Name(s): _____________________________________________________ ________
For Busch & Tasa 10th ed. Fall 2015 - Circle Section (Tue=1) (Thur=2)
Lab 4: Rock-Forming Processes and the Rock Cycle - I
This lab is worth 140 points and 2% of the total marks for the course. The aim of this lab is to
introduce you to rocks and some physical characteristics of rocks which help you infer how the rocks
formed. Rocks are comprised of minerals or of pre-existing rock fragments made of minerals. Silicates
are most common, but carbonates, oxides, sulfides and mineral-like substances such as agate, opal or
coal. Rocks are really mineral assemblages which reflect the kinds of minerals which occur together
because of the processes which assembled them, or are thermodynamically stable together because of
the rocks chemical composition and physical conditions of formation. Finding and identifying some of
the minerals present provides clues as to which other minerals might also occur (what is missing from
this card deck?). Identifying the minerals that make up a rock relies on the physical and chemical tests
you have already worked with in addition to the optical characteristics and fine scale textures visible
under a petrographic microscope in thin section.
To do this successfully, you must be able to identify their constituent minerals so use some of the
lab time to ensure that you KNOW the major rock-forming minerals. In addition to the tables in Lab
4 to help you recognize and distinguish the 3 main rock groupings: Igneous, Sedimentary and
Metamorphic, you will need to refer back to the mineral identification tables in Ch 3 and to look ahead
to the more detailed tables for each group in the subsequent 3 chapters: 5, 6, and 7.
Turn to Lab 4, and read p. 89-99 in the lab manual. Read the information and view the diagrams.
Inspect the rock kits for each of the 3 rock groups and the larger labeled specimens. Learn the principal
differences among the groups such as where they occur, how they form, what minerals comprise them
and what special textures they exhibit. An example of each of the 3 rock types follows. 1) Shales are a
fine grained sedimentary rock with pronounced fine scale layering on the scale of a few mm. They are
made of clay minerals and other very fine grain sized mineral particles of quartz, calcite, iron oxides.
They are found in basins along with other sediments. They form by the weathering and transport of other
pre-existing rocks. 2) Ignimbrites or welded tuffs are fine grained partly glassy and partly finecrystalline explosive volcanic rocks. They are typically flow folded and often contain flattened glass
shards shaped like the letter Y and flattened gas pockets called vesicles. They were deposited subaerially
by explosive ring-of –fire, gas-rich rhyolitic volcanoes in subduction zone settuings. 3) Skarns are
unusual coarsely crystalline but usually equant textured metamorphic rocks formed in hydrothermal and
ore deposit settings. They typically contain quartz, epidote, garnet, diopside and often metal-sulfide ore
minerals like chalcopyrite or pyrrhotite. These form on the geological contact between intermediate
plutonic rocks and dirty or clay and sand bearing limestones. While these examples give you a lot of
new terms to consider, each rock has special minerals, textures and settings in which they occur or
processes by which they are formed. Other rock and mineral guides are helpful to get you started.
Activity 4.1: Rock Photos and the Rock Cycle
Weathering and Sediments: All sediments form from pre-existing detritus or dissolved ions in
waters at Earth’s surface. Any rock may weather but what will this one weather to? Limestones may
easily and entirely dissolve to ions and saltiness in the sea (caves anyone?) but shales, schists or granites
tend to mechanically weather to rock fragments. What kind of residual particles will each rock type
make for future sediments? Quartz will last and feldspar might for a while. Most ferromagnesian
minerals react to clays, iron oxides (red dirt) & dissolved salts. Sediments are low pressure rocks
deposited on top of the earth’s surface and only shallowly buried if at all. As a result they have abundant
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open pore spaces and are often filled with economically important resources like ground water,
petroleum or natural gas.
Partial Melting: All igneous rocks originated by partial melting, moved upwards in the crust by
buoyancy and either intruded or extruded by flow and cooled to form solid rocks. The mantle peridotite
partially melts to make basalt at about 1250°C. The gabbro or amphibolite lower crust can partially melt
to make rhyolite at about 1000°C. Pure quartz doesn’t melt until 1700°C. Calcite usually doesn’t
survive to get hot enough to melt because it decrepitates to carbon dioxide (CO2 (g) ) and Calcium Oxide
(CaO) or another calcium bearing mineral at temperatures above 750°C. Most rocks can partially melt if
they get hot enough but what kind of melts could each particular rock generate? Be specific: Rhyolite,
Basalt, Carbonatite, etc.
