Metamorphic Rocks

OBJECTIVES

• Increase your understanding of Igneous and Sedimentary rocks by review
• Understand the processes that produce Metamorphic rocks
• Identify the common features seen in Metamorphic rocks: Foliated and Non-Foliated
• Be able to name a possible protolith (parent rock) of a metamorphic rock
• Be able to estimate the grade/level of metamorphism as low/medium/high
• Name the common metamorphic rocks

MATERIALS

• The same suite of Igneous and Sedimentary rocks seen in the last two labs.
• Metamorphic Rocks: Slate, Phyllite, Schist (2), Gneiss, Marble, Quartzite, Anthracite Coal
• Hand lens
• A few minerals such as garnet, mica, quartz and the feldspars

IGNEOUS AND SEDIMENTARY ROCKS REVIEW

Remove all three rock types from the bin and separate them into the previously seen Igneous and Sedimentary rock groups putting aside the new Metamorphic rocks.

Once the instructor has checked the groupings, begin a review of the Igneous rocks with explanations and names and then review the sedimentary rock samples. Use the rock cycle again as your guide.

Igneous review: Recreate the Texture and Chemical compositional chart to review Igneous rocks.  Use the graph on the next page as a guide.  Be able to explain how each texture is formed.  What minerals are included or excluded from each chemical/color level? Name of each Igneous rock.

Sedimentary review: Distinguish between the Detrital/Clastic Sedimentary rocks from the Biochemical/Evaporite Sedimentary rocks.  Looking carefully at grain size, shape, sorting to distinguish the Detrital/Clastic samples.  Consider the energy associated with the various grain sizes.  Separate and identify the biochemical and the evaporite Sedimentary rocks.  Using the chart provided, try to identify the unique materials and name the samples.

SEDIMENTARY ROCKS CHART – Fill in the Names

INTRODUCTION TO METAMORPHIC ROCKS

Metamorphic rocks form when preexisting rocks such as Igneous, Sedimentary or other Metamorphic rocks are subjected to increases in temperature and pressure.  These conditions can cause the rock and its minerals to change as a solid.  The temperature must stay below the melting point, if you recall from the Igneous lab and the rock cycle, once a rock melts then cools it becomes an Igneous rock. This is why in any introductory physical geology textbook the chapter on Metamorphic rocks always follows the chapters on Igneous and Sedimentary rocks. Metamorphic rocks are therefore recycled rocks (figure 1).

All rocks are formed at certain temperatures and pressures on or more commonly, beneath the earth’s surface (figure 2).  Rocks are the most stable at the conditions under which they form. Therefore, changing the temperature and/or pressure conditions may lead to a different rock, one that changed in order to be stable under new conditions. This new rock that forms in response to changes in its physical and chemical environment is called a Metamorphic rock.  The word metamorphism means to change form, and for rocks this means a recrystallization of minerals (crystals) under sub-solidus (temperatures too low for melt production 200-800 0C) conditions and/or with great pressure. A metamorphic change can also occur if the rock’s composition is altered by hot, chemically reactive fluids, causing a change in the mineral content of the rock.

QUESTION: WHAT COULD DO THIS TO A ROCK??

Figure 1. A highly metamorphosed Gneiss.   Image S Earle opentextbc.ca

Figure 2. The Rock Cycle.  Note that Metamorphic rocks form below the Earth’s surface as the result of deep burial combined with heat and pressure. Images CC license oer.galileo.usg.edu

AGENTS THAT CHANGE A ROCK

Metamorphism refers to the changes in the solid state of a pre-existing rock, which commonly occurs deep within the lithosphere (the crust). The conditions that can change one rock into another type of rock are called Agents of Change.  These are typically increased Temperature, increased Pressure and Chemically active fluids or gasses.

