Metamorphic rocks are the second of the three major rock-type groupings. As
described in the discussion of igneous rocks, the most general classification
system is based upon processes of rock formation. In this case, the word
metamorphic describes the process responsible for the origin of these rocks –
meta means change, while morph means shape. Thus, metamorphic rocks are
those who’s shape has been changed by geologic processes. The question
remains, however, what is the “shape” that is changed by metamorphism?
In order to understand the types of changes that occur during metamorphism, it
is necessary to recall the two criteria by which all rocks are classified within three
broad process-based groupings. The two classifying criteria are mineral (or
chemical) composition and the texture of the crystals or mineral grains within the
rock. Changes that occur during metamorphism are driven by changes in the
physical parameters temperature and pressure. Thus, as temperature and
pressure conditions change, alterations in mineral composition and texture
ensue. Importantly, these are changes that occur below the melting point of the
rock. Thus, all metamorphic reactions involve solid-state transformations of
minerals.
The role of Temperature
Changes in temperature conditions during metamorphism causes several
important processes to occur - first, temperature is a measure of energy. With
increasing temperature, and thus higher energy, chemical bonds are able to
break and reform. Thus, the breaking and making of crystal bonds drive the
chemical reactions that change a rock's chemistry during metamorphism.
Secondly, increasing temperature results in the growth of crystals. Within a
rock, a small number of large crystals have a higher thermodynamic stability
than do a large number of small crystals. As a result, increasing temperature
during metamorphism, even in the absence of any chemical change, will
generally result in the amalgamation of small crystals to produce a coarser
grained rock. Thirdly, individual minerals are only stable over specific
temperature ranges. Thus, as temperature changes, minerals within a rock
become unstable and transform through chemical reactions to new minerals.
This property is very important to our interpretation of metamorphic rocks. By
observing the mineral assemblage (set of minerals) within a metamorphic rock, it
is often possible to make an estimate of the temperature at the time of formation.
That is, minerals can be used as thermometers of the process of metamorphism.
Finally, increasing temperature tends to cause chemical reactions to occur at a
faster rate such that those reactions that are chemically favorable will occur with
greater speed during metamorphism.
The Role of Pressure
Pressure, the second of the two physical parameters controlling metamorphism,
occurs in two forms. The most widely experienced type of pressure is lithostatic.
This “rock-constant” pressure is derived from the weight of overlying rocks.
Lithostatic pressure is experienced uniformly by a metamorphic rock. That is,
the rock is squeezed to the same degree in all directions. Thus, there is no
preferred orientation to lithostatic pressure and there is no mechanical drive to
rearrange crystals within a metamorphic rock experiencing lithostatic conditions.
Conversely, directed pressure, the second type of pressure associated with
metamorphism, is the pressure of motion and action. Plate tectonics provide the
underlying mechanical control for all forms of directed pressure. Thus,
metamorphism is closely linked to the plate tectonic cycle and many
metamorphic rocks are the products of tectonic interactions.
As was the case with changes in temperature, changes in pressure, either
lithostatic or directed, have important impacts upon the stability of minerals.
Every mineral is stable over a range of pressures, if pressure conditions during
metamorphism exceed a mineral’s stability range the mineral will transform to a
new phase. Many of these solid-state reactions involve polymorphic
transformation – changes between minerals with the same chemistry and
different crystallographic structures. Just as with temperature, mineral
assemblages within a metamorphic rock can be used as a barometer to measure
pressure at the time of formation. Additionally, the application of directed
pressure to a metamorphic rock results in the alignment of grains. Such
alignment will occur when mineral grains are either elongate or platy in form.
As we will learn, this alignment is responsible for producing the textures
observed in a large group of metamorphic rocks.
Metamorphic Grade
Geologists describe the intensity of a metamorphic event through the use of the
concept of metamorphic grade. With increasing depth in the Earth, ambient
temperature and pressure conditions rise steadily. Thus, within the continental
crust, temperatures vary from approximately 200 °C at 5 kilometers to 800 °C at
35 kilometers. While these temperatures are extreme relative to our everyday
experiences, they are significantly below the melting point of most rocks.
Likewise, lithostatic pressure increases with increasing depth. At 5 kilometers
the pressure is approximately 2 kilobarrs, or about 2000 times atmospheric
pressure. Deeper within the crust, at about 35 kilometers, the pressure increases
to some 10 kb. This trend of increasing temperature and pressure within the
Earth is defined by a region of commonly encountered metamorphic conditions.