Metamorphism: Any rock may get strained and re-crystallized by changes of pressure,
temperature, stresses and fluids that collectively add up to metamorphism. Judging from the minerals
already present what new minerals would form from the bulk composition of each rock pictured or in
the specimens provided? Clays may re-crystallize into micas and aluminosilicate minerals (kyanite,
sillimanite, andalusite). Pure quartz or calcite just stay as quartz or calcite but a marly sediment
containing calcite, quartz and clays make Ca-rich alumino-silicate rocks or coarse grained skarns with
minerals like tremolite, actinolite, diopside or grossularite garnet. Shales may regionally metamorphose
into schists or gneisses, or get baked to hornfels where cut by intrusions. Granites may metamorphose
into gneisses. Volcanic rocks may metamorphose into greenstones (low temperatures and pressures) or
blueschists (low temperatures and very high pressures). The final rock depends both on the starting
minerals and on the physical conditions (Strain, P, T, fluids). Like cooking, the same starting
composition may yield very different products depending on what happens along the way.
Sample#
Rock Properties
Classification/Name
Origin/Formation
Future Changes
A
Glassy fractured
devitrified and
discoloured
IGN-Obsidian
Quenched fast from
subaerial volcano
Clay, salt
Schist
Remelt
E
I
Crystalline, non
foliated, calcite
MET-Marble
Fine grained leaf
fossil hematite
cement
SED-Redbed
Fossiliferous Siltstone
Recrystallized
limestone
convergent marg
Salts
Clastic lacustrine or
riverine on land
Mud & salts
Marble
Schist
Rhyolite melt
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Activity 4.3:Practice with Abundant Rock Forming Minerals
Mineral name
Formula
Group
1. Olivine
2. Serpentine
3. Kaolinite
4. Muscovite
5. Chlorite
6. Biotite
7. Gypsum
8. Orthoclase
9. Andesine
10. Halite
11. Augite
12. Hornblende
13. Garnet
14. Calcite
15. Quartz
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Rock Type Geological Setting
4.4: Recognizing Rock Textures
Rock textures like mineral assemblages and grain sizes result from the physical and chemical conditions
present at the time the rock crystallized, recrystallized or collected and cemented as a pile of individual
grains. Most textures are unique to a single class of rocks: igneous, metamorphic or sedimentary.
Examine the photos of the 12 rocks on p 124 and fill the boxes in each column with the terms and
textures that apply. Some columns may get no term and others might get more than one. For example an
igneous rock with an interlocking crystalline texture does not have any clastic grains at all and
metamorphic rocks will rarely appear glassy or vesicular but might have deformed amygdules (vesicles
filled by secondary minerals. Do not lose points by putting terms where none belong.
(30)
Rock name
(Ig, Met or Sed)
Red-Green-Yel
Glassy
Fine
Crystalline:
<1mm
Vesicular
Heterogranular
Coarse Equigranular
>1mm
1. Gneiss
2. Coquina
3. Lava
4. Granite
5.Peridotite
6.Obsidian
7.Sandstone
8.Gneiss
9.Breccia
10.Shale
11.Schist
12Conglomerate
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Clastic:
Composition: Layers:
Angular
Bioclastic
Rounded Mineral
Clasts
Flat
Laminated
Foliated
Activity 4.5: Introduction to Rocks and the Rock Cycle
A-1) Inspect Figure 4.2 on p. 114. Complete this circular rock cycle figure from p. 127 by colouring the
arrows for the processes which make or characterize each of the 3 rock types. Colour any arrow which
results in a sediment or creates a sedimentary rock yellow. Colour any arrow which results in a
metamorphic rock green. Colour any arrow which results in an igneous rock red. Note: it is the process
which counts here and the final product, and not necessarily what it was to begin with! The colour
matches the process and the final product, not the starting rock. For example, an obsidian can weather at
the surface to dissolved salts and residual clays to make runoff and soils. Because of the surface
weathering process and the sediments it generates, the process is sedimentary so should be coloured
yellow.
(11)
A-2) Complete the table below the figure and place an x in the column if some of that type of rock
contains this feature or is formed by this process. This requires some reading and is a bit tricky as you
can lose points for not putting in an x where you should or for putting in an x where you shouldn’t! (15)
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4.5-B:
Analyzing Rocks and Interpreting Rock Cycle Changes
In column 1 list at least 2 specific terms for textures, minerals, fossils, cements. In column 2 name the
rock. In the 3rd column, be specific as to the rock’s conditions of formation or original geological setting.
Petrologists do this when the look at rocks to see not only what is before their eyes, but also they
interpret the environment that created it. Finally in the last column, put 1 realistic example of a specific
sedimentary, metamorphic and igneous process that could befall each rock in the rock cycle. The rocks
a-e are all pictured on p. 120. Equivalent specimens in the lab are labeled 4.5a through 4.5e. (30)
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