TEMPERATURE 

When high temperature, without the presence of significant pressure, a rock undergoes unique changes similar to cooking.  This type of metamorphism is called Thermal or Contact metamorphism and typically occurs when rocks are heated by an intrusive igneous magma source (figure 3).  As a result of coming into contact with such elevated temperatures, the cooler surrounding rock is literally cooked.  The rock becomes drier, more brittle, darker in color and develops a dull matte surface look, much like cooking a piece of bread in the toaster. The rock surrounding the intrusion where the contact occurs is called a metamorphic aureole and produces a unique rock called a Hornfels (figure 4). Hornfels tend to be very uniform in appearance and can be difficult to identify without microscopic examination or direct knowledge of the heat source (magma intrusion).

Figure 3. The extreme heat from an intruding igneous magma source provides significant energy in the form of heat to thermally alter the surrounding rocks.  This contact metamorphism produces a zone (aureole) of metamorphic rocks around the igneous magma called hornfels. Image:

https://commons.wikimedia.org/wiki/File:Rock_contact_metamorphism_eng_big_text.jpg

Jasmin Ros, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Figure 4. The contact or thermally metamorphosed rock is called a Hornfels.  Hornfels tend to be very dull and uniform looking. The color can range from beige to black. Image: http://skywalker.cochise.edu/wellerr/rocks/mtrx/hornfelsG.htm

PRESSURE

All rocks beneath the surface of the earth experience an increase in pressure due to the weight of the overlying sediment and rock layers, and with increasing depth there is a corresponding increase in pressure. This increased pressure does not necessarily cause a rock to become metamorphic, because this particular pressure is typically equal in all directions and is known as lithostatic or confining pressure. Lithostatic pressure is similar to hydrostatic pressure, such as the pressure on the eardrums a swimmer will experience as they dive deep under water. Lithostatic pressure on rocks below the earth’s surface may cause a change (usually reduction) in overall rock volume. If the pressure on a rock is unequal and the rocks become squeezed in one direction more than another direction it is known as differential pressure, and it can result in a significant change in the appearance of a rock. Figure 5 demonstrates how a mineral can change shape due to differential pressure, in this case with the greatest pressures from the top and bottom (as demonstrated by the large gray arrows). Two initially rounded mineral grains (Figure 5A) within a sedimentary rock are experiencing the greatest amount of pressure at the contact between the grains (see arrows in the figure), and the bonds linking the atoms in this grain will break. The atoms will migrate into the area of lesser pressure and reform a bond with other atoms in the mineral grain (Figure 5B). As a result, the grains have a flattened shape that is perpendicular to the direction of greatest pressure (Figure 5C) (Figure 6). Imagine squeezing a rounded ball of Silly Putty until it is flattened. This very common type of force creates Regional metamorphic rocks.  Most of these have a parallel alignment texture called foliation (figure 7).

Figure 5. Directional stress creates differential pressure.  Grains spread out to adjust to the greatest direction of stress.  Can create a foliated texture.  Image CC license oer.galileo.usg.edu    

Figure 6. An Igneous rock Granite shows randomly oriented mineral grains (left) while the metamorphic version called a Gneiss shows the development of oriented foliated texture due to directional stresses. Image CC BY opengeology.org

Figure 7. Strong foliation known as slaty cleavage is seen in the metamorphic rock Slate. This texture forms from strong directional stresses.

Image: Karen Tefend oer.galileo.usg.edu

CHEMICALLY ACTIVE FLUIDS AND GASES

The phrase chemically active refers to the dissolved ions in a fluid phase that may react with minerals in a rock. These ions may take the place of some of the atoms in the mineral’s structure, which may lead to a significant change in the chemical composition of a rock. Sometimes these fluids are quite hot, especially if they are fluids released from a nearby magma body that is crystallizing while cooling. This type of alteration commonly occurs at Divergent boundaries on the ocean floor, as abundant fluid and heat are both available to alter the Basaltic rocks of the ocean floor.  Metamorphism due to such fluids is known as hydrothermal metamorphism – literally ‘hot water’. Rocks altered in this manner are called Metasomatic or Skarns (figure 8).

