Name 1______________Name 2__________ Section
Geoscience 100 Lab 5 Fall 2011: Igneous Processes and Rocks
This part of your lab work is worth 2% of the course total. In addition, inspect
laboratory specimens of igneous rocks in the ward’s kit and hand specimens to see real
examples corresponding to the photos in 5.5 – 5.11, in order to practice the identification
of texture, contained minerals and other constituents, and to classify the rocks.
Remember that there will be a rock quiz in a few weeks. It is useful also to compare
igneous rocks with other rocks (sedimentary & metamoprphic). In order to interpret the
nature and origin of igneous rocks it is very important to KNOW the textures especially
those which are particular to igneous rocks. Also know how to identify the minerals and
other constituents using your hand lens, hardness kits and the tables in Chapter 2. The
first step is to use colour index Figure 5.2.1 and mineral proportions in the nomogram
Fig.5.2.2, then to examine textures as per figure 5.2.3. The rock names are correlated to
textures, minerals and colour index and these characteristic properties are combined
classification & texture chart in Fig. 5.3.3. This figure is a good summary but remember
that it is generalized. For example, although a distinction is made between intrusive and
extrusive rocks, be aware that texture can vary within a given intrusion or extrusion,
dependent on size, composition, temperature, viscosity, etc. of the magma or lava.
Thermal gradients can vary as can cooling times and even the composition of the original
melt. One rock name does not describe a whole batholith or even the entire thickness of a
thick lava flow, let alone a single volcano. Often many closely related rocks occur
together in close map or stratigraphic association.
Activity 5A
Igneous rocks are classified and interpreted by their mineralogy and texture. Look at Fig.
5.2 and answer the following questions giving both word and numerical values for each
colour index. Eg. CI = Sum (all dark “MAFIC” minerals). For (1%Biotite + 2%
Amphibole + 3% Magnetite), CI = 6 and the rock is Felsic. If its texture is fine grained
with a few large phenocrysts, it is a porphyritic rhyolite.
Use the General names on the top row of Fig 5.2.1 for 1 & 2 below. For the rest give the
most precise rock name from Fig 5.2.3.
1. If a rock contains 3% Biotite and 5% Amphibole, what is its numerical colour
index? CI =_____ & general classification using the top row _____________. (2)
2. A rock with 0% Quartz, has a colour index_____ & classification __________.(2)
3. A fine grained rock with both Quartz & Pyroxene is classified as a/an _______.(2)
4. A phanertic-porphyritic rock with predominant large phenocrysts of Amphibole &
Plagioclase is called a/an ______________ __________. (2)
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5. Examine figure 5.2 for classification and 5.3 for the approximate temperature
according to the 1st mineral to crystallize in Bowen’s Reaction Series for each
rock type below. The percentages of constituent minerals and textures come from
examining thin sections. Recall that the CI is only the sum of the mafic or dark
coloured minerals, not the felsic ones! Give the CI and identify the corresponding
names of the rocks.
(a) Aphanitic rock
18% Amphibole, 19% Pyroxene, 59% Plagioclase,
1% K-Feldspar, 3% Quartz
CI ____ Name ______________ (2)
(b) Pegmatitic rock 8% Amphibole, 5% Biotite, 9% Ca-Plagioclase,
55% K-Feldspar, 4% Muscovite, 25% Quartz
CI ____ Name __________ (2)
(c) Coarse pyroclastic
8% Biotite, 12% Amphibole, 30% Plagioclase,
3% K-feldspar, 16% Quartz
CI ____ Name _______________ (2)
(d) Plutonic rock 9% Amphibole, 35% Pyroxene, 51% Olivine, 5% Plagioclase
CI ____ Name _____________ (2)
Activity 5C: Intrusion & Eruption of Volcanic Landforms & Reading Maps
6. If the rock in question 5(d) came from a 1m-wide dike, how would the texture vary
across the intrusion? Examine Figure 5.4 to figure out what mineral crystallizes first,
next, … last. Make a careful drawing labeling the minerals & texture across this intrusion
from its cold walls to its hot core. Since the dyke comes in hot, it loses the most heat first
at the walls. It solidifies at the walls first then cools inwards. Use the crystallization
temperatures from Bowen’s Reaction Series on Fig 5.3. (5)
Temperature profile
(draw T°C profile)
Wall Rock
_|___________Dyke_______________|__ Wall Rock_____
Texture
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7. Examine intrusive and extrusive shapes and names in Fig 5.1 & Fig. 5.5. In which type
of igneous intrusive body (intrusive shape name) and geological setting (tectonic) would
the rock in Question 2 (above) most likely occur? Keep in mind that this is the most
common igneous rock type and structure on the planet!
