are less than ... Plutonic rocks Igneous rock formed at great depth. ... – PowerPoint PPT presentation

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Title: Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks

Chapter 3
  • Igneous Rocks, Intrusive Activity, and the Origin
    of Igneous Rocks
  • Index ?

Picture on pg. 53
  • ? ?

Igneous Rocks
  • Igneous Rocks A rock formed or apparently
    formed from solidification of magma.
  • Igneous rocks may be either extrusive if they
    form at the earths surface (e.g., basalt) or
    intrusive if magma solidifies underground.
  • ? ?

How do we know?
  • Unlike the volcanic rock in Hawaii, nobody has
    ever seen magma solidify into intrusive rock. So
    what evidence suggests that bodies of granite
    (and other intrusive rocks) solidified
    underground from magma?
  • ? ?

Proof I
  • Mineralogically and chemically, intrusive rocks
    are essentially identical to volcanic rocks.
  • Volcanic rocks are fine-grained.
  • Experiments have confirmed that most of the
    minerals in these rocks can form only at high
    temperatures. More evidence comes from examining
    intrusive contacts, such as shown in Fig. 3.1 and
    Fig. 3.2. (A contact is a surface separating
    different rock types.
  • Preexisting solid rock, country rock, appears to
    have been forcibly broken by an intruding liquid,
    with the magma flowing into the fractures that
    developed. Country rock is an accepted term for
    any older rock into which an igneous body
  • ? ?

Proof II
  • Close examination of the country rock immediately
    adjacent to the intrusive rock usually indicates
    that it appears baked close to the contact with
    the intrusive rock.
  • Rock types of the country rock often match
    xenoliths, fragments of rock that are distinct
    from the body of igneous rocks in which they are
  • In the intrusive rock adjacent to contacts with
    country rock are chill zones, finer-grained rocks
    that indicate magma solidified more quickly here
    because of the rapid loss of heat to cooler heat.
  • ? ?

Different Types of Igneous Rocks
  • Fine-grained rocks A rock in which most of the
    mineral grains are less than one millimeter
    across (igneous) or less than 1/16 mm
  • Plutonic rocks Igneous rock formed at great
  • Coarse-grained rocks Rock in which most of the
    grains are larger than 1 millimeter (igneous)
    or 2 millimeters (sedimentary). (Fig. 3.3)
  • ? ?

Identification of Igneous Rocks
  • Igneous rock names are based on texture (notably
    grain size) and mineralogical composition (which
    reflects chemical composition). Mineralogically
    (and chemically) equivalent rocks are
    granite-rhyolite, diorite-andesite, and
    gabbro-basalt. The relationships between igneous
    rocks are shown in Fig. 3.4.
  • Table 3.1 Fig. 3.5
  • ? ?

Intrusive Bodies
  • Intrusions, or intrusive structures, are bodies
    of intrusive rock whose names are based on their
    size and shape, as well as their relationship to
    surrounding rocks. They are important aspects of
    the architecture, or structure, of the earths
    crust. The various intrusions are named and
    classified on the basis of the following
    considerations (1) Is the body large or small?
    (2) Does it have a particular geometric shape?
    (3) Did the rock form at a considerable depth or
    was it a shallow intrusion? (4) Does it follow
    layering in the country rock or not?
  • ? ?

Volcanic neck
  • A volcanic neck is an intrusive structure
    apparently formed from magma that solidified
    within the throat of a volcano. One of the best
    examples is Ship Rock in New Mexico. (Fig. 3.6)
  • ? ?

Dikes and Sills
  • Dike A tabular, discordant intrusive structure.
  • Fig. 3.7 Fig. 3.8
  • Discordant Not parallel to any layering or
    parallel planes.
  • Sill A tabular intrusive concordant with the
    country rock.
  • Fig. 3.9
  • Concordant Parallel to layering or earlier
    developed planar structures.
  • ? ?

