Geology 125 Mineralogy and Petrology
Igneous Rocks Major Concepts
Goals
After completing this section of the course you should
be able to
1) recognize the prominent characteristics of
igneous rocks,
2) recognize and classify the major igneous rock types,
3) write a professional description of the major igneous
rock types.
4) have general knowledge with respect to how igneous
rocks form.
Igneous Rock Description
What are the major rock characteristics that can be used
to adequately describe a rock?
-
Grain Type (i.e., mineral identification)
-
Modal Abundance (i.e., volume percentages of the minerals
which comprise the rock)
-
Grain Size
-
aphanitic vs phaneritic
-
pegmatitic, coarse-, medium-, fine-grained, aplitic etc.
-
equigranular vs inequigranular
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Grain Shape
-
euhedral
-
subhedral
-
anhedral
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Grain Distribution
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preferred alignments, fabrics
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Special features and textures
-
e.g., granophyric, orbicular etc.
Phanerites (plutonic igneous rocks)
Felsic Phaneritic (coarse grained felsic plutonic rocks)
are classified on the basis of the modal abundance of four key minerals:
-
Alkali feldspar
-
Plagioclase
-
Quartz
-
Feldspathoids
The color index of the rock is the volume percentage of the
rest of the minerals that comprise the rock (e.g., biotite, amphibole etc.)
and the following terms can be used to describe the color index of igneous
rocks
This classification scheme is presented in Figure
4-8 & 4-9 of your text and was presented as a handout. This classification
scheme was also discussed in lab. Given the modal abundance of the minerals
that comprise a rock you should be able to correctly classify the rock.
Ultramafic Phaneritic (coarse grained mafic plutonic rocks)
are classified on the basis of the modal abundance of five key minerals:
-
Olivine
-
Orthopyroxene
-
Clinopyroxene
-
Plagioclase
-
Amphibole
An example of one of these classification diagrams is presented
in Figure 4-8 & 4-20 of your text, and as a handout in class. Given
the modal abundance of the minerals that comprise an ultramafic rock you
should be able to correctly classify the rock.
Aphanites and Aphanite Porphyries (shallow plutonic
and volcanic igneous rocks)
We discuss the inherent problems of modal classification
schemes for fine- to glassy (vitric) rock types. A chemical classification
scheme based on the chemical composition of the rock using the abundance
of Total Alkalis vs Silica was presented in class. The agreement between
this classification scheme and the modal classification scheme is quite
good. Thus, it is still worthwhile to be familiar with the modal scheme
which was presented as a handout in class.
Tables 4-4 and 4-5 are both helpful in classifying these
types of igneous rocks.
Pyroclasts, Tephra and Pyroclastic Rocks (volcanic
igneous rocks)
We discussed the origin of tephra and pyroclastic rocks
and their classification. Classification schemes are presented in Figures
4-34 & 4-35 and in Table 4-6 of your text. This material was also distributed
in class.
These rocks form from material violently injected into
the atmosphere from erupting from volcanoes. The material then returns
to the Earth surface to be deposited, and thus shares many characteristics
of sedimentary rocks (e.g., sorting).
The material is comprised of glass shards, crystal fragments,
and rock fragments, and can also be classified with respect to the size
of this material (bombs, lapilli) and the physical state of the material
during eruption (magma, rock).
Important modes of deposition are from nuee ardentes,
lahars, ash-fall, and ash-flow tuffs. The rocks can also be described in
terms of the extent of welding.
Petrogenesis of Igneous Rocks
Igneous rocks crystallize from magma. How can we explain
the diversity in the composition of igneous rocks? Do we need a distinct
magma composition to produce each igneous rock?
Igneous rocks represent a past thermal anomaly, and thus
require a heat source to induce melting.
Possible heat sources include:
-
Decompression melting (typical in extensional environments
such as the mid-oceanic ridge)
-
Introduction of a fluid (typically H2O-rich) to lower drastically
lower the solidus (melting point) of rocks.
-
Injection of high temperature magmas (e.g., basaltic magma)
into rocks with considerably lower melting points (e.g., felsic rocks of
the continental crust, and by the way I spelled that word all on my own.....)
to induce melting.
The composition of the Source Material (gotta have something
to melt) has a significant effect on the composition of the melt produced.
-
melting of mafic rocks (e.g., lherzolites) which comprise
the mantle produces mafic magmas, which in turn rise up to intrude the
lithosphere and crystallize to form mafic igneous rocks (e.g., gabbros
beneath the surface and basalts at the surface).
-
melting of felsic rocks of the continental crust gives rise
to felsic magmas which in turn crystallize to form felsic igneous rocks
(e.g., granite beneath the Earth's surface and rhyolite at the Earth's
surface).
The composition of magma can be changed after it segregates
form the source region by "magmatic processes"
-
Fractional Crystallization-seperation of early formed crystals
from the residual melt during solidification (e.g., as a result of gravitational
separation) can give rise to igneous rocks of distinct compositions. For
example basalt can undergo extreme fractional crystallization to produce
a liquid of rhyolitic composition, although the yield will be small (approximately
10%).
-
Magma mixing-blending of a liquid of basaltic composition
with one of a rhyolitic composition to yield a liquid of intermediate composition
which then crystallizes to form an "intermediate" igneous rock type (e.g.,
quartz monzonite).
The tectonic setting (compressional vs extensional) can impart
a distinct geochemical signature on the resulting igneous rock.
-
Igneous rocks that formed as a result of subduction zone
processes are enriched in elements soluble in a fluid phase (e.g., Ba,
Sr, etc.) and depleted in insoluble elements (Nb, Ta, etc.)
Movie "In the Path of A Killer Volcano" Employee's
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