Recitation 4 - Rocks-1
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Pittsburg State University *
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160
Subject
Geology
Date
Jan 9, 2024
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7
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1
Name: ______________________________
Recitation Section: ___________________________
Recitation 4: Rocks
Introduction:
All rocks are formed by the processes outlined in the
Rock Cycle
. The different environments rocks form in lead to the
three broad categories of rock types: Igneous, Sedimentary, and Metamorphic. Within each of the three rock types,
rocks can be further categorized based on their texture and mineralogy (composition). Texture refers to the size, shape,
and distribution of mineral grains within a rock. Examples include coarse-grained, fine-grained, microcrystalline, and
glassy. Mineralogy refers to the types and abundance of minerals found within a rock.
Igneous rocks crystallize directly from molten rock, either from magma deep below the Earth’s surface (
intrusive
) or
from lava erupted to the surface (
extrusive
). Sedimentary rocks form from the products of chemical and physical
weathering of pre-existing rocks. Metamorphic rocks form when pre-existing rocks are changed in response to changes
in pressure, temperature, and/or hydrothermal fluids. The different environments that each rock type forms in leads to
distinct textures and mineral assemblages.
Goals:
Learn to recognize these textures in igneous, sedimentary, and metamorphic rocks.
Learn to recognize by appearance and common co-occurrence the eight common minerals.
Learn to identify the common igneous rock types.
Learn to recognize the sedimentary rocks most commonly found on the Earth’s surface.
Learn to recognize the metamorphic rocks most commonly found on the Earth’s surface.
Skills Developed:
This lab hones your ability to make careful observations of natural materials in order to identify and
understand igneous, sedimentary, and metamorphic rocks. This recitation provides practice in learning to look closely at
rocks to gather the observations needed to identify them. In addition, it gives you a taste of moving from basic
identification to interpretation, which is what makes geology in the field fun.
Part A: Igneous Rocks
A.1 Bowen’s Reaction Series and Rock Composition
Bowen’s reaction series is an icon of Geology. In the early 20
th
century, N.L. Bowen carried out a series of experiments
in which he slowly cooled mafic magmas and observed which minerals formed as a function of temperature. He found
that olivine and Ca-rich plagioclase always formed first. With further slow cooling, olivine dissolved and recrystallized
to form pyroxene. With more cooling, some of the pyroxene recrystallized to form amphibole, and ultimately some of
the amphibole converted into biotite. Full conversion of one mineral to the next depends on there being enough of the
required elements to make the new minerals (e.g., amphibole and biotite need enough Na, K, Al, and water in the
magma). Meanwhile, the plagioclase also changes with temperature: Ca-rich plagioclase forms first. With progressive
cooling, plagioclase progressively richer in Na and poorer in Ca grows around the first-formed crystals.
For this class, the main value of the Bowen reaction series diagram is that it summarizes which minerals are found in
which rocks. For example, look in the field marked “
Mafic
”. Olivine, pyroxene, and Ca-rich plagioclase fall within this
field, as do the rocks gabbro and basalt. Remember that the lack of key ingredients in the starting melts, as well as too-
rapid cooling, may prevent the expected minerals from forming in a given sample.
2
Examine Samples #1-7 and determine the identity. Samples #2 and #6 have already been identified. List what minerals
make up each sample (Hint: consider Bowen’s Reaction Series in the handouts). Then answer the question following the
table.
Intrusive
Extrusive
Mafic
Gabbro
Basalt
Sample: ________________
Sample: ________________
Minerals:
Intermediate
Diorite
Andesite
Sample:
#2
.
Sample:
#6
.
Minerals:
Amphibole, biotite, plagioclase feldspar (Ca and Na rich), maybe
some pyroxene
Felsic
Granite
Rhyolite
Sample: ________________
Sample: ________________
Minerals:
Question:
1.
Which mineral makes the red granite so much darker than the white granite?
_____________________________________________
A.2 Extrusive Rock Textures
Examine Samples #5 (identified in
A.1
), #8 (scoria), #9 (pumice), and # 10 (obsidian) and answer the following
questions.
1.
Which rock do you think is best for making arrowheads and spear points?
_____________________________________________
2.
