Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Lab 2: Crystal Growth and Igneous Textures
The identification and classification of an igneous rock is typically a two fold process that requires
the determination of the type of cooling unit that the rock sample was taken from in the field and an
estimation of its mineralogical and chemical composition. The identification of the cooling unit is
best done on the basis of structures and textures in the field and we will look at this aspect in more
detail in the next lab. The first order question you must decide for any igneous rock is whether it is
a volcanic or plutonic rock. After the field context of the rock sample, this is best answered on the
basis of textures in hand specimen and thin section.
Volcanic (extrusive) rocks cool quickly and crystallize rapidly on the surface and are thus
typically relatively fine-grained and have compositions that approximate that of the liquids from
which they formed. Volcanic rocks are commonly phenocrystic, however, containing some
larger euhedral crystals that formed at slower cooling rates before eruption. They also
commonly contain vesicles or amygdules (filled vesicles) formed by exsolving volatiles, and
may exhibit reddening and/or brecciation along flow boundaries if they are subaerial. The
presence of pillow structures with glassy margins is indicative of sub-aqueous eruption.
Plutonic (intrusive) rocks crystallize more slowly below the Earth’s surface and are typically
coarser-grained and more equi-granular than volcanic rocks. Their grain-size is indicative of
both the rate of cooling, and therefore the depth of crystallization, and the viscosity of the
magma, which is a function of composition and water content. Plutonic rocks are commonly in
part cumulate, that is their compositions reflect the mechanical accumulation of crystals rather
than a frozen liquid. Some plutonic rocks are almost completely cumulate in nature, while others
reflect a mixture of accumulated crystals and frozen interstitial liquid.
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Station A – Mafic Volcanics – Pillow lavas:
These rocks are specimens of the margins of pillow lavas that erupted into water. They
cooled very rapidly and 'quenched in' a virtual snapshot of the magmas as they erupted. They
are ideal samples with which to determine the phenocryst assemblage of the magma at the
time of eruption.
Specimens AS-7a & WG-3 are very old (~1.9 Ga), and their primary mineralogy has been
replaced by greenschist facies metamorphic minerals, which is responsible for their green colour.
The primary pillow rim texture, however are still well preserved.
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Stations B1 and B2 – Mafic Volcanics - Lava Flows
The 2 sets of 4 rocks at stations B1 and B2 are samples taken across single lava flows:
B1 - the darker set was taken across an alkaline olivine basalt flow from the Canadian
cordillera that is 4 metres in thickness. Samples A, B, C, D
B2 - the lighter set was taken across a tholeiitic olivine basalt flow from Baffin Island
that is 5 metres in thickness. Samples Pd-48, PD-41, PD-46, PD-45, and AK-42.
The samples from each flow have approximately the same chemical composition, and
mineralogy, but differ texturally because of their different cooling histories.
For each sample set, determine the location of each of the samples in their respective flows by
examining their textures in hand specimen.
Describe the variation in the textures of the minerals and vesicles across the same flow and
briefly rationalize this variation in terms of the cooling history of the lava flow.
Station C: Mafic to Intermediate Volcanics - Lava Flows
The samples at this station are random pieces of lava flows with mafic to intermediate
compositions. Examine each specimen to determine what part of the lava flow it represents,
using the criteria developed in Section B. Identify any phenocrysts in each specimen, and
qualitatively estimate where the specimen lies in the mafic to intermediate compositional
spectrum.
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Station D1: Felsic Volcanics – Lava Flows
Felsic volcanic rocks commonly have the lightest coloured matrices, if they are holocrystalline.
However, silica-rich magmas will often quench to a glass that is dark in colour and similar in
appearance to basaltic glass, unless examined through a thin chip, in which case basaltic glass
has a darker brown colour. It is unusual, however, for an entire sample of basalt to be glassy
because it is harder to quench silica-poor mafic magmas. In addition, glassy felsic lavas
frequently exhibit spherulites, small spherical bodies composed of radiating felsic crystals that
are though to have grown by the devitrification of glass.
spherulites
Extremely fine-grained varieties of felsic lavas will also be relatively dark in colour and easily
confused with more mafic lavas in the field. The presence of concoidal fracture in a extremely finegrained dark matrix that cannot be scratched with a knife is typically diagnostic of a felsic
composition. No matter how dark the fresh surface, the weathering surface of felsic rocks will
typically be light coloured compared to that of mafic to intermediate rocks.
obsidian
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Felsic volcanic flows are commonly flow banded. Flow banding is defined by alternating
layers of glass and finely crystalline material, sometimes coalescing spherulites, which
define the slip planes along which the viscous felsic magma flowed. The crystalline layers
are thought to be those along which volatile bubbles have concentrated during lamellar flow.
These volatiles aided the crystallisation of the magma in these layers, which tend to be
lighter coloured than the darker layers that are depleted in such bubbles and quenched to
glass. This banding is frequently preserved even when the felsic volcanic rock has been
completely recrystallized. This type of banding is not observed in mafic to intermediate
volcanic rocks. Furthermore, the margins of felsic flows are always fragmented, and their
brecciated margins constitute a larger proportion (thicker) of the flow than in the case of
mafic aa flows. Ropy or pahoehoe tops are not observed in felsic lavas.
