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Metamorphic Rock Identification

Metamorphic Rock Identification

Introduction: Long before whiteboards, interactive whiteboards, and tablets, slate blackboards were the standard classroom tool for teachers and students. Slate is a hard, fine-grained, metamorphic rock that tends to break in flat planes. These properties made slate ideal for writing on with soft pieces of chalk (usually the mineral gypsum). This also made slate a good choice for roof shingles, paving stones, and even tombstones. Interestingly, slate typically forms from shale, a soft sedimentary rock made from mud. So how does a soft rock become a rock that is both hard and durable, yet able to break into smooth sheets? 

Metamorphic rocks form when preexisting rocks change under heat and pressure. These chemical and physical changes are called metamorphism. Metamorphism usually occurs when a rock becomes buried deep within the earth and its minerals recrystallize and/or become deformed. It is important to understand that although metamorphic rocks are exposed to extreme temperatures and pressures, the rocks and their minerals do not melt.

Geologists classify metamorphic rocks based mainly on their mineral composition and texture. There are two major categories of metamorphic textures: foliated and nonfoliated. 

Rock - Metamorphic - SchistFoliated rocks form when flat minerals (like micas) realign into sheets. This makes the rock look like it has layers that are flat or folded. Do not confuse foliation with sedimentary layering! The greater the degree of metamorphism the rock undergoes, the more visible the foliation becomes. Slate (shown at top of lesson), is a low-grade metamorphic rock. It has fine foliation and forms under lower temperatures and pressure. Schist (shown to the right) forms under higher temperatures and pressures. It has more easily visible foliation. Highly metamorphosed rocks can show banding, or alternating layers of light and dark minerals. 

Rock - Metamorphic - MarbleNonfoliated rocks don't tend to show patterns in mineral alignment. Since nonfoliated rocks don’t have the textures of foliated rocks, try to identify the major mineral in the rock. For example, marble (shown to the left) can be metamorphosed limestone. Like limestone, marble is mainly made of the mineral calcite. Using a metamorphic rock chart like the one shown can help with the process of rock identification.
Chart - Metamorphic Rocks


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