Chapter 8 Metamorphism and Metamorphic Rocks

Metamorphism means to "change form." Describe how a rock may change during metamorphism:

- leads to changes in the mineralogy, texture, and sometimes the chemical composition of rocks - takes place when preexisting rock is subjected to a physical or chemical environment that is significantly different from that in which it initially formed. In response, the rock gradually changes until a state of equilibrium with the new environment is reached - most metamorphic changes occur at the elevated temperatures and pressures that exist in the zone beginning a few kilometers below Earth's surface and extending into the upper mantle

Slate and phyllite resemble each other. How might you distinguish one from the other?

Both are derived by regional metamorphism of shale or mudstone. Slate forms at lower temperatures and often exhibits well-developed rock cleavage. The aligned mica and chlorite grains are far too small to be visible to the naked eye, and the fracture cleavage surfaces show, at most, a dull sheen. Phyllite develops at somewhat higher temperatures; the mica and chlorite are fine grained but usually visible to the naked eye, sometimes with difficulty. The foliation surfaces exhibit a bright sheen caused by light reflecting from the aligned cleavage planes of the mica and/or chlorite grains.

Each of the following statements describes one or more characteristics of a particular metamorphic rock. For each statement, name the metamorphic rock that is being described.

Calcite-rich and nonfoliated - marble Foliated and composed mainly of granular minerals - gneiss Represents a grade of metamorphism between slate and schist - phyllite Very fine-grained and foliated; excellent rock cleavage - slate Foliated and composed of more than 50 percent platy minerals - schist Often composed of alternating bands of light and dark silicate minerals - gneiss Hard, nonfoliated rock resulting from contact metamorphism - hornfels

Distinguish between contact metamorphism and regional metamorphism. Which creates the greatest quantity of metamorphic rock?

Contact metamorphism is restricted to the thermal halo (aureole) surrounding a pluton, batholith, or other intrusive magma body (Fig. 7.14). The effects of metamorphism are limited to a specific volume of wall rock around the magma body; the metamorphic episode is over once the magma body is cooled and crystallized. In regional metamorphism (Fig. 7.16), very large volumes of sedimentary, volcanic, and mid- to upper-crustal rocks are squeezed at convergent plate margins, deeply buried, heated by the Earth's geothermal heat, and invaded by hot, metamorphic fluids. The metamorphic episode is long lasting and ceases only when the compressive deformational event ends and the rocks are tectonically uplifted and cooled. Thus regional metamorphism generates by far the larger volume of metamorphic rock.

What is foliation? Distinguish between slaty cleavage and schistosity.

Foliation describes a preferred, parallel orientation of sheetlike (platy) mineral grains, mainly micas and chlorite, in a metamorphic rock. This parallel orientation of platy mineral grains also accounts for slaty (rock) cleavage and schistosity. The strong tendency of slate to split along parallel cracks, forming plate-shaped fragments with a dull, surface luster, is called slaty or rock cleavage. The cracks open parallel to the plane of the tiny, aligned mica and chlorite grains. Thus in some areas, slate is still used as roofing material (Fig. 7.7). The strong foliation in metamorphic rocks imparted by concentrations of visible, aligned mica and/or chlorite grains is called schistosity because it characterizes virtually all schists.

What feature would easily distinguish schist and gneiss from quartzite and marble?

Foliation is much better developed in most schists and gneisses than in most quartzites and marbles. Schist and gneiss are composed dominantly of silicate minerals. Marble is an all calcite rock and quartzite is composed dominantly of quartz. Calcite is much softer than most rock-forming silicates and reacts vigorously with dilute acids. Quartzite is very hard and generally lacks foliation, especially if micas are absent. Thus the presence or absence of foliation, as well as the mineralogy, are very helpful in telling schist and gneiss from marble and quartzite.

What is meant by the statement, "Every metamorphic rock has a parent rock"

Metamorphic rocks are produced from preexisting igneous, sedimentary, or other metamorphic rocks. So every rock has a rock from which it was formed (parent rock).

What is metamorphism? What are the agents that change rocks?

Metamorphism is a substantive change in mineral composition and/or texture in a rock as a response to one or more of the following: higher temperatures, higher pressures, and contact with metamorphic fluids (water-dominated solutions at elevated temperatures and pressures that contain dissolved, silicate mineral components). Metamorphism also accompanies localized, mechanical fragmentation and melting such as occur due to fault-zone shearing and melting in rocks subjected to enormous, transient, over pressures from impacts of extraterrestrial objects such as meteorites, comets, and asteroids.

Are migmatites associated with high-grade or low-grade metamorphism?

Migmatites (Fig. 7.20) form under high-grade conditions. They form by partial melting under pressure-temperature conditions in the melting range for granitic compositions. Migmatites are streaky, layered rocks composed of alternating, dark-colored, residual minerals of the original parent rock and light-colored streaks and veins that crystallized from the melted granitic fraction.

List some changes that might occur to a rock in response to metamorphic processes?

Mineral grain size could increase with metamorphic recrystallization, as in the change of limestone into marble. Some minerals decompose and new ones grow (crystallize), causing changes in both the mineral composition and in the rock texture.

Briefly describe the textural and mineralogical differences among slate, mica schist, and gneiss. Which one of these rocks represents the highest degree of metamorphism?

Slate, derived from shale or mudstone, is a very fine-grained, metamorphic rock with well-developed rock cleavage, and the mineral grains are not visible to the naked eye. Slate forms at the lowest metamorphic grade of the three. Schist, also derived from shales and mudrocks, is formed at much higher metamorphic grades than slate. Micas and/or chlorite are abundant and coarse grained; their parallel grain alignment produces a strong foliation. Gneiss is a coarse grained, foliated, metamorphic rock rich in equidimensional grains of feldspar and quartz. Micas are sparse and are concentrated into thin, discontinuous layers. These mica-rich zones accentuate a strong foliation, easily seen as alternating bands or layers of darker- and lighter-colored minerals. Gneiss generally forms under the highest metamorphic grades; however, many coarse grained schists form under equivalent metamorphic conditions.

With what type of plate boundary is regional metamorphism associated?

The high pressures generated by deep burial and elevated temperatures generated by deep burial and magma intrusion are associated with convergent boundaries (Figs. 7.16 & 7.21). In oceanic-oceanic and oceanic-continent collisions, the oceanic slab and some of its sedimentary cover can be subducted to great depths, generating very high pressures and eventually, high temperatures. In continent-continent collisions, marine sedimentary strata in the closing marine basin between the converging continents are squeezed laterally, folded, thrust faulted, and overthickened. Continued shortening and thickening can generate the deep burial and elevated temperatures required for regional metamorphism. In addition, the crystalline-rock crusts of one or both continents are shortened and thickened by thrust faults, resulting in even deeper burial for some rocks and strong uplift for others. Partial melting at depth leads to magma generation and intrusion of granitic batholiths at mid to upper crustal levels.