Download Geology 103 Name(s): Lab 2: Igneous and sedimentary rocks, and

Geology 103
Name(s):
Lab 2: Igneous and sedimentary rocks, and plate tectonics
The two major rock types you will use to tell the story of western Washington are
igneous and sedimentary. In this lab, you will figure out some methods to
identify the name of these rocks. Plate tectonics is the theory of the Earth’s
behavior as an “active” planet. In this lab, you will use a plate tectonic map to
think of the connection between the rocks the Earth is made of, and plate
tectonics.
Needed (at various points):
• the minerals and rocks in drawers 4 – 17, 19, 21 – 34
• the Circum-Pacific Plate Tectonic map
• the Mineral Identification kits
Igneous rocks
Igneous rocks form when magma (molten rock) cools and crystallizes. Volcanic
eruptions take place when magma reaches the surface before it solidifies. The
magma flows onto the surface as lava or erupts explosively as rapidly expanding
gas propels bits of lava and rock outward. The rocks resulting from eruptions are
called volcanic or extrusive igneous rocks. On the other hand, the magma can
crystallize before it reaches the surface, in which case they are called plutonic or
intrusive igneous rocks.
Historical note: Vulcan was the blacksmith god of the Romans, forging the tools of
the gods inside volcanoes. Pluto was the Roman god of the underworld.
Igneous rocks are classified based on their mineral composition and their mineral
grain texture. This classification system is interpretive because it says something
about the source of the magma and the conditions under which the rocks were
formed.
The mineralogy of an igneous rock is controlled by the chemical composition of
its parent magma. Most igneous rocks are composed primarily of the following
eight elements:
• Silicon (Si)
• Oxygen (O)
• Sodium (Na)
• Potassium (K)
• Magnesium (Mg)
• Aluminum (Al)
• Iron (Fe)
• Calcium (Ca)
For reasons that are a bit beyond this class, the elements in the first column,
especially a combination of silicon and oxygen called silica (which is chemically
written in ionic form as SiO42–), are less dense and generally lighter-colored than
the elements in the second column.
All of these elements occur in the crystal structure of eight common igneous
rock-forming minerals. These minerals in turn make up 95% of the volume of all
igneous rocks. The names and chemical formulae for these minerals:
Olivine
Plagioclase
Pyroxene
Amphibole
Biotite
Orthoclase
Muscovite
Quartz
(Mg,Fe)2SiO4
NaAlSi3O8 (sodium-rich) CaAl2Si2O8 (calcium-rich)
Complex Ca-Mg-Fe-Al silicates
Complex water-bearing Na-Ca-Mg-Fe-Al silicates
K(Mg,Fe)3AlSi3O10(OH)2
KAlSi3O8
KAl2(AlSi3O10)(F,OH)2
SiO2
The list of minerals is in decreasing order of freezing (melting) temperature of the
mineral; in other words, olivine freezes from a magma first as the magma cools,
and quartz last.
Finally, those minerals made mostly of elements from the first column of elements
on the previous page are called felsic, and those minerals made mostly of elements
from the second column of elements on the previous page are called mafic (look at
the first initials of the top three element symbols in the second column).
1. Using the attached mineral ID charts (Tables A-1 through A-3), identify the
eight common igneous rock-forming minerals. I don’t care how you do it, but do
list the features of the mineral that most distinguished it to you.
Mineral # Mineral name
M1
M5
M6
M11
M13
M14
M15
M17
Distinguishing characteristic(s)
2. One way to separate minerals is according to elemental composition, which,
fortunately for us, is roughly associated with color, as noted above. The division is
between darker, mafic minerals, and lighter, felsic minerals. The exceptions will be
calcium-rich plagioclase, which is considered to be mafic, and biotite, which is
considered to be felsic.
Color
Darkcolored
(mafic)
Minerals
Average specific
gravity (density)
Diagnostic
chemistry
3.3
Magnesium,
aluminum and
iron-rich
2.8
Silica-rich
calcium-rich plagioclase
biotite
Lightcolored
(felsic)
3. Look at rock samples R4 and R6. Identify the minerals that are present in each
sample (a hint is given as to the color of each mineral), then determine if the samples as a rock are mafic, felsic or intermediate, based on the table in question 2.
Sample
Minerals present
Pink =
R4
White =
Translucent =
Dull black =
R6
Shiny black =
Dull green =
4. So what is the difference between a mineral and a rock?
Mafic, felsic or
intermediate?
5. An igneous rock may be identified by either its texture or its characteristic
mineralogy and grain size. Mineralogy means “are the minerals in the rock
generally felsic, generally mafic, or even amounts of both (intermediate)?”. Grain
size means “what is the predominant size of the minerals in this rock? Sand-size?
