Download Lab #__: IGNEOUS

GG 101L
Semester: _______
NAME _________________________
SECTION (Weekday and time)____________________
Lab #__: IGNEOUS ROCKS
I Introduction:
Igneous rocks are rocks that solidify from molten rock. When molten rock is underground we
call it magma whereas when it is above ground we call it lava, even though it is the same stuff.
Geologists classify igneous rocks because the different types indicate different tectonic settings,
different volcanic plumbing systems, different sources for magma, and many other things. Two
main characteristics are used to classify igneous rocks: 1) texture (the size of the mineral grains in
the rock; and 2) composition (often determined by what the actual minerals are). Most geologists
first determine the texture and then the composition, and with these two bits of information fit the
rock into an established igneous-rock classification scheme (of which there are many).
There are hundreds of different igneous rock names. Fortunately, many of these are quite rare
and often restricted to only a few places on Earth, so most geologists ignore worrying about
identifying them. Identifying common igneous rocks therefore becomes much more of a tractable
problem, especially because you can do it using only 7 of the 8 most common minerals (that you
learned to identify last week, right?). Identifying these minerals in a rock, combined with assessing
the rock’s texture, makes it possible to do a very decent job of classifying probably 90% of all
igneous rocks.
The 7 minerals that you need to be able to identify in order to classify igneous rocks are quartz,
potassium feldspar (sometimes called alkali feldspar), plagioclase feldspar, mica, amphibole,
pyroxene, and olivine. The hints from last week for identifying these minerals is attached to the end
of this lab handout. The two textures that are used for classification are called phaneritic and
aphanitic. A third texture, porphyritic, is kind of a hybrid between aphanitic and phaneritic.
Porphyritic texture isn’t used in most rock-name classification schemes, but it is important to
identify because if present, it tells you something about the history of the igneous rock.
II Texture:
Texture is basically a measure of how big the crystals in an igneous rock are, and is important
because it gives an indication of how quickly or slowly the rock solidified. This in turn, tells you
whether or not it solidified partially or wholly below the surface or whether it solidified on the
surface. The basic rule is that the slower the magma or lava solidifies, the larger and more
interlocked the mineral grains will be. We call rocks that solidified underground plutonic or
intrusive and those that solidified above ground volcanic or extrusive. Note that there are a few
exceptions to the fine-grained = extrusive and coarse-grained = intrusive rules, so it is important to
not automatically equate texture with solidification location.
Rocks are good insulators so if a magma solidifies underground, it will cool very slowly. This
slow cooling allows individual mineral grains go grow to large sizes. The texture that results is
called phaneritic texture, and is defined as a rock where all the grains are large enough for you to see
with your naked eyes. This is why rocks that solidify underground and thus exhibit phaneritic
texture are called plutonic rocks or intrusive rocks. Typically, the mineral grains in plutonic rocks
have irregular shapes because as they grow, they eventually start to bump into their neighbors and
therefore can’t assume their ideal shapes. Instead, they form irregular, interlocking shapes. The
interlocking of all these individual minerals is what gives many plutonic rocks (such as granite) their
strength. Remember, plutonic (or intrusive) is the rock type, phaneritic is the texture that plutonic
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(or intrusive) rocks exhibit. The exception to this situation (which is common in Hawai‘i, actually)
is thin dikes, which cool very rapidly. They are intrusive (because dikes intrude pre-existing rock),
but because they are so thin, they cool quickly, and are extremely fine-grained.
If a rock cools above ground without the benefit of insulating surrounding rocks, it will solidify
rapidly, meaning that there is little time for crystals to grow. Instead of a few large crystals there
will be many more, small crystals. This is aphanitic texture, and is defined as a rock where any or
all of the mineral grains are too small to be seen with the naked eye. Rocks that solidify on the
surface and thus exhibit aphanitic texture are called volcanic rocks or extrusive rocks. Volcanic (or
extrusive) is the rock type, aphanitic is the texture that volcanic (or extrusive) rocks exhibit. The
exception to this situation is very thick lava flows that may take weeks to months to solidify. They
are extrusive (because they erupted on the surface), but because they are thick, the central parts cool
slowly, and they can be reasonably coarse-grained, although never as coarse grained as a typical
granite. A special class of aphanitic rocks cools so fast that there isn’t enough time for any crystals
to grow. Instead, what is formed is natural glass, such as obsidian and pumice. We will talk about
obsidian and pumice later.
