Chapter 8: Metamorphism and Metamorphic Rocks
- Definition of Metamorphism: Metamorphism comes from the Greek words meta, meaning "change," and morpho, meaning "shape." It refers to the process whereby rocks are subjected to sufficient heat, pressure, and fluid reactivity to change their mineral composition, texture, or both, forming new rocks that generally do not resemble the original rock.
- Metamorphic Rocks: The rocks formed from pre-existing or parent rocks via metamorphism are called metamorphic rocks.
- Three Principal Agents of Metamorphism:
- Heat
- Pressure
- Fluid Activity
- The texture and mineral content of metamorphic rocks depend on several factors:
- Parent Rock Composition: Typically, no new material is added to the rock during metamorphism; thus, the resulting metamorphic rock usually has a similar composition to the parent rock.
- Temperature and Pressure During Metamorphism: These variables significantly impact the formation of metamorphic rocks.
- Effects of Tectonic Forces: The physical forces acting on the rocks can influence their structure.
- Effects of Fluids, Such as Water: The presence of fluids is essential to many metamorphic processes.
- Role of Heat: Heat is crucial because it increases the rate of chemical reactions, potentially producing minerals that differ from those in the original rock.
- Temperature Stability: All minerals have stable types over finite temperature ranges; should the temperature exceed this range, new minerals will form.
- Melting Point: If temperatures reach high levels, melting of the rock may occur.
- Lithostatic Pressure: The stress exerted on a rock in Earth's crust is uniform in all directions; this is also called confining pressure.
- Example: Being submerged in water experiences equal pressure from all sides.
- Pressure Depth Relation: Pressure within the Earth is directly proportional to depth.
- High-Pressure Minerals: At high pressures, minerals become more compact and dense.
- Differential Pressure: In addition to lithostatic pressure from burial, rocks may experience differential pressure from uneven stress.
- Compressive Stress: This type of stress causes flattening perpendicular to the applied force.
- Shearing: This involves sliding parallel to the stress applied, leading to flattening.
- Foliation: This term refers to the planar rock texture of aligned minerals created by differential stress.
- Formation: Foliation is formed as a result of differential stress acting on the rocks.
- Role of Fluids: Fluids enhance metamorphism by accelerating the pace of chemical reactions. The essential chemically active fluids arise from three primary sources:
- Water trapped in the pore spaces of sedimentary rocks during their formation.
- Volatile fluids derived from magma.
- Dehydration of water-bearing minerals, such as gypsum and some clays.
- Time Factor: Metamorphism, especially under high pressures, can take millions of years. Longer durations enable the growth of larger, newly stable minerals and increased foliation.
- Example: Seawater entering hot basaltic rock in the oceanic crust can transform olivine into the metamorphic mineral serpentine.
- Geologists identify three main types of metamorphism:
- Contact (Thermal) Metamorphism
- Dynamic Metamorphism
- Regional Metamorphism
- Definition: This type occurs adjacent to intrusive igneous rocks, where temperature rises due to hot magma intruding into pre-existing rock.
- Characteristics:
- The primary factor is temperature, leading to the formation of non-foliated rocks.
- Rocks in this category do not exhibit foliation, as they are not affected by pressure changes or differential stress.
- Typically occurs in a narrow zone, referred to as the contact aureole, about 1-100 m wide.
- Examples include fine-grained rocks, such as hornfels, and coarse-grained rocks like marble and quartzite.
Hydrothermal Alteration Processes
- Hydrothermal Processes: These involve rocks being altered by, or precipitated from, hot water, common at divergent plate boundaries.
- Metasomatism: This is the change in the composition of a rock due to the introduction or removal of chemical constituents.
- Ore Deposits Formation: Water passing through rocks can precipitate new minerals in the cracks and pore spaces, often forming metallic ore deposits, such as copper, gold, iron, tin, and zinc.
- Definition: Associated with faults where rocks undergo high levels of differential stress, resulting in a specific type of metamorphic rock known as mylonites.
- Characteristics: Mylonites are tough, dense, fine-grained rocks identified by their laminated appearance.
- Description: Most metamorphic rocks result from regional metamorphism, occurring extensively across large areas deep within the Earth's crust.
