MINERAL STABILITY AND METAMORPHISMA mineral that does not decompose or change in other ways, no matter how much time passes, is a stable mineral. Millions of years ago, weathering processes may have formed the clay mineral used by the potter to create her vase. They were stable and had remained unchanged since they formed. A stable mineral can become unstable when environmental conditions change. Three types on environmental change cause metamorphism: rising temperature, rising pressure, and changing chemical environment. Evert mineral is stable only within a certain temperature range. In a similar manner, each mineral is stable only within a certain pressure range. In addition, a mineral is stable only in a certain chemical environment. If fluids transport new chemicals to a rock, those chemicals may react with the original minerals to form new ones that are stable in the altered chemical environment. If fluids remove chemical components from a rock, new minerals may form for the same reasons. Metamorphism occurs because each mineral is stable only within a certain range of temperature, pressure, and chemical environment. If temperature of pressure rises above that range, or if chemicals are added to or removed from the rock, the rock’s original minerals may decompose and their components recombine to form new minerals that are stable under the new conditions.
METAMORPHIC CHANGES
Metamorphism commonly alter both the texture and mineral content of a rock.
TEXTURAL CHANGES As a rock undergoes metamorphism, some mineral grains grow larger and others shrink. The shapes of the grains may also change. For example, metamorphism transforms limestone into a metamorphic rock called marble. Like the fossiliferous limestone, the marble is composed of calcite, but the texture is now one of large interlocking grains, and the fossils have vanished.
MINERALOGICAL CHANGES As a general rule, when a parent rock (the original rock) contains only one mineral, metamorphism transforms the rock into one composed of the same mineral but with a coarser texture. The mineral content does not change because no other chemical components are available during metamorphism. Limestone converting to marble is one example of this generalization. Another is the metamorphism of quartz sandstone to quartzite, a rock composed of recrystallized quartz grains. In contrast, metamorphism of a parent rock containing several minerals usually forms a rock with new and different minerals and a new texture. For example, a typical shale contains large amounts of clay, as well as quartz and feldspar. When heated, some of those minerals decompose, and their atoms recombine to form new minerals such as mica, garner, and a different kind of feldspar. If migrating fluids change the chemical composition of a rock, new mineral invariably form.
METAMORPHIC GRADE
Metamorphic grade expresses the intensity of metamorphism that affected a rock. Because temperature is the most important factor in metamorphism, metamorphic grade closely reflects the highest temperature attained during metamorphism. Geologists can interpret the metamorphic grade of most rocks because many metamorphic minerals form only within certain temperature ranges. The rate at which temperature increases with depth is called geothermal gradient. Consequently, the metamorphic grade of many rocks is related to the depth to which they were buried. Low-grade metamorphism occurs at shallow depths, less than 10 to 12 kilometers beneath the surface, where temperature is below 350° C. High-grade conditions are found deep within continental crust and in the upper mantle, 40 or more kilometers below the Earth’s surface. The temperature in these regions is 600° C or hotter and is near the melting point of rock. High-grade metamorphism can occur at shallower depths, where magma rises to a shallow level of the Earth’s crust.
THE RATE OF METAMORPHISM
The speed of a chemical reaction doubles with every 10° C rise in temperature. Thus, reactions occur slowly in a cold environment, but rapidly in a hot one. For the same reason, metamorphic changes occur slowly at low temperature, but much faster at high temperature. Geologists are able to produce metamorphic reactions in the laboratory at temperatures above 500°C in a few days. The upper limit of metamorphism is the point at which rocks melt to form magma. That temperature varies depending on rock composition, pressure, and amount of water present, but it is between 600° and 1200°C for most rocks. A rock heated to its melting point creates magma, which forms igneous rocks when it solidifies. Metamorphism refers only to changes that occur without melting.
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