The third texture, porphyritic, is kind of a hybrid between phaneritic and aphanitic. A
porphyritic rock is one where one set of mineral grains is much larger than all the others. These
larger grains are called phenocrysts. In the case of a porphyritic intrusive rock, there will be large,
well-formed phenocrysts in a matrix of smaller (but still visible), interlocking grains. In the case of
a porphyritic volcanic rock, there will be large, well-formed phenocrysts in a fine-grained matrix in
which the individual crystals are too small to see. Porphyritic texture tells you that the igneous rock
underwent two stages of cooling, first slow, then fast. The larger grains formed and grew during the
early, slow cooling, for example while magma sits in a volcanic magma chamber. The finer-grained
matrix formed during later, but more rapid cooling, for example after the magma erupted onto the
surface.
From these brief descriptions you can see that much of an igneous rock’s history can be
determined merely from an assessment of its texture. Of course in nature there is infinite variability,
so there are always going to be rocks that fall on the boundary between the textures defined here. A
rock with grains that you can barely see might be a fine-grained plutonic (intrusive) rock or a coarsegrained volcanic (extrusive) rock. In those cases a geologist needs to look for other evidence to
determine whether the rock is plutonic or volcanic.
1. For Rock Set 1, determine the textures of igneous rocks A-H using the texture terminology
discussed above, in your lab book, and by your TA. Also state what you think the cooling history
was of each rock (slow, fast, slow-then-fast, etc.).
Rock Texture
1A
1B
1C
1D
1E
1F
1G
1H
Cooling History
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III Chemical Composition:
Chemical composition is the other major criterion that geologists use to classify igneous rocks.
There are numerous schemes that have been devised to do this classification, some of which are
simple and others of which are exceedingly complex. Here, we’ll deal with two of the simple
classifications. The simplest classification divides igneous rocks into three groups, felsic (also
called silicic), intermediate, and mafic (also called basic).
The word felsic comes from “feldspar and silica in combination”, and refers to rocks that consist
mainly of potassium feldspar and quartz. Potassium feldspar and quartz are light-colored (usually),
so felsic rocks tend to be light-colored. Common examples include rocks such as granite (plutonic)
and rhyolite (volcanic).
The word mafic comes from “magnesium and iron in combination”. Most magnesium- and
iron-bearing minerals (olivines, pyroxenes, amphiboles) tend to be dark, so mafic rocks are usually
quite dark as well. The most common plutonic example is gabbro, and the most common volcanic
example is basalt.
Intermediate rocks, as you might guess, are rocks that fall somewhere between felsic and mafic.
Table 3.2 of your lab book lists only one set of intermediate rocks (andesite and diorite), but there
are others that deserve to be mentioned. For example, Pinatubo and Mt. St. Helens erupt dacite most
of the time. Dacite is a volcanic composition that lies between rhyolite and andesite. Additionally,
much of the so-called granite that makes up the Sierra Nevada mountains in the Western USA
(including Half Dome and El Capitan in Yosemite) aren’t actually granite. Instead, these mountains
are granodiorite, a plutonic composition that, as its name implies, lies between granite and diorite.
The table below and Figure 1 on the next page show a more complete scheme for igneous rock
classification. Notice that for each composition there is a volcanic and an intrusive rock.
Cooling History
Volcanic
Plutonic
Felsic
Rhyolite
Granite
Intermediate
Dacite
Andesite
Granodiorite
Diorite
Mafic
Basalt
Gabbro
2. For Rock Set 2, determine whether the rock is felsic, mafic, or intermediate
Rock
2A
2B
2C
2D
2E
2F
2G
2H
Felsic, Mafic, or Intermediate?
The only way to really place a particular igneous rock into it’s correct classification is to run
chemical analyses on it. This involves grinding the rock into powder and subjecting it to a battery of
chemical tests. This is not what most geologists do in the field, obviously, and not what we will do
in this lab.
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The only way to really place a particular igneous rock into it’s correct classification is to run
chemical analyses on it. This involves grinding the rock into powder and subjecting it to a battery of
chemical tests. This is not what most geologists do in the field, obviously, and not what we will do
in this lab.
Classification by Mineralogy:
Instead, we will classify the igneous rocks based on what minerals are present. Although not as
definitive as using chemical analyses, using mineralogy works most of the time because the specific
minerals that form are a direct reflection of the chemical composition of the magma.
Table 3.2 in your lab book lists minerals and their percentages in different igneous rocks.
Unfortunately, as noted above, it ignores some important intermediate rock types. Figure 1 below
shows a more complete complement of rock types (below) and the typical minerals found in each
(above). So, for example, if you find an extrusive rock with lots of K-spar, some quartz, some
plagioclase feldspar, some muscovite, and some biotite, you can be pretty sure that it’s a rhyolite.
Likewise, if you find an intrusive rock with lots of plagioclase feldspar, some amphibole, and some
pyroxene, it is probably a diorite.
