Chapter 29 - PS100 Home Page

Chapter 29: Earth Materials
Introduction
Now that we have a basic understanding of chemistry and physics, as well as the
earth’s internal structure and plate tectonics, we are ready to fully appreciate the
significance of what the earth is made of. We intuitively appreciate that the earth is a
rocky planet, but what are these things we call rocks made of? In this chapter, we will
discover that rocks are made of minerals. Beyond this, rocks, loose sediment, and soil
vary greatly in their texture, chemical composition, and mode of origin.
In the broadest sense, we can place rocks into three broad but interrelated
categories: igneous, sedimentary, and metamorphic. Igneous rocks are those that form
by cooling from a molten state. Molten rock is known as magma. Sediment is material
that has been removed from bedrock by chemical and physical processes and transported
and deposited in another location. If sediment is lithified (i.e., compacted and cemented
together), it can be turned into a sedimentary rock. If a preexisting igneous or
sedimentary rock is heated or compressed, the minerals within it can recrystallize in the
solid state to form a metamorphic rock.
This is pretty straightforward, but it implies something deeper is at work and other
concepts need to be identified. First, sedimentary and metamorphic rocks, by their very
existence, imply that other solid earth materials came before them. Sediment cannot be
eroded and deposited if there were nothing to be eroded. A metamorphic rock cannot
recrystallize without a protolith (proto=precursor; lith=rock). And igneous rocks have to
form because something melted. Thus, these three rock categories must be linked
together. It must be possible for one rock type to be converted into another through some
sort of a cycle.
Figure 1 illustrates the concept of a rock cycle with complex linkages. For
example, an igneous rock can be eroded to form a sedimentary rock. Either a
sedimentary or igneous rock can recrystallize at elevated temperature and pressure into a
metamorphic rock. And finally, a metamorphic rock can get hot enough to melt or
eroded at the surface to form new sediment. You should keep these interrelationships in
mind as we discuss igneous, sedimentary, and metamorphic rocks separately.
Finally, you must also remember that the basic laws of chemistry and physics we
have discussed all semester allow us to understand what is happening in nature in the
rock cycle. We will see, for example, that recrystallization in metamorphic rocks is
driven by the minimization of free energy and maximization of entropy. We will also see
that magma rises to the surface because of convection and buoyancy, or that chemical
bonds dictate how minerals form. The science of geology is, to a large degree, the study
of physics and chemistry applied to the materials that make up our planet.
Figure 1.
Minerals
There are a number of definitions for a mineral. We often think of “mineral
water” in which certain chemical elements are dissolved. We may use the word as a
synonym for ore, a necessary nutrient in our diet, or some other inorganic substance. We
need to be more precise, however, and we need to think of minerals in two ways. First,
rocks (with a few exceptions we will not discuss) are made of one or more minerals.
Minerals are what rocks and therefore most of the earth is made of.
But our definition must be a little deeper than this. Thousands of different
mineral have been identified, although the list of minerals that are abundant in the crust
and mantle is surprisingly small and limited to a couple of dozen or so. So you may
wonder what features of a mineral like quartz make it so very different from diamond?
And what makes a mineral different from other solid materials that we will exclude from
our definition?
For our purposes a mineral:
1. Is naturally occurring
2. Is an inorganic solid
3. Has a fixed or narrowly limited chemical composition
4. Has a definite internal crystal structure
5. Has limited stability in the face of varying pressure, temperature, or in the
presence of water
Let’s use our definition to determine whether or not some common materials are
minerals. How about ice? It occurs naturally, it is inorganic, and has a fixed chemical
composition of H2O. We all know that ice melts at 0°C, so its stability is limited. But
does it have a definite internal structure? In fact it does. Figure 2 is a photograph of a
single snowflake. Although we often hear that no two are alike, they all have some
things in common. They all have 6 arms or sides that reflect a sub-microscopic regular
crystal structure or pattern in which the H2O molecules are linked together. Ice is, in fact,
a mineral by our definition and glacial ice would be considered a rock!
Figure 2.
What about the glass in a window? This is not a mineral because it fails our
definition on two accounts. First, humans manufacture window glass. Second, it does
not have atoms that are bonded together in a regular pattern. Rather, the atoms are
bonded irregularly, and this is the key characteristic of a glass. It fails our definition on
two of the five criteria.
