Now that we have a better understanding of minerals, we are ready to discuss rocks.
Minerals and rocks form in response to environmental forcing, with important
variables being pressure, temperature, and chemistry of the starting material.
Here, we will discuss the crystallization of magma and the importance of its chemistry in determining
a rock’s mineralogy. There are three types of rocks: igneous, sedimentary, and metamorphic,
and these differ in their mechanism of formation. We will begin our discussion with igneous rocks,
which were the first type of rocks to appear on Earth after its formation and subsequent cooling.
Igneous rocks are formed when a mass of melted rock crystallizes. There are two terms geologists
use for molten rock. When it is underground, it is called magma, up here it is called lava. So,
where does this magma come from? Most igneous rocks were once mantle rock that, at some point,
melted and moved toward the surface, where it cooled and solidified. You may now be wondering
why lava isn’t erupting everywhere on Earth. It turns out that under normal conditions, the mantle
is perfectly happy as a solid and cannot melt; and volcanoes are an exception to the rule. So,
what sort of process is going on here that allows the mantle to melt? In fact, there are two ways
to melt the mantle in large quantities: decompression melting and flux melting.
Let’s start with decompression melting. This is exactly what it sounds like, melting the mantle
by lowering its pressure. Decompression melting is responsible for the volcanism above both mid-ocean
ridges and hotspots. Beneath these environments, hot mantle rock is moving upward due to mantle
convection. Since hot material is less dense than cold material, the weight of a column of hot
material will be less than that of cold material. Therefore, at a given depth, the pressure is lower
beneath a mid-ocean ridge or hotspot than it is beneath a regular piece of crust.
Let’s follow an ascending piece of mantle rock as it rises beneath a mid-ocean ridge. At a depth of
about 30 kilometers, our piece of mantle has a temperature of 1400°C. Its pressure can be
calculated using the following equation: P = ρgz, where ρ is its density, g is acceleration due to
gravity, and z is the depth below Earth's surface. Since ρ and g are more or less constant for this
example, the main variable affecting the pressure is the depth, which should be fairly intuitive:
the more mass above a given area, the higher the pressure will be. The two black arrows
are representative of the downward pressure. The upper right section shows a phase diagram
corresponding to the state of our piece of mantle in pressure-depth space. The green dashed curve is
the average geotherm, which shows how temperature changes with depth. The red solid line is the
melting point of mantle rock. Notice how the two curves are nearly parallel, this illustrates the
requirement for abnormal conditions to get the mantle to melt. As our mantle rock ascends,
it experiences lower pressure, since there is now less mass above it. But notice that the
temperature did not change, it is still 1400°C. This is an example of an adiabatic process,
meaning heat has not been added or removed from the system. Essentially, the rising motion of
the mantle occurs at sufficient speed that we can assume that heat is not lost to its surroundings
during ascent. Therefore, the key to decompression melting is the advection of hot material upward,
to a lower-pressure environment. Finally, at a pressure of 2 kilobars, the mantle is melting,
with the purple dashed line signaling the onset of melting. So long as there is upward-moving mantle,
our magma factory will persist - this is decompression melting. The vast majority of
volcanism on Earth, both past and present, is due to decompression partial melting.
Flux melting involves the addition of water to mantle rock, which lowers its melting
point. This is responsible for the volcanism at subduction zones. Here, the subducting oceanic
lithosphere releases water into the mantle above it, which induces melting. It is not
actual liquid water that is transported to the mantle, but hydroxide anions that are released
from the high-pressure breakdown of hydrous minerals, like serpentine, or Mg3Si2O5(OH)4.
This chemically bound water is released around the same depth everywhere on Earth,
about 100 kilometers, which explains why volcanoes occur in a line, or arc at subduction zones.
Since most igneous rocks are derived from the mantle, which is relatively chemically
homogeneous, we might expect igneous rocks to also be this way, but they aren’t. To understand this,
we must first discuss the crystallization process, and more importantly, crystallization sequence. In
a cooling magma, crystallization order is dependent upon the chemistry of the magma
and the relative melting points of the stable minerals. Since most magmas are derived from
melting the same stuff, the mantle, we can ignore the influence of chemistry for these purposes.
In 1928, pioneering geologist N. L. Bowen created an experimental model
for the crystallization order of a standard magma as it cools and chemically evolves. In
Bowen’s Reaction Series, the first mineral to crystallize is olivine, followed by pyroxene,
then amphibole, then biotite, and finally potassium feldspar, muscovite, and quartz.
Plagioclase can exist at all stages of Bowen’s Reaction Series, as it is the most abundant
mineral in the crust. We may recall that the first mineral to crystallize, olivine, an orthosilicate,
has the smallest amount of silica of all the silicate minerals, while the last group contains
potassium feldspar and quartz, which are framework silicates, the minerals with the most silica. This
has important implications for the composition of a cooling melt. Let’s consider an average basalt,
having 50% SiO2, 15% Al2O3, 20% MgO, 10% CaO, and 5% alkalis. Now, let’s start to crystallize out
forsterite, the magnesium endmember of olivine, which is composed of 57% MgO and 43% SiO2. What
is going to happen? Well, the olivine crystals, which are more dense, will sink to the bottom
of the magma chamber, becoming isolated from the upward-moving magma. Since olivine has a
smaller fraction of silica in comparison to the melt, the melt will become enriched in silica
as olivine crystallizes. In fact, the melt will become enriched in everything but MgO,
which gets depleted making olivine, since olivine has a higher fraction of MgO than the melt. The
result is a melt that becomes enriched in silica, calcium, alkalis, and aluminum as it cools and
moves upward. This process is then repeated for pyroxene, amphibole, and biotite, which depletes
the melt in everything but silica, aluminum, and the alkalis, which become concentrated in the
cooling melt. This results in the last minerals to form being those with silicon, aluminum, sodium,
and potassium in their chemistry, such as quartz, muscovite, and alkali feldspar.
Solid solution further complicates things. For a general rule of thumb, magnesium-rich endmembers
crystallize before iron-rich endmembers, and calcium-rich endmembers form before
sodium-rich endmembers. This explains the right side of Bowen’s Reaction Series;
since anorthite has a higher melting point than albite, the first plagioclase that crystallizes
will be calcium-rich and the last will be more sodium-rich. The same thing happens with olivine,
pyroxene, and amphibole; the first minerals to form will be magnesium-rich, with crystals
becoming progressively iron-enriched during cooling. These processes are together referred
to as differentiation, which describes how a magma changes, or becomes different, as it cools.
The amount of differentiation a magma undergoes is generally related to the
amount of crust that it had to move through to make it near the surface. Therefore, the coolest
and most differentiated magmas commonly erupt in areas where the crust is relatively thick,
such as on continents. This is easy to see when comparing Hawaii and Yellowstone, as they are both
hotspots. The volcanoes of Hawaii erupt basaltic, or mafic lava at around 1300 degrees Celsius,
whereas Yellowstone erupts cool, felsic magma that is so thick and viscous, that it clogs up the
volcano, causing extremely explosive eruptions. What is the difference? The Yellowstone hotspot
is beneath thick continental crust, while the Hawaii hotspot is beneath thin, oceanic crust. We
have more to discuss regarding igneous rocks, so let’s move forward and learn more about them now.