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.