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Geology 101
Name(s):
Lab 4: Sedimentary and metamorphic rocks
More sedimentary rocks
1. Now consider the actual mineral grains in rock samples R18, R19, R24 and
R25. Because sand-sized dark minerals are very hard to identify, sedimentary
petrologists (much to the horror of igneous petrologists) use the term “dark
lithic fragments” to categorize lots of little dark minerals.
Sample
R18
Most common mineral
R19
R24
R25
Sedimentary rocks can be classified in a number of ways. For our purposes, the
first division to be made is between clastic sedimentary rocks (those that are made
of weathered and eroded grains) and non-clastic or other sedimentary rocks (these
include sedimentary rocks of biological and chemical origin).
You will use the Sedimentary Rock ID flow chart on the next page.
One other thing: fossils (Latin for “dug up”) are the remains of living organisms. If
the fossil is literally the body of the organism (or parts such as skeleton or shell), it
is called a hard parts fossil; if the fossil merely records the shape of an organism
(like a leaf impression in silt) or the passage of an organism (like preserved
footprints), then it is called a trace fossil. For any sedimentary rock, if it contains
any fossils, use the adjective fossiliferous in front of the rock name.
Flow chart for identifying sedimentary rocks — If the rock is made of grains or other
materials which have been deposited by wind, water or ice, or else was
generated by biological or surface chemical activity, it's a sedimentary rock.
First step.
If the rock is made of broken up bits of
rock (including extremely fine grains)
→ GO TO Second step alternative A.
Else → GO TO Second step alternative
B.
Second step alternative A. Consider the most common grain size in the rock from
the following list.
cobble or
pebble
sand
silt
clay
> 2 mm
easily visible to naked eye; "grains" may
contain identifiable minerals
0.062 — 2 mm
visible to naked eye
0.005 — 0.062 mm not visible but can be felt between fingers or
across teeth
< 0.005 mm
not visible; cannot be felt between fingers or
across teeth
If the most common grain size is cobble or pebble → conglomerate
If the most common grain size is sand → sandstone
If the most common mineral is quartz → arenitic sandstone
If the many dark minerals are present → arkosic sandstone
If the most common grain size is silt → siltstone
If the most common grain size is clay → claystone (shale)
Second step alternative B. Identify the most common mineral in the specimen
(use mineral ID chart if necessary).
If the most common mineral is quartz → chert
If the most common mineral is halite → rock salt
If the most common mineral is gypsum → rock gypsum
If it is black-colored, not very dense and flaky → coal
(also look for plant fibers)
If it fizzes, the most common substance is calcium carbonate, usually in the
form of the mineral calcite (be careful you are not fizzing the cement)
If the rock is not very dense and pure white → chalk
If the rock is made of broken-up shells → coquina
If the rock is dense and white, gray or black → limestone
2. Fill in the following table for clastic sedimentary rocks. Begin by determining
the average grain size of the clasts in the rock (use the grain size terms in the
flow chart), then the grain sorting (the choices are: well-sorted, moderately
sorted, poorly sorted and unsorted) and the grain roundness (the choices are;
well-rounded, sub-rounded, sub-angular and angular). See the diagrams to
determine which type of rounding and sorting the grains have. Under fossils,
your choices are none, hard parts or trace fossils. Finally, identify the rock, using
the flow chart.
Clastic sedimentary rocks
Sample #
Grain size
Grain
sorting
Grain
roundness
Fossils
Rock name
R18
R19
R20
R21
R22
R23
3. Fill in the following table for “other” sedimentary rocks. Begin by determining
the rock’s mineral composition. Then add any other details that help identify it.
Under fossils, your choices are none, hard parts or trace fossils. Finally, identify
the rock, using the flow chart.
“Other” (chemical and biological origin) sedimentary rocks
Sample #
Mineral
composition
Other defining
details
Fossils
Rock name
R24
R25
R26
R27
R28
4. Return to R18 and R19 and circle the correct answers:
a. Which rock contains the most stable mineral clasts?
(at the Earth's surface)
b. Which rock is composed of rounder grains?
R18
R19
R18
R19
c. Which rock is more well-sorted?
R18
R19
d. Based on a-c, which sample was deposited furthest
from its source (and thus is called mature)?
R18
R19
5. The energy of the system (how much force is behind the medium of transport
(air or water)) can be characterized by the size of the particles the system can
carry. For instance, high-energy systems can carry large grains; low-energy
systems can carry small grains. Examine and rank rocks R18, R21 and R20 in
order from highest energy to lowest energy depositional system.
