Name 1___________________Name 2_______________ Section
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Geoscience 100 Lab 5 - Winter 2014: 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.
B. Describe what happened to the viscosity and temperature as the flow moved and
hardened.
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?
D. Observe the texture of the cooled “candy lava” with a hand lens. Note the vesicles
of trapped air or steam. What prevented them from leaving the lava?
7. There are 5 large igneous rock specimens and 3 thin sections to inspect in detail
in the lab. Describe the 5 hand samples and 3 thin sections and for each specimen:
(i)
Carefully draw a representative part of its texture, indicating a scale
bar on your diagram;
(ii)
Label the mineral and textural components that you can identify; eg.
plagioclase as tabular phenocrysts, quartz infilling vesicles
(iii)
Suggest the geological environment for each rock and how it formed.
e.g. an aphanitic glassy tuff would be formed by a subaerial volcanic
eruption.
8. Fill out descriptions of the rock photos in figures 5.6-5.18 on table 5.24.
Part II:
Drawing specimens (40 points)
Draw & label 5 hand specimens and 3 thin sections including a scale bar & correct terms.
Hand Spec. A
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Hand Spec B
Hand Spec C
Hand Spec D
Hand Spec. E.
Thin Section
No. ______
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Thin Section
No. _______
Thin Section
No. ______
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