Metamorphic Rock Characteristics

Why This Matters

Metamorphic rocks are Earth's geological storytellers—they record the intense pressures and temperatures deep within the crust and reveal the forces that build mountains and reshape continents. When you study metamorphic rock characteristics, you're learning to decode evidence of plate tectonics, mountain-building events, and thermal processes that have shaped our planet over billions of years. These concepts connect directly to understanding rock cycle dynamics, convergent plate boundaries, and the conditions that exist at various depths within Earth's interior.

You're being tested on your ability to read rocks like a geologist: identifying what conditions formed them, what parent rock they came from, and what processes transformed them. Don't just memorize rock names—know why foliation develops, how mineral assemblages reveal pressure-temperature conditions, and what distinguishes contact from regional metamorphism. Master the underlying mechanisms, and you'll be ready for any question they throw at you.

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Processes That Drive Metamorphism

Metamorphism occurs when existing rocks are subjected to conditions different from those under which they originally formed, causing physical and chemical changes without melting.

Pressure and Temperature Effects

• Heat and pressure are the twin engines of metamorphism—they destabilize existing minerals and drive the formation of new, more stable mineral structures
• Directed pressure (differential stress) causes minerals to rotate and realign perpendicular to the stress direction, creating foliation
• Confining pressure increases rock density, while elevated temperatures provide the energy needed for atoms to migrate and reorganize into new crystal structures

Recrystallization

• Recrystallization transforms mineral structures without changing bulk chemistry—atoms rearrange into larger, more stable crystals under metamorphic conditions
• Crystal growth occurs as smaller, less stable grains dissolve and reprecipitate onto larger crystals, a process called Ostwald ripening
• The resulting rock becomes denser and more interlocking, which explains why metamorphic rocks are often harder and more durable than their parent rocks

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Textures and Structures

The arrangement, size, and orientation of mineral grains in metamorphic rocks provide direct evidence of the conditions and stresses present during formation.

Foliation

• Foliation is the parallel alignment of platy or elongate minerals—it develops when differential pressure causes minerals like micas to rotate perpendicular to the maximum stress direction
• Foliation intensity increases with metamorphic grade—from slaty cleavage (fine-grained) to schistosity (visible mica flakes) to gneissic banding (segregated light and dark layers)
• Non-foliated textures occur when the parent rock lacks platy minerals or when pressure is equal in all directions (confining pressure only)

Texture Changes

• Grain size generally increases with metamorphic grade—higher temperatures allow atoms to diffuse farther, growing larger crystals
• Porphyroblastic texture features large crystals (porphyroblasts) like garnet or staurolite set in a finer-grained matrix, indicating minerals that grew rapidly under specific conditions
• Original sedimentary or igneous textures are progressively destroyed as metamorphism intensifies, making high-grade rocks harder to trace back to their protolith

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Indicators of Metamorphic Conditions

Geologists use mineral assemblages and metamorphic grade as "geological thermometers and barometers" to reconstruct the pressure-temperature history of a rock.

Mineral Assemblages

• Specific mineral combinations form only under particular P-T conditions—the presence of certain minerals tells you the pressure and temperature range during metamorphism
• Index minerals like chlorite, biotite, garnet, staurolite, kyanite, and sillimanite appear in sequence as grade increases, serving as metamorphic thermometers
• Garnet, kyanite, and sillimanite indicate high-grade conditions, while chlorite and muscovite indicate low-grade metamorphism

Metamorphic Grade

• Metamorphic grade describes the intensity of metamorphism—low-grade rocks form at approximately 200-400°C, while high-grade rocks require temperatures exceeding 600°C
• Grade is assessed by examining mineral assemblages and textures—fine-grained rocks with chlorite are low-grade; coarse-grained rocks with sillimanite are high-grade
• Progressive metamorphism describes the sequence of changes a rock undergoes as temperature and pressure increase, often visible across a mountain belt

Metamorphic Facies

• Metamorphic facies are sets of mineral assemblages that form under similar P-T conditions—they represent equilibrium states for specific environments
• Greenschist facies (low-grade, characterized by chlorite and epidote) contrasts with amphibolite facies (medium-high grade, dominated by hornblende) and granulite facies (highest grade)
• Blueschist facies indicates high pressure but relatively low temperature—a signature of subduction zone metamorphism

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Types of Metamorphism

The geological setting determines whether metamorphism is localized around a heat source or regionally extensive across mountain belts.

Contact vs. Regional Metamorphism

• Contact metamorphism occurs adjacent to igneous intrusions—heat from magma bakes surrounding rock, creating a metamorphic aureole that grades outward from high to low intensity
• Regional metamorphism affects vast areas during mountain-building events—rocks are subjected to both elevated temperatures and directed pressures from tectonic forces
• Contact metamorphic rocks are typically non-foliated (no directed pressure), while regional metamorphic rocks usually display strong foliation from tectonic stress

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Parent Rocks and Their Products

The protolith's original composition constrains what metamorphic rock can form—you can't create minerals from elements that weren't there to begin with.

Parent Rock Influence

• The protolith determines the available chemical ingredients—shale (clay-rich) produces mica-rich rocks, while basalt (mafic) produces amphibole-rich rocks
• Texture of the parent rock influences initial metamorphic response—fine-grained rocks recrystallize more readily than coarse-grained rocks at the same conditions
• Common protolith-product pairs include shale → slate → schist → gneiss, limestone → marble, sandstone → quartzite, and basalt → greenschist → amphibolite

Common Metamorphic Rocks

• Slate forms from shale under low-grade conditions—its excellent slaty cleavage (flat, parallel fracture planes) makes it ideal for roofing tiles and chalkboards
• Schist represents medium-grade metamorphism with visible, aligned mica crystals—schistosity creates a sparkly, wavy foliation often containing garnet porphyroblasts
• Gneiss is a high-grade rock with distinct compositional banding—alternating light (quartz, feldspar) and dark (biotite, hornblende) layers indicate extensive mineral segregation

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Quick Reference Table

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Self-Check Questions

1. Which two metamorphic rocks both form from shale but represent different metamorphic grades, and what textural differences distinguish them?

2. A rock sample contains abundant hornblende and plagioclase with strong foliation. What metamorphic facies does this suggest, and was it likely formed by contact or regional metamorphism?

3. Compare and contrast the formation of marble and quartzite—what do their non-foliated textures tell you about their parent minerals and the type of pressure involved?

4. You find a metamorphic rock with large garnet crystals surrounded by fine-grained mica. What is this texture called, and what does the presence of garnet indicate about metamorphic conditions?

5. An FRQ asks you to explain how geologists use mineral assemblages to determine the pressure-temperature history of a mountain belt. Which concept—metamorphic grade or metamorphic facies—provides more specific information, and why?