Figure 1. Good rock-classification scheme (from Exploring Earth by Davidson, Reed, and Davis).
So, in order to classify an igneous rock, you first decide whether it has a phaneritic or aphanitic
texture. Next, you look at the minerals that are in the rock and their abundance. Then, using Figure
1, you give the rock a name.
This is not super-easy, and many geologists (not to mention geology students) have trouble
getting rocks right all the time. Not only do you have to get the mineral identifications right, you
also have to estimate their percentages. Most people tend to over-estimate abundances. Figures 2
and 3 are diagrams that show what actual abundances look like.
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Figure 2. Diagrams for estimating mineral abundances:
mafic minerals
Figure 3. Diagrams for estimating mineral abundances: felsic
minerals
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3. For Rock Set 3, give each rock a name and list minerals you found in it and/or justify your answer
Rock
3A
3B
3C
3D
3E
3F
3G
3H
Name
Minerals Present and/or justification
If you want to see a really comprehensive rock-ID scheme, check out Figure 4 (don’t worry, you
won’t have to use it).
Figure 4. Rock classification scheme based on the abundances of quartz, K-spar, plagioclase
feldspar, and a family of minerals called feldspathoids (from Rocks and Minerals by Detrich and
Skinner). Note that the shading shows the relative abundances of each type on Earth.
IV Glassy Rocks:
Obsidian
Obsidian is a type of natural glass. Some geologists consider glass to technically be a liquid
because its molecular structure is completely random. Thus, instead of a regular, repeating structure,
as you expect for a true mineral, there is no pattern at all. Natural glass forms when lava or magma
cools so quickly that the atoms and molecules don’t have enough time to organize themselves into
the right locations to form minerals. Instead, they are quenched in their random, liquid positions.
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Natural glass is closest in texture to aphanitic rocks, and can be porphyritic if there are
phenocrysts in a matrix of glass. Natural glass can be any composition (see below), but the term
obsidian is usually reserved for felsic compositions. Obsidian is typically very dark in color.
4. The thickness of window glass in old medieval churches is greater at the bottom of the panes than
it is at the top. What is one possible explanation for this?
5. Cleavage planes in a mineral are flat surfaces, and they tell you that there are preferred planes of
weakness in that mineral. Conchoidal fracture, on the other hand, is curved, and doesn’t follow any
preferred orientation. Why do you think that conchoidal fracture is common in obsidian?
Pumice
Pumice is also natural glass, but it is full of vesicles, which are quenched bubbles. The bubbles
form in the magma as it rises and the pressure confining it decreases. The bubbles form in the same
way that bubbles form in a bottle of beer (or soda if you’re under 21 - maybe we should just say
carbonated beverage) when you take off the cap. There are molecules of gas floating around in the
magma. In most magmas they are molecules of CO2, SO2, and H2O. When the magma is under
high pressure, these molecules don’t attach to each other, and instead they are just floating around.
They are said to be dissolved in the magma. In a bottle of carbonated beverage, the gas molecules
are CO2, and as long as the cap is on, the pressure in the bottle is high, and the CO2 molecules are
dissolved in the beverage.
When the pressure on a magma is decreased, usually by the fact that the magma is moving
upward in a conduit so that there is less and less material pressing down on it, the gas molecules
start to become able to glom together into little regions of pure gas. We call these little regions of
pure gas bubbles. This process is called exsolution - the gas is said to have exsolved. If there is
initially a lot dissolved gas, and it exsolves rapidly, you will end up with a lot of bubbles. If they
can’t float up through, and escape out of, the magma, they will get trapped in it when it solidifies. A
solidified bubble is called a vesicle. Pumice is an extreme form of this natural, glassy, foam. By
definition, the vesicles in pumice are not connected. This means that pumice doesn’t get
waterlogged and can therefore float.
There is often an intimate mixing of pumice and obsidian, meaning that the gas in some parts of
a magma was able to exsolve, whereas that in other parts wasn’t. Pumice is commonly light in
color.
Scoria and Reticulite
Two other types of very bubbly rocks are scoria and reticulite. Both of these typically form in
eruptions of mafic lava (see below), when there are high lava fountains. Scoria is sometimes called
cinder, and it is what lots of people buy to mix in with potting soil for their gardens. It can be as
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high as 75% vesicles, but this isn’t high enough to overcome the relatively high density of typical
mafic lava compositions, so scoria doesn’t float.
