Name 1_______________________________Name 2___________________________ Section
Geoscience 100 Lab 5 – Winter 2016: Igneous Rocks and Processes
Over 90% of the Earth’s crust and all of the mantle formed via igneous processes. These rocks make up
over half the volume of the planet. 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 the AGI lab manual, 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 learn
the names and to be able to interpret the significance of 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. To do this rock lay you’ll need to work with the
figures and tables in the manual so start by reading pages 129 through 142 in chapter 5.
Where Igneous Rocks Originate and 3 Dominant Processes Which Affect Them:
Igneous rocks are the result of 3 processes: partial melting, buoyant rise and solidification via cooling.
Igneous rocks all originated by partial melting usually in the upper mantle asthenosphere of peridotites
or sometimes in the lower crust via partial melting of amphibolites, gneisses or gabbros). Once the
partial melt forms, it separates by wetting mineral grain surfaces, fracturing source rocks, shear and
compaction (due to local directed forces) to buoyantly rise up into or even through the crust. The most
common magma in all igneous settings is mafic basalt from > 1250°C in the upper mantle. Sometimes
the lower crust in arcs or hotspots gets heated enough by basalts to stat melting in its own right ~
1050°C to produce rhyolites or dacites. Magma buoyancy varies with: 1) temperature (due to thermal
expansion), 2) with composition such that silica rich magmas (felsic) are lighter and more
ferromagnesian rich magmas are denser and 3) by gas or fluid content ((H2O and CO2) such that gassy
magmas are lighter and can even resemble foams. The final step is cooling which increases viscosity,
slowing flow and causing a sequence of minerals to crystallize. If the cooling is super fast, the magma or
lava at the surface quenches as a glass. If cooling is slower, crystals have time to nucleate and grow.
When gases (fluids) are present, they act like solvents and thin the magma permitting faster flow,
diffusion and crystal growth. Because the amount of magma is limited in volume and the crust, sea or air
are cold, so cooling and solidification of magma or lava to rock is inevitable. The 2 main environments
are plutonic for intrusive rocks and volcanic for extrusive rocks.
Analyzing Rocks: Textures, Mineralogy and Mafic Colour Index:
Review figures 5.4 (p.136) for general textures, names and rock forming environments. Review figure
5.5 (p. 137) for igneous rocks derived from silicate melts to see how their names relate to textures,
percentage of dark minerals (Mafic Colour Index CI) and general chemical composition. Review figure
5.6 (p.138) for Bowen’s Reaction series to understand the order in which minerals crystallize from
cooling melts. Rock colour is an approximate indication of iron content in minerals or quenched glasses.
Colour Index means the sum total of dark ferromagnesian minerals. The most common ones in igneous
rocks are: Olivine, Pyroxenes, Amphiboles, Biotite and Iron-Titanium Oxides, but other iron bearing
silicates and metal oxides or sulphides also count as dark minerals too: Tourmaline, Epidote, Sphene
(Titanite), Hematite, Rutile, Pyrrhotite, Cassiterite etc. You may not be able to identify all of these but if
it is dark add it into the total percentage of dark minerals to calculate the colour index. Texture and
crystal size relates to cooling rate, because most crystal growth only happens within 100-150° below the
liquidus temperature (point at which the first crystals appear upon cooling). Phaneritic (coarsely
crystalline rocks with grain sizes > 1 mm) are easiest to describe and analyse. They cooled slowly
enough at depth inside the Earth for crystals to nucleate and grow. We may see their phaneritic texture
but form this we can interpret that they originated as intrusive magmas deep in the Earth so we give
them the environmental name “Plutonic” or simply “Intrusive”. When igneous rocks extruded from
volcanoes, they cool quickly and once they cool by 100 to 150°C they cool through the glass transition
temperature and they solidify and cease to flow. Because of this, crystals can only nucleate and grow
when magmas are hotter and closer to their liquidus temperatures. When volcanic rocks have broken
angular clasts or fragments they are collectively termed pyroclastic. When they are wholly glassy they
are termed obsidian. Most volcanic rocks erupted and cooled more slowly to form many smaller or tiny
crystals. These fine grained sugary textured rocks are called aphanitic and we interpret them to have
cooled quickly in an extrusive environment. Rocks with a few large crystals and the majority smaller
ones are called porphyritic. This means they had a 2 stage cooling history, slowly at first to form the big
phenocrysts and fast at the end as they hit the cold ground, air or water. Shallow intrusions within
volcanoes also have porphyritic textures especially for small plugs or thin dykes so we interpret them to
have cooled in a hypabyssal (not very deep) environment.
Activity 5.1 Igneous Rock Inquiry:
Find numbered rocks in the lab like those in the table below and pictured in your lab manual on p. 143.
