Metamorphic Rocks Overview

Metamorphic Rocks

• Metamorphic rocks are rocks that have changed due to heat, pressure, or chemical reactions in fluids.
• Any type of rock (igneous, sedimentary, or metamorphic) can be transformed into a metamorphic rock.
• The changes occur without melting the rock.
• Increased pressure, temperature, and chemical reactions in fluids cause new mineral and texture formations.
• Metamorphic rocks are classified based on texture and composition into foliated and non-foliated types.

Metamorphism

• Metamorphism is the transformation of existing rock to a rock with a different mineral composition or texture.
• Metamorphism takes place at temperatures exceeding 150 to 200 °C (300 to 400 °F), often with elevated pressure or chemically active fluids.
• The rock remains mostly solid during transformation.

Types of Metamorphism

• Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism.
• These differ in their characteristic temperatures, pressures, and the rate at which they occur, along with the involvement of reactive fluids.
• Prograde metamorphism occurs when pressure and temperature increase.
• Retrograde metamorphism occurs when pressure and temperature decrease.

Textural Changes

• As a rock undergoes metamorphism, some mineral grains grow larger while others shrink.
• The shapes of the grains can also change.
• Limestone is transformed into marble during metamorphism.
• Marble has large interlocking grains with no fossils.

Minerals in Metamorphic Rocks

• Key minerals found in metamorphic rocks include andalusite, garnet, sillimanite, kyanite, and staurolite.
• These are examples of minerals characteristic of metamorphic rocks.

Controlling Factors in Metamorphic Rocks

• Parent rock composition is a key factor.
• No new material is added during metamorphism.
• The resulting metamorphic rock has a composition similar to the original rock.
• If the parent material contains only one mineral, the metamorphic rock will have only that mineral.
• Minerals will often be recrystallized leading to alterations in the texture.

Texture Changes Examples

• Limestone has fragments of shells, while marble has interconnecting grains and no fossils.

Parent Material with Many Minerals

• If the parent material contains multiple minerals, the old minerals will combine to create new metamorphic minerals, like garnet or others.

Temperature during Metamorphism

• Heat originates from the Earth's interior.
• Minerals are stable within a temperature range.
• Temperatures exceeding this stable range can cause melting.

Pressure during Metamorphism

• Pressure in the Earth is equal in all directions throughout the depth of the Earth.
• Pressure increases by approximately 1 kilobar (kbar) per 3.3 kilometers of depth
• Increased pressure causes grains to pack together, decreasing the overall volume and producing more compact mineral grains.

Differential Stress during Metamorphism

• Tectonic forces create forces that are unequal in all directions—differential stress
• Compressive stress (pushing together) causes flattening that is perpendicular to stress.
• Shearing, (one surface sliding past another) causes flattening that is parallel to the stress direction

Fluids during Metamorphism

• Hot water (or water vapor) is the most significant fluid component involved in altering existing minerals.
• Heat causes unstable minerals to release water.
• Water reacts with surrounding rocks and transports dissolved minerals, altering the chemical composition over extensive periods of time.

Time during Metamorphism

• Metamorphism can take millions of years.
• Longer times allow minerals to grow larger, creating coarser-grained rocks.

Classification of Metamorphic Rocks

• Metamorphic rocks are categorized based on their texture.
• Foliated rocks display layered textures (e.g., slate, shale, gneiss).
• Non-foliated rocks have non-layered textures (e.g., marble, quartzite, hornfels).

Foliated Metamorphic Rocks Examples

• Examples include slate, shale, phyllite, schist, gneiss, and migmatite.
• Foliated rocks are formed through differential stress.

Non-Foliated Metamorphic Rocks Examples

• Examples include marble, quartzite, and hornfels.

Types of Contact Metamorphism

• Contact metamorphism occurs near magma bodies (intrusion) resulting in a zone of contact known as an aureole.
• Fine-grained rocks (e.g., hornfels) or coarse-grained rocks (e.g., marble) are typical outcomes.

Types of Regional Metamorphism

• Regional metamorphism occurs over extensive areas, especially in mountain ranges.
• Folded and deformed rocks yield foliated metamorphic types. Pressure and temperature conditions in these regions promote the creation of new and altered minerals.

Partial Melting in Metamorphism

• Migmatites are a product of partial melting during metamorphism.
• These high-grade metamorphic rocks exhibit both intrusive and metamorphic textures.

Shock Metamorphism

• Shock metamorphism is associated with impact events and high pressures.
• Rocks around impact craters exhibit "shocked" features.

Hydrothermal Metamorphism

• Hydrothermal metamorphism occurs when cold seawater interacts with hot basalt from intrusions.
• This creates alterations to the chemical and mineral composition of the basalt in altered zones.

Plate Tectonics and Metamorphism

• Regional metamorphism is associated with convergent plate boundaries.
• This process involves increasing pressure and temperature with depth
• These variations result in different metamorphic conditions that lead to different degrees or grades of metamorphism.