Another type of very bubbly natural glass, also associated with high lava fountains of mafic
lava, is called reticulite, and it can be as much as 95% vesicles! In fact, the only thing holding
reticulate together is the thin septa between bubbles. Reticulite is pretty spectacular when viewed
with a hand lens (hint: check it out). The vesicles in reticulite are interconnected so if you put a
piece of it in water it will eventually become waterlogged and sink. Reticulite is very fragile, so
please handle it with care.
Pele’s Hair
Early western geologists were fascinated by thin strands of basalt-composition glass that they
found lying around on Kīlauea. They (not Hawaiians) named this material Pele’s hair. Pele’s hair
consists of thin, flexible strands of glass that is usually golden to green-brown in color. It forms in
high lava fountains when a piece of molten lava is pulled apart as it flies through the air. A thin
filament gets pulled out between the two pieces, and it eventually breaks as they separate. The
pieces of lava (scoria, usually) fall directly to the ground whereas the Pele’s hair may be carried
many kilometers by the wind before it lands. Pele’s hair can also form when a stream of lava issues
from a break in a tube. If the tube is flowing full, the surface of the lava will drag against the
ceiling, and parts of it will be pulled into long strands of Pele’s hair.
6. For Rock Set 4, categorize each sample as a type of glassy rock. For each, explain what
properties led you to your decision.
Rock Name
4A
4B
4C
4D
4E
4F
Reason for Answer
V Ultramafic Rocks:
Ultramafic rocks are usually easy to identify because they commonly consist of only 1 or two
minerals (olivine and pyroxene). Both these minerals are quite dense, so ultramafic rocks are also
quite dense. Although there are lots of ultramafic rock names, you can get by with only 3:
Mineral assemblage
>90% olivine
olivine+pyroxene
>90% pyroxene
Ultramafic rock name
Dunite
peridotite
Pyroxenite
Most ultramafic rocks are coarse-grained, but they didn’t crystallize out of a magma the same
way that real igneous rocks do, so most geologists would probably disagree with considering them
as phaneritic. Dunites, for example, often consist of olivines that formed in a magma chamber and
sank to the bottom where they collected and got stuck together. Even though the olivines
crystallized out of a magma, there was never a magma that consisted of pure molten olivine.
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There are no fine-grained equivalents of ultramafic rocks any more, because magmas of this
ultramafic composition don’t exist any more. However, back in the Archean (~3 billion years ago),
lavas of this composition were erupted on Earth. They are called komatiites.
Although ultramafic rocks are very rare in the Earth’s upper crust, they probably make up
almost all of the mantle. How do we even know what’s down there? The main way is through
xenoliths “foreign rocks”, that are brought up in some volcanic eruptions. The ultramafic rocks in
Rock Set 5 were all brought up by eruptions at Hawaiian volcanoes.
7. So finally, for Rock Set 5, classify these using the table above.
Rock
5A
5B
5C
5D
5E
Name
Minerals Present
VI. IMPORTANT ROCK-FORMING MINERALS (repeat from last week for reference)
Unless you’re really totally lucky (or you’re robbing a gem store), you are not going to be finding
museum-quality or even lab-quality samples of individual minerals lying around. Instead, there will be
mineral grains within rocks, and in most cases you won’t be able to pull them out for the types of
analyses you’ve been doing so far. This part of the exercise will give you practice in identifying
minerals in more normal situations.
Table 2.1in the lab manual has a column for Rock-Forming Minerals. These are the minerals that are
important to know if you are going to start classifying rocks. Fortunately, this list is short, and in reality
you can identify most rocks with only 8 of them. In Hawai‘i we are lucky because the list shrinks to
only 4 important rock-forming minerals!
Here are the 8 minerals you need to be able to identify in order to classify ~90% of all rocks. Those
marked with a * are the ones that occur in Hawai‘i:
quartz
potassium feldspar (sometimes called K-spar or alkali feldspar)
*plagioclase feldspar
*olivine
*pyroxene
amphibole
mica (muscovite or biotite are the most common types)
*calcite and aragonite
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What follows are some simple rules for identifying these 8 important minerals (these same rules were
presented in last week’s lab). These are the rules you might use out in the field where you don’t have
access to fancy analytical equipment. Many times these particular properties are visible only with a
hand lens.
Quartz: Quartz is hard, and it will scratch glass and steel (a pocketknife, for example). Quartz is
almost always very clear (you can see into an individual grain), fresh-looking, and shiny (but not
metallic). Also, because quartz does not have cleavage, the shiny surfaces are not flat, and instead are
usually slightly curved (this is conchoidal fracture). Particularly if it is set into a recess where not much
light can get in, quartz may look like kind of a dark, clear, gray. Quartz can come in all kinds of other
colors but these are relatively uncommon. Quartz grains typically have irregular shapes.