Use the terms from the guide figures and tables mentioned in the paragraph above to practice observing
and interpreting: rock texture names (size, shape and arrangement of crystals, groundmass, phenocrysts,
vesicles or rock fragments. CI or mafic colour index as your manual calls it is a percentage equal to the
sum of all dark minerals present. There is a tendency to overestimate dark things and underestimate light
ones. Imagine as you look at a rock that you could take a little broom and sweep all the dark mineral
crystals to one corner of the rock and compare the area they would cover to 25% of the circle or square
area you are staring at to tell whether it would take up all 25% or only a fifth, quarter, third, half of that.
Once you have number for the CI you can decide whether the rock is felsic, intermediate, mafic or ultra
mafic. Next compare the crystal sizes, at first using your hand lens, but eventually with your eyes alone.
Distinguish coarse grained (> 1 mm) from fine grained or glassy rocks and additionally note whether
they are pyroclastic and fragmental versus interlocking crystalline in their textures. After a few practice
rocks you’ll be able to see a coarse grained intermediate colour index rock like a diorite and be thinking
of the pluton it came from deep in the crust. Also rocks with glass fragments, shards and vesicles should
cause you to think of volcanoes. Fill out the table below for the rocks pictured in 5.1 on p. 143.
Rock Name
1. __________ Diorite
Composition Texture
Black & White
2. ____________
Felsic Glass
3. Olivine
Olivine, ____ ______ with
Phenocrysts &
Of Formation
Plutonic or Intrusive
4. Granite
5. Tourmaline bearing
Granite Pegmatite
6. Scoria
Volcanic or ______
Activity 5.2 Minerals which form Igneous Rocks:
The find samples of the minerals pictured on page 144 in labeled trays 5.2.1-10. These minerals make up
more than 99% of common igneous rocks crystallize in an orderly sequence. These are the primary rock
forming minerals. Rarely small amounts of partial melts form in short lived isolated small environments
when lightning strikes to fuse sands at the surface to glassy fulgurites, meteorite strikes to form impact
glasses such as tektites or lechatelierite, coal seams burning sediments to clinker and slag. Other more
unusual minerals may form there owing to different starting compositions than the common peridotite in
the upper mantle or the gabbro of the lower crust. Because minerals crystallize over a range of
temperatures (see Bowen’s Reaction Series), the simple act of their crystallization changes the
composition of the residual magma. Also because magmas flow, early formed crystals may separate by
settling or floatation. When magmas rise up into crustal rocks, depending on what minerals are present
in the roof or wall rocks such as quartz, calcite, shales, or salts contamination drives the magma in new
compositional directions (silica rich explosive, calcium rich plagioclase or pyroxene, peraluminous, or
peralkaline). In active magmatic regions new magmas may rise into older cooler magma chambers
mixing magmas or even heating things up enough to cause eruptions. As these processes occur, igneous
rocks may solidify from one or 2 minerals (Olivine  Dunite, Augite, enstatite  Pyroxenite,
Plagioclase  Anorthosite) whose composition is rather different from the starting magma. Because of
this there are many different igneous rocks, and more rock types than the simpler, fewer parental
magmas they crystallized from. Complete the table below keeping in mind the igneous rocks these
minerals comprise. Plagioclase feldspar ranges from the high temperature Ca-Al rich Anorthite through,
Bytownite, Labradorite, Andesine, Oligoclase to low temperature Na-Si rich Albite. Volcanics may
contain Sanidine or Anorthoclase instead of Sanidine. 9-12 are minor accessories.
Mineral Name
containing it
1. Olivine
2. Muscovite
3. Quartz
4. Biotite
5. Augite
6. Plagioclase
7. Orthoclase
8. Hornblende
9. Magnetite
10. Ilmenite
11. Apatite
12. Zircon
Describe in your own words to
Activity 5.3 Igneous Rock Compositions:
The first step is to use colour index Figure 5.5 p. 137 shows 5 minerals on the vertical axis and Mafic
Colour Index on the horizontal axis. Notice dark minerals are all in shades of green or brown at the
bottom of the graph. Igneous rocks become lighter in CI and cooler for their melting interval as they
contain less Fe and Mg and the dark minerals which crystallize from them. You may also use the grey
scale bar or dot bar at the top of the page. The Geo tools in the back have dot cards for the critical
divisions at 85%, 45%, 15% and 5% as well. To use this chart to classify a rock first total up all of its
darl minerals as a % area of a face on the rock or thin section. See whether this CI % is < 15 % for
Felsic, between 15-45% for Intermediate, between 45-85% for Mafic or > 85% for Ultramafic. Next,
determine the grain size of the crystals. Phaneritic or coarsely crystalline rocks get a name from the
Yellow “Intrusive” or Plutonic part of the chart below. Aphanitic or fine grained rocks get a pink
Extrusive or volcanic name from the bottom of the chart. Vesicular rocks were blown to foams by
superheated expanding hot gases (H2O, CO2). Scoria can be grey, black or oxidized red-brown from
hematite or limonite. 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
proximity or stratigraphic association.
