Download igneous rocks reading, a supplement to the lab manual

REMINDER GUIDE TO ROCK TYPES FOR THE CAMPUS TOUR EXERCISE
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
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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 (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. 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. We will talk about
obsidian later. 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.
The third texture, porphyritic, is kind of a hybrid between phaneritic and aphanitic. A
porphyritic volcanic 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 finegrained 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 coarse-grained volcanic (extrusive) rock. In those cases a geologist needs to
look for other evidence to determine whether the rock is plutonic or volcanic.
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
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(usually), so felsic rocks tend to be light-colored. Examples include rocks such as granite,
granodiorite (both of which are plutonic) and rhyolite and dacite (both of which are 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. Plutonic examples include diorite and gabbro, and volcanic examples
include andesite and basalt.
Intermediate rocks, as you might guess, are rocks that fall somewhere between felsic and mafic.
In Table 3.2, your lab book lists only one set of intermediate rocks (andesite and diorite), but as
we’ll see in a moment, there are other important intermediate rocks as well.
Intermediate-composition igneous rocks are quite common. Most of the Andean volcanoes
erupt andesite, and in fact that’s where the name andesite comes from in the first place! Table 3.2
includes andesite but doesn’t include some equally common and important rock types. 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 shows a
more complete scheme for igneous rock classification. Notice that for each composition there is a
fine-grained (almost always volcanic) rock name and a coarse-grained (almost always intrusive)
rock name.
Texture
Fine-grained
Rhyolite
Dacite
Andesite
Basalt
Coarse-grained
Granite
Granodiorite
Diorite
Gabbro
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.
The top row of 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 shows the full 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.
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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 have trouble getting rocks right all the time. One
of the most difficult parts is estimating the percentages of different minerals (assuming you have
identified them correctly). 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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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.
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.
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 reserved for high-silica compositions. Obsidian is typically very dark in color.
Pumice
Pumice is also natural glass, but it is full of bubbles. These 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) when you take off the cap. There are molecules of
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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 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. 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 be dragged by
the ceiling, and parts of it will be pulled into long strands of Pele’s hair.
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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 question 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.
There are no fine-grained equivalents of ultramafic rocks any more, because magmas of this
ultramafic composition don’t exist. 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.
SEDIMENTARY ROCKS
Sedimentary rocks are common in continental parts of the Earth, but not very common here in
Hawai‘i. In this lab you will learn how to identify some of the more common sedimentary and
metamorphic rocks, and also what these rocks can tell you about the conditions under which they
formed. Unlike igneous rocks, which can form just about anywhere, sedimentary rocks form in
particular environments and under particular conditions. They are therefore very useful for
unraveling Earth’s history.
There are two main types of sedimentary rocks – those that are made up of pieces of other
rocks (sometimes called detrital or clastic sedimentary rocks), and those that form by precipitating
of minerals out of water by either biological or non-biological processes (sometimes called
chemical or biochemical sedimentary rocks). It can get a bit confusing because there are some
sedimentary rocks that are form from a combination of both clastic and precipitating processes.
Importantly, however, all of these sedimentary rock types form under normal conditions of
temperature and pressure with regard to the Earth’s surface.
Detrital Sedimentary Rocks
These are probably the most well-known types of sedimentary rocks, and include sandstones,
shales, breccias, mudstones, conglomerates, and many others. Detrital sedimentary rocks are
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important because they contain a record of the starting rock type out of which the particles were
eroded, the transport mechanism and distance that the particles moved under, and the conditions
under which the particles were deposited, and finally the conditions under which the particles
were sedimented together (lithified) to form the eventual rock. Detrital (or clastic) sedimentary
rocks are classified by a number of characteristics, including the size of the particles, their shape,
whether or not they’re all the same size and composition, etc. Your lab book discusses these
characteristics in more detail.
Chemical and Biochemical Sedimentary Rocks
Chemical and biochemical sedimentary rocks either form by non-biological precipitation out
of chemical-laden water, or by biological processes, or by some combination of these. The
convention is to consider any rock that at some time had of life involved in it’s formation to be a
biochemical sedimentary rock. Thus, even if it is made up of a whole bunch of broken seashell
pieces (and thus the animals were actually dead before the pieces became a rock), it is still
considered to be a biochemical sedimentary rock rather than a detrital sedimentary rock.
Characteristics of Sedimentary Rocks
The following sections describe different properties of sedimentary rocks that are used in
classifying them. Keep in mind that the ultimate goal in geology is not to come up with a rock
name, it is to understand the circumstances under which the rock formed. The rock name is just a
shortcut way to describe the rock so that other geologists will be able to readily understand the
rock’s significance and where it fits into a geological story.