Body ____________________ & Tectonic Setting ________________________. (2)
8. Look at Figure 5.3 of Bowen’s Reaction Series and note the relationship of igneous
minerals to their respective ranges of crystallization temperature. Note that most common
magmas have a crystallization or melting range of about 200°C between the liquidus and
the solidus. Once a magma cools by about 200° C it is too stiff to flow or for diffusion to
let ions move around and grow any more crystals. This is an “idealized model” subject to
“real-world” modifications. Notice the sequence of crystallization from the top of the
diagram down and how this relates to the position of minerals as read from right to left on
Fig. 5.2.
Look at the geology maps of “Victoria” and “Fraser River” on the walls of F300.
On the Victoria map there is a legend on the left-hand side of the map.
Locate (i) Tm Metchosin volcanics and Tsg Sooke Gabbro
(ii) Jg Island Intrusions
The detailed descriptions of each of these rock types is given in the legend block on the
left hand side of the map.
Use Figs 5.2, 5.3 and the map to complete the table:
T°C _____ to T°C_____
Rock_______|______Minerals Present |__Texture | Crystallization Range
Tm
Tsg
Jg
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Activity 5.7
Shiprock NM. Miocene Volcanic Centre in Stereo air photos. Examine the air photos in
fig 5.13 then find the corresponding intrusive feature on the block diagram, Fig 5.12.
a. What kind of body is labeled A? ___________________ (1)
b. What kind of body is labeled B? ___________________ (1)
Activity 5.8
Geological map of SE Pennsylvania, Fig 5.14. Examine the inset for the geographic
location near the western margin of the Atlantic Ocean, which you’ll recall started
forming when Pangea began to rift at about 200 Ma. The brown-tan areas are old
Paleozoic basement rocks from a previous Wilson Cycle. The green areas are subhorizontal, 220-200 Ma Permian continental red beds sediments of sands and muds
deposited in narrow river valleys, shed off the Paleozoic. The pink areas are 190 Ma
Jurassic mafic magmatic rocks.
a. Describe the shapes of the pink igneous bodies, interpret their 3 dimensional
geometry from the map view and discuss the reasons for your answers: (2)
b. Notice the igneous bodies labeled B. Describe these shapes and interpret their
3 dimensional geometry. Explain what do you think they are and discuss your
reasons. (2)
c. What other landforms must have been present at 180 Ma in this active igneous
landscape? Give your reasons. (2)
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Activity 5.1: Glassy and Vesicular Texture of Igneous Rocks (Candy Lava Flow)
Examine the laboratory demonstration put on by your instructor using Sugar, Water, A
hot plate and an aluminum foil “river bed” for your lava flow to move down.
A. Describe the initial viscosity and temperature of the molten sugar candy lava
flow. (2)
B. Describe what changes occurred to the viscosity and temperature as the flow
moved and hardened and explain how you can tell. (3)
C. Compare the final broken texture of the sugar lava flow to 2 objects in the lab.
What can you infer about the cooling history of those other items? (3)
D. Cut or break off a piece of candy-lava and examine with your hand lens. Note the
vesicles. What do they contain inside? What stopped it from escaping to the
atmosphere? (2)
E. When a saturated sugar solution evaporates slowly, large orderly interlocking
crystals grow. What 2 things prevented the sugar lava flow from doing the same
thing as it cooled on the Aluminum foil? (2)
F. Look through the photos in Fig 5.1, the wards kit of igneous rocks and the lab
specimens for more detailed description. Find one or more samples which have
textures like 5C . Explain the processes that led to this texture. (2)
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G. Look through the photos in Fig 5.1, the wards kit of igneous rocks and the lab
specimens for more detailed description. Find one or more samples which have
textures like 5D . Explain the processes that led to this texture. (2)
Activity 5.2 Crystalline Textures of Igneous Rocks: Phaneritic, Pegmatitic,
Porphyritic, Phenocrysts and Groundmass. Phaneritic means wholly crystalline
and made of crystals visible to the naked eye (>1-2 mm in size). Pegmatitic means
very coarsely crystalline (generally with the majority of crystals > 1 cm). Porphyritc
means there are 2 distinct crystal sizes, larger and smaller, without restrictions as to
how big either one is. All porphyritc rocks have phenocrysts of some kind. Thus,
there can be porphyritic-phaneritic, porphyritic-aphanitic and porphyritic-hyaline
textures. In the latter 2, the fine grained or glassy material is called groundmass. Use
the figures on p 120 to answer the following questions.