A. Fill out the following table for rocks pictured and provided as hand specimens 5.3-A.1-A.4
Rock name M.C.I.% Compositional Group Texture Geological Environment
B. Discuss the relative ease or difficulty of classifying rocks via visual estimates versus point counts for:
1. phaneritic (3 & 4) versus _________________________________________________________ (1)
2. aphanitic or glassy (1 & 2) ________________________________________________________ (1)
C. (Off book) Use the textural descriptions provided and mineral content % to classify the
following rocks or to answer the question. Look at Fig. 5.5 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.
1. If a rock contains 3% Biotite and 5% Amphibole, what is its numerical colour index? CI =_____ &
general compositional group using the top row of Figure 5.5.1 _____________________________. (2)
2. A rock with 0% Quartz, has a Colour Index______ & general compositional group ___________. (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 ________________ ________________.
D. (Off book) Use the textural descriptions provided and mineral content % to classify the
following rocks or to answer the question. Examine figure 5.5 for classification and 5.6 for the
approximate temperature according to the 1st mineral to crystallize in Bowen’s Reaction Series for each
rock type below. As magmas cool, silica tetrahedral link up (polymerize) and make minerals richer in
SiO2 and more connected (chains after lone tetrahedral, sheets after double chains etc. The percentages
of constituent minerals and textures come from examining thin sections and point counting on grids
instead of dashed lines like you tried above. 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.
1. Aphanitic rock: 18% Amphibole, 19% Pyroxene, 59% Plagioclase,
1% K-Feldspar, 3% Quartz
CI ____ Name ______________ (2)
2. Pegmatitic rock: 8% Amphibole, 5% Biotite, 9% Ca-Plagioclase, 55% K-Feldspar, 4% Muscovite,
25% Quartz. CI ______ Rock Name _____________________ __________________________ (3)
3. Coarse pyroclastic 8% Biotite, 12% Amphibole, 30% Plagioclase, 3% K-feldspar, 16% Quartz
CI ________ Rock Name _________________________ ________________________________ (3)
4. Plutonic rock: 9% Amphibole, 35% Pyroxene, 51% Olivine, 5% Plagioclase
CI ________ Rock Name ___________________________________________________________ (2)
Activity 5.4: 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 (resistance to flow or deformation, shear stress/shear strain) and
temperature of the molten sugar candy lava flow. How did the boiling lava behavior change as the water
(solvent) boiled off? ________________________________________________________________ (2)
B. Describe what happened to the viscosity and temperature as the flow moved and hardened. _______
_________________________________________________________________________________ (2)
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. 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? _________________________________________
_________________________________________________________________________________ (2)
E. When sugar solutions slowly evaporate at room temperature they crystallize to relatively large rock
candy crystals. Since crystal formation depends on ordering and diffusion of atoms or molecules, what 2
processes or conditions prevented sugar crystals from forming in the candy lava? _________________
_________________________________________________________________________________ (2)
F. Find a labelled igneous rock pictured in the book and one like it in the lab that have a texture
resembling the candy lava from step C above. Record its number _______ & name ___________ . (2)
G. Find a labelled igneous rock pictured in the book and one like it in the lab that have a texture
resembling the candy lava from step D above. Record its number _______ & name ___________ . (2)
Activity 5.5: Crystalline Textures of Igneous Rocks
Crystallization experiments of a model compound (mint alcohol): Menthol C10H3(OH)
Quench 2 minutes 2mm x 20 mm
Quench 5 minutes 5 mm x 25 mm
A. Describe how the crystal size and shapes above vary with quench rate or cooling time. _________
________________________________________________________________________________ (2)
B. Examine the 3 rock pictures and lab specimens. Describe the relative cooling rate for the following
crystallinity or textures:
1. Aphanitic: _____________________________________________________________________ (1)
2. Phaneritic: _____________________________________________________________________ (1)
3. Pegmatitic: _____________________________________________________________________ (1)
C. Examine the specimens with porphyritic texture and describe how they might have formed. Consider
that more than one process or sequence of events might be possible: 2 cooling events, gas build up,
crystal floatation or settling…
1. Porphyritic Granite (K-Spar)
3. Porphyritic Oceanite-Basalt (Olivine)
2. Porphyritic Andesite (Andesine & Hornblende)
4. Porphyritic Peridotite (Olivine-Dunite)
Largest sizes are respectively: 1) 5 cm, 2) 2 cm, 3) 8 mm 4) 12 cm
Label as aphanitc or phaneritic & describe possible origins for the porphyritic texture for each rock:
1) _____________________________________________________________________________ (2)
2) ____________________________________________________________________________ (2)
3) ____________________________________________________________________________ (2)
4) ____________________________________________________________________________ (2)
Activity 5.7: Bowen’s Reaction Series & Crystallization Sequence from Thin Section Petrography
Look at Figure 5.6 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
interval 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.
