Published on January 14, 2008
MetamorphismandMetamorphic Rocks: Metamorphism and Metamorphic Rocks Metamorphic Rocks: Metamorphic Rocks Metamorphism means to “Change form” Metamorphism takes place where/when: preexisting rock is subjected to temperatures and pressures unlike those in which it formed. Mineral composition or texture of the rock is changed by heat or pressure Metamorphism: Metamorphism Metamorphism ranges: from slight changes (low – grade metamorphism) to substantial changes (high-grade metamorphism) Grade metamorphism: Grade metamorphism Low-grade metamorphism – difficult to distinguish sedimentary rock becomes a metamorphic rock is somewhat arbitrary. i.e. Shale becomes more compact under low grade metamorphism to become slate. Grade metamorphism: Grade metamorphism High-grade metamorphism a transformation can be so complete that the parent rock cannot be determined. Features like bedding planes, and fossils in the parent rock are obliterated. At high temps, the ductile rock can fold into intricate folds (but still a solid!) Two Types of Metamorphism and Their Settings: Two Types of Metamorphism and Their Settings Contact or Thermal metamorphism rock is intruded by a magma body. Rise in temperature results in a change in the rock. The rock is “baked”. Regional metamorphism during mountain building, great quantities of rock are subjected to directed pressures and high temperatures meta aureole: meta aureole Regional Metamorphism: Regional Metamorphism Many mountain belts have significant amounts of metamorphic rocks at their core Even stable continental interiors are underlain by metamorphic basement rocks Slide11: Metamorphism cont, and Sedimentary Rocks HW-2 assigned – Due Oct 23rd in class Wednesday 11th Complete Sedimentary Review Monday 16th – Film on Hazards of volcanoes – included for Exam-2 Wednesday 18th – Exam 2 Metamorphism Changes Rocks in Two Ways: Metamorphism Changes Rocks in Two Ways Composition new minerals stable at higher temperatures or pressures form (i.e. clays are changed to micas, Chlorite, and Garnet at temps between 150°C & 200°C) Texture (term used to describe the size, shape, and arrangement of grains within a rock) Foliation – a texture that gives a rock a layered/banded appearance. Caused by pressure (stress) on a rock. Pressure (Stress): Pressure (Stress) Confining pressure – As dept increases – pressure increases Confining pressure is similar to water pressure, forces are equal in all directions. Causes spaces between mineral grains to close, producing a more compact rock with a greater density. Can cause minerals to recrystallize into new minerals that display a more compact crystalline form. Pressure (Stress): Pressure (Stress) Differential stress – forces are unequal in different directions. Mainly acts along one plane. Shortened in the direction that pressure is applied; and elongated in the direction perpendicular to the force. At low temperatures rocks are brittle and tend to fracture At high temperatures rocks are ductile. Mineral grains tend to flatten and elongate when subjected to differential stress. Chemically active fluids: Chemically active fluids Hot fluids aid in the recrystallization of mineral grains by exchanging ions The rock is still solid, not a liquid Metamorphic Textures: Metamorphic Textures Foliated - crystals will recrystallize perpendicular to the direction of the compressional force. Resulting in a layered or banded appearance. Non-foliated – composed of only one mineral that forms equidimensional crystals Regional metamorphic rocks: Regional metamorphic rocks foliated Slate Phyllite Schist gneiss nonfoliated Quartzite Marble Slide20: Fig. 3-35, p.66 foliation: foliation Slate: Slate Slate: Slate Schist: Schist Gneiss: Gneiss Marble: Marble Sandstone: Sandstone Quartzite: Quartzite Schist: Schist Gneiss: Gneiss Shale: Shale Marble: Marble Metamorphic rocks in daily life: Metamorphic rocks in daily life Strong and weather-resistant - slate for roofing Valued for appearance - garnet (January's birthstone), verde antique, Carrara marble Used for buildings, sculpture, and common commodities - slate, jadeite, talc, asbestos Sites used for some valuable mineral deposits - hydrothermal metamorphism will concentrate precious metals such as gold, platinum, as well as others such as copper and lead. Potential hazards from metamorphic rocks - since some metamorphic rocks develop a strong foliation they may be a hazard, say for landslide potential, if the foliation is oriented in the wrong direction What to know…: What to know… What is metamorphism and what are metamorphic rocks What are the primary agents which change rock and how do they change it Major kinds of metamorphism and in what kinds of plate tectonic environments they are found Major classes of metamorphic rocks and rock textures, and examples of each Metamorphic grade and order of increasing degree of metamorphose Contact metamorphism: Contact metamorphism Happens when rock is intruded by a magma body Change is driven by a rise in temperature within the rock surrounding the molten igneous body i.e. shale to hornfels Foliated Metamorphic Rocks: Foliated Metamorphic Rocks platy minerals slate--slaty cleavage, mostly clays Phyllites -- phyllitic sheen, tiny mica crystals Schists --schistocity flakiness large mica crystals. Gneisses have bands of different minerals.