Metamorphic Rocks Tour
HOME CREDITS INTRODUCTION
PROCESSES OF METAMORPHISM METAMORPHIC ROCKS
Types of Metamorphism
cataclastic metamorphism contact metamorphism
INTRODUCTION The three different types of metamorphism have different settings and metamorphic processes. Please keep in mind that, as with other aspects of metamorphism and metamorphic rocks, the distinctions between the types of metamorphism is sometimes unclear and subjective. Note that metamorphic processes of recrystallization, neomorphism and metasomatism referred to below are described in the PROCESSES OF METAMORPHISM page.
TYPES OF METAMORPHISM
cataclastic metamorphism The grinding of rocks as they shift along an active fault pulverizes the rocks; hence the term cataclastic (cata- for catastrophic and -clastic for fragmentation) metamorphism. Rocks that experience cataclastic metamorphism are subjected to sudden high-pressure and low-temperature conditions, resulting mainly in a change in texture of the original rock. Repeated movement of rocks along a fault can crush mineral grains into a fine powder, called fault gouge. Since cataclastic metamorphism occurs where rocks grind past each other along a fault, it is a common feature along most plate boundaries, where plates are interacting with each other generating tremendous stresses and resulting movements of the crust.
These are views of the Punchbowl Fault, just south and east of the Devils Punchbowl County Park, southern California. The first image shows the fault cutting through the San Gabriel Mountains, from middle right to upper left of the image. The second image is a closeup-view of the fault. Note the fragmentation of the granite as it has shifted and fractured due to movement along this active fault. See the black camera-lens cap (two inches in diameter) for scale in the second image.
Fault gouge developed due to long-term movement along the Elsinore Fault. This exposure is within a roadcut on Montezuma Road near Anza Borrego State Park, southern California.
A closer view of Steve and the fault gouge from the image above. Steve could easily dig his fingers into the crushed rock, whereas the gneiss on each side of the fault is still very solid and hard.
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contact metamorphism As a body of magma invades the upper portion of Earth's crust, its heat will cause recrystallization and possibly neomorphism of the surrounding, cooler rocks. Metasomatism may also occur as hydrothermal fluids circulate through the surrounding rock. The zone of contact-metamorphosed rock surrounding the body of magma is called an aureole. Many profitable mines are situated in metal-rich aureoles formed by contact metamorphism.
A view of Vulcan Mine which was active in the 1940's. The aureole of the mine consists of an iron zone formed by metasomatism (shown in the lower portion of the image), and a light-colored marble zone formed by recrystallization of calcite (shown in the middle of the image). The original, parent rock, the Bonanza King Limestone is visible in the distance.
Another view of Vulcan Mine showing more clearly the iron and marble zones of the aureole. The iron removed from this open-pit mine in the early 1940's was required for industrial and military needs during World War II. The mine was abandoned at the end of the war due to its remote location in the Mojave Desert of southern California.
Small igneous intrusions, like these igneous dikes within the rocks of Canyon Sin Nombre, Anza Borrego State Park, will cause only a little contact metamorphism in the rocks directly adjacent to the intrusion.
This closeup of one of the dikes from the image above shows that little change occurred to the original rock that was invaded by this small amount of magma.
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regional metamorphism Sometimes alterations in the textures and compositions of rocks buried deep underground occur to a large volume of Earth's crust, not just along a fault as is the case with cataclastic metamorphism, or only adjacent to an igneous intrusion as happens with contact metamorphism. To develop regional metamorphism of Earth's crust requires one of the following:
1) the intrusion of huge quantities of magma into the crust (typical at a subduction plate boundary, such as along the western edge of South America at the Peru-Chile Trench),
2) the gradual buildup of very high pressure and temperature conditions (typical at a collision plate boundary, such as the collision of India with Asia which is expressed at the surface by the Himalaya Mountains), or
3) the massive movement of hydrothemal fluids through pores and fractures within rocks (associated with subduction-generated magmatic intrusions as well as with the cycling of ocean water through hot oceanic crust near divergent plate boundaries, such as the East Pacific Rise and the Mid-Atlantic Ridge).
The end product of any of the options above is medium to high-grade metamorphism of the parent rock(s), sometimes to the degree that it is very difficult to tell exactly what the parent rock was. The effects of recrystallization, neomorphism and metasomatism may be pervasive and intertwined. The images below illustrate some of the effects of regional metamorphism.
A portion of this mountain range in Alsaka was invaded by hydrothermal fluids derived from magma that was produced by subduction. The result was the massive invasion of copper-rich fluids and the precipitation of copper within the rocks.
Diane demonstrates the directions of pressure application (squeezing) that must have existed deep underground when this sample of gneiss was formed. Such well-developed foliation is typical of rocks that have undergone regional metamorphism.
The brown to reddish-colored rocks exposed within Canyon Sin Nombre of Anza Borrego State Park is very ancient schist and gneiss of unknown origin. Here it is difficult to say exactly what kind of regional metamorphism occurred; maybe all three versions.
A closer view of the canyon. The lighter colored streaks are much younger igneous intrusions. Nicole is younger still.
Jill and Paul ponder the metamorphic rocks exposed within the Inner Gorge of Grand Canyon due to the combination of uplift of the Colorado Plateau and downcutting by the Colorado River, which is out of view at the bottom of the gorge.
A closeup of the same image above. The black rocks are high-grade metamorphic rocks formally called the Vishnu Group. This group is composed of gneiss,schist and quartzite that contains relic features that indicate that it was originally a body of sedimentary rock include\ing limestone and sandstone which became buried deeply underground forming the roots of ancient mountains. This was probably due to a collision of continents approximately 1.7 billion years ago. The reddish-colored rocks within the Vishnu Schist are large igneous dikes that are somewhat younger than the schist.
This is a dizzying view of the Inner Gorge Vishnu Group and igneous intrusions, as well as a footbridge that spans the Colorado River near Bright Angel Creek. The photo is courtesy of Dr. Stanley Finney, CSU Long Beach.
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