Build up of solvents (volatile fluids) or contamination by salts extends this up to another 150°C. 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. 5.5.
Label all 5 minerals on the Plane light image on the right: Ol, Au, Pl, Ul, Cr.
Lunar Basalt – Mare Procellarum – Apollo 12 Mission. 40X – 4mm wide
Left: Plane Polarized Light. Large opaque euhedral Ulvospinel Fe2TiO4 .
Clear, high relief, fractured, subhedral forsteritic Olivine. Pale pinkbrown pyroxene Augite with 2 cleavages. Clear white, low relief calcic
Plagioclase. Both Plagioclase and Pyroxene have inclusions of Olivine
and small opaque cubic Chromite FeCr2O4 .
Right: Cross Polarized Light. Same image and power. Fractured olivine
crystals have high birefringence colours (blue, pink, yellow, green &
black) depending on crystal orientation and refraction. Augite appears
pale white to beige or black. Plagioclase shows albite twinning and
blue-white to black colours. Ulvospinel and chromite are opaque , so
they are black.
A.1 Look at Bowen’s Reaction Series and explain what process caused the Plagioclase and Pyroxene
(Augite) crystals to form together?
______________________________________________________________________________ (2)
A.2 Examine a thin section of Precambrian Keewanawan Basalt from 1.1 Ga (North American Mid
Continent Rift), when the continent almost split apart. Notice the red oxidation of glass to hematite,
vesicle infillings of quartz and calcite and some residual primary plagioclase and pyroxene (high
birefringence colours). The Lunar Basalts from Mare Procellarum are over 2.4 Ga, more than twice as
old. Describe what caused the difference between these 2 mafic lavas in thin section?
______________________________________________________________________________ (2)
Left: Cross Polarized photo. Andesite tends to be crystal rich and have fragments of broken crystals,
here 25-30 % of the volcanic rock. The crystals often show concentric zonation in composition from Carich cores to Na-rich rims. Aggregates of 2-3 different minerals form clots. Plagioclase is albite twinned
in white-grey-black. Hot birefringence colours are hornblende. The groundmass is feldspar and brown
(black) glass. Some black crystals are extinct silicates, others are opaques like magnetite.
Right: Cross polarized Dacite Porphyry with oscillatory zoned plagioclase from Ca-rich Labradorite to
Na-Rich Albite. Note some of the zones have well formed crystal outlines while others seem to be
embayed or resorbed. Groundmass is dark glass, and fine grained feldspar and quartz.
B.1 How hot was the magma on the left when it crystallized hornblende and intermediate plagioclase?
______________________________________________________________________________ (1)
B.2 Explain what caused the multiple zones in the plagioclase crystal on the right? _______________
_______________________________________________________________________________ (2)
B3. Examine the thin section of the Hornblende andesite over on our microscope. Is the brown
hornblende uniform in composition? circle one (yes or no) Describe or draw how it looks. Where are the
magnetites in this rock? What do you think caused these textures? ___________________________ (3)
C.1 Explain what caused the intergrowth of quartz and alkali feldspar? ________________________
________________________________________________________________________________ (2)
D. For the last 4 rocks: A1, B1, B2, C1 write their letters and names (basalt etc.) on the chart above
where they formed. ______________________________________________________________ (8)
Activity 5.9
Geological map of SE Pennsylvania, Fig 5.15. 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 sub-horizontal, 220-200 Ma Permian continental red beds sediments of sands and
muds deposited in narrow river valleys, shed off the Paleozoic highlands in the middle of Pangea. The
pink areas are 190 Ma Jurassic mafic magmatic rocks which outcrop from Nova Scotia to New Jersey
and in Morocco on the other side of the Atlantic.
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 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)
5.10 (rocks in Lab) Drawing & Labelling specimens (40 points)
Draw & label 5 hand specimens including a scale bar & correct terms for minerals, textures. Interpret
the igneous name for each rock and describe where and how it formed. Diameter of field of view for
low power brown objective is 7.0mm, medium power yellow is 1.4 mm.
(5 points each – 25 total)
5.10-A. Peridotite
5.10-B. Gabbro-Pegmatite
5.10-C. Scoria
5.10-D Diorite
5.10-E Welded Tuff