Silt-size? Clay-size?” Texture means “overall, what sort of distinguishing
irregularities occur over the whole rock? Bubbles (vesicles)? Shattered bits of
other rock?” Using Tables B-1a (grain size and mineralogy) and B-1b (texture),
identify the following rocks:
(BIG hint: use Table B-1a for R4 – R9 and use Table B-1b for R10 – R14)
Rock #
Felsic, mafic or
intermediate?
Predominant
grain size
Rock name
R4
R5
R6
R7
R8
R9
Rock #
R12
R13
R14
R15
R16
Texture
Rock name
The viscosity of a fluid is its ability to resist flow; higher viscosity means that the
fluid doesn't flow very fast. You will easily be able to outrun a viscous lava flow;
you will not be able to outrun a non-viscous (runny) lava flow. The reason for
this lies in the chemical composition of the lava: the silicate ion is not as mobile
(in other words, tends to stick to other silicate ions) as metal (e.g., iron,
magnesium, aluminum) ions.
6. Answer the following questions with mafic, felsic, intermediate or any:
a. Which magma contains more silica?
b. Which magma will be erupted at an initially lower temperature?
c. Which magma will have higher viscosity?
d. Which magma will have higher volatile (gas) content?
e. Which magma will likely erupt explosively?
f. Therefore, which magma will likely end up as a widespread ash?
Table B-1a. Igneous rocks which are identified by their composition/grain size
Felsic
COMPOSITION
(Minerals present)
Intermediate
quartz
Na-plagioclase
orthoclase
amphibole
biotite
pyroxene
Na-plagioclase
biotite
Mafic
Ca-plagioclase
olivine
pyroxene
TEXTURE
Coarse-grained
(mostly visible
grains)
Fine-grained
(mostly invisible
grains)
GRANITE
DIORITE
GABBRO
RHYOLITE
ANDESITE
BASALT
Table B-1b. Igneous rocks which are identified by their texture
1. Is the rock glassy on any fresh surface?
Yes — Obsidian
No — Go to #2.
2. Is the rock vesicular (containing gas bubbles)?
Yes — Go to #3
No — Go to #4
3. Is the rock dark-colored on a fresh surface? Yes — Scoria
No — Pumice
4. Is the rock composed of large pieces of broken rocks?
Yes — Breccia
No — Go to #5
5. Is the rock well-cemented (doesn't crumble easily)? Yes — Welded tuff
No — Tuff
Tuff and pumice are sometimes difficult to tell apart; however, a simple test
distinguishes them: pumice floats on water and tuff sinks.
Sedimentary rocks
Sedimentary rocks are deposited. Using the agents of transport (wind, water or ice),
the loose material (called sediment) is compacted (compressed so that the pore
space is removed) and cemented (what pore space remaining is filled with a
different chemical) into a sedimentary rock.
Sediment is made by the erosion of weathered rocks. Weathering is the process that
either chemically or physically breaks up fresh rock material; an example of chemical weathering is a rock reacting with acid rain, while an example of physical weathering is when water freezes with a crack in the rock and makes the crack bigger.
Sedimentary rocks fall into two categories: clastic sedimentary rocks, which are
those that are made of weathering products of pre-existing rocks, and chemical or
biochemical sedimentary rocks, which are the result of either evaporation or
precipitation of mineral-rich water or the accumulation of the remains of biological
organisms. To sum up: “Some sedimentary rocks are born, not made, but all are
deposited”.
Many of the same elements that occur in the igneous rock-forming minerals also
occur in sedimentary minerals. However, since weathering reactions occur, the
elements will rearrange and form new crystal patterns, which will have different
mineral names. In addition, the surface environment contains compounds such
as water (H2O) and carbon dioxide (CO2) that will react and become part of the
sedimentary minerals.
Sedimentary minerals are the most common minerals found on the earth’s
surface because sedimentary rocks make up most of the earth’s surface. The
names and chemical formulae for these minerals:
Quartz
Calcite
Gypsum
Halite
Pyrite
Magnetite
SiO2
CaCO3
CaSO4•2H2O
NaCl
FeS2
Fe3O4
Some minerals you will not see in lab, because they are clay minerals and thus
difficult to identify without more sophisticated equipment:
Kaolinite
Illite
Smectite
Al2Si2O5(OH)4
Complex hydrated potassium and aluminum sheet silicates
Complex hydrated sodium and aluminum sheet silicates
7. Again using the mineral ID charts and any means possible, identify the six
common sedimentary rock-forming minerals (i.e., the list given above, except for
the clay minerals). Note that one of these will be the same as one you saw in the
igneous minerals portion of this lab. Give a distinguishing characteristic of each
mienral.