Figure 8. The metasomatic skarn is called Wollastonite skarn and forms from the hydrothermal alteration of the limestone (calcium carbonate) within a sandstone (mostly silica) is heated with fluids to migrate the Calcium ions from the limestone to the silica. This process releases carbon dioxide. This mineral is added to plastics to give it flexibility and strength as well as ceramics as a source of calcium and silica.  The sample is from Willsboro, NY. Image: http://skywalker.cochise.edu/wellerr/mineral/wollastonite/wollastonite3.htm

TECTONIC SETTINGS DETERMINE THE CONDITIONS OF METAMORPHISM

 The characteristics of the metamorphic rock indicate the tectonic setting of formation (Figure 9). Metamorphic conditions can occur in tectonic collision zones, subduction zones, or adjacent to igneous intrusions deep below Earth’s surface.  This creates contact, regional and subduction metamorphism (Table 1).

                                        Contact Metamorphism around

hot magma bodies                

Compression causing mountain

belts and Regional metamorphism        Hydrothermal alteration of the ocean floor

                        Zone of high pressure/low temperature at subduction zones

Figure 9. Different tectonic settings produce the various conditions necessary to metamorphose a rock, such as high temperature, high pressure or chemically active fluids. Image CC oer.galileo.usg.edu.

Table 1. The characteristic textures seen in metamorphic rocks with tectonic setting and type of metamorphism produced.

Images: Hornfels: https://upload.wikimedia.org/wikipedia/commons/8/8e/Corneenne_dielette_manche.jpg

Blueschist image: By Arlette1 - document personnel, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2193415

https://en.wikipedia.org/wiki/Blueschist#/media/File:Schistes_bleus.jpg Gneiss, Quartzite, Skarn R weller skywalker.cochise college.edu

METAMORPHIC ROCKS ARE RECYCLED FROM EXISTING ROCKS

PROTOLITH

To distinguish between the pre-existing rock (original) and the new metamorphic one, the term protolith or parent rock is used to describe the pre-existing rock.  The protolith (parent rock) along with the agents of change determine the resultant metamorphic rock. For example, if a protolith contains mostly quartz silica-rich minerals such as a sedimentary sandstone and is then metamorphosed, the limited mineral chemistry will typically create a metamorphic Quartzite.  The protolith provides the unique combination or restricted ingredients that can then reform new minerals. The elements that make up the protolith are the same elements in the metamorphic rock.  If the protolith has a wider variety of original chemistry such as a sedimentary mudstone or an igneous granite, the heat and pressure of metamorphism can cause new minerals to form from the original chemistry. It is important to note that this reaction occurs in solid state (no melting), as ions migrate and re-stabilize in the new higher pressure-higher temperature conditions. The new usually denser minerals that form are called index minerals and can be used to determine the chemistry and conditions that the rock has experienced.

INDEX MINERALS

Index minerals are minerals that form under specific temperature and pressure ranges within a specific chemistry of a protolith.  The new minerals can be used to determine the temperature/pressure of metamorphism (figure 10).  The protoliths overall chemistry can determine which new minerals can form. For example, if the protolith has sufficient Iron or Magnesium along with abundant Silica the common metamorphic mineral Garnet can form (figure 11). So, if you can identify Garnet in a rock it is a good indicator that this is a metamorphic rock.  Remember that the elements in the protolith are the same as in the new metamorphic rock. They are just rearranged to be stable at the new conditions.  

Figure 10. Metamorphic Index minerals and their approximate temperature ranges providing the necessary elements/minerals are present in the protolith. Image CC BY S.A. 4.0 international license S. Earle.

Figure 11. A metamorphic Schist with visible Garnet minerals. The garnets only form at specific temperature and pressure ranges. They can then be used as an index mineral. Image: R Weller Skywalker.cochise college.edu

METAMORPHIC TEXTURES AND CLASSIFICATION

When protolith mineralogy is combined with metamorphic conditions the development of unique metamorphic textures can form. Metamorphic rocks can be classified into two broad textural groups: Foliated – the most common type and Non-Foliated.

Foliated - Foliation is defined as the parallel or linear alignment of grains in a rock in an interlocking crystalline form. Most commonly the minerals that align are mica and amphiboles. Foliation has several grades or levels based on the duration and extent of the metamorphic conditions (Figure 12). Foliated rocks typically appear as if the minerals are stacked like pages of a book or stretched.  This produces a platy look.  Certain minerals have a single direction of growth and produce a specific type of foliation called lineation (Figure 13), as though a bunch or pencils or straws were lined up.

