Metamorphism / Metamorphic Rocks

Transcription

1 Metamorphism / Metamorphic Rocks Metamorphism: occurs when rocks are subjected to heat, pressure, and/or other environmental conditions - The rock remains a solid during this time period - Why Should You Study Metamorphic Rocks? - Exposed metamorphic rocks make up large parts of continents - Certain minerals in metamorphic rocks give clues about the conditions existing when they formed - Used widely as building materials and manufacturing!1

2 - Why does Metamorphism occur? - Minerals are only stable only within certain temperature and pressure ranges. - Minerals will decompose and rearrange to form different minerals that are stable at the new temperature and pressure!2

3 - Factors that Cause Metamorphism * Temperature and Pressure!* - Temperature - The deeper you go, the hotter it gets - Geothermal gradient = C/Km - Depth Temp - Increases the rate of chemical reactions that produce different minerals - Heat sources - intrusive magma - deep burial along subduction zones!3

4 - Pressure - The deeper you go the more pressure there is - lithostatic pressure results from the weight of overlying rocks - Depth Temp - Expressed in kilobars - 1 kilobar = 1000 atmospheres - 1 atmosphere = 14.7 lb/in 2-1 kilobar = lb/in 2 - Pressure increases.3 kilobar/km - Mineral grains are more closely packed - Recrystallization may occur - Differential pressure results from unequal forces applied to the rock Lithostatic pressure is applied equally in all directions in Earth s crust due to the weight of overlying rocks. Thus, pressure increases with depth, as indicated by the sloping black line.!4

5 ! A similar situation occurs when 200-ml cups composed of Styrofoam are lowered to ocean depths of approximately 750 m and 1,500 m. Increased water pressure is exerted equally in all directions on the cups, and they consequently decrease in volume while maintaining their general shape. - Deformation: Caused by Differential Pressure. Changes in the shape of the rock due to mechanical forces. Results in foliation (layering). Foliation is perpendicular to pressure. Platy minerals *The layers are said to have Slaty Cleavage*!5

6 - Fluid Activity - water and carbon dioxide are almost always present in some amount in metamorphic regions - these fluids enhance metamorphism by increasing the rate of chemical reactions - Fluid Source - Textural Changes - grain size increases - water trapped in the pore space of sedimentary rocks (groundwater) - from cooling magma (juvenile water) - dehydration of water-bearing minerals that are subjected to heat and pressure; water released due to metamorphism (metamorphic water) - ex. limestone marble (fine grained) (coarse grain) - Mineralogical Changes! Fossiliferous Limestone - 2 types of metamorphic reactions Marble 1) No change in mineral type, just bigger grains - limestone to marble!6

7 2) Original mineral decompose new mineral - atoms rearrange to gain stability - ex. shales to hornfels Shale Hornfels How are Metamorphic Rocks Classified? 1) Foliated: minerals arranged in a parallel fashion - subjected to differential pressure - examples: slate, phyllite, schist, gneiss 2) Nonfoliated: minerals that are not layered - most form from contact or regional metamorphism from non-platy or non-elongated minerals. - examples: marble, quartzite, greenstones, hornfels, anthracite!7

8 Three Main Types of Metamorphism 1) Contact Metamorphism: produced when a body of magma alters the surrounding country rock - causes old minerals to decompose and new ones to form, causes metamorphism to occur - Factors in contact metamorphism - initial temperature and size of the intrusion - presence and chemistry of fluids - Metamorphic aureoles (halo) - zones of mineral assemblages surrounding intrusion! Contact metamorphic aureole or halo surrounding a pluton!8

9 Metamorphic Aureole: A metamorphic aureole, the area surrounding an intrusion, consists of zones that reflect the degree of metamorphism. Contact Metamorphism from a Lava Flow A highly weathered basaltic lava flow near Susanville, California, has altered an underlying rhyolitic volcanic ash by contact metamorphism. The!9

10 red zone below the lava flow has been baked by the heat of the lava when it flowed over the ash layer. The lava flow displays spheroidal weathering, a type of weathering common in fractured rocks (see Chapter 6). - Rock Characteristics - Nonfoliated - Little to no deformation - Typical Contact Metamorphism Rocks - Hornfels: dark to black - Skarn (tactite): Contact metamorphism of carbonate rocks: limestone or dolomite - usually large grained or crystals!10

11 2) Dynamic Metamorphism - occurs along fault zones - fault: fracture along which movement has occurred - rocks are subjected to differential shear pressure.!! Mylonite: An outcrop of mylonite from the Adirondack Highlands, New York. Mylonites result from dynamic metamorphism, where rocks are subjected to high levels of differential pressure. Note the thin laminations that are characteristic of many mylonites.! 11

12 3) Regional Metamorphism - occurs when 2 lithospheric plates collide and squeeze and deform rocks as a result - highly deformed - associated with mountain building - produces most metamorphic rocks; makes up the shields - results from extreme heat and pressure generated by the processes at convergent plate boundaries - covers large geographic areas, and usually shows a gradation of deformation corresponding to areas of the most intense heat/pressure - certain minerals are known to form under specific conditions of temperature and pressure: index minerals - low, intermediate, and high grades of metamorphism are recognized, based on conditions and the resulting mineral assemblages!12

13 - Foliated Metamorphic Rocks - Heat and differential pressure cause minerals to arrange themselves in a parallel fashion called foliation - size and shape of crystals determines texture; fine to coarse - with increasing grain size, common foliated metamorphic rocks include: - Slate! - Phyllite!13

14 ! - Schist - Gneiss!14

15 Metamorphic Grade Change in mineral assemblage and rock type with increasing metamorphism of shale. When a clay-rich rock such as shale is subjected to increasing metamorphism, new minerals form, as shown by the colored bars. The progressive appearance of certain minerals, known as index minerals, allows geologists to recognize low-, intermediate-, and high-grade metamorphism.!15

16 Metamorphic Zones and Facies - metamorphic zone: area that has the same metamorphic minerals - occur because metamorphic minerals are only stable at certain pressures and temperatures - metamorphic facie: assemblages that were formed under similar pressures and temperatures - rocks that have the same minerals, have same geologic history!16

17 Metamorphic Zones in the Upper Peninsula of Michigan The zones in this region are based on the presence of distinctive silicate mineral assemblages resulting from the metamorphism of sedimentary rocks during an interval of mountain building and minor granitic intrusion during the Proterozoic Eon, about 1.5 billion years ago. The lines separating the different metamorphic zones are isograds.!17

18 Metamorphic Facies and Their Associated Temperature Pressure Conditions. A temperature pressure diagram showing under what conditions various metamorphic facies occur. A metamorphic facies is characterized by a particular mineral assemblage that formed under the same broad temperature pressure conditions. Each facies is named after its most characteristic rock or mineral.!18

19 - Regional Burial Metamorphism (subset of regional) - occurs when rock is buried deeper than 2 Km - Lots of pressure; 1 cubic foot of granite equals 168 lbs - Non-foliated - Common rocks - Marble: parent rock is limestone!! Limestone Marble - Quartzite: parent rock is quartz sandstone!! Quartz sandstone Quartzite!19

20 - In sandstone (A) fractures run between the sand grains. In quartzite (B) fractures run through the grains!20