This notion of the regular patterns of bonds and arrangement of atoms in solids is
not new to us. This was discussed quite thoroughly in Chapter 24, so in this sense we
need not go into much more detail. In fact, the regularly repeating patterns in silicate
materials was introduced with this discussion in mind. You should remember that the
SiO44- tetrahedron can be linked in several ways: a) as isolated entities, b) as single
chains, c) as double chains, d) as sheets, and e) as 3-dimensional networks. In fact there
are other ways that tetrahedra can be linked that we have not even mentioned. As a
result, there are thousands of different silicate minerals, and even though the number of
abundant minerals is limited, relatively few of them make up the large majority of the
crust and mantle. Silicate minerals are supremely important in the study of the solid
earth.
Igneous Rocks
We already know that igneous rocks represent magma, or molten rock, that has
cooled. This magma is nearly always a mixture of silicate material and other metals. We
could simply stop our study of igneous rocks here. However, nearly all of humanity lives
on continental crust, which is comprised of a mixture of igneous, sedimentary, and
metamorphic rocks. However, igneous processes are absolutely critical to the formation
of continental crust. The generation of magma in the mantle and its ascent into the crust
is the fundamental process by which the continents have grown in the past and continue
to grow. This is true even if much of this original igneous material has been recycled and
reformed, perhaps repeatedly, into sedimentary and metamorphic rocks. No magma? No
crust. It is also useful to apply the basic principles of chemistry and physics we have
already learned to understand how and why magmas form, how and why they are
transported from great depth to near the surface of the earth, and what their textures can
tell us about their history.
Igneous rocks, and in turn the magma from which they crystallize, can very
greatly in terms of their chemical composition, or in other words in the proportions of
elements present. Even so, only about 10 elements are important in most rocks and
include: H, C, O, Na, K, Ca, Mg, Fe, Si, and Al. The other 80 or so naturally occurring
elements in the periodic table are present in trace quantities in most cases. Although we
could discuss the chemical variation of igneous rocks in great detail, we will not consider
this topic further.
Figure 3.
A first question to consider is: How do rocks melt? It is natural for us to think
that things melt because they get hotter. But how can rocks melt if the earth formed 4.5
billion year ago and has been cooling for that long ever since? Figure 3 will help us get
beyond this paradox.
First, carefully examine the axes. Pressure, or depth within the earth, is shown to
increase downward. Temperature increases toward the right. Now examine the two red
curves (ignore the blue lines for now). The area to the left of the left curve represents the
range of pressure and temperature below which the earth’s mantle is solid. It may be hot
and pressure may be high, but if the mantle is in this area it is nonetheless solid. The area
to the right of the right curve represents pressures and temperatures at which the mantle
would be completely melted. The area between the curves represents pressures and
temperatures at which both magma and solid mantle exist together. The closer we get to
the right curve, the greater proportion of magma present. In other words, the mantle does
not melt at one temperature—it melts over a range of temperatures, pressures, or both.
Second, examine the square. We can cross the “beginning of melting” or left
curve in two ways. We can increase temperature or we can decrease pressure. So, if we
can find a way to decrease pressure in the mantle, we can cause it to melt without raising
the temperature at all! This is how magmas are formed at hot spots and beneath midocean ridges. As mantle rises as a flowing plastic material it decompresses and then
melts.
Third, there is one other way, in addition to decompression, that magmas form.
To understand this, we need to refer again to the square in Figure 3. It is in the “all solid”
region relative to the red curves. However, what if there were a way to change the shape
and location of the curves? In fact, there is. If there is a way to add hot water as a fluid
(so-called supercritical water) to the mantle, the curves change from the shape of the red
to the blue lines. So, if water can be added to the mantle, we can lower its melting point
without changing pressure or temperature at all!
This is how melting happens at subduction zones. When oceanic crust descends
back into the mantle it contains minerals that have water bound in their crystal lattices.
When these minerals heat up, they break down and release water, which is less dense and
rises from the subducting ocean crust into the overlying mantle (Fig. 29.x). This water
lowers the melting temperature of the mantle and melts it. Now we also understand why
volcanoes at subduction zones often erupt violently! The magma is full of water that
boils out vigorously when it reaches the surface.