Flow-banded rhyolite
Although glassy and crypto-crystalline felsic volcanic rocks are relatively dark in colour,
most old felsic lavas have recrystallized, and thus are light in colour reflecting the fact
that their compositions are rich in felsic minerals (E8733, E8969, 2.7 Ga).
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Station D2: Felsic Fragmental Volcanics – Pyroclastic rocks
The majority of felsic volcanic rocks are not lava flows, but pyroclastic rocks produced
by the accumulation of clasts or fragments produced in an explosive volcanic eruption.
Felsic magmas commonly erupt explosively because their Si-rich compositions make
them so viscous that exsolving volatiles can not escape. Because of the combination of
high melt viscosity and high gas contents, the majority of felsic magmas erupt as
pyroclastic deposits than rather than effusive lava flows. Mafic magmas can also produce
fragmental volcanic rocks during explosive eruptions, but typically erupt as more effusive
lava flows. Some petrologists view pyroclastic rocks as sedimentary rather than igneous
rocks.
This station contains a wide variety of fragmental volcanic rocks. The classification of
fragmental rocks requires the consideration of a number of features: grain size, matrix
versus clast support, degree of sorting, presence of layering or grading, monomictic
versus polymictic character, signs of aerial or hot emplacement. Examine each specimen
in terms of these features and classify them according to the first figure in this lab.
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Air-fall tuffs are deposited relatively cold and may or may
not be layered. Water-lain tuffs are more typically layered
and exhibit better sorting than air fall tuffs. Air fall tuffs
are commonly reworked by wind and water, showing
structures such as fine-scale cross-bedding. Such rocks are
technically sedimentary, rather than volcanic, and they are
sometimes referred to as epiclastic as opposed to
pyroclastic rocks. If you don’t know which a rock is, it is
safer to use the term fragmental volcanic or volcanoclastic
rock can be used – a clastic rock made up of fragments of
volcanic rock and/or broken crystals.
Some of the samples in this section are welded tuffs or ignimbrites. They are tuffs that have
between deposited by pyroclastic flows.
They are characteristically poorly sorted
and lack well defined layering, although
they commonly exhibit grading. In some
cases, they are hot enough that the
fragments are plastic and welded together
after emplacement. Signs of hot deposition
include flattened pumice or glass fragments
known as fiamme, columnar joints,
alteration rims around larger fragments, and
the presence of a hard glassy matrix.
Strongly welded rocks are typically
physically hard to break and when they do,
fiamme
they tend to break across fragments. Many
ash flow tuffs are not welded, however, and are recognized on the basis of lack of layering
and/or poor sorting. In some cases, the interior of ash flows are welded, while the margins are
not, or ash flows are welded near their source, but not welded far from their source.
Pyroclastic Surge deposits are distinguished from pyroclastic flow deposits by the presence of
well developed planar or wavy layering and low-angle, large-scale dune cross bedding. They are
typically thinner than pyroclastic flows deposits.
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Intro. Petrology EPSC 212, Lab. 2, Francis 2014
Station E: Felsic Intrusive Rocks
Intrusive felsic dykes can range from aplites, characterized by a fine-grain sugary texture
thought to represent rapid crystallization due to the loss of volatiles, to very coarsegrained pegmatites that crystallize slowly from volatile rich melts.
High-level felsic dykes are commonly porphyritic, characterized by euhedral quartz
and/or feldpsar phenocrysts in a very fine-grained felsic matrix (QFP). More deepseated felsic intrusive rocks tend to be more coarse-grained and equigranular.
Depending on crystallization conditions, quartz and feldspars can develop a variety of
intergrowths. Graphic texture is an intergrowth of K-feldspar with cuneiform or patchy
quartz, formed by selective nucleation of quartz along the feldspar crystal edges and
corners. The K-feldspar in coarse-grained deep-seated felsic plutons commonly exhibit
perthitic exsolution lamellae of albite and K-spar due to slow cooling that has enabled
the feldspar to re-equilibrate at relatively low temperatures.
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Station F: Mafic Plutonic Rocks
This station contains examples of mafic plutonic rocks. They range from high-level (near
surface) porphyritic dykes, whose grain-size is virtually indistinguishable from mafic
volcanic rocks in the interiors of flows (seen at station B and C) to coarser-grained
cumulate rocks formed in large deep-seated intrusions.
High-level mafic dykes are dark in colour and commonly lack vesicles, but may be
difficult to distinguish from mafic lavas. The presence of finer grained chilled margins
against their host rock is diagnostic.
Deeper-seated cumulate rocks are typically coarser-grained and rarely porphyritic. They
may sometimes be recognized in outcrop by the presence of mineral layering, or the
alignment of large tabular crystals such as plagioclase. In some cases they are
characterized by a restricted mineralogy, for example cumulates of olivine are called
dunites, cumulates of plagioclase are termed anorthosites. Another characteristic
feature of cumulate rocks is the presence of oikocrysts, large crystals of pyroxene or
plagioclase enclosing smaller equant crystals of earlier formed olivine (chadocrysts).
These oikocrysts are thought to crystallize from the interstitial liquid trapped between
mechanically accumulated crystals.
Examine the mafic intrusive rocks at station F, comparing them to the mafic volcanic
rocks that you have seen at station B, and C. Identify all the minerals you can and look
for features indicative of cumulate rocks, such as layering or oikocrysts.
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