Grainsize
Detrital sedimentary rocks are classified most generally by the size of the particles (usually
called grains) making up the rock. Table 4.1A in your lab book illustrates the size classifications
of sediment, and Table 4.4 compares grainsize to the eventual sedimentary rocks that develop
from these sediments. From finest to coarsest, the sediments (and corresponding sedimentary
rock) are: clay (claystone, shale), silt (siltstone, shale), sand (sandstone), granules + pebbles +
cobbles + boulders (conglomerate or breccia).
Thus just by measuring the dominant size of the grains in a sedimentary rock, you can go a
long way toward identifying the rock. Do geologists go around with straight-edges measuring
individual grains in rocks? Not usually. Instead, there are some general rules that they follow.
For example, it is pretty easy to tell if the grains are bigger than 2 mm across and therefore telling
pebbles from sand is easy. Additionally, if the grains are too small to be seen individually, they
are finer than sand and must therefore be silt or clay. How do you tell the difference between silt
and clay? Your lab book suggests grinding a bit between your teeth. If it feels gritty, it is silt. If
it feels smooth it is clay. However, in GG101 labs, it is pretty common for students to test every
single rock with HCl, so perhaps you don’t want to go around biting all the fine-grained rock
samples. Instead, try scraping the rock with your fingernail instead of your teeth.
Particularly for water-borne sediments, grain size is often an indication of the distance that the
sediment traveled. This is because if you followed any stream or river from its headwaters to
where it enters an ocean or lake, you would notice that the flow velocity would decrease along the
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way. There is a direct relationship between the velocity of flowing water and the size of particles
that it can carry, with faster water being able to carry only larger particles. Thus a single stream
may be characterized by swiftly flowing water at its head that is pushing boulders and cobbles
down relatively steep stream bed. Farther downstream, slower flow carries only pebbles. Even
farther downstream there may be some sand, and eventually, when the stream has merged with
other streams into a slow-flowing lazy river, only silt or clay will be transported. Of course the
character of streams and rivers changes depending on whether it is flooding or not, so the size of
the particles that can be transported may also change.
Grainsize is not as diagnostic for chemical and biochemical sedimentary rocks. However,
there are some cases where grainsize is part of the characteristic used to identify them. For
example, coquina consists of a coarse-grained aggregate of shell fragments whereas micritic
limestone consists of clay-sized particles of calcite and clay. Some chemical and biochemical
sedimentary rocks don’t really have a sedimentary “grainsize” at all because they precipitated
directly from water. Instead of being fragments of previous rocks they are a crystalline aggregate,
sometimes having a texture that looks more like an igneous rock.
Sorting
Sorting refers to the variety of grain sizes in a rock. A well-sorted sedimentary rock is one in
which all the grains have the same size. A poorly-sorted sedimentary rock contains grains of
many different sizes. Sorting is important because it is closely tied to the environment under
which the sediment was deposited, to the mechanism that carried the sediment to its final resting
place, and to the distance that the sediment traveled before becoming lithified. Wind, for
example, is very good at moving sand-sized grains, tends to blow silt and clay particles away, and
doesn’t often move pebbles. Wind-deposited sandstones therefore tend to be very well sorted.
Meanwhile that silt and clay that is blown away eventually lands somewhere to form a different
(but also well sorted) rock called loess.
Certain transport environments are not very good at sorting particles by size. Rapidly flowing
streams, mudflows, landslides, and glaciers, are capable of transporting everything from boulders
to clay particles. If all the particles are deposited directly from one of these high-energy
environments, the resulting sedimentary rock will be very poorly-sorted.
Grain Shape
If the grains are big enough, their shapes hold important clues to the amount of energy in the
environment that the sediments were deposited in and also to the amount of time and/or distance
over which the sediments were transported. High-energy streams crash boulders and pebbles
together so they often have broken, angular shapes. On high-energy ocean and lake beaches,
pebbles and boulders are constantly ground and smashed against each other so they end up with
well-rounded shapes. Sand grains hit against each other while being blown by the wind, and
whether or how they break depends on what they consist of. Quartz sand grains, being very hard,
rarely fracture all the way through. Instead, tiny chips are broken off when they crash together
and the eventual result is a frosted surface much like beach glass. Feldspar sand grains are much
more likely to break apart when they collide (usually along cleavage planes), so they become
smaller and smaller while blowing in the wind. Eventually, all the feldspar will be broken into
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tiny pieces that get blown away completely, leaving behind a sandstone consisting only of round,
frosted quartz grains.
