Download
1 / 25
350 likes | 726 Vues
Metamorphism and Metamorphic Rocks. A. Metamorphic rocks: Form at conditions between igneous and sedimentary rocks. Metamorphism and Metamorphic Rocks. B. Metamorphism
E N D
Metamorphism and Metamorphic Rocks A. Metamorphic rocks: Form at conditions between igneous and sedimentary rocks
Metamorphism and Metamorphic Rocks B. Metamorphism Heat, pressure, and chemical reactions deep within the Earth alter the mineral content and/or structure of preexisting rock without melting it Any rock (igneous, sedimentary, metamorphic) can be metamorphosed Some atomic bonds are broken so atoms and ions can rearrange themselves into new minerals. If all the atomic bonds break, the rock would melt.
Metamorphism and Metamorphic Rocks B. Metamorphism Derived from older rocks through solid-state crystallization. Does not involve a significant melt phase. The lack of melting distinguishes igneous from metamorphic processes.
Metamorphism and Metamorphic Rocks • B. Metamorphism • Less stable minerals will become more stable minerals. • Clay transforms into mica • 2. Relatively stable minerals (e.g., quartz) will recrystallize or change structure. • Can’t witness metamorphism on the Earth’s surface. • Occurs under all the rocks we see at the surface, we see metamorphic rocks when the area is uplifted and eroded.
Metamorphism and Metamorphic Rocks I. Factors controlling metamorphism A. Heat: most important factor this drives chemical reactions Subject rock to a change in temperature 1. Bury Rocks Geothermal gradient = 75F/mile (average 25°C/km) So bury rocks deep enough (either by tectonic movement or deep burial under sediments) and they will metamorphose Deepest mine: 2.5 miles deep—rock is hot enough to burn human skin Deepest well: 7.7 miles, temperature is 473F (212F is boiling) 2. Burial and geothermal gradient
Metamorphism and Metamorphic Rocks I. Factors controlling metamorphism A. Heat: most important factor this drives chemical reactions Geothermal gradient = 75F/mile (average 25°C/km) What would be the temperature at 5 km depth within the interior of North America? Every point within the Earth we could measure or calculate a T value. If nothing perturbed our simple system, then we could easily predict how T changed with depth. However, it not that simple.
Metamorphism and Metamorphic Rocks I. Factors controlling metamorphism A. Heat: most important factor this drives chemical reactions Geothermal gradient = 75F/mile (average 25°C/km) Temperature gradient in volcanic arcs and mid-ocean ridges is significantly greater than in the interiors of continental areas. Volcanic regions - the geothermal gradient can reach 50°C/km or more. The geothermal gradient in regions oceanward of arcs is only about 10°C/km.
Metamorphism and Metamorphic Rocks I. Factors controlling metamorphism B. Pressure 1. Confining pressure (lithostatic pressure) Deep in the Earth Equal pressure from all directions Rock will be compressed into a smaller volume (no change in rock shape) Ions migrate within minerals from high pressure to low pressure regions
Metamorphism and Metamorphic Rocks • I. Factors controlling metamorphism • B. Pressure • Confining pressure(lithostatic pressure) • Pressures are given in kilobars (kbars) or MPascals (MPa) • For every ~3 km increase in depth pressures increases by ~1 kbar. • The continental interiors, the geobarometric (or geopressure) gradient is ~0.33 kbar/km. • What would be the pressure at 5 km depth?
Metamorphism and Metamorphic Rocks • I. Factors controlling metamorphism • B. Pressure • 1. Confining pressure(lithostatic pressure) • Deep in the Earth • Equal pressure from all directions • Rock will be compressed into a smaller volume (no change in rock shape) • Ions migrate within minerals from high pressure to low pressure
Metamorphism and Metamorphic Rocks • I. Factors controlling metamorphism • B. Pressure • 2. Directed pressure • Pressure is greater in one direction that in others • Occurs at plate boundaries • Changes the shape of a rock • Flattens a rock in the direction of greatest pressure • Minerals growing align perpendicular to the pressure foliation • May also stretch, bend, or fold existing features
I. Factors controlling metamorphism • C. Circulating Fluids • Predominantly water with dissolved ions • Increases potential for metamorphic reactions • Aids in migration of unbonded atoms and ions; ions move easily in fluids • Water sources: percolated from Earth’s surface trapped in subducting slab between sediment grains or cracks • Release from water rich minerals (amphiboles, clays) • Carry in new ions to the rock and carry away ions so they can change a rocks overall chemistry Metamorphism and Metamorphic Rocks
Metamorphism and Metamorphic Rocks I. Types of Metamorphism: heat, pressure, and fluids interact differently in different geological settings to produce different metamorphic rocks A.Contact Metamorphism Solid rock near magma may be baked by heat High temp, low pressure (no foliation) Rocks not good conductors of heat, so area of metamorphism is localized Extent depends on size of intrusion: Small dike or sill centimeters to meters or Large batholith several kilometers
Metamorphism and Metamorphic Rocks II. Types of Metamorphism: Heat, pressure, and fluids interact differently in different geological settings to produce different metamorphic rocks A. Contact Metamorphism Solid rock near magma may be baked by heat High temperature, but low pressures. Near cooling magma (igneous intrusions)
Metamorphism and Metamorphic Rocks II. Types of Metamorphism: A. Contact Metamorphism
Metamorphism and Metamorphic Rocks II. Types of Metamorphism: A. Contact Metamorphism
Metamorphism and Metamorphic Rocks II. Types of Metamorphism: B. Regional Metamorphism Alters rock for thousands of square kilometers 1. Continental Collision Zones (rocks once at surface get buried) Directed pressure foliation 2. Burial--Sediment gets buried in deep sedimentary basins Confining pressure no foliation Louisiana, Mississippi River delta, bottom deposits are 12 km down This is high temperature and high pressure
Metamorphism and Metamorphic Rocks II. Types of Metamorphism: B. Regional Metamorphism
Metamorphism and Metamorphic Rocks II. Types of Metamorphism: C. Subduction Zone Blueschistmetamorphism Low temperature, high pressure In a subduction zone environment What is the geothermal gradient Low or High? ~5°-10°C/km
II. Types of Metamorphism: D. Hydrothermal Metamorphism: chemical alteration of preexisting rocks by hot water Usually happens at divergent plate boundaries—beneath ocean floors, where seawater percolates down, warms up by magma and alters rock High temperature, low pressure Similar tectonic setting to contact metamorphism Metamorphism and Metamorphic Rocks
Metamorphism and Metamorphic Rocks High pressure and medium pressure metamorphic rocks in the Caledonides of western Norway. These rocks all have broadly basaltic compositions, and were metamorphosed in the Paleozoic. The high pressure rocks were brought rapidly enough to the surface so that these high pressure rocks scarcely have any evidence of retrograde metamorphism.
