Metamorphic Rocks in the Field provides an illustrative introduction to metamorphic rocks as observed in their natural settings. This guide is tailored for earth science and geology students, from advanced high school to graduate levels, aiming to rapidly develop their observational skills. Beyond field photographs, the book features numerous diagrams and examples of metamorphic features showcased in thin sections. The thin section photos are scaled and contextualized to relate directly to field observations achievable with a hand lens.
Learning about metamorphic rocks presents unique challenges compared to igneous and sedimentary rocks. Metamorphic rocks originate from pre-existing igneous and sedimentary rocks, inheriting their complex chemical, mineralogical, and structural attributes. Metamorphism then introduces new characteristics, often altering rock shape, mineralogy, grain size, and chemical composition. These rocks contain clues about metamorphic processes and the original igneous and sedimentary materials. Mastering the interpretation of metamorphic rock records requires extensive practice. Metamorphic Rocks in the Field aims to accelerate this learning process.
Understanding Metamorphism: Key Concepts
The Basics: Metamorphism involves the transformation of pre-existing rocks (protoliths) under elevated temperature, pressure, and chemically active fluids. This process results in new mineral assemblages and textures without melting the rock entirely.
Temperature, Pressure, and Metamorphic Grade: Metamorphic grade refers to the intensity of metamorphism. Higher temperatures and pressures generally indicate a higher metamorphic grade. Different minerals are stable under different temperature and pressure conditions, allowing geologists to infer the metamorphic conditions from the mineral assemblage present.
Movement of Rocks Through P-T Space: Rocks can experience changes in pressure (P) and temperature (T) over time as they are buried, uplifted, or subjected to tectonic forces. This movement through “P-T space” dictates the metamorphic path a rock undergoes and the resulting mineralogical and textural changes.
Preserving the Prograde Assemblage: The prograde path refers to the increase in temperature and pressure during metamorphism. Sometimes, the mineral assemblage formed during the peak of metamorphism (the prograde assemblage) is preserved, providing valuable information about the highest temperatures and pressures experienced by the rock.
Metamorphic Field Gradient: A metamorphic field gradient describes the change in metamorphic grade over a geographic area. This can be observed by mapping the distribution of different metamorphic minerals or assemblages.
Rock Strain: Foliation, Cleavage, and Lineation: Directed pressure during metamorphism can cause minerals to align, resulting in foliation (planar alignment of minerals), cleavage (tendency to break along parallel planes), and lineation (linear alignment of minerals). These features provide information about the stress field during metamorphism.
Chemical Flux: Fluids play a significant role in metamorphism, acting as a medium for chemical transport. The introduction or removal of chemical components by fluids can significantly alter the rock’s composition and mineralogy.
Recrystallization: Metamorphism often involves recrystallization, where existing minerals dissolve and re-precipitate as new, more stable minerals under the new temperature and pressure conditions.
Metamorphic Rock Types: A Visual Exploration
The following sections offer a pictorial guide to identifying common metamorphic rock types in the field.
Pelitic Rocks: These rocks originate from clay-rich sedimentary rocks like shale. Common metamorphic products include slate, phyllite, schist, and gneiss, characterized by increasing metamorphic grade and coarser grain size. They are typically rich in mica minerals.
Quartzites: Formed from quartz-rich sandstones, quartzites are hard, durable rocks with a massive texture. Metamorphism causes the quartz grains to interlock, increasing the rock’s strength.
Marbles: Derived from limestones or dolomites, marbles are composed primarily of calcite or dolomite. They often exhibit a sugary texture and can display a variety of colors due to impurities.
Calc-silicate Rocks: These rocks form from impure limestones or dolomites that contain silica and other elements. They are characterized by the presence of calcium-silicate minerals like wollastonite, diopside, and grossular garnet.
Mixed Sedimentary Rocks: Metamorphism of mixed sedimentary rocks results in a diverse range of compositions and textures, reflecting the complexity of the original sediment.
Conglomerates: Metamorphosed conglomerates retain evidence of their original rounded clasts, which may be stretched or deformed by the metamorphic process.
Gneisses: Gneisses are high-grade metamorphic rocks characterized by distinct banding of light and dark minerals. The banding is a result of mineral segregation during metamorphism.
Basaltic Rocks (Low and Intermediate Pressure): Metamorphism of basaltic rocks under low to intermediate pressure conditions results in rocks like greenschist and amphibolite, characterized by the presence of green minerals like chlorite and amphibole.
Blueschists: Formed under high-pressure, low-temperature conditions, blueschists contain blue amphibole minerals like glaucophane, giving them a distinctive blue color.
Eclogite: Eclogite is a high-pressure metamorphic rock composed primarily of garnet and omphacite (a green pyroxene). It is typically formed at depths greater than 45 km.
Ultramafic Rocks: Metamorphism of ultramafic rocks (rocks rich in magnesium and iron) produces rocks like serpentinite, which is often associated with altered oceanic crust.
Contact Metamorphic Rocks: Contact metamorphism occurs when magma intrudes into surrounding rocks. The heat from the magma causes localized metamorphism in the adjacent rocks, forming a metamorphic aureole.
Fault Rocks: Fault rocks are formed by the intense deformation and frictional heating that occur along fault zones. Examples include mylonite and cataclasite.
Metamorphic Features: A Closer Look
Foliation: The parallel alignment of platy minerals like mica, creating a layered or banded appearance.
Folds: Bending or warping of rock layers due to ductile deformation during metamorphism.
Porphyroblasts: Large, conspicuous crystals that grow within a finer-grained matrix during metamorphism. Garnets are a common example.
Boudins: Elongated, sausage-shaped segments of a competent layer (e.g., a vein) that have been stretched and broken during deformation.
Veins: Fractures in rocks filled with minerals that have precipitated from hydrothermal fluids.
Metasomatism: Chemical alteration of a rock by the introduction or removal of chemical constituents via fluids.
Partial Melting: The process where some minerals in a rock melt while others remain solid. This can lead to the formation of migmatites, which are rocks that exhibit both igneous and metamorphic characteristics.
Retrograde Metamorphism: The process where a rock that has already been metamorphosed is subjected to lower temperatures and pressures, resulting in the formation of new minerals that are stable under the new conditions.
Relict Pre-Metamorphic Features
Relict Sedimentary Features: Original sedimentary structures, such as bedding or cross-bedding, that are partially preserved in the metamorphic rock.
Fossils in Metamorphic Rocks: Although rare, fossils can sometimes be found in metamorphic rocks, providing evidence of the rock’s sedimentary origin.
Relict Igneous Features: Original igneous textures, such as phenocrysts or vesicles, that are partially preserved in the metamorphic rock.
By understanding these key concepts and utilizing this pictorial guide, earth science and geology students can develop a solid foundation for identifying and interpreting metamorphic rocks in the field. This knowledge is crucial for unraveling the complex geological history of our planet.