Composition
The composition of the particles in detrital or chemical/biochemical sedimentary rocks is an
important clue to their origin, and therefore important for classifying them. Calcite, for example,
is produced almost exclusively by biochemical processes, so any sedimentary rock consisting of
calcite is classified as chemical or biochemical (even if it may happen to look like a sandstone).
As we noted above, depending on the distance over which wind-blown sand is carried, the overall
composition can change from a mixture of quartz and feldspar eventually to pure quartz. Thus,
looking at the composition can tell you something about the distance that detrital sediments were
carried, which is important if your goal is to reconstruct ancient environments.
Fossils
Fossils, evidence of organisms, are almost exclusive to sedimentary rock (as opposed to
igneous and metamorphic rocks), and indeed some sedimentary rocks consist wholly of fossils.
Sometimes a detrital sedimentary rock will contain fossils of some sort, and it will therefore be
considered “fossiliferous”. Many chemical and biochemical sedimentary rocks are also
fossiliferous, but some are not. There are chemical and biochemical sedimentary rocks that owe
their existence to fossils that once existed but which have been completely dissolved away.
Limestone is a common biochemical sedimentary rock that is derived mostly from ancient reefs.
Some limestones look basically like a live reef today, with shells, worm casings, coral fragments,
sand grains, etc. Other limestones, however, have been completely recrystallized, meaning that all
the calcite that once made up the shells, work casings, coral fragments, and sand grains has been
dissolved and then re-precipitated; no fossils remain. And of course there is every gradation in
between these extremes.
Fissile/Layered
Some sedimentary rocks are layered. This can be layering inherited from the initial deposition
process, and includes bedding in sandstones and siltstones. Fine layering, particularly if it is
uniform in thickness, is called lamination. Other layering, particular to the finest-grained
sedimentary rocks, derives from the parallel orientation of mica minerals. This parallel
orientation is partly due to the deposition processes that took place, but mostly due to the
development of clay minerals that occurs after deposition takes place. This type of layering
imparts a characteristic weakness in the rock that is manifested by thin, platy layers that may flake
off. Such rocks are said to be “fissile”, and the best example is shale. Shales can be mostly siltsized or clay-sized so there are silty shales and clay shales. Sedimentary rocks that are neither
layered nor fissile are “massive”.
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Identifying the Characteristics of Sedimentary Rock
Table 1 (next page) lists a number of characteristics of common sedimentary rocks. Tables
4.1, 4.4, and 4.5 of your lab manual give more details.
Grainsize choices
>2 mm (pebbles, cobbles, or boulders)
visible to 2 mm (sand)
gritty (silt)
really fine (clay)
looks crystalline (interlocking grains of various
or very small sizes)
Sorting choices
well-sorted
moderately sorted
poorly sorted
Grain shape choices
rounded
sub-rounded
sub-angular
angular
Composition choices (can be more than one)
quartz
feldspar
calcite
can’t tell
mix
Fossil choices
no fossils
few fossils
lots of fossils
Fissile/layered choices
fissile
layered (or laminated)
massive
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Table 1: Characteristics of Common Sedimentary Rocks
Rock #
Grainsize
Sorting
>2
mm
Poorly sorted
Breccia
Grain Shape
angular
Composition
Can be lots of things
Fossils?
Usually no
Fissile/Layered?
massive
Conglomerate
>2 mm
Poorly sorted
rounded
Can be lots of things
sometimes
massive
Quartz
Sandstone
Arkose
Sandstone
Greywacke/
Wackeystone
Siltstone
Vis-2 mm
Well sorted
rounded
quartz
no
Massive or layered
Vis-2 mm
Rounded to subrounded
Sub-rounded to
sub-angular
Can’t tell
Massive or layered
Usually no
massive
sometimes
massive
Silty shale
Fine (gritty)
Well sorted
Can’t tell
sometimes
fissile
Claystone/
Mudstone
Very fine
Well sorted
Can’t tell
Quartz +
K-feldspar
Quartz, feldspar,
lithics, clay
Can’t really tell, but
mostly clay minerals
Can’t really tell, but
mostly clay minerals
Can’t really tell, but
mostly clay minerals
no
Fine (gritty)
Well sorted to
moderately sorted
Moderately sorted
to poorly sorted
Well sorted
sometimes
massive
Clay shale
Very fine
Well sorted
Can’t tell
sometimes
fissile
Coquina
>2 mm
Moderately sorted
variable
Crystalline
Limestone
Micritic
Limestone
Chalk
Crystalline, can see n.a.
crystals
Crystalline, can’t
n.a.
see crystals
Very fine
Well sorted
n.a.