Mineral #
Mineral name
Distinguishing characteristic(s)
M2
M7
M8
M10
M18
M20
Properly speaking, magnetite is also formed as an igneous mineral, but not a
major one.
Note the abundance of elements like oxygen (O) and sulfur (S) in sedimentary
minerals; these elements help stabilize metal ions at the Earth’s surface and will
play a large role in the preservation of fossils, and other historical geology issues.
8. Find the drawer with the weathering samples. Samples W1 and W2 are
igneous rocks from the first part of the lab. Sample W3 is a partially weathered
igneous rock. Sample W4 is beach sediment.
a. Identify W1 and W2 (hint: use the Igneous Rock ID chart).
b. Identify the minerals in the clasts breaking off of W3.
c. Did sample W3 start off as W1 or W2?
d. Look at sample W4. Did this sediment come from W1 or W2?
e. When sample W4 compacts and lithifies, what will its rock name be?
9. a. As mentioned above, after compacting sediment, in order to make a
sedimentary rock, the sediment needs to be cemented by another chemical agent.
If there are only the following three cement materials available, how would you
identify each cement (think of a test for each)?
calcite:
iron oxide:
silica:
b. Look at rock samples R18 and R19. What is the cement that holds each rock’s
grains together?
R18 =
R19 =
10. a. How would you be able to tell apart a sedimentary rock made of calcite
from one that was made of quartz grains cemented by calcite? (Hint: is the acid
test useful here? What other test might be useful?)
b. Sedimentary rocks will be identified, in large part, to the materials they are
made of. Fill in the table with the most common mineral of each rock. Note that
this question is not asking you about the cement of each rock!
Sample
R23
Most common mineral
R24
R25
11. Fill in the following table using the Sedimentary Rock ID flow chart in the back.
Note there are separate tables for clastic and chemical/biochemical sedimentary
rocks. For clastic rocks:
• Determine the most common grain size
• Determine the degree of grain rounding (choose from: angular, subangular,
subrounded or rounded)
• Determine the degree of sorting of the grains that occurs in the rock (choose from:
well-sorted, moderately-sorted or poorly-sorted)
• Then, based on your observations and the ID table, identify the rock.
Clastic sedimentary rocks
Sample #
R18
R19
R20
R21
R22
R23
R24
Grain size
Grain rounding
Grain
sorting
Rock name
For chemical or biochemical sedimentary rocks:
• Write the name of the greatest percentage mineral component
• Determine if the sample was chemical in origin or biological in origin
• If it was chemical, deduce what sort of depositional environment (like hot
springs or tidal flats) the rock was deposited
• If it was biological, deduce what organism’s remains make up the rock (this
can be very general, such as “clam”)
• Then identify the rock, using your observations and the ID table. Remember, if
there are fossils, add the adjective fossiliferous before the rock name!
Chemical and biochemical sedimentary rocks
Sample #
Composition
Biological or
chemical?
Organism or
depositional
environment
Rock name
R25
R26
R27
R28
R29
R30
R31
R32
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, it's a sedimentary
rock. For any sedimentary rock, if it contains any fossils, use the adjective
fossiliferous in front of the rock name.
One. If the rock is made of broken up bits of rock (including extremely fine
grains) then the rock is clastic, and go to two; otherwise go to three.
Two. 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 (it may contain other
grain sizes in its matrix) Æ conglomerate
If the most common grain size is sand Æ sandstone
If the most common mineral is quartz Æ arenitic sandstone
If the most common mineral is feldspar (orthoclase or plagioclase
— rock may look gray or pink) Æ arkosic sandstone
If there are many dark rock fragments and a significant volume of
the rock is silt or clay sizeÆ graywacke
If the most common grain size is silt Æ siltstone
If the most common grain size is clay Æ claystone (shale)
Three. Identify the most common mineral in the specimen (use mineral ID chart
if necessary).
If the most common mineral is quartz Æ chert
If the most common mineral is halite Æ rock salt
If the most common mineral is gypsum Æ rock gypsum
If it is black-colored, not very dense and flaky Æ 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, or
mistaking the fizzing for the fluid entering holes in the rock)
If the rock is not very dense and pure white Æ chalk
If the rock is made of small, round “clasts” Æ oolitic limestone
If the rock is made of almost exclusively broken-up shells Æ coquina
If the rock is banded Æ travertine
If the rock is buff colored, does not fizz readily, but the powdered
rock fizzes readily Æ dolostone
Else Æ limestone
12. Return to R18 and R19 and circle the correct answers:
a. Which rock contains the most stable mineral clasts?
