Metamorphic Rocks in the National Parks

OBJECTIVES

• • 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
  • Name the common metamorphic rocks
  • Connect several national parks with unique metamorphic rock exposures

MATERIALS

• • Metamorphic Rocks: Slate, Schist, Gneiss, Marble, Quartzite, Anthracite Coal
  • Hand lens

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 – they used to be another rock (figure 1). This is why the rock cycle is so important.

All rocks are formed at certain temperatures and pressures on or more commonly, beneath the earth’s surface. 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 (figure 2).

Figure 1. 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

QUESTION: WHAT COULD DO THIS TO A ROCK??

Figure 2. Folded rocks with granite veins on Mt. Rushmore National Memorial South Dakota. Image: NPS Public Domain. https://www.nps.gov/moru/learn/nature/geologicactivity.htm

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) of the Earth. 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. We will focus on the combination of increased Temperature and Pressure as this produces the largest volume of Metamorphic rocks.

TEMPERATURE - Alone

When high temperature, without the presence of significant pressure, occurs 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 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 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 & TEMPERATURE combined

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). 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 6).

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

TECTONIC SETTINGS DETERMINE THE CONDITIONS OF METAMORPHISM

The characteristics of a metamorphic rock indicate the tectonic setting of formation (Figure 7). 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 7. 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.

In the United States of America there are many National Parks that contain tectonically created or exposed metamorphic rocks. The diverse and widely distributed Appalachian Mountain ranges formed during several episodes of continental collision during the Paleozoic Era. Parks such as Acadia NP in Maine to the vast Shenandoah NP located in the Blue Ridge mountains of Virginia to Arkansas Hot Springs NP to Big Bend NP in southern Texas (figure 8).

Figure 8. The widespread distribution of National Parks extending from as far north as Acadia NP in Maine to Big Bend NP in southern Texas. Image NPS Public Domain

https://www.nps.gov/subjects/geology/plate-tectonics-collisional-mountain-ranges.htm

Figure 9. Images of Shenandoah National Park, Virginia. Rugged Appalachian Mountains are eroded remnants of much higher mountains that formed as continents collided 300 million years ago (left) and Big Bend NP in Texas which had a wide variety of geologic episodes in the park. Images: NPS photo public Domain https://www.nps.gov/media/photo/gallery-item.htm?pg=3533793&id=547D251E-155D-451F-67165B304A34224E&gid=56AC6441-155D-451F-670E4DDB882AEB40 Big Bend image credit Ann Wildermuth. https://www.nps.gov/media/photo/gallery-item.htm?pg=2567920&id=2D217FD0-155D-451F-67D2195DCE0779F7&gid=F4348BE2-155D-451F-6760A36937D99BC4

The Shenandoah Mountain National Park shows vast expanses of folded and folded mountains in figure 10.

Figure 10. Mountain view from Turks gap overlook in Shenandoah National Park in Virginia. Image: public domain. https://www.nps.gov/media/photo/gallery-item.htm?pg=3533793&id=54D257B6-155D-451F-67C3DC7F3931A2AF&gid=56AC6441-155D-451F-670E4DDB882AEB40

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 RECYLCED 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 11). 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 12). 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 11. Metamorphic Index minerals and their approximate temperature ranges providing the necessary elements/minerals are present in the protolith. Image opentextbc.ca CC BY S.A. 4.0 international license S. Earle.

Figure 12. A metamorphic Schist with visible Garnet minerals from North Cascades NPS Complex. The garnets only form at specific temperature and pressure ranges. They can then be used as an index mineral. Image: Image: Peter Misch Geology Collection NOCA 45665 https://nps.maps.arcgis.com/apps/MapJournal/index.html?appid=8e33939d87a645a7bd4587fa13452d18

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 13). Foliated rocks typically appear as if the minerals are stacked like pages of a book or stretched. This produces a platy look.

Figure 13. An example of foliation with a layered texture from Boulder Creek near Glacier Peak in Glacier Peak wilderness in Mount Baker-Snoqualmie National Forest, Washington. Image credit Tabour et al 2009 https://nps.maps.arcgis.com/apps/MapJournal/index.html?appid=8e33939d87a645a7bd4587fa13452d18

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 forms 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.

Metamorphic rocks can experience increasing levels of temperature and pressure due directly to mountain building forces associated with plate tectonic movements. Common sedimentary rocks such as claystones and mudstones are metamorphosed into slates and schists (figure 18).

Figure 17. Gneissic texture from Grand Tetons NP in Wyoming that is 2.7 billion years old. Bands may be horizontal or wavy or folded. They are usually distinct color bands. Image NPS Public Domain by P. Sasnett https://www.doi.gov/blog/earth-rocks

Figure 18. The increasing levels of pressure and temperature lead to increasing grades of metamorphism. Image public domain https://gotbooks.miracosta.edu/geology/images/meta_shale_gneiss.jpg

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 (Figures 19 & 20), and the metamorphic rocks that they make are named by their composition, not by foliation type. For example, Figure 19 shows two quartzite samples, a metamorphosed quartz-rich sandstone. Figure 21 shows 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 21, you will see the calcite crystals are fairly large compared to the quartz crystals in the quartzite in Figure 19; 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 19. Quartzite. Notice the uniform non-foliated texture due to the monomineralic protolith sandstone. Rosey Quartz image is from the Black Hills National Forest in South Dakota, near Mt Rushmore NM. Image NPFlynn

Figure 20. The sedimentary rock called sandstone is metamorphosed into Quartzite. Image: public domain https://gotbooks.miracosta.edu/geology/images/meta_sand_quartzite.jpg

Figure 21. Marble from the canyon walls of Marble Canyon Death Valley NP in California and Nevada. 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 NPS Public Domain Dan Kish https://www.nps.gov/subjects/geology/metamorphic.htm

Figure 22. Limestone (sedimentary) is transformed into Marble. Image: public domain https://gotbooks.miracosta.edu/geology/images/meta_lime_marble.jpg

Anthracite Coal

One final non-foliated rock type that should be mentioned is anthracite coal (Figure 23). 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. The National Parks service is home to national Coal Heritage Area (NCHA). The NCHA’s mission is to preserve, protect and interpret lands structures and communities associated with the coal mining heritage of West Virginia, which include 13 counties with 3 National Park regions Bluestone National Scenic River, Gauley River National Recreation Area, and New River Gorge National River. There are many other national and state preserved regions intent on preserving the culture and history of American Coal mining.

Figure 23. 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.

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 opengeology.org, 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.

Try to identify the 2 images below

This 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/

2 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 other don’t?