Figure 12. An example of foliation with a layered texture. Image CC BY S.A. 4.0 international license S. Earle.opentextbc.ca

Figure 13. An example of metamorphic lineation texture in a Gneiss. Image CC BY S.A. 4.0 international license S. Earle. Opentextbc.ca

FOLIATED TEXTURES AND ROCKS

SLATY - There is one foliation type that is defined by the alignment of minerals that are too small to see, yet the foliation can still be visible. This type of foliation is only seen in the metamorphic rock called slate; slate forms by the low temperature and low-pressure alteration of a shale protolith (Sedimentary). The clay sized minerals in the shale re-crystallize into very tiny micas which are larger than the clay minerals, but still too small to be visible. However, because these tiny micas are aligned, they control how the metamorphic rock (slate) breaks, and the rock tends to break parallel to the mica alignment. Therefore, even though we cannot see the aligned minerals that define the foliation, we can use the alignment of the rock fracture pattern, as the rock is cleaved or split. For this reason, the foliation is called a slaty cleavage, and a rock displaying this type of foliation is called a slate. Figure 14 is an example of the foliated slate displaying slaty cleavage; notice that this rock has retained its original sedimentary layering (depositional beds), which in this case is quite different from the foliation direction. The only protolith for slate is shale, and the fact that original sedimentary features and even some fossils in shale may be preserved and visible in slate is due to the low temperatures and pressures that barely alter the shale protolith, making slate an example of a low-grade metamorphic rock. Slate has great economic value in the construction industry; due to its ability to break into thin layers and impermeability to water, slate is used as roofing tiles and flooring.

Figure 14. Slaty foliation seen in the metamorphic rock Slate.  Slate often form from shales, and mudstones and comes in a wide variety of colors from beige to reddish to black. Image (left) CC BY-SA 3.0 license oer.galileo.usg.edu Karen Tefend (right) R Weller skywalker.cochise college.edu

PHYLLITIC – Phyllitic texture results from the increased level of metamorphism of slates, more pressure and temperature.  The already formed and aligned mica mineral grains continue to grow in size in response to increased pressure and temperature until they become large enough to make the slate very shiny.  The single grains may not be individually noticed but in large amounts they produce a distinctly foliated (layered looking) texture (Figure 15).  Phyllites break more easily than slates as the foliated texture produces layers of weakness.  As a result, they are not as valuable as slates for building materials. Many have kinked crenulations due to variable stress directions.

Figure 15. Phyllite showing phyllitic strongly foliated textures along with a shiny appearance due to the formation and alignment of many tiny mica minerals. Image(left) cc license oer.galileo.usg.edu, (right) R Weller skywalker.cochisecollege.edu

SCHISTOSE - Another type of foliation is defined by the presence of flat or platy minerals, such as muscovite or biotite micas. Metamorphic rocks with a foliation pattern defined by the layering of platy minerals are called schist; the rock name is commonly modified to indicate what mica is present. For example, Figure 16 is a photo of a muscovite schist, however it also has garnet present, so the correct name for the rock pictured in Figure 16A is a garnet muscovite schist. By convention, when naming a metamorphic rock, the mineral in the lowest quantity (garnet, in this case) is mentioned first. Notice that the muscovite micas define a very wavy foliation in the rock; this textural pattern of wavy micas is called a schistose foliation (Figure 16B). The sedimentary rock shale is usually the protolith for schist; during metamorphism, the very tiny clay minerals in shale recrystallize into micas that are large enough to see unaided. Temperatures and pressures necessary for schistose foliation are not as high as those for gneiss and amphibolite; therefore, schists represent an intermediate grade of metamorphism.

