Metamorphic Rocks Overview

Jul 6, 2025

Overview

This lecture explains metamorphic rocks, their formation processes (regional and contact metamorphism), how to identify them, and their role in the rock cycle.

Formation of Metamorphic Rocks

  • Metamorphic rocks form when pre-existing rocks undergo intense heat and/or pressure.
  • The original rock that transforms is called the parent rock.
  • Metamorphism does not involve melting; if the rock melts, it becomes igneous.

Types of Metamorphism

  • Regional metamorphism involves high pressure, typically at plate boundaries, and produces foliated rocks.
  • Contact metamorphism involves high temperature (from nearby magma) and results in nonfoliated rocks.

Foliated vs. Nonfoliated Rocks

  • Foliated rocks show mineral grains aligned in bands or layers due to pressure (e.g., slate, schist, gneiss).
  • Nonfoliated rocks have randomly arranged grains, formed mainly by heat (e.g., marble, quartzite).

Examples of Parent Rocks and Metamorphic Products

  • Shale (sedimentary) under pressure → Slate (foliated)
  • Granite under pressure → Gneiss (foliated)
  • Basalt under pressure → Schist (foliated)
  • Limestone under heat → Marble (nonfoliated)
  • Sandstone under heat → Quartzite (nonfoliated)

Physical Properties and Uses

  • Marble (from limestone) reacts with acids and is softer, making it less durable for countertops.
  • Quartzite (from sandstone) is harder and more resistant to weathering and scratching, suitable for surfaces.

Metamorphic Continuum

  • Increasing heat and pressure causes rocks to progress through different metamorphic stages (e.g., shale → slate → phyllite → schist → gneiss).
  • Continuous cycling changes the rock until it potentially melts to become igneous.

Metamorphic Rocks in the Rock Cycle

  • Metamorphic rocks are part of the rock cycle, linking igneous, sedimentary, and tectonic processes.
  • Their presence can indicate past or current tectonic or volcanic activity.

Key Terms & Definitions

  • Metamorphism — Alteration of rocks by heat, pressure, or both.
  • Foliation — Alignment of minerals in parallel bands within a metamorphic rock.
  • Regional metamorphism — Metamorphism caused primarily by high pressure over large areas.
  • Contact metamorphism — Metamorphism caused by high temperature near igneous intrusions.
  • Parent rock — The original rock before undergoing metamorphism.

Action Items / Next Steps

  • Review the rock cycle and key features of metamorphic rocks.
  • Prepare for Module 4 on earthquakes.