Can’t really tell, but
mostly clay minerals
Calcite (shell
fragments)
calcite
Essentially all
fossils
sometimes
Massive, maybe
layered
massive
n.a.
Calcite
sometimes
Usually massive
Can’t tell
Soft calcite
Usually massive
Fossiliferous
Limestone
Oolitic
Limestone
Chert
Variable
variable
Fossil fragments
Essentially all
fossils
lots
rounded
Calcite (oolites)
sometimes
n.a.
Silica (microcrystalline sometimes
quartz)
Vis-2 mm, ± fines
variable
1-2 mm
crystalline
n.a.
Massive, maybe
layered
Massive, maybe
layered
Usually massive
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METAMORPHIC ROCKS
Metamorphic rocks result from alteration of pre-existing rock types by combinations of heat,
pressure, and the chemical action of fluids and gases. This metamorphosis often takes place deep
within the crust where temperature and pressure are high, and by studying metamorphic rocks we
can make inferences about the conditions under which they formed. We know, for example, that
certain metamorphic minerals only form at particular pressures (equivalent to particular depths).
If such minerals are found in a rock, they indicate that at one time that rock was at that particular
depth.
As rocks are metamorphosed, they change mineralogically and texturally to become stable
under the new environment they find themselves in. Minerals contained in the parent rock often
are unstable at temperatures and pressures found deeper in the Earth. With time, therefore, they
react in such a way as to produce new mineral assemblages that are stable under the new
conditions; metamorphism is the collective name for these reactions.
The effects of metamorphism include: 1) chemical recombination and growth of new
minerals; 2) deformation and rotation of mineral grains; and 3) recrystallization of minerals into
larger grains. The net result is a rock of greater crystallinity and hardness, possessing new
structural features and textures which commonly exhibit flowage or other expressions of
deformation.
Contact Metamorphism
There are two general types of metamorphism, contact and regional. Keep in mind, however,
that they are not mutually exclusive so that many gradations between them occur. Contact
metamorphism is usually of a local nature (i.e. not over a huge area), and is concentrated near the
contacts between intrusive igneous bodies and the surrounding country rock. Heat and chemical
activity are the principal agents of contact metamorphism, and the rocks affected generally
recrystallize into hard, massive bodies. The effects of contact metamorphism diminish with
distance from the intrusion. Indeed, if you find an area where the amount of contact
metamorphism increases in a particular direction, you are probably heading in the direction of the
intrusion that caused the metamorphism (a useful observation for geological interpretation).
Regional Metamorphism
Regional metamorphism is a result of the combined forces of high temperature and high
pressure, and it is common in areas that have been subjected to large-scale deformational stresses
such as the collision of two tectonic plates. Regional metamorphism therefore often extends
across large areas. The rocks produced in regional metamorphic processes are usually folded,
faulted, crushed, sheared, or stretched (or all of these).
Metamorphic Grade
The degree to which a rock has been metamorphosed is referred to as its metamorphic grade.
Each metamorphic grade corresponds to a particular combination of temperature and pressure, and
is characterized by a particular mineral assemblage. Figure 5.12 (5.10 in 4th edition) of your lab
book illustrates some of the minerals associated with the different metamorphic grades. Note that
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there are assemblages of minerals (as opposed to individual minerals) that indicate metamorphic
grade.
Regionally metamorphosed areas are exceedingly complex, and metamorphic grades may vary
in all kinds of patterns. It is tempting to say that they show semi-parallel bands of metamorphic
grade that are roughly perpendicular to the main direction of compression associated with the
metamorphism, however, there are probably as many exceptions to this rule as there are examples
that follow it.
Contact metamorphic grade patterns are usually simpler, tending to be roughly concentric
around the intrusion that provided the metamorphic heat. The intrusion itself may not even be
exposed, but a geologist can figure out where it must be (or have been) by mapping the
metamorphic rocks nearby.
Foliation
Metamorphic textures hold evidence about the degree of metamorphism that occurred as well
as about the nature of the original rock that was metamorphosed (called the protolith). Foliation
develops when there are directed stresses involved in the metamorphism, meaning that the
pressure is greater in one direction than in others (as opposed to pressure that is the same in all
directions). This greatest pressure direction may be the collision direction of two continents, but
as noted above, regionally-metamorphosed areas are terribly complex, so a geologist would not
want to jump to such a conclusion without considerable other evidence. Foliation is a general
term for the preferred orientation and growth of certain minerals, and the type of foliation
indicates the degree to which the rock was metamorphosed. You should note that the
development of foliation involves both the actual rotation of mineral grains as well as their growth
in particular orientations. Note also that not all protoliths contain the right mineral and chemical
constituents to develop foliation, even under high directed stresses.