Up to this point we have implied that processes in the mantle drive all igneous
systems. Although there may be some exceptions to this, we will consider them to be
rare. But how does magma get out of the mantle and into the crust and sometimes all the
way to the surface? Once again, the answer lies in buoyant forces. If magma is less
dense than the crust or mantle that surrounds it, it will seek a way to the surface.
Magma that cools slowly within the crust loses heat at a slow enough rate that the
crystals growing from the magma can become quite large (Fig. 4a). Magma that erupts at
the surface cools quickly. It can be completely made of crystals, but they may be so
small that you cannot see them without a microscope (Fig. 4b). In other words,
something as seemingly unimportant as the size of grains in an igneous rocks tell us
something important about the conditions that formed it.
Figure 4.
Let’s put all these concepts in terms of the fundamentals we understand. Rising
mantle in a plume or beneath a spreading ridge ascends as a plastic solid because it is less
dense than the mantle around it. In other words, it rises because of buoyant forces.
Rising mantle is less dense because it is a little hotter than the mantle that surrounds it.
Remember that most materials expand when they are heated. In other words, hot buoyant
mantle that rises is a form of convective heat transport. When it crosses the beginning of
melting curve, magma forms spontaneously. We know that this must be a process in
which entropy is increasing. As minerals melt to form magma, molecular bonds are
being broken to form a liquid with much less order.
Once it is formed, magma that rises to the surface and erupts from a volcano does
so because it is less dense than the crust that surrounds it. This is also a form of
convection driven by buoyant forces. As we have seen, as magmas cool and solidify, the
size of individual minerals reveals something about the rate of cooling. But there are
even more principles we have learned reflected in igneous rocks. Remember the
principle of energy barriers in chemical reactions? If the energy barrier for the formation
of particular mineral (say olivine, see Chpt. 24) in a magma is small, many such crystals
will form as it cools. Olivine will be abundant and small in most rocks where it is found
because in terms of energy it is easier to form many new crystals than for a few crystals
to grow very large. Olivine is not rare in the earth. In fact most of the upper mantle is
made of small olivine crystals! The gem variety of olivine, called peridot (the August
birthstone) is valuable not because it is rare, but because it is rarely large. Now you
know why—it has a small energy barrier to new crystal formation in nature. Once again,
everything involved in the formation of igneous rocks can be understood in terms of the
fundamental laws of chemistry and physics we have discussed in this class. The same is
true of soil, sedimentary and metamorphic rocks.
Soil and Sedimentary Rocks
Nature is the ultimate recycler. Natural chemical and physical processes erode
rocks to create sediment, which is then transported, deposited, compacted, and cemented
to form new sedimentary rock, and according to the rock cycle, these can eventually be
melted or metamorphosed into new igneous or metamorphic forms. Today’s sand on the
beach or the silt in a river delta is the result of the breakdown of pre-existing igneous,
sedimentary, or metamorphic rocks. But what will that sediment be a million or a billion
years from now?
We first need to understand how sediment is created through a process called
weathering. The mechanical breakup of rocks (physical weathering) is usually
accomplished by the freezing and melting of ice in fractures (Fig. 5). In regions where
the temperature rises above and below freezing many times every year, the expansion of
water results in large contact forces that wedge fractures open. Where this happens in
mountainous regions, large masses of rock can be freed from cliff faces to fall and
accumulate as cone shaped masses of rock debris called talus. Contact forces exerted by
expanding ice remove a piece of rock from a cliff face. The gravitational potential
energy of the rock is converted to kinetic energy. As the rock strikes the ground, the
kinetic energy is converted to heat and the rock is further broken. Does this sound
familiar? It should. Ordered forms of energy are spontaneously being converted to less
ordered forms. Finally, although ice and gravity are probably the most important agents
of physical weathering, the roots of trees and shrubs growing in fractures can also pry
rocks apart.
Figure 5.
Many minerals, especially those in igneous and metamorphic rocks, are
metastable. This means that they were thermodynamically stable (had maximized
entropy) at the elevated temperatures and pressures at which they formed, but are not
stable at the earth’s surface, at least not in a strict sense. Metastable minerals will convert
to something else if they can overcome the energy barrier required for their reaction.