Figure 16. Schist. A. The index mineral garnet is visible; therefore, this schist example is correctly termed a garnet schist. B. Wavy foliation is illustrated. Right image shows the large flaky mica minerals and the garnets.  It is correctly named: garnet mica schist and is from Gassett Vermont.  Image (left) CC BY-SA 3.0 license oer.galileo.usg.edu.Karen Tefend. Right R Weller skywalker Cochise college.edu

GNEISSIC – One type of foliation is described as a layering of dark and light-colored minerals, so that the foliation is defined as alternating dark and light mineral bands throughout the rock; such a foliation is called gneissic banding (Figure 17), and the metamorphic rock is called gneiss (pronounced “nice”, with a silent g). In Figure 17A, the layering in this gneiss is horizontal, and the greatest pressures were at right angles to the gneissic bands. Note that these bands are not always flat but may be seen contorted as in Figure 17B; this rock is still considered to have gneissic banding even though the bands are not horizontal. The typical minerals seen in the dark colored bands are biotite micas and/or amphiboles, whereas the light-colored bands are typically quartz or light-colored feldspars. The protoliths for gneiss can be any rock that contains more than one mineral, such as shale with its clay minerals and clay-sized quartz and feldspar, or an igneous rock with both dark-colored ferromagnesian minerals and light-colored non-ferromagnesian minerals. For gneissic foliation to develop, temperatures and pressures need to be quite high; for this reason, gneiss rocks represent a high grade of metamorphism.

Figure 17. Gneissic texture. A. Bands may be horizontal. B.Bands may be wavy or folded. They are usually distinct color bands.  Image (left) CC BY-SA 3.0 license oer.galileo.usg.edu Karen Tefend. Right R Weller skywalker cochise college.edu

NON-FOLIATED TEXTURES

Quartzite and Marble

If the protolith rock is mono-minerallic (composed of one mineral type), such as limestone, dolostone, or a sandstone with only quartz sands, then a foliated texture will not develop even with differential pressure. Why? The calcite mineral in limestone, the dolomite mineral in dolostone, and the quartz sands in sandstone are neither platy minerals, nor are there different colored minerals in these rocks. These minerals (calcite, dolomite, and quartz) recrystallize into equigranular, coarse crystals due to their limited overall chemistry (Figure 18), and the metamorphic rocks that they make are named by their composition, not by foliation type. For example, Figure 18 shows two quartzite samples, a metamorphosed quartz-rich sandstone. Figure 19 shows three examples of marble; note that color can vary for marble, as well as for the quartzite. As a result, Quartzites and Marbles may be hard to identify based on appearance, therefore you must rely on the properties of the minerals that comprise these rocks; you may recall that quartz is harder than glass, while limestone and dolomite are softer than glass. Also, marble will react (effervesce) to acid, but quartz will not react. If you zoom in for a close view of the marble in Figure 19, you will see the calcite crystals are fairly large compared to the quartz crystals in the quartzite in Figure 18; this can be attributed to the temperature of metamorphism, as higher temperatures result in larger crystals. These rocks are also of economic importance; marble and quartzite are used for dimension stone in buildings and for countertops in many homes. Furthermore, marble is commonly used for statues and sometimes grave markers.

Figure 18. Quartzite. Notice the uniform non-foliated texture due to the monomineralic protolith sandstone. Image (left) CC BY-SA 3.0 license oer.galileo.usg.edu Karen Tefend. Right R Weller skywalker.cochise college.edu

Figure 19. Two samples of Marble.  The color can vary due to impurities in the protoliths limestone and dolostone. Impurities in the protolith create a variety of colors and patterns common to Marble. Image (left) CC CC BY-SA 3.0 license oer.galileo.usg.edu Karen Tefend. Right R Weller skywalker.cochise college.edu

Anthracite Coal

One final non-foliated rock type that should be mentioned is anthracite coal (Figure 20). As you may recall, coal is a sedimentary rock composed of fossilized plant remains. This sedimentary coal is called Lignite or Bituminous coal; under higher temperatures and pressures bituminous coal can lose more of the volatiles typical of coal (water vapor, for example), but the carbon content is enriched, making metamorphic coal (anthracite coal) a hotter burning coal due to the higher carbon content. Anthracite coal can be distinguished from sedimentary coal by the shinier appearance, and is somewhat harder than bituminous coal, although both coal types are of low density due to their carbon content. Note that this particular metamorphism is not a recrystallization event, per se, as coal is mostly organic remains. This is why it is called a fossil fuel.

