LESSON 4 METAMORPHISM

METAMORPHISM

Definition

• A geologic process in which rocks change in form, composition, and structure due to intense heat, pressure, and sometimes the introduction of chemically active fluids.

Conditions

• Takes place tens of kilometers below the Earth's surface where temperatures and pressures are high enough to metamorphose rocks without melting them.

Temperature's Role in Metamorphism

Effects of Heat

• Heat affects the rock’s chemical composition, mineralogy, and texture, leading to recrystallization.

• Increased temperature correlates with a higher grade of metamorphism until reaching the melting point, at which magma forms.

Geothermal Gradient

• Geothermal gradient varies by tectonic settings:

  • Subduction zones: low-temperature metamorphism.

  • Converging crustal plates: high-temperature metamorphism.

Pressure's Role in Metamorphism

Types of Pressure

• Pressure impacts the composition, mineralogy, and texture of rocks.

• High-pressure metamorphism: commonly found in subduction zones.

• Moderate-pressure metamorphism: occurs at collision zones between continental crusts.

Types of Stress

• Vertical Stress/Confining Pressure: stress exerted equally in all directions, can fracture or deform rocks.

• Directed/Differential Pressure: uneven pressure causes rocks to form folds and changes the shape and orientation of new crystals, contributing to foliation.

  • Foliation is a set of flat or wavy parallel cleavage planes created through deformation under directed pressures.

TYPES OF METAMORPHISM

Metasomatism

• Alteration process by hydrothermal fluids catalyzing chemical reactions, causing changes in mineral compositions, sometimes replacing minerals without affecting textures.

• Example: hydrothermal fluids in mid-ocean ridges lead to serpentinization where peridotites react with ocean water.

Regional Metamorphism

• High temperature and pressure result in foliated rocks such as Gneiss and Schist, particularly where continental plates collide.

• Foliation results from new alignment of minerals during recrystallization due to confining and directed pressures.

Contact Metamorphism

• Occurs when surrounding rocks are heated by magma intrusion.

• Results in formations like Marble and Quartzite with large crystals near igneous intrusions. Crystal size decreases with distance from the heat source.

Shock Metamorphism

• Takes place from the heat and shock waves of meteor or asteroid impacts, transforming nearby rocks.

• Example: graphite transforms into diamond, quartz into coesite under high shock pressures.

Burial Metamorphism

• Occurs below the continental crust at lower temperature and pressure, transforming sedimentary rocks into low-grade metamorphic rocks.

• Involves partial alteration of mineralogy and texture.

High-Pressure Metamorphism

• Found in subduction zones with conditions rarely exposed on Earth's surface.

• Rock example: eclogite forms under pressure exceeding 28 kbar at depths around 80 km.

CHECK-UP SEATWORK

• Types of metamorphism and changes in rocks based on geographic location.

Metamorphic Rock Changes

1. Alignment of minerals perpendicular to force. 2-20. Transition of minerals, change in mineralogy and texture, crystal size increase, etc.