Potassium feldspar: Potassium feldspar (sometimes abbreviated to K-spar) almost always has kind of a
milky, non-transparent look to it, and often you can see little wavy, milky veins (more properly called
“exsolution lamellae”). There is a good photo of these lamellae in Fig. 1.21 of your lab book. Note that
these lamellae are almost always lighter in tone than the surrounding part of the mineral, and although
they may be somewhat parallel to each other, they are not perfectly straight or parallel. Potassium
feldspar is usually whitish or reddish. It is common for potassium feldspar to exhibit one or more
cleavage planes, and/or to be in grains with reasonably nice crystal forms. You can also find potassium
feldspar grains that have completely irregular forms. Potassium feldspar is reasonably hard - it might
scratch your knife.
Plagioclase feldspar: Fresh plagioclase feldspar has a translucent to shiny-white appearance, so it may
be confused with white potassium feldspar or clear quartz. However, a very key property of plagioclase
feldspar is very straight and very parallel striations (lines) that are visible in all but the smallest grains.
The far-right photo in Figure 2.1 shows these striations nicely. Plagioclase feldspar is about as hard as
potassium feldspar. Plagioclase feldspar can form irregularly-shaped grains, sometimes with a cleavage
plane visible, or it can form elongate, prismatic-looking crystals with the elongation direction parallel to
the cleavage and the striations. If plagioclase feldspar is weathered, it becomes chalky-white rather than
translucent or shiny-white.
Olivine: Olivine has a very distinctive olive-green color. Fresh olivines have a shiny clear appearance,
but it doesn’t take much exposure to surface conditions and weathering for them to look a bit dull on the
surface. These weathered surfaces may also have a slight red tint or they may also show the same play
of colors that you see when there is oil on a wet road. Really weathered olivines are yellow-brown to
rust colored. If it is an iron-rich olivine it may be almost black, but these are uncommon. Olivine
crystals are can be irregularly shaped, but sometimes they show good form as slightly elongate, stubby
crystals.
Pyroxene: Pyroxene is actually a very dark green color but in most cases it appears black. It is
typically somewhat shiny, and sometimes shows two cleavage planes that meet at ~90°. The cleavage
planes are not really nice and smooth, however, and look more like lots of little, stepped, planes. Some
pyroxene crystals show very nice ~diamond-like or triangular forms. Pyroxene weathers much more
slowly than olivine and it is common to find fresh-looking pyroxenes and weathered-looking olivines in
the same rock.
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Amphibole: Amphibole is also actually very dark green but almost always appears to be shiny and
black. Unlike pyroxene, however, amphibole commonly forms elongate grains that are at least 2-3
times as long as they are wide. They sometimes look like short fat slivers. If you can view an
amphibole crystal end-on, you should see a diamond shape formed from two cleavage planes that meet
at 56° and 124° angles (see Figure 1.10 B of your lab book). Other times, amphibole just looks like
little irregular black blobs filling in spaces between other minerals. In these cases it might be difficult
to tell from biotite mica viewed on edge, except that biotite is very soft and can be scratched or broken
easily.
Mica: Micas are usually easy to identify because they show one perfect cleavage plane that causes the
grains to break into thin shiny sheets. Hold a rock at arms length and move its orientation - if you see
bright winks of light, these are probably mica cleavage planes. Even small grains show these sheets and
they can be seen with a hand lens. Also, micas are soft, and you can scratch them sometimes with your
fingernail and always with a pocket knife. The two most common micas are biotite, which is very dark
brown to black, and muscovite, which is clear to dull silver (although it is not a metallic mineral).
Micas viewed perpendicular to the cleavage plane often look hexagonal - think of mica as stacks of
hexagonal sheets that you may be seeing face-on, edge-on, or any angle in between.
Calcite and Aragonite: These two minerals are two forms of the same chemical composition - calcium
carbonate. Calcite and Aragonite can be formed by organic processes (limestone reefs and most sea
shells are Aragonite to begin with, but change naturally to calcite over time), and also by inorganic
processes (beautiful calcite crystals often form in veins where water percolates through limestone). In
the organic cases, individual mineral grains are almost always too small to distinguish, and the best
thing to do to see if calcite or aragonite are present is to put a small drop of dilute HCl (hydrochloric
acid) on the rock. If it fizzes, then probably you have some calcite or aragonite. Sometimes also, on a
weathered limestone surface, you will see what looks like craggy ridges and grooves, kind of like a
fingerprint (but more straight). This is often called “elephant-skin texture” and is diagnostic of calcite.
Inorganic calcite has a distinctive rhombohedral form, and you may be able to detect parallel lines
similar to those in plagioclase feldspar.