CE11 - Lecture 5 - Metamorphism and Metamorphic Rocks

Metamorphism

The process that alters a rock’s minerals, texture, and structure due to heat, pressure, and fluids

Metamorphic Rocks

Rocks formed through metamorphism of pre-existing igneous or sedimentary rocks

Parent Rock

The original rock from which a metamorphic rock forms (e.g., mudstone → slate → schist)

Solid-State Reaction

Mineral changes during metamorphism occur mostly in solid form, not molten

Closed-System Metamorphism

Rock is altered using only its original chemical components

Open-System Metamorphism

Magmatic fluids add new elements during metamorphism

Regional Metamorphism

Large-scale metamorphism caused by deep burial and tectonic pressures over large areas

Contact Metamorphism

Local metamorphism caused by heat from nearby magma intrusions

Temperature (in Metamorphism)

Influences which minerals are stable and thus which form during metamorphism

Pressure (in Metamorphism)

Affects rock texture and can lead to foliation if directed

Equal Confining Pressure

Pressure applied uniformly in all directions

Directed Pressure

Greater pressure from specific directions, causing deformation and foliation

Shear Stress

Type of pressure where opposing forces slide past one another, affecting rock fabric

Polymorphs

Minerals with the same chemical formula but different crystal structures depending on pressure and temperature

Fluid Availability

Fluids like water aid ion transport and speed up metamorphic reactions

Time (in Metamorphism)

Metamorphic changes occur over millions of years, allowing crystal growth and mineral changes

Foliation

Layering or banding in metamorphic rocks caused by directed pressure or shear stress

Non-Foliated Rocks

Metamorphic rocks formed under low or equal pressure, showing no visible layering

Slaty Texture

Fine-grained foliation, rock splits into flat sheets (e.g., slate)

Phyllitic Texture

Slightly coarser than slaty, with silky sheen due to aligned mica crystals (e.g., phyllite)

Schistose Texture

Medium to coarse-grained, visible aligned mica and other minerals (e.g., schist)

Gneissic Texture

Banded foliation with alternating light and dark mineral bands (e.g., gneiss)

Slate

Low-grade foliated metamorphic rock from shale, splits along slaty cleavage

Phyllite

Foliated rock with a wavy, silky appearance, higher grade than slate

Schist

Coarse-grained foliated rock with visible mica, quartz, and feldspar

Gneiss

High-grade metamorphic rock with banded appearance, forms from further metamorphosed schist or granite

Quartzite

Non-foliated rock formed from sandstone, hard and durable

Marble

Non-foliated rock formed from limestone, softer and used as building material

Amphibolite

Metamorphic rock dominated by amphibole minerals, can show gneissic texture

Anisotropy

Variation in rock strength or properties depending on direction, common in foliated rocks

Engineering Consideration (Foliation)

Weakness planes from foliation can affect stability in excavations and slopes

Creep and Slides

Movement along foliation planes in metamorphic rocks

Building Uses of Metamorphic Rocks

Quartzite is used as railroad ballast, marble is used in construction and decoration

Fresh Metamorphic Rock

Typically strong and sound for engineering use

Weathered Metamorphic Rock

May show reduced strength, especially along foliation planes