Intrusives That Crystallize at Depth
  • Pluton An igneous body that crystallize deep
  • Stock A small discordant pluton with an
    outcropping area of less than 100 square
  • Batholith A large discordant pluton with an
    outcropping area greater than 100 square
    kilometers. (Fig. 3.10)
  • Diapir Bodies of rock (e.g., rock salt) or
    magma that ascend within the earths interior
    because they are less dense than the surrounding
    rock. (Fig. 3.11)
  • ? ?

Geothermal Gradient
  • Geothermal Gradient The rate at which
    temperature increases with increasing
    depth beneath the surface It is, on the
    average, to be about 3oC for each 100
    meters (30oC/km) of depth in the upper part
    of the crust.
  • Fig. 3.13
  • ? ?

Factors That Control Melting Temperatures
  • Pressure
  • The melting point of a mineral generally
    increases with increasing pressure. Pressure
    increases with depth in the earths crust, just
    as temperature does.
  • Water Under Pressure
  • If enough gas, especially water vapor, is present
    and under high pressure, a dramatic change occurs
    in the melting process. Water vapor sealed in
    under high pressure by overlying rocks helps
    break down crystal structures. High water
    pressure can significantly lower the melting
    points of minerals. (Fig. 3.14)
  • Effect of Mixed Minerals
  • Two metals as in solder can be mixed in a
    ratio that lowers their melting temperature far
    below that of the melting points of the pure
    metals. (Fig. 3.15)
  • ? ?

Differentiation and Bowens Reaction Theory
  • Differentiation is the process by which different
    ingredients separate from an originally
    homogenous mixture. In the early part of the
    twentieth century, N. L. Bowen conducted a series
    of laboratory experiments demonstrating that
    differentiation is a plausible way for silicic
    and mafic rocks to form from a single parent
  • Bowens reaction series is the sequence in which
    minerals crystallize from a cooling magma, as
    demonstrated by Bowens laboratory experiments.
    In simplest terms, Bowens reaction series shows
    that those minerals with the highest melting
    temperatures crystallize from the cooling magma
    before those with lower melting points. However,
    the concept is a bit more complicated than that.
  • ? ?

  • Crystallization begins along two branches, the
    discontinuous branch and the continuous branch.
    In the discontinuous branch, one mineral changes
    to another at discrete temperatures during
    cooling and solidification of the magma. Changes
    in the continuous branch occur gradationally
    through a range in temperatures and affect only
    the one mineral, plagioclase. Crystallization
    takes lace simultaneously along both branches.
  • ? ?

  • A very hot magma may melt some of the country
    rock and assimilate the newly molten material
    into the magma (Fig. 3.18) This is like putting a
    few ice cubes into a cup of hot coffee. The ice
    melts and the coffee cools as it becomes diluted.
  • ? ?

Mixing of Magmas
  • If two magmas meet and merge within the crust,
    the combined magma will be compositionally
    intermediate. (Fig. 3.19)
  • ? ?

Fig. 3.22
  • Pg. 71
  • ? ?

Fig. 3.23
  • Pg. 72
  • ? Back to
    the Beginning

Fig. 3.1
  • Pg. 54
  • Back

Fig. 3.2
  • Pg. 54
  • Back

Fig. 3.3
  • Pg. 55
  • Back

Fig. 3.4
  • Pg. 56
  • Back

Table 3.1
  • Pg. 56
  • Back

Fig. 3.5
  • Pg. 57
  • Back

Fig. 3.6
  • Pg. 60
  • Back

Fig. 3.7
  • Pg. 61
  • Back

Fig. 3.8
  • Pg. 61
  • Back

Fig. 3.9
  • Pg. 62
  • Back

Fig. 3.10
  • Pg. 62
  • Back

Fig. 3.11
  • Pg. 63
  • Back

Fig. 3.13
  • Pg. 64
  • Back

Fig. 3.14
  • Pg. 65
  • Back

Fig. 3.15
  • Pg. 66
  • Back

Bowens Reaction Series
  • Pg. 3.16
  • Back

Fig. 3.18
  • Pg. 69
  • Back

Fig. 3.19
  • Pg. 69
  • Back

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