Download 04-Rock-Cycle-AGI

Name(s): _____________________________ & _____________________________.
For Busch & Tasa 10th ed.
Winter 2017- Circle Section (Tue=1) (Wed=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 the most common, but carbonates, oxides, sulfides and mineral-like substances such as agate, opal or
coal also count. Rocks are usually 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
there (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 a 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 rocks 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 settings. 3) Skarns
are unusual coarsely crystalline, but usually equant textured metamorphic rocks formed in ore deposit
settings. Skarns typically contain quartz, epidote, garnet, diopside, magnetite and often metal-sulfide ore
minerals like chalcopyrite or pyrrhotite. These rocks 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 shale, schist or granite
tends 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 persist for a while. Most ferromagnesian
minerals react to clays, iron oxides (red dirt) & dissolved salts. Sediments are low pressure rocks
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deposited on top of the earth’s surface and only shallowly buried if at all. As a result they have abundant
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
(55)
Rock Type Geological Setting
Peridotite
1. Olivine
2. Serpentine
3. Kaolinite
4. Muscovite
Fe-Mg Silicates
5. Chlorite
6. Biotite
Evaporites, Playa Lakes
7. Gypsum
8. Orthoclase
9. Andesine
10. Halite
NaCl
11. Augite
12. Hornblende
13. Garnet
14. Calcite
15. Quartz
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4.4:A 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
Glassy
(Ig, Met or Sed)
Vesicular
Red-Green-Yel
Fine
<1mm
Crystalline:
Clastic:
Composition: Layers:
Heterogranular
Angular
Bioclastic
Coarse Equigranular
>1mm
1.Gneiss
2. Coquina
3. Lava
4. Granite
5.Peridotite
6.Obsidian
7.Sandstone
8.Granodiorite
9.Breccia
10.Shale
11.Schist
12Conglomerate
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Rounded Mineral
Clasts
Flat
Laminated
Foliated
4.4: B
Recognizing Rock Textures and Weathering
Rocks formed at depth, higher pressure, higher temperatures, in the absence of free oxygen or in the
absence of cold acidic water react when they are eroded and exposed at Earth’s surface. Weathering can
happen in place (in-situ) where rocks are still attached to the crust below and this process continues
during sedimentary transport. Mechanical weathering such as freeze thaw, thermal expansion,
decompression causes minerals to separate into individual mineral grains or smaller rock fragments. A
weathering product of these processes is grus or mineral gravel. Chemical weathering is a differential
process that selectively exchanges or removes certain elements from the mineral lattice by acidic
leaching, ion exchange, hydrolysis, hydration and dissolution. Mineral surfaces lose their original luster
and appear duller, earthy or tarnished. This type of chemical weathering etches and pits mineral surfaces
making them rough to the touch. Tear pants weathering is notable in limestones. When rocks or their
minerals contain iron, it tends to oxidize in shades of red, yellow or orange producing iron oxide or
hydroxide residues (limonite, goethite etc..) in place of primary pyrite or ferromagnesian silicates. Rocks
with granular and poorly cemented textures tend to expand, lose cements and become friable (crumbly).
Examine freshly broken and weathered examples as pairs of rocks for each of the following and note
changes which occurs and classify them as mechanical or chemical weathering.
B. Fill out the following descriptive table to note the contrast between primary and weathered features
for closely related Fresh and Weathered rock pairs.
(12 points)
a. Fresh Rock
Mechanical
Chemical
b. Weathered Rock
1a. Granite
1b. Granite Pegmatite
2a. Shelly Sandstone
2b. Orange Sandstone
3a. Massive sulfide core
3b. Limonite breccia
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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 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.
(25)
Sample
Rock
Textures
Classification Rock Name
Example
4.1.B p121
Broken fossil
clasts & lime
mud
Sedimentary
Process of 3 Different Rock Cycle
Formation Processes
Fossiliferous Shallow
limestone,
warm
Biomicrite
tropical
seas
4.5 a.
4.5 b.
4.5 c.
4.5 d.
4.5 e.
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1. Weather to mud & ions
2. Recrystallize to marble
3. Melt to carbonatite magma