Metamorphism: Definition and Agents

Definition

Metamorphism: The process of changing a pre-existing rock (the protolith) into a new rock (a metamorphic rock) in the solid state, due to changes in Temperature (T), Pressure (P), or chemically active fluids.

This is not melting. If the rock melts, it becomes igneous. Metamorphism happens *before* melting.

Agents of Metamorphism

1. Heat (Temperature): The most important agent. Heat provides the energy for chemical reactions to occur, causing minerals to recrystallize or form new minerals. Sources: Geothermal gradient, magma intrusions.
2. Pressure: Pressure has two types:
   • Confining (Lithostatic) Pressure: Equal pressure in all directions, like air pressure. Caused by burial. It compacts rock and forms denser minerals.
   • Directed (Differential) Stress: Pressure that is stronger in one direction than another. Caused by tectonic plate collisions. This is the agent that causes foliation (alignment of minerals).
3. Chemically Active Fluids: (e.g., water, CO₂). These hot, high-pressure fluids flow through rock, transporting ions and speeding up chemical reactions. If they change the rock's bulk chemical composition, the process is called Metasomatism.

Factors Controlling Metamorphism

The type of metamorphic rock that forms is controlled by several factors:

1. Protolith Composition: This is the most important factor. You cannot form a marble (CaCO₃) from a sandstone (SiO₂). The original rock determines the "ingredients" available.
   • Shale (pelitic) → Slate, Phyllite, Schist, Gneiss
   • Sandstone (psammitic) → Quartzite
   • Limestone (calcareous) → Marble
   • Basalt (mafic) → Greenschist, Amphibolite
2. Temperature and Pressure (Metamorphic Grade): The T and P conditions determine which *new* minerals are stable.
   • Low Grade: Low T, Low P (e.g., Slate, Greenschist)
   • High Grade: High T, High P (e.g., Gneiss, Amphibolite)
3. Fluids: The presence or absence of water can determine which reactions happen.
4. Time: Metamorphic reactions are slow and require millions of years to complete.

Types of Metamorphism

Contact Metamorphism

• Cause: High Temperature, Low Pressure.
• Setting: Occurs when magma intrudes into cool "country rock." The magma "bakes" the rock immediately surrounding it.
• Key Feature: Creates a "metamorphic aureole" or "baked zone." Directed stress is low, so rocks are non-foliated.
• Typical Rock: Hornfels (a hard, dense, fine-grained non-foliated rock).

Regional Metamorphism

• Cause: High Temperature AND High Pressure (especially directed stress).
• Setting: Occurs over very large areas (thousands of km²) during mountain-building (orogeny), where plates collide and rocks are deeply buried and squeezed.
• Key Feature: Produces strongly foliated rocks.
• Typical Rocks (in order of increasing grade): Slate → Phyllite → Schist → Gneiss.

Fault Zone Metamorphism (Dynamic)

• Cause: High directed stress (shear stress) along fault zones.
• Setting: Occurs in a narrow band along a fault.
  • Near surface (brittle): Rock is crushed → Cataclasite or Breccia.
  • Deeper (ductile): Rock is sheared → Mylonite (a fine-grained, foliated rock).

Impact Metamorphism (Shock)

• Cause: Instantaneous, ultra-high Pressure and Temperature from a meteorite impact.
• Setting: Occurs at impact craters.
• Key Feature: Produces "shocked" minerals (e.g., shocked quartz) and shatter cones that are unique to this environment.

Structures and Textures of Metamorphic Rocks

Texture refers to the size, shape, and arrangement of mineral grains. Structure refers to large-scale, field-level features.

Metamorphic Textures

Textures are broadly grouped into Foliated (layered) and Non-Foliated (massive).

Foliated Textures

A planar arrangement of mineral grains or structural features in a rock, caused by directed stress. This is a progressive texture that develops with increasing metamorphic grade.

• Slaty Cleavage: Very fine-grained alignment of clay/mica minerals. Rock breaks into perfect, flat sheets. (Rock: Slate).
• Phyllitic Texture: Fine-grained micas give the rock a satiny lustre or sheen. The cleavage is often wavy. (Rock: Phyllite).
• Schistosity: Medium-to-coarse grained. Platy minerals (micas, chlorite) are visible to the naked eye and are all aligned. (Rock: Schist).
• Gneissic Banding: Coarse-grained. Minerals have segregated into alternating light-colored (felsic) and dark-colored (mafic) bands. (Rock: Gneiss).

Non-Foliated Textures

Rocks with no preferred mineral alignment. Forms when directed stress is absent (contact metamorphism) OR when the protolith has no platy minerals (e.g., pure quartz or calcite).

• Granoblastic: Grains are interlocking, roughly equal-sized, and randomly oriented.
  • Marble: Protolith = Limestone (CaCO₃)
  • Quartzite: Protolith = Sandstone (SiO₂)
• Hornfelsic: Fine-grained, hard, dense. (Rock: Hornfels).

Other Textures

• Porphyroblastic: A texture where large metamorphic crystals (porphyroblasts) have grown within a finer-grained matrix. (e.g., Garnet-Mica Schist, Staurolite Schist).

Metamorphic Structures

These are large-scale features, often related to deformation.

• Folds: Bends and warps in the rock layers (foliation) caused by ductile compression.
• Lineation: A fabric element where linear minerals (like amphibole) or stretched features are aligned.
• Boudinage: A structure where a competent (hard) layer is stretched and broken into sausage-shaped blocks ("boudins") within a less competent (softer) rock.