Metamorphism Explained — Types of Metamorphic Rocks, Foliation, and Tectonic Settings | Chapter 8 from Earth: Portrait of a Planet

How do rocks transform without melting? Chapter 8 of Earth: Portrait of a Planet by Stephen Marshak uncovers the science of metamorphism—the process by which pre-existing rocks are altered under the influence of heat, pressure, stress, or hydrothermal fluids. For a comprehensive podcast summary, watch the full video on YouTube, or explore the detailed written breakdown below.

What Is Metamorphism?

Metamorphism is the solid-state transformation of a protolith (pre-existing rock) into a new type of rock with a different mineral assemblage or texture, without melting. This process is driven by changes in temperature, pressure, differential stress, or the presence of hydrothermal fluids.

Key Metamorphic Processes

• Recrystallization: Changes the size and shape of grains without changing mineral identity.
• Phase Change: Transforms a mineral into another with the same composition but a different structure.
• Neocrystallization: Growth of new minerals from existing components.
• Pressure Solution: Dissolution at grain boundaries under pressure, followed by precipitation elsewhere.
• Plastic Deformation: Minerals change shape without breaking, under high temperature and pressure.

Foliation and Preferred Orientation

Foliation is the parallel alignment of platy minerals or banding in metamorphic rocks, produced by differential stress during metamorphism. Structures like gneissic banding and preferred mineral orientation provide clues to deformation conditions deep within Earth’s crust.

Types of Metamorphic Rocks

• Foliated Rocks: Slate, phyllite, schist, gneiss, and metaconglomerate—characterized by layered or banded appearance.
• Nonfoliated Rocks: Marble (from limestone), quartzite (from sandstone), hornfels, amphibolite—lacking a planar structure.
• Special Types: Migmatite (partly melted rock), mylonite (formed by intense shear).

Metamorphic Grade, Facies, and Index Minerals

Metamorphic rocks are classified by grade (low, intermediate, high), determined by temperature and pressure conditions. Index minerals (like garnet, staurolite, kyanite) and isograds help geologists map metamorphic zones and understand depth and temperature at the time of formation. The pressure-temperature-time (P-T-t) path and metamorphic facies reveal the history of rocks within tectonic settings.

Tectonic Settings of Metamorphism

• Regional Metamorphism: Occurs over large areas during mountain building at convergent plate boundaries.
• Contact Metamorphism: Localized alteration near igneous intrusions (aureoles).
• Burial Metamorphism: Due to deep burial of sediments in basins.
• Dynamic Metamorphism: Shearing at faults creates mylonites.
• Hydrothermal Metamorphism: Hot, chemically active fluids alter rock (often at mid-ocean ridges).
• Shock Metamorphism: Meteorite impacts generate high-pressure features.

Exhumation and the Rock Cycle

Exhumation brings deeply buried metamorphic rocks back to the surface through uplift, erosion, and extension. These rocks are often exposed in shields and ancient continental cores. The rock cycle continues as these materials weather, erode, and start the cycle anew.

Conclusion: The Importance of Metamorphism

Metamorphism reveals how Earth’s interior processes transform rocks and recycle materials through time. Understanding foliated and nonfoliated rocks, index minerals, and tectonic settings helps us interpret the dynamic nature of our planet.

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