6. a. Some limestones (R33) are dense, fine-grained and black. So is basalt (R5).
What test can you perform to tell them apart, and how does each behave in the
test?
b. By the way, in general, why didn’t you worry about fossils in igneous rocks?
The place in which the sediment is deposited or the organisms lived is called the
depositional environment. Examples of depositional environments include
terrestrial environments (like lakes, deserts and rivers), transitional
environments (like beaches and tidal flats) and marine environments (like
continental shelves and the abyss). Note that, over time, a beach area may be
uplifted by plate tectonics so that you will find a transitional depositional
environment quartz-rich sandstone deep in a mountain range!
7. In what depositional environment did rock R25 form? Hint: these kinds of
rocks are called evaporites. Please explain how they form.
8. Look at sedimentary structure S1, which is an example of ripple marks. Are
they symmetrical or asymmetrical? Based on that answer and on the wavelength
of the ripples, is it more likely that these ripples were originally deposited in a
desert, a river, or a tidal flat? How are they preserved so that you can see them
today?
Metamorphic rocks
Metamorphic rocks have been subjected to sufficient heat and/or pressure to
melt some of their constituent minerals, but not all of them. As a result of this
selective mobilization of chemicals, only certain chemical reactions can occur,
and so a whole new set of metamorphic minerals are crystallized.
Throw in the presence of fluids such as water and carbon dioxide (yes, at these
pressures, even carbon dioxide can be a liquid), and nature has the means to
create even more metamorphic minerals and therefore metamorphic rocks. Note
that metamorphic rocks must be formed at depth; metamorphism is not a surface
process, and so is distinguishable from mere sedimentation.
Rocks that have foliation (a sort of wavy layering, though it can resemble
horizontal layering) are metamorphic rocks; the foliation indicates that
directional pressure was applied to the rock while the mineralogical changes
were occurring. On the other hand, some metamorphic rocks are not foliated;
they appear crystalline, like coarse-grained igneous rocks. These metamorphic
rocks were subjected to isotropic, or nondirected, pressure.
Because there are so many metamorphic minerals (of which you have seen but a
few), there are all sorts of ways to name metamorphic rocks. We will concentrate
on naming rocks by their metamorphic grade (that is, by the maximum degree of
heat and pressure they were subjected to, and not their mineral composition), or,
in some unusual cases, by their apparent composition (for instance, rocks like
marble, quartzite or metaconglomerate, from which you cannot determine the
metamorphic grade).
The protolith of a metamorphic rock is the original rock that was
metamorphosed into what you see today. As you can see from Table 4.1, the
protolith’s minerals really do determine the resulting metamorphic rock’s
composition. Note the differences in mineralogy even at the same grade.
A metamorphic facies is a name of a set of metamorphic minerals which is
uniquely created at a particular pressure and temperature. So, in addition to a
metamorphic grade, a rock can belong to a particular metamorphic facies as well!
Confused? You bet! However, realize that these terms all have their uses.
Table 4.1— Mineralogy of metamorphic rocks related to protolith and grade
MetamorFacies
Protolith
phic grade
Basalt
Shale
Low
Zeolite
Calcite, chlorite, zeolite Zeolite, sodium-rich
micas
Medium
Greenschist
Chlorite, amphibole,
Chlorite, muscovite,
plagioclase, epidote
plagioclase, quartz
Amphibolite
Amphibole, garnet,
Garnet, biotite,
plagioclase, quartz
muscovite, quartz
High
Granulite
Pyroxene, plagioclase, Biotite, orthoclase,
garnet
quartz, andalusite
One other consideration: there are three different types of metamorphism,
related to the particular tectonic setting of the metamorphism. As you are aware,
the deeper rocks are drawn into the lithosphere, the higher the temperatures and
pressures the rocks are subjected to. This is called regional metamorphism.
However, there are two other sets of conditions.
Convergence-type metamorphism occurs under high-pressure but lowtemperature (high P, low T) conditions. Contact metamorphism occurs under
high-temperature but low-pressure (high T, low P) conditions. This means that,
depending on the tectonic setting, three different metamorphic rocks could arise
from the same protolith. Table 4.2 summarizes these types.
Table 4.2 — Mineralogy of metamorphic rocks related to metamorphic type
Meta.
Facies
Protolith
type
Basalt
Shale
Regional
See table 4.1
Convergence Blueschist
Blue amphibole,
Blue amphibole,
(low grade)
chlorite, Ca-silicates
chlorite, quartz
Convergence Eclogite
Pyroxene, garnet,
not observed
(high grade)
kyanite
Contact
Hornfels
Pyroxene, plagioclase
Andalusite, biotite,
orthoclase, quartz
Needed: rock samples R34 through R44.
9. Look at rock sample R34, a regionally-metamorphosed shale.
a. Name three minerals in R34.
b. What grade of metamorphism do those ranges imply (use table 4.1)?
c. In what metamorphic facies is R34?
One way that metamorphic petrologists try to quantify the conditions of
metamorphism for various rocks is to draw a pressure/temperature (P/T)
diagram as shown in the figure on the next page. The field of the graph shows
the ranges of various metamorphic facies. The vertical axis shows the depth of
the metamorphism and the equivalent pressure in kilobars (kb). 1 bar is 100000
Pa or approximately 1 atmosphere of pressure, and therefore 1 kb is about 1000
atmospheres of pressure. The horizontal axis shows the temperature of the
metamorphism in degrees Celsius.
10 a. Use the named minerals from question 9b to determine the range of
possible maximum pressures and the range of possible maximum temperatures
at which R34 formed. Use units of °C for temperature and kbar for pressure.
b. Suppose another area where this rock is found was subjected to less than 1
kbar of pressure during metamorphism. Name two minerals (besides the ones
you named in part a) you would expect to find.
As you have seen, some minerals are quite useful in determining the grade or type
of metamorphism because they can only form under certain metamorphic
conditions. These are called index minerals. The next figure shows some ranges
under which certain minerals will form under regional metamorphic conditions,
and some of the associated rocks from a shale protolith.
11. You are given the following information about a metamorphic rock:
Mineral composition: pyroxene, garnet, kyanite
Chemical composition: silicon dioxide 50.24%, aluminum oxide 13.32%,
calcium oxide 10.84%, iron oxide 9.85%, magnesium oxide 8.39%
Which type of composition is more useful in determining the grade and
protolith of metamorphism and why? Or do both lists give equivalent
information?
12. a. Now look at R35. List some minerals in this regionally-metamorphosed rock.
b. What is its metamorphic grade? What is its protolith?
c. How does R35’s grade compare to R34’s grade?
Protolith
Intensity of metamorphism
Low grade
shale
rhyolite
granite
basalt
limestone
sandstone
High grade
slate
schist
gneiss
amphibolite
marble
quartzite
13. Now find R40 and R41; both of these rocks achieved the same grade of
regional metamorphism as R34 and R35 did. Determine the protoliths of these
rocks (hint: the standard mineral tests work well here).
R40’s protolith:
R41’s protolith:
14. In what two ways would you be able to distinguish rock R38 from its
protolith (note that rock R38's protolith is also in the same drawer)? (Hint: ignore
color. Look at the shape and “brokenness” grains within the rocks).
15. Why doesn't rock R40 have a foliation? (Hint: look at its mineralogy)
16. What changes in foliation thickness and mineral grain size would you
expect to see in a shale as it is subjected to greater temperatures and pressures
during metamorphism? (Hint: compare, in order, R37, R34, R36)
17. Fill in the following table (you have most of the answers by now). For
composition, your choices are: a mineral name, "clay minerals" and "rock
fragments". For foliation, write "F" for rocks with observed foliation and "NF" for
non-foliated rocks. For texture, your choices are slaty cleavage, schistosity,
gneissic banding or crystalline (meaning “unfoliated”); you decide the criteria.
Finally, identify the rock.
Rock
Composition
Foliation
Texture
Rock name
R34
R35
R36
R37
R38
R39
R40
R41
Plate Tectonics and Metamorphic Rocks
18. a. Rock R42 is blueschist, a unique type of metamorphic rock that forms
under conditions of high pressure and low temperature. Label the area on the
cross-section on the next page where you might expect blueschist to crystallize.
b. So, if you were to find blueschist as you walked along the Appalachian Trail in
North Carolina, what could you infer about the history of the East Coast of the US?
19. Rock R43 is serpentinite, which blueschist often becomes over time. A key
mineral in blueschist is forsterite, a form of olivine, with the chemical formula
Mg2SiO4. A key mineral in serpentinite is (surprise) serpentine (chemical
formula: Mg3Si2O5(OH)4). How does serpentinite form from blueschist? (Hint:
consider readily available simple molecules at metamorphic depths and the
difference between the two chemical formulae)
20. a. Rock R44 is hornfels, a unique type of metamorphic rock that forms under
conditions of low pressure and high temperature. Label the area on the crosssection where you might expect hornfels to crystallize.
b. What is hornfels' protolith? Or is there a unique protolith?
c. Why is contact metamorphism such an appropriate term for this type of
metamorphism?