Think of a beautiful, expensive diamond. It is metastable at the earth’s surface, whereas
graphite is the stable form. If you want to prove this you could take a diamond, leave it
under vacuum in a high-temperature furnace overnight, and recover a lump of graphite
the next day. The heat of the furnace overcomes the energy barrier that prevents this
from happening spontaneously.
Chemical weathering reactions dissolve rocks and minerals and produce new,
stable minerals at the earth’s surface. We will look at two types of weathering reactions,
with an emphasis on learning what they teach us about processes rather than simply
memorizing reactants and products. But first, we need to learn why water in soils and
shallow rocks are often acid. Consider:
Simply stated, dissolved CO2 generates acid waters. This is why carbonated drinks are
acid. Microorganisms in soil and plant roots all produce CO2 in the soil. In fact, CO2 is
often 25 times higher in soil atmosphere than in the air we breathe. We expect soil
moisture to be acid in many instances.
The first type of weathering reaction is called dissolution. Here is an example:
This reaction says that the mineral calcite will dissolve in acid waters. Calcium ions will
be released to the water in the soil and the carbonic acid, or at least some of it, will be
neutralized to bicarbonate (HCO3-). Marble and limestone are made of the mineral
calcite and they will be dissolved by groundwater through this reaction. If this were not
true, there would be no caves for spelunkers to explore.
However, most minerals in most rocks are silicates. The following reaction
illustrates the breakdown of the framework silicate mineral plagioclase, and although
many of you may never have heard of it before, it is one of the most common minerals in
the earth’s crust:
Let’s examine what is essential in this reaction. Acidic water reacts with silicate
minerals. The result of the reaction is the release of chemical species to the water (i.e.,
dissolved “stuff” like Na+, H4SiO4), neutralization of the carbonic acid (formation of
HCO3-), and the formation of a brand new clay mineral. The new clay is
thermodynamically stable at the earth’s surface, whereas the plagioclase it was produced
from was metastable. Remember our example of the diamond? If you take a crystal of
plagioclase and keep it dry, it will remain unchanged forever. However, if you add
necessary reactants like water and carbonic acid it will begin to break down into clay.
Just think, the clay used to make the bowl that held last night’s Ramen noodles
may have started out as plagioclase crystals in granite that cooled deep under the earth.
Erosion stripped off the overlying rock and brought the granite to the surface.
Weathering reactions turned the plagioclase into clay and streams transported and
deposited the clay somewhere else. The material that made your bowl could have been
born in the bowels of the crust by material transported from the mantle by convection and
then recycled by weathering reactions at the surface. The important point to take away
from this discussion is this: Chemical weathering causes the minerals in rocks to dissolve
into surface or ground waters, and to be transformed into new, stable minerals in soils.
Without chemical weathering, there would be no soil. Without soil, we would all starve.
There are two major categories of sedimentary rocks: clastic and chemical. A
clast is simply a fragent, so clastic rocks are simply made up of fragments that have been
produced by chemical and physical weathering. However, most clasts do not want to
stick together on their own. They need to be compacted by burial and cemented together
by minerals that precipitate from ground water in the empty spaces between the grains.
Think of sand castles you made at the beach when you were little. To get the sand to
stick together you had to compact it and you had to make sure it was wet. The water
acted as a “cement,” at least until it evaporated. The water exerted contact forces that
held the grains together rather than pushing them apart.
Figure 6.
Figure 6 is a photograph of sandstone taken through a microscope. Grains labeled
“Q” are quartz clasts and “C” is a mineral or cement (calcite in this case) that holds the
quartz grains together. Burial compacted the grains and calcite precipitated from water
between the grains. What was once loose sand at the beach is now solid rock.
Clastic sedimentary rocks are classified according to the size of grains. If a rock
contains clasts >2 mm in diameter, it is called conglomerate. Rocks with grains <2 but
>1/16 mm are called sandstones. Rocks with grains <1/16 and greater than 1/256 mm are
siltstones, and shale has particles <1/256 mm. Chemical sedimentary rocks, an entire
separate category, contain minerals that precipitated out of water. Limestone, gypsum,
and halite (natural rock salt) are important examples of these.
Whatever their origin, chemical or clastic, most sediments are deposited by wind
and especially water in horizontal layers, or beds called strata (Fig. 7). The different
layers are produced by a change in the size or kind of material that is deposited. If the
layers we see are no longer horizontal we know that some sort of tectonic activity like
folding or faulting has disturbed them.
