Metamorphism and Igneous Rocks: Minerals, Processes & Examples

Metamorphism: Definition and Agents

Metamorphism: Metamorphism is the set of physical and chemical processes that modify rocks into a changed form.

Agents of metamorphism: Temperature, pressure, and the addition or removal of substances (fluid activity) from the original rock.

Types of Metamorphism

• Thermal metamorphism: Metamorphism in which the dominant agent is almost exclusively temperature.
• Dynamic metamorphism: Metamorphism in which the dominant agent is mechanical pressure or stress (often associated with faulting and deformation).
• Metasomatism: This occurs when the original rock's chemical composition is altered by the gain or loss of chemical components carried by fluids.
• Regional (general) metamorphism: Metamorphism that acts over large areas, where the three agents (temperature, pressure, and fluid activity) commonly act together.

Metamorphic Minerals

Some minerals are common to igneous, sedimentary, or metamorphic rocks; others are characteristic and form only by metamorphic action.

• Andalusite group: These minerals are aluminum silicates (e.g., andalusite, kyanite, sillimanite).
• Garnets: Composed of different metal cations; garnets are complex silicates commonly formed during metamorphism.
• Chlorite, serpentine, and talc: Hydrated magnesium silicates typical of low- to medium-grade metamorphism.
• Graphite: A metamorphic mineral that is essentially pure carbon.

Metamorphic Rocks

Metamorphic rocks are also called stratocrystalline rocks; they often retain the layered arrangement of the original sedimentary rock. Some well-known metamorphic rocks include:

• Marble: Constituted by interlocking crystals of the mineral calcite.
• Quartzite: Composed almost exclusively of quartz grains.
• Slate: Originates from low-grade metamorphism of mud and silt.
• Micaceous schists (micacitas): Originate from relatively intense metamorphism of certain clay-rich sediments and contain abundant mica.
• Gneiss: Consists of quartz, feldspar, and mica organized in banded structures.

Igneous Minerals and Magmatic Rocks

Igneous minerals and rocks: Result from the solidification of magma (molten rock).

Magmatic minerals: They can be classified as silicates, oxides, phosphates, and halides (fluorides).

Magmatic rock types: Magmatic (igneous) rocks are commonly divided into plutonic rocks, hypabyssal (filonianas) rocks, and volcanic (effusive) rocks; they are formed mainly from silicate minerals.

• Plutonic rocks: Solidified at depth in the interior of the Earth after slow cooling, producing coarse-grained textures.
• Hypabyssal (filonianas) rocks: Formed by the solidification of magma within dikes, sills, or fractures near the surface.
• Effusive/volcanic rocks: Formed by rapid solidification when magma reaches the surface during volcanic eruptions.

Volcanic Features and Examples

Classic volcanic deposit — lava: Lava is the molten rock that flows onto the surface and can form a thin mantle covering the volcanic edifice.

Lava cones: When lavas are viscous, they tend to accumulate around the crater and build cones.

Examples of volcanic rocks: Volcanic glass (obsidian), pumice, and other products of rapid cooling and gas-rich eruptions.