(= lowest freezing point of the minerals in the rock)
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?
R18
R19
e. Therefore, which is the more mature rock?
R18
R19
13. 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 R24, R27 and R28 in
order from highest energy to lowest energy depositional system.
The connection between rocks and plate tectonics
Plate tectonics is the theory that describes the mechanism by which the Earth’s
surface changes, both in terms of mountain and continent growth and
movement, and the distribution of materials across the surface. Thus, it is the
cornerstone of all fields of geological inquiry.
14. a. Find a copy of the “Circum-Pacific Plate Tectonic Map” (look in the map
cabinet). What plate are we (that is, residents of Seattle) on?
b. What plate do we (again, the Seattle part of the map) border to the west?
c. What is the name given to the feature at the boundary between these two
plates?
d. Describe the type of interaction that is occurring at this boundary. Hint: use
the Explanations section of the map.
15. Find the Pacific Plate, and note the arrows that indicate the direction in
which the plate is moving. You will want to find out the rate at which the Pacific
Plate is moving. Do you use the red arrows or clear arrows? What’s the
difference between them? Hint: Again use the Explanations section.
16. a. What is the current rate of Pacific Plate motion near Hawaii? Don’t forget to
include the units of motion! In what direction is it moving?
b. Measure the distance from the big island of Hawaii to Kammu Seamount, off
to the west (remember to measure this distance as kilometers). Hint: You don’t
necessarily need to use a ruler; an edge of a piece of paper will do.
c. Using the conversion factor 1 km = 100,000 cm, calculate the length of time in
years represented by the seafloor between Kammu Seamount and Hawaii,
assuming the Pacific Plate has moved at a constant rate.
This length of time is significant because of how the volcanoes in this “chain”
formed. Under Hawaii is a hotspot, an active area of the mantle of the Earth that
feeds magma to volcanoes on the surface above it. All of the islands in the
Hawaiian “chain” (also called “the Hawaiian arc”) were active at one point; the
critical issue is that each island (or seamount) was active at different times —
right when that island (or seamount) was positioned directly over the hotspot.
The hotspot stays put, and different volcanoes become active as the plate they are
riding on moves across and over the hotspot.
17. a. During what period on the geologic time scale was Kammu Seamount
active?
b. Now, how can we know that? (Rhetorical question). Look at the bar-code like
column near the Explanations section. What does this column represent? What
do the numbers on either side of the column represent?
c. Look on the Pacific Plate for some of the numbers on the left side of the
column. Where are the youngest rocks on the Pacific Plate? To what feature are
these rocks parallel?
d. Where are the oldest rocks on the Pacific Plate? What does this imply about
the direction of motion of the Pacific Plate? Is this consistent with your answer to
question 16a?
e. How is it that this technique works? (Rhetorical question). What mineral
you’ve seen in this lab would be able to record what the column in question 17b
purports to record? When does that mineral have the freedom to move? When
does it lose the freedom to move, thus preserving its orientation forever?
18. a. Going back to your answer to question 14d, describe exactly what is
happening to each plate at our plate boundary. A sketch might be helpful here,
especially if it is a cross-section, and it is properly labelled with plate names and
orientation directions and arrows of movement. Hint: the teeth on the boundary
will be important in making your drawing correct.
b. What does your drawing suggest about the density of the rocks of each plate?
In other words, are the rocks of each plate the same density? What does this
imply about the identity of rocks of each plate (same or different)?
c. Revisit question 2. Describe the rocks of each plate in terms of the words used
in question 2.
19. a. At the plate boundary near us, how fast is crustal material being destroyed
(hint: do you use the red or the clear arrows?)?
b. Since the Earth isn’t shrinking or expanding, one cannot simply destroy large
areas of the Earth’s crust without replacing it. What plate tectonic feature is
replacing the destroyed crust near us? Hint: you don’t have to look too far away,
and the Explanations section will help, and this should be similar to part of your
answer to question 17c.
20. (Payoff question) a. The word “extrusive” is used to describe igneous rocks
that are erupted or reach the surface (the ocean floor counts) as a liquid. The
word “intrusive” is used to describe igneous rocks that freeze deep
underground. Will the plate boundary you mention in questions 17c and 19b
have intrusive or extrusive igneous rocks?
b. Is there any reason to believe the continental rock will ever be subducted on a
large-scale? For reasons to be given later, this rock is intrusive.
c. Fill in the blanks with rock names from question 5: At a ocean-continent
subduction zone, ___________________ is subducted beneath ________________ .