8. METAMORPHIC ROCKS

METAMORPHISM SHALE Metamorphism is the solid-state change in rocks produced by increased heat (T) & Pressure (P). GNEISS P (K bar) The formation of metamorphic minerals and textures takes place very slowly (thousands to millions of years). 1- 15- 25-

Clay minerals (made of elements such as Al, Si, O, K, Mg, etc.) Gneiss Shale Changes involve: Rock composition; texture/structure SHALE GNEISS Composition Clay minerals (made of elements such as Al, Si, O, K, Mg, etc.) Garnet, Plagioclase, Amphibole, Pyroxene Texture & Structure Soft, light in weight, made of very small to microscopic minerals Hard, dense, coarse-grained, sometimes folded

METAMORPHIC REACTION (NEOCRYSTALLIZATION) involves chemical reactions that decompose older minerals and produce new minerals. Example: SiO2 + CaCO3  CaSiO3 + CO2 Quartz Calcite Wollastonite Wollastonite (metamorphic mineral) Impure Limestone

Example 1: Limestone  Marble RECRYSTALLIZATION: changes the shape and size of minerals without changing the chemical composition of the minerals. Example 1: Limestone  Marble CaCO3 CaCO3 Example 2: Sandstone  Quartzite SiO2