- Dominant Factor: High pressure is the primary cause, typically yielding foliated textures in the resulting rocks.
- Common Locations: This type of metamorphism is prevalent in severely deformed mountain ranges and can occur over a wide range of temperatures.
- Definition: Produced by the rapid application of extreme pressure, often as a result of meteor impacts.
- Characteristics: Shocked rocks are found surrounding and beneath impact craters due to the intense forces experienced during such events.
- Metamorphic Grade: This term describes the degree to which a rock has undergone metamorphism, characterized by the stability range of certain minerals, known as index minerals.
- The boundaries defining grades are not strictly defined but allow for a general understanding of metamorphic intensity.
- Low-Grade Metamorphism: Involves lower temperatures and pressures.
- Example: The transformation of shale, a common sedimentary rock, into slate.
- High-Grade Metamorphism: Involves high temperatures, strong compressional forces, and the transformation of more significant structures.
- Example: The metamorphosis of granite into gneiss due to intense conditions.
Index Minerals and Their Significance
- Formation of Chlorite: In clay-rich rocks such as shale undergoing regional metamorphism, chlorite starts to crystallize at relatively low temperatures (~200°C), indicating low-grade metamorphism.
- Continued Transformation: As temperature and pressure continue to rise, chlorite is replaced by minerals more stable under these conditions, which help in determining the metamorphic grade.
- Nonfoliated metamorphic rocks are classified based on composition:
- Marble: A coarse-grained rock composed of interlocking calcite crystals.
- Quartzite: Formed from the welding together of grains from quartz sandstone.
- Greenstone: A compact, dark-green altered mafic igneous rock.
- Hornfels: A fine-grained rock often comprising microscopically visible micas originating from clay particles in shale.
- Anthracite: A shiny, hard form of coal, containing a high percentage of fixed carbon and a low percentage of volatile matter, valued for heating and power.
- Foliated rocks are typically categorized based on the type of foliation:
- Slate: A very fine-grained metamorphic rock displaying slaty cleavage, resulting from the regional metamorphism of shale. Variances in color arise from inclusions of graphite (black), iron oxide (red/purple), and chlorite (green).
- Phyllite: Contains a similar composition to slate but is coarser and noted for a glossy sheen, distinguishing it from slate.
- Schist: Often forms from clay-rich sedimentary rocks and exhibits schistose foliation, leading to a wavy parting structure when split.
- Gneiss: A high-grade metamorphic rock characterized by distinct alternating bands of light and dark minerals; it most often results from the recrystallization of clay-rich rocks and sometimes from granite.
- Amphibolite: A dark rock predominantly comprising hornblende and plagioclase formed through intermediate to high-grade metamorphism of basalt or mafic rocks.
- Migmatite: Known as “mixed rocks,” these display both igneous and high-grade metamorphic characteristics due to exposure to extreme temperatures.
- Metamorphic rocks are organized into different zones based on distinctive silicate mineral assemblages recognized by index minerals, which indicate degrees of metamorphism.
- Metamorphic Facies: Defined as a group of metamorphic rocks showcasing particular mineral assemblages formed under comparable temperature and pressure conditions.
- Example: The green mineral chlorite, which develops under relatively low temperatures and pressures, belongs to the greenschist facies.
- Isotherms: These lines on maps connect points with equivalent temperatures at a specific time or averaged over a designated period, bowing down where oceanic plates sink and bowing up where magma rises.
- Relationship with Facies: At oceanic-continental convergent plate boundaries, blueschist facies are encountered where there are low temperatures and high pressures. The presence of blueschist facies rocks provides evidence of ancient subduction zones.
- Metamorphic Facies Assignment:
- Examine diagrams matching each facies with associated temperature/pressure ranges and interpret how those facies correspond to diagrams of an oceanic-continental convergent plate boundary.
- Rock Cycle Diagram Assignment:
- Focuses on visualizing, labeling, and explaining the major components and processes of the rock cycle, illustrating the interconnections between various rock types through Earth processes. Create a diagram in Canva, ensuring all requirements are met, and answer questions elaborating on the rock cycle in detail.