A. The molten Thymol which crystallized over 1 minute had crystals that were ___
mm in size. (1)
B. The molten Thymol which crystallized over 3 minutes had crystals that were ___
mm in size. (1)
C. The rock shown has a porphyritic texture. Find one like this in our wards’ kits and
lab samples. The largest white plagioclase crystals are called “phenocrysts”. Look
back at the rock classification chart Fig 5.2 and Bowen’s reaction Series, Fig 5.3
and assume that the phenocrysts are the hottest and earliest mineral to crystallize.
What minerals do you think the groundmass is made of? Interpret how this texture
might have formed and relate it to the temperature and viscosity variations as the
magma cooled to become a rock. (4)
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Activity 5.3: Rock Analysis and Textural interpretation for assigning rock names.
Refer to Figures 5.1, 5.2 the Ward’s kits and other specimens in the lab to analyse these
photos and their look-alikes in our rock collection. Keep in mind that the term CI for
Colour index is the total sum of the dark coloured ferromagnesian minerals (Olivine to
Biotite in Bowen’s Reaction series plus any Fe-oxides or Fe-sulfides). In classifying a
phaneritic textured plutonic igneous rocks the minerals are all large enough to see and
count. For aphanitic or glassy textured volcanic rocks there are very few mineral crystals
for any kind of mineralogical classification so we tend to use textural names like
obsidian, tuff or breccias depending on the texture of the groundmass. Tuffs have ash to
lapilli sized fragments (< a few mm). Volcanic breccias have angular rock fragments
(larger than a few cm). Fill out the table on p 121 provided. (21)
Activity 5.4 Thin Section Analysis and Bowen’s Reaction Series
5.4.1 Read the text, examine the photos and look at thin sections of gabbro and biotite
granite in our own collection then answer the 3 questions A, B, C on p 122 provided. (6)
Additionally, observe 3 igneous rocks in thin section from our collection: a glassy or
tuffaceous volcanic one like the rhyolites, dacites or welded tuffs; a porphyritic one like
the hornblende andesite or trachyte with flow aligned phenocrysts and a phaneritic one
like a diorite or granodiorite.
5.4.2 Below, draw and label 3 thin sections, saying what sample number you used, draw a
scale bar. The field of view’s diameter is 7 mm for low power or 1.4 mm for high power.
Also label each of the minerals or textures you observe in plane PPL or crossed polarized
light XPL. Be sure to name the rock and interpret whether it is mafic, intermediate or
felsic according to Figure 5.2. (15)
Thin Section #: _________ Rock Name: ___________________
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Thin Section #: _________ Rock Name: ___________________
Thin Section #: _________ Rock Name: ___________________
5.4.3 Below, draw hand specimens of corresponding or similar igneous rocks in hand
specimen using 3 of the lettered specimens for the description sheets in 5.5. Draw a scale
bar, label any minerals or textures present, name the rock and interpret whether it is
felsic, intermediate or mafic. (15)
Hand Specimen #: _________
Rock Name: ___________________
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Hand Specimen #: _________
Rock Name: ___________________
Hand Specimen #: _________
Rock Name: ___________________
Activity 5.5 Igneous Hand Specimen Analysis
Fill out the table, on p 123 provided for the lettered specimens A-H. Once you have
decided what minerals are present and what textures the rock has, you can name it and
interpret where and how it formed. For example: Rock N cooled slowly in the lower
ocean crust or Rock V exploded, quenching glass and ash particles that fell down and
compacted while they were still partially molten. (35)
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