Geologists describe low temperature and pressure setting as low-grade
metamorphism, while high temperature and intense pressure is known as highgrade metamorphism. This characteristic of varying intensity of physical
conditions results in the production of a variety of metamorphic rocks.
Textures of Metamorphic Rocks
As was the case with igneous rocks, the primary parameter of classification
among the metamorphic rocks is texture. Textures exhibited by metamorphic
rocks are broadly defined as either foliated or non-foliated. Those rocks
described to possess a foliated texture are formed from aligned mineral grains.
Such alignment of elongated or platy grains will only occur under the influence
of directed pressure. Thus, foliated metamorphic rocks are the product of the
interaction of lithospheric plates and as such compose a significant part of the
Earth’s major mountain chains.
Non-foliated metamorphic rocks are formed from mineral grains lacking any
particular alignment. Such random arrangement of minerals occurs during
metamorphism in two ways. First, when rocks are exposed to elevated
temperatures – in the absence of any directed forces – mineral grains are free to
grow with any orientation. Thus, non-foliated rocks are always produced by
lithostatic pressure. However, non-foliated rocks can also form in another way.
If the precursor rock consists of equant sized grains – grains with shapes that
have similar lengths, widths, and heights – then the application of directed force
fails to produce any preferential orientation. Thus, rocks containing largely
spherical or cubical shaped grains will not produce a foliated texture, no matter
the intensity of the directed pressure. This is a critical difference. Foliated
metamorphic rocks only form under conditions of directed pressure. Lithostatic
pressure will only produce non-foliated metamorphic rocks. However, non-
foliated metamorphic rocks can also form under conditions of directed pressure,
when the precursor grains are equant in shape.
Foliated Metamorphic Rocks
Temperature and pressure are the two controlling physical parameters
responsible for altering the texture and composition of a metamorphic rock.
However, there is a third critical factor that determines a rock’s lithologic
characteristics – the texture and composition of the precursor rock. All
metamorphic rocks are formed from some preexisting rock – either sedimentary,
metamorphic, or igneous. As such, the physical and chemical characteristics of
the original rock have a profound influence upon the nature of the rock after
metamorphism.
The largest and most important groupings of metamorphic rocks are those
displaying a foliated texture. Formed under conditions of directed stress,
foliated metamorphic rocks exhibit four distinct types of aligned textures. At the
lowest metamorphic grade is the foliated metamorphic rock slate. Slate
generally forms by the realignment of clay minerals in the precursor rock shale at
low temperatures and pressures. Slate is denser, and more resistant to erosion,
than is shale. Additionally, the alignment of the clay crystals causes the rock to
break along extremely flat and smooth surfaces – a property described as rock
cleavage. In order of increasing metamorphic grade, the second rock type is
phyllite. These chlorite-rich foliated rocks are commonly dark green in color,
with a slight sheen or luster due to the cleavages of the sheet silicate mineral
cholorite. One of the most abundant and important foliated metamorphic rocks
is schist. Schist consists predominantly of micas but also commonly contains
several important accessory minerals useful as thermometers and barometers of
metamorphic conditions. Finally, at highest metamorphic grade is the rock
gneiss. This foliated rock consists of alternating bands of quartz, feldspar, and
micas. There are two important trends within these foliated metamorphic rocks.
With increasing metamorphic grade the intensity of the foliation increases.
Likewise, at higher grade, the rocks are typically coarser grained.
Non-foliated Metamorphic Rocks
As previously described, non-foliated metamorphic rocks can form either under
conditions of lithostatic or directed stress. Perhaps even more than any other
type of metamorphic rock, the non-foliated rocks are highly dependent upon the
composition of their precursor. The most well known of all metamorphic rocks
is marble. This commonly used architectural and decorative stone is composed
of the carbonate minerals calcite or dolomite. In either case, marble’s precursor
rock is limestone, a common type of sedimentary rock. The equant shape of the
carbonate crystal structure results in a homogeneous (non-foliated) texture even
under conditions of directed stress. Similarly, the rock type quartzite is formed
from the sedimentary rock sandstone. In this case, the quartz sand grains are
nearly spherical in shape and as such, produce a dense, hard, non-foliated
metamorphic rock. The third common and important type of non-foliated
metamorphic rock is formed only during the metamorphism of shale under
conditions of lithostatic pressure. The product of metamorphism under these
conditions is a dense, brittle rock known as hornfels. This rock is generally
similar in appearance and analogous in origin to pottery or porcelain created by
the firing of clay.
