Metamorphic Rocks

Metamorphic rocks form when existing rocks are transformed in the solid state by heat, pressure, and chemically active fluids, recording the conditions deep within mountain belts and the crust.

Definition

A metamorphic rock is a rock produced by the solid-state transformation of a pre-existing igneous, sedimentary, or metamorphic rock in response to changes in temperature, pressure, or fluid composition, without complete melting.

Scope

This topic introduces metamorphic rocks from a general-geology standpoint: the agents and types of metamorphism, the contrast between foliated and non-foliated rocks, and the concept of metamorphic grade as a record of changing temperature and pressure. Detailed phase equilibria and metamorphic petrology are treated in mineralogy-and-petrology.

Core questions

• What agents and settings produce metamorphism?
• Why are some metamorphic rocks foliated and others not?
• How do mineral assemblages record metamorphic grade?

Key theories

Metamorphic grade and index minerals

Barrow recognized that distinctive index minerals appear in a predictable order with increasing metamorphic intensity, allowing the mapping of metamorphic zones that record progressively higher temperature and pressure.

Types of metamorphism

Metamorphism is driven by heat (contact metamorphism near intrusions), by combined heat and pressure over wide regions (regional metamorphism in mountain belts), and by directed stress and fluids, producing characteristic textures including foliation.

Mechanisms

When rock is subjected to elevated temperature, pressure, or reactive fluids, its minerals recrystallize and react to form new, more stable minerals while remaining solid. Directed stress aligns platy and elongate minerals into foliation, while rocks lacking such minerals recrystallize without foliation. The resulting mineral assemblage records the peak temperature and pressure, expressed as metamorphic grade or facies.

Clinical relevance

Metamorphic rocks supply valuable building and ornamental stone such as marble and slate, host graphite, talc, and some metallic deposits, and their mineral assemblages let geologists reconstruct the depth, temperature, and tectonic history of ancient mountain belts.

History

George Barrow mapped metamorphic zones in the Scottish Highlands in the 1890s, establishing the concept of progressive metamorphism with index minerals. Pentti Eskola introduced the metamorphic-facies concept in the early twentieth century, providing the framework that links mineral assemblages to temperature and pressure.

Key figures

• George Barrow
• Pentti Eskola

Seminal works

• barrow1893

Frequently asked questions

What is the difference between foliated and non-foliated metamorphic rocks?

Foliated rocks, such as slate and schist, have a layered or banded texture caused by minerals aligning under directed pressure, while non-foliated rocks, such as marble and quartzite, lack such alignment because they recrystallized from minerals that do not form platy or elongate crystals.