Figure 20. Anthracite coal.  This is a metamorphic version of Lignite or Bituminous coal. It is generally very shiny and light weight. Image (left) CC license oer.galileo.usg.edu. Right https://www.usgs.gov/media/images/anthracite-coal - Donna Pizzarelli

Metamorphic Grade

Not all metamorphic rocks are recrystallized to the same degree. The intensity of metamorphism, called metamorphic grade, depends on how much pressure and heat have been applied. Minerals tend to grow in size with increasing grade.  Also, some rocks change into other metamorphic rocks depending on the grade. For example, sedimentary shale can become metamorphic slate and igneous granite can become metamorphic gneisses. However, certain rocks do not appear to change much with increasing metamorphic grade other than increases in grain size due to annealing (e.g. marble and quartzite) (Table 2).

Table 2. Metamorphic Grade with related pressure and temperature conditions.

OTHER LESS COMMON TYPES OF METAMORPHISM

There are several other types of geologic conditions that can produce metamorphic rocks.  Conditions that occur uniquely as the oceanic crust is subducted - Blueschist, impact cites where a meteor strikes the earth the resulting rock is called shock metamorphism (figure 21) as well as the unique high stress with low temperature conditions that occur along fault zones, and Mylonites (figure 22) which are formed when rocks become plastic due to high heat and pressure.  Many of these rocks are very important to geologists as we try to understand the changing and dynamic nature of our planet and its 4.6-billion-year history.

Figure 21. Winslow crater near Winslow Arizona. Approximately 50,000 years ago a 40 to 50 meter iron-nickel meteor struck, producing a 1.2 km diameter and 170 meter deep crater. The rocks around have been shock metamorphosed. Image: https://www.nasa.gov/sites/default/files/thumbnails/image/asteroid-meteor-crater.png

Figure 22. A mylonite showing stretched augens of quart pebbles. Arrows show direction of forces. Image CC BY Peter Davis from opengeology.org/textbook

Table 3. Metamorphic rocks classified by texture

Images: Slate, Schist K. Tefend, Phyllite oer.galileo.usg.edu, Gneiss, Marble R weller skywalker cochise college.edu, Quartzite Pete Davis, Anthracite https://www.usgs.gov/media/images/anthracite-coal – Donna Pizzarelli

To summarize:

1. AGENTS OF CHANGE: Metamorphism is the process by which a pre-existing rock (the protolith) is altered by a change in temperature, pressure, or by contact with chemically reactive fluids, or by any combination of these three parameters.  The agents of change are directly connected to unique plate tectonic conditions such as continental collisions.
2. ROCK CHANGES: The alteration process is a recrystallization event, where the initial rock’s minerals (crystals) have changed size, shape, and/or composition in response to these new external conditions. This produces unique textures such as foliation and sometimes new index minerals.  The index minerals can give us clues to the amount or grade of the metamorphism.
3. PROTOLITH: What metamorphic rock you end up with is strongly dependent on what rock you started with before the metamorphic event. Images below CC license BY Pete Davis, Manishwiki15 

Try to identify the 4 images below

1 https://upload.wikimedia.org/wikipedia/commons/e/e3/Garnet-chlorite_schist_%28Alabama%2C_USA%29_%2849155351038%29.jpg

2 R Weller skywalker chochise college.edu

3This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
Based on a work at all-geo.org.

Simon wellings

https://all-geo.org/metageologist/author/metageologist/

        4 NPHFlynn

LET’S PRACTICE IDENTIFYING THE METAMORPHIC ROCKS

Using the samples provided by our instructor:

Determine if the sample has a Foliated or Non-foliated texture.  

Then try to identify any specific index minerals, level or grade of metamorphism.

Then consider a protolith.  

Think about the following questions:

How can you tell Quartzite from Marble?

Why do some metamorphic rocks have foliation and others don’t?

ROCK QUIZ NEXT WEEK: Try to identify each samples’ unique features and name them.