Types of Metamorphism
Processes of metamorphism – changes in temperature and pressure – occur in a
variety of geologic settings. There are two major processes, and a several
important, but less common secondary processes. The most important largescale metamorphic process is known as regional metamorphism. First described
and studied in northern Scotland, this form of metamorphism is the product of
directed pressures generated during convergent tectonics. Commonly exhibiting
regionally extensive bands of foliated metamorphic rock of varying grade,
regional metamorphic terrains form the cores of major mountain chains. The
second major process is known as contact metamorphism. Generally constrained
to small geographic areas, this form of metamorphism occurs under conditions
of elevated temperature and lithostatic pressure. The emplacement of intrusive
igneous rock bodies (batholiths or stocks) results in the formation of a strong
temperature gradient between the molten magma and the surrounding country
rock. As heat is conducted from the magma chamber to the rocks in contact with
the magma, chemical reactions occur. Contact metamorphism is characterized
by concentric zones displaying high metamorphic grade close to the magma
chamber. As is the case with all metamorphism under conditions of lithostatic
pressure, contact metamorphism only results in the creation of non-foliated
metamorphic rocks.
There are three other significant, yet less common, forms of metamorphism, each
with its own unique set of characteristics. Cataclastic metamorphism occurs
along fault planes when rocks grind against each other. This is the only type of
metamorphism that results in a reduction of grain size. Faults are regions of
high fluid flow, and the fine grain sizes produced by grinding along the fault
during cataclastic metamorphism allows for the chemical alteration of the rock.
Perhaps the most pervasive of the secondary processes is hydrothermal
metamorphism. During the emplacement of a magma chamber fluids within the
surrounding country rock to begin to circulate. The temperature gradient
between the hot batholith and the cold country rock creates a convection cell that
moves fluid from colder regions to warmer regions. This circulation of rock,
along with the subsequent increase in temperature as it approaches the batholith,
causes numerous mineral-water chemical reactions. This process is responsible
for the formation of much of the worlds precious metal deposits as well as many
economically significant metal sulfide ore deposits. The third minor
metamorphic process occurs as sedimentary rocks are slowly buried. Burial
metamorphism occurs at relatively low temperature and under lithostatic
pressure. Thus, all burial metamorphic transformations are classified as lowgrade and produce non-foliated rocks.
Metamorphic Facies
One of the most significant concepts to be drawn from the study of metamorphic
rocks is the notion of metamorphic facies. A facies (face – ees) is a grouping of
metamorphic rocks of various compositions that have formed under the same
grade of metamorphism. By evaluating the changes in texture and composition
within metamorphic rocks it is possible to delineate regions of high and low
metamorphic grade. That is, the geometries of similar metamorphic rocks
provide insights to the processes of metamorphism. There are two key points to
be drawn from the analysis of metamorphic facies. First, different kinds of
metamorphic rocks are formed from different parent rocks at the same grade.
Thus, compositional variation in the precursor rocks is inherited by the
metamorphic rocks. Second, different kinds of metamorphic rocks are formed
from the same precursor at different metamorphic grades.
Through the application of the concept of metamorphic facies, the compositions
and textures of metamorphic rocks can be utilized to estimate temperature and
pressure at the time of metamorphism. Changes in the precursor rock occur as
temperatures and pressures increase. Yet, it is important to consider how it is
geologists are able to evaluate the maximum conditions of metamorphism. The
answer lies in the fact that metamorphic reactions are generally irreversible.
Changes in texture and mineralogy generally occur only as the temperature and
pressure increases during metamorphism. That is, changes get locked into the
rock at the maximum conditions. Occasionally, however, reverse (retrograde)
reactions occur during the cooling or depressurization of a metamorphic rock.
Geologists study the properties of metamorphic minerals through
experimentation. From laboratory studies it is possible to understand the
responses of minerals to changes in temperature and pressure during
metamorphism. As previously described, minerals are stable over limited ranges
of temperature and pressure. When metamorphic conditions exceed those limits,
mineral transformations occur. One of the most important forms of
mineralogical changes that occurs within rocks is the transformation among
mineral polymorphs. Three polymorphs of the alumino-silicate composition
Al2SiO5 are commonly used as pressure-temperature indicators within
metamorphic rocks. The high pressure – low temperature polymorph of Al2SiO5
is Kyanite, the low pressure phase is Andalusite, while the high temperature
polymorph is Sillimanite. Each of these minerals occur naturally over specific
ranges of temperature and pressure. The boundaries between the mineral
stability fields are pressure-temperature combinations where both adjoining
phases are stable. At the center of the phase diagram is a single combination of
temperature and pressure where all three polymorphs are stable. This triple
junction is never sustained in nature, but is an interesting theoretical aspect of
phase relationships within metamorphic rocks.