Finally, sedimentary rocks are extremely valuable and useful to humanity. The
vast majority of the energy we consume is derived from fossil fuels, and virtually all
fossil fuels are formed in and extracted from sedimentary rocks. 80% of the electricity in
the U.S. is generated by burning coal and natural gas. The U.S. also uses about 21 million
barrels of crude oil per day. At a price of $100 per barrel, this equates to a cost of $770
billion dollars a year before a single drop of it is even refined.
Figure 7.
Metamorphic Rocks
Metamorphism literally means to change (meta) form (morphology). In modern
English we use these root words improperly. We say that one thing “morphs” into
another. However, it would be more correct to say that rock A “metas” into rock B. In
geology, metamorphism is the recrystallization of a rock in the solid state at elevated
pressure and temperature. The fact that it happens in the solid state means that melting is
not involved, otherwise the end product would be an igneous rock. Spontaneous
recrystallization tells us that free energies are lowered and entropies increased once
energy barriers are overcome during metamorphism. At least this must be true at the
pressures and temperatures at which the rocks formed, if not where we find them at the
surface.
Now the important question could be: How do rocks experience elevated pressure
and temperature? Typical continental crust is 40 km thick. Temperature commonly
increases at depth in the crust at a rate of about 20°C for every km increase in depth.
This increase in heat with depth is due primarily to two factors: a) heat transfer from the
deep earth by conduction and convection, and b) heating by radioactive decay by natural
isotopes of uranium, thorium, and potassium. Rocks at the base of the earth’s crust have
temperatures of about 800°C. Given that rocks begin to metamorphose at about 200°C,
most of the rocks in the continental crust are metamorphic by virtue of being buried.
Thus, we didn’t really ask the right question. We can’t visit the deep crust where
temperatures are >200°C. Rather, we should have asked how deeply buried rocks get to
the surface for us to examine. The answer to that question lies in understanding plate
tectonics. The forces that thicken the crust at convergent plate boundaries result in the
erosion of mountain belts from the top down, eventually bringing metamorphic rocks to
the surface. Folding and faulting at convergent plate boundaries also brings rocks from
depth to near the surface without the direct involvement of erosion. So, in addition to the
igneous activity that occurs at convergent boundaries, we should think of the crust above
subduction zones as “factories” where metamorphic rocks are made (Fig. 29.x).
Two other features exhibited by many metamorphic rocks are important for us to
appreciate. First, every metamorphic rock has a protolith that describes what the rock
was before metamorphism. We often know or can make educated guesses of the
precursor for most metamorphic rocks. Marble has limestone and slate has shale as
protoliths, for example.
Second, many metamorphic rocks exhibit layering. That layering can come in
many forms such as how the rock fractures, bands of light and dark colored minerals, or
other features. Whatever the origin, the layering tells us something about the forces that
produced the rock. This layering is called foliation and should not be confused with the
layering of strata in sedimentary rocks. Metamorphic rocks that lack this layering are
considered “non-foliated.”
On the left in Figure 8 we see a hand specimen of a foliated rock. The reflection
of light from the top of the rock is due to the presence of a white, shiny mineral called
muscovite. It is actually hard to see the layering in this rock because the top and bottom
of the specimen are parallel to the tabletop and the top and bottom of the rock are
produced by foliation. On the right in Figure 8 is a photograph taken through a
microscope of a rock similar to that on the left. The gray grains are quartz and the
elongate and vibrant blue, yellow and pink grains are foliated muscovite crystals. The
red arrows show that pressure applied to the rock was greater in a direction perpendicular
to the foliation. In other words, foliation reveals the direction of past, unbalanced forces
within the earth!
Let’s synthesize what we know about metamorphic rocks. The minerals in a
metamorphic rock reveal the equilibrium temperatures and pressures present when the
rock formed. Minerals present in the rock can tell us how long ago the episode
metamorphism happened through applied studies of natural radiation, or absolute dating.
Foliation in the rock will tell us the direction that unbalanced forces were applied. We
can learn a lot about past tectonic activity, past mountain ranges that have been formed
and completely eroded away, by the careful study of the metamorphic rocks left behind
from their roots.