Slaty cleavage
Slaty cleavage is characterized by closely-spaced fractures that cause the rock to split along
parallel planes. Slaty cleavage results from the parallel orientation of microscopic (i.e., too small
to see with your naked eye) mica minerals. These flat minerals grow with their flat surfaces
perpendicular to the direction of the directed pressure. Because micas have one perfect cleavage
direction, the rock ends up also having a cleavage direction. A rock with slaty cleavage is called
slate. It may be difficult to tell slate from shale, but slates typically are harder and break along
flatter planes (in the olden days, chalkboards were made of slate).
Phyllitic texture
Phyllitic texture develops when the mica minerals have grown a little more than they have in
slaty rocks, and this indicates a slightly greater amount of metamorphism has occurred. These
slightly larger grains are still microscopic, but they are able to reflect light a little better, giving
phyllites (the rocks that show phyllitic texture) an obvious sheen. Phyllites sometimes break
along preferred planes similar to slates.
Schistosity
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Schistosity continues the trend in that the individual mica grains are now visible to the naked
eye. A schist thus has a sparkly look to it, with the sparkles coming from the individual mica
grains. Schists tend to break in one direction, but the fracture planes are usually less well-defined
than in slates. Because big mica grains are not very hard, schists can often be pulled apart with
your hands or fingernails.
Gneissic layering
At even higher amounts of metamorphism, rocks are getting pretty close to actually melting.
This allows extreme rearrangement of ions and minerals to take place. Minerals tend to grow in
layers along with others of their same kind. This produces distinct layers of different minerals,
also perpendicular to the direction of applied pressure. The rock type is called a gneiss (rhymes
with nice). The minerals in a gneiss are often the same minerals that you might find in a plutonic
rock (quartz, feldspar, amphibole, pyroxene, etc.), but the layering is usually the giveaway that it
is a metamorphic rock rather than an igneous rock.
Non-foliated metamorphic rocks
Not all metamorphic rocks are foliated. Instead, they appear massive. This may occur
because there is only one mineral in the protolith (limiting the ability to form layers of different
minerals under gneissic conditions, for example). Non-foliated metamorphic rocks may indicate
that the metamorphism was contact rather than regional, however, as noted above, you wouldn’t
want to jump to this conclusion.
Metaconglomerate
As the name suggests, a metaconglomerate is a conglomerate that has been metamorphosed.
Sometimes it looks just like a conglomerate whereas other times it is obvious that all the pebbles
or cobbles have been stretched and squoshed. In a metaconglomerate the once clay-rich matrix
recrystallizes into other, harder, minerals. Thus unlike a true conglomerate, a metaconglomerate
will fracture across the pebbles just as easily as around the pebbles.
Metabreccia
As the name suggests, a metabreccia is a breccia that has been metamorphosed. Sometimes it
looks just like a breccia whereas other times it is obvious that all the clasts have been stretched
and squoshed. In a metabreccia the once softer matrix recrystallizes into other, harder, minerals.
Thus unlike a true breccia, a metabreccia will fracture across the clasts just as easily as around the
clasts.
Quartzite
Quartzite is the result of metamorphosing a quartz-rich protolith, usually quartz sandstone.
Other possible protoliths for quartzite are high-silica igneous rocks such as rhyolite. Because
quartz is so stable at all kinds of temperatures and pressures, there isn’t much that metamorphism
can do to it. About all that happens is that the quartz grains and silica cement (if the protolith is a
sandstone) get pressed together a little bit and start to recrystallize. Thus, what were once
individual round sand particles in a matrix becomes interlocked grains of pure quartz. As with
metaonglomerates, quartzites will fracture right across grains rather than around them.
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Marble
Marble is the result of metamorphosing a limestone or dolostone. As with forming a
quartzite, the main process that takes place is the intergrowth of larger and larger calcite crystals.
Additionally, any pore space that was present in the original limestone gets squeezed out, leaving
a rather dense, nearly pure calcite rock. Because it is still calcite, however, marble is no harder
than limestone, and it also fizzes with HCl (although you may have to powder it a little bit first).
Hornfels
Hornfels is a type of non-foliated metamorphic rock that is usually very dark and fine grains.
It consists mainly of amphibole (the most common amphibole is called hornblende, hence the
name hornfels).