Download Lab 6

Geology 101
Name(s):
Lab 6: Sedimentary and metamorphic rocks
Identifying sedimentary rocks
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).
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. (Clastic rocks)
Else → GO TO Second step
alternative B.
(Chemical or biological rocks)
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
0.062 — 2 mm
0.005 — 0.062
mm
< 0.005 mm
easily visible to naked eye; "grains" may
contain identifiable minerals
visible to naked eye
not visible but can be felt between fingers or
across teeth
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 (arenite)
If the most common mineral is quartz → quartz arenite
If the rocks is medium gray to red and well-sorted → arkose
If the rock is dark-colored and has much fine grain-size material in its
matrix → greywacke
If the most common grain size is silt or clay or a combination of both:
If the rock splits into irregular or regular layers → shale
If the rock is massive (no layering) → mudstone
The terms siltstone and claystone are also used on occasion.
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
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.
Needed: sedimentary rock samples R18 – R27 (Tubs 20 – 29), R33 (Tub 35) and S1
(Tub 36)
1. 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; wellrounded, 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.
Grain sorting:
Grain rounding:
Clastic sedimentary rocks
Sample #
Grain
size
Grain
sorting
Grain
roundness
Fossils
Rock name
R18
R19
R20
R21
R22
2. 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 #
R23
R24
R25
R26
R27
Mineral
composition
Other
defining
details
Fossils
Rock name
Sedimentary rock properties and depositional environments
3. Return to R18 and R19 and circle the correct answers:
a. Which rock contains the most stable mineral clasts?
(at the Earth's surface)
R18
R19
b. Which rock is composed of rounder grains?
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
4. 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.
5. 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. Which rock has fossils? 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!
6. In what depositional environment did rock R25 form (see diagram above)?
Hint: these kinds of rocks are called evaporites. Explain how they form.
7. Look at sedimentary structure S1, which is an example of preserved 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?
Properties of 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 6.1, the protolith’s minerals
really do determine the resulting metamorphic rock’s composition. Note the
differences in mineralogy even at the same grade.
Table 6.1— Mineralogy of metamorphic rocks related to protolith and grade
MetamorFacies
phic
grade
Low
Zeolite
Greenschist
Medium
Amphibolite
High
Granulite
Basalt
Protolith
Shale
Calcite, chlorite, zeolite Zeolite, sodium-rich
micas
Chlorite, amphibole,
Chlorite, muscovite,
plagioclase, epidote
plagioclase, quartz
Amphibole, garnet,
Garnet, biotite,
plagioclase, quartz
muscovite, quartz
Pyroxene, plagioclase, Biotite, orthoclase,
garnet
quartz, andalusite
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.
Note that not all minerals in a given cell in the table above will show up in every
specimen of that grade/facies/protolith, but all minerals in the specimen will be
named in the cell!
Metamorphic rock identification
Needed: Samples M18 and M 19 (Tub 37), R34 through 45 (Tubs 38 – 49)
8. Some minerals are made under metamorphic conditions. You have seen a few
already in Lab 3 (for instance, talc and graphite). Identify these two other
metamorphic minerals:
Mineral #
Distinguishing features (color, cleavage,
hardness, magnetism, density, etc.)
Mineral name
M-18
M-19
9. Look at rock sample R34, a regionally-metamorphosed shale.
a. Name two minerals in R34 (hint: you did this already in Lab 3)
b. Given that muscovite is present in R34 but hard to see, what grade of
metamorphism does this mineralogy imply (use table 5.1)?
c. Still using that table, what metamorphic facies is R34?
d. So what is the name of the rock? To find this, see the diagram on the next page,
or table 7.1 (page 145) in the text.
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 6.2 summarizes these types.
Table 6.2 — Mineralogy of metamorphic rocks related to protolith and grade
Meta.
type
Regional
Convergence
(low grade)
Convergence
(high grade)
Contact
Facies
Protolith
Shale
See table 5.1
Blue amphibole,
Blue amphibole,
chlorite, Ca-silicates
chlorite, quartz
Pyroxene, garnet,
not observed
kyanite
Pyroxene, plagioclase
Andalusite, biotite,
orthoclase, quartz
Basalt
Blueschist
Eclogite
Hornfels
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 below. 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 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 facies from question 12c 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 the protolith was found was subjected to less than 1
kbar of pressure but the same temperature range during metamorphism. Name one
mineral (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.
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, which is the same metamorphic grade as R34. What are the
mineralogical differences? (In other words, what minerals show up in R34 but not R35?
In R35 but not R34?)
b. But what is the name of this rock, anyway? Hint: kind of a trick question.
13. In fact, for many metamorphic rocks, the most common mineral in the rock is
used as an adjective in front of the rock name. Fill in the appropriate mineral name
for the samples below, using the suggested test given:
Sample #
Test
Rock name
R34
Cleavage
_____________ schist
R35
Obvious mineral
_____________ schist
R36
Color
_____________ schist
R37
Scratch
_____________ schist
Protolith
shale
rhyolite
granite
basalt
limestone
sandstone
conglom.
Intensity of metamorphism
Low grade
grade
slate
High
phyllite
schist
gneiss
amphibolite
marble
quartzite
metaconglomerate
14. 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, R38, R39, R34, R40)
15. So fill in the following rock names, using your answer to part a and the fact that
each sample represents a different metamorphic grade:
Sample #
Metamorphic grade
Rock name
R38
R39
R40
16. R41 and R42 are nonfoliated metamorphic rocks (the text calls them
“granoblastic rocks”); both of these rocks achieved the same grade of regional
metamorphism as R34 and R35 did. Identify the rock names using the hints
suggested in the characterization column; identify their protoliths from the table on
the previous page.
Sample #
Characterization
R41
Glass plate
R42
Acid bottle
Rock name
Rock protolith
Plate Tectonics and Metamorphic Rocks
17. a. R43 is blueschist, a unique type of metamorphic rock that forms under
conditions of high pressure and low temperature. Label the area on the crosssection 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?
18. R44 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)
19. a. R45 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?