Which textural feature links the basalt, scoria, and pumice?
_____________________________________________
3
3.
Despite appearances, the pumice and obsidian are both felsic rocks. (Tiny grains of magnetite give the obsidian
glass the black color typical of mafic rocks.) Imagine that you have a sample of magma the same mass and
volume as your obsidian sample. Now image that your magma erupts and suddenly forms pumice. The mass of
material remains constant, but how does the volume change as a result of bubbles forming in your magma
sample?
_____________________________________________
4.
One measure of the volume taken up by bubbles is density, and one way to estimate density is to see what
floats on water.
a.
Of the basalt, scoria, and pumice, which is most likely to float on water?
_____________________________________________
b.
Which is least likely to float on water?
_____________________________________________
Part B: Sedimentary Rocks
B.1 Clastic Sedimentary Rocks
Clastic sedimentary rocks are formed from the products of physical weathering, e.g. the broken pieces (sediments) of
pre-existing rocks, that have been cemented together. Clastic rocks are identified using composition and grain size. The
mineralogy of clastic rocks is typically dominated by grains of quartz, K-feldspar, mica, and clay minerals.
Examine Samples #11-16 and
arrange the clastic rocks going from smallest to largest grain sizes. Use the chart below
to classify each sample (Note: assume that all shale samples are the same grain size). Then answer the questions
following the table below.
Grain Size
Rock Type
Corresponding Sample Numbers
Most grains greater than 2 mm
Conglomerate
Most grains between
0.06 and 2 mm
Sandstone
Most grains too small to see (<0.06
mm)
Shale
Questions:
1.
Can you spot any fossils in any of these samples? One sample should have some fossils in it. If your samples
seem barren, check out a neighbor’s kit. Sketch and identify the fossil below:
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2.
Three things make identifying minerals in clastic rocks tough: 1. Weathering and transport make fresh minerals
look dull and rounded, so luster and cleavage are tough to spot; 2. Thin coatings of iron oxides give many
minerals a dull, pale orange/tan color; 3. Grain sizes are often so small that it takes a hand lens and an
experienced eye to make a positive ID.
a.
In which samples can you spot quartz?
_____________________________________________
b.
List any samples that contain muscovite:
_____________________________________________
c.
Which ones are (one assumes) dominated by clay minerals?
_____________________________________________
B.2 Chemical Sedimentary Rocks
Chemical sedimentary rocks are made from the products of chemical weathering, i.e. the ions dissolved from pre-
existing rocks. Biochemical sedimentary rocks are formed from biological organisms utilizing the dissolved ions to
make shells, etc. Organic sedimentary rocks are formed from the carbon-rich remnants of plants that utilized the
dissolved ions from the environment to grow. Chemical rocks (gypsum, halite) are produced by inorganic precipitation
of dissolved ions from water. Some sedimentary rocks can be produced by multiple pathways; for example, limestone is
both a biochemical rock made from calcite/shells and a chemical rock produced by inorganic precipitation of calcite
from water.
Examine Samples #17-19 and answer the following questions.
1.
You have in front of you two contrasting examples of limestone. Which do you think is best described as a
microcrystalline limestone and which as a bioclastic limestone? (As you might infer, ‘microcrystaline’ means
made of tiny crystals and ‘bioclastic’ means made up of shell debris.) Limestone is made of calcite (CaCO
3
).
a.
Microcrystalline limestone:
_____________________________________________
b.
Bioclastic limestone:
_____________________________________________
Note:
Many of our samples of microcrystalline limestone (called ‘micrite’ for short) have a tan weathered
surface and a gray fresh surface. The weathered surface shows the original layering, whereas the fresh surface
hides this layering. Weathering often reveals surprisingly interesting sedimentary features in wild rocks.
2.
In what sedimentary environment does coal form?
_____________________________________________
a.
Why is coal appropriately called a fossil fuel?
_________________________________________________________________________________________
_________________________________________________________________________________________
5
3.
You can commonly find evaporite deposits (e.g., halite, gypsum) exposed in deserts, but you rarely see them
exposed in wetter areas like Pennsylvania. Why is this?
_________________________________________________________________________________________
_________________________________________________________________________________________
Part C: Metamorphic Rocks
C.1 Classifying Metamorphic Rocks
Foliated metamorphic rocks display aligned mineral grains or roughly parallel bands of light and dark minerals.
"Parallel" does not always imply "planar". Many foliated rocks display wavy, undulating, or contorted foliation textures.
Foliation caused by microscopic mineral grains can cause a rock to break into pieces with parallel tops and bottoms.
Non-foliated rocks display no such mineral textures, although they may display banding related to impurities in the non-
foliated mineral grains (e.g., cross-bedding in quartzite or organic matter or clays in marble).
Examine Samples #20-24. Use the chart below to classify each sample as foliated or non-foliated. Then answer the
questions following the table below.
Foliated
Non-foliated
Sample: ________________
Sample: ________________
Sample: ________________
Sample: ________________
Sample: ________________
Sample: ________________
Questions:
1.
Quartzite is made from quartz. Marble is made from calcite. Can you figure out which sample is which?
Hint
:
The easiest way is to look for tiny cleavages with a hand lens. Do you remember which mineral has them and
which doesn’t? And don’t mistake a muscovite for a quartz or calcite cleavage!
a.
Quartzite:
_____________________________________________
b.
Marble:
_____________________________________________
c.
Which physical property/properties did you use to distinguish the two?
_________________________________________________________________________________________
_________________________________________________________________________________________
6
2.
In the back of the recitation classroom (SRCC 113) is a large polished slab of the Morton Gneiss. The Morton
Gneiss is actually a migmatite. This rock is 3.5 billion years old.
a.
Is the migmatite a foliated or non-foliated rock?
_____________________________________________
b.
Which are generally coarser (larger) grained, the felsic or mafic minerals?
_____________________________________________
C.2 Metamorphic Grade
The metamorphic grade of a rock refers to the degree and intensity of metamorphism and provides insight into the
relative temperature and pressure conditions under which metamorphic rocks form. Foliated rocks are handy because
they help us to distinguish between low, medium, and high grade metamorphic conditions. The progression from low to
high grade is marked by an increase in the average size of mineral grains.
Examine Samples #20-24 and identify the rock kit samples of slate, schist, and gneiss in the following table. Then
answer the questions following the table.
Low Grade: Slate
Medium Grade: Schist
High Grade: Gneiss
Sample: ________________
Sample: ________________
Sample: ________________
Questions:
1.
How does the metamorphic grade of a migmatite, like the Morton Gneiss, compare to the metamorphic rocks
in the previous table?
_________________________________________________________________________________________
_________________________________________________________________________________________
2.
What would have happened to the Morton Gneiss if the metamorphic conditions had increased further?
_________________________________________________________________________________________
_________________________________________________________________________________________
Part D: Mystery Rock Identification
From the plastic tubs of mystery rocks, select
at least
one rock from each tub (Igneous, Sedimentary, Metamorphic).
You will need to identify a total of 4 mystery rocks, with at least one sedimentary rock, one metamorphic rock, and one
igneous rock; the identity of the final rock is up to you and must be listed in the following table. Identify the proper
name for each rock.
Note:
The metamorphic rocks mystery kits include a sample of phyllite. Identify the phyllite based on its metamorphic
grade being between slate and schist, if you choose this sample.
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Specimen care
: Please
carefully
place the samples back in the box when you are done. Rocks may seem tough but
handling them roughly damages the surface and makes them harder for future people to work with. Some rocks are
pretty hard and tough, but these also tend to damage the softer ones!
Sample Number
Rock Type
Specific Rock Identity
Igneous
Sedimentary
Metamorphic
Mystery Rock Options (Rocks Encountered in Recitation 4)
(May see more than once!)
Igneous
Granite
Gabbro
Diorite
Andesite
Rhyolite
Basalt
Obsidian
Pumice
Scoria
Sedimentary
Conglomerate
Sandstone
Shale
Microcrystalline Limestone
Bioclastic Limestone
Coal
Metamorphic
Foliated
Slate
Low Grade (low T/P)
Phyllite
Schist (aligned muscovite, garnet)
Gneiss (banded light/dark)
High Grade (high T/P)
Non-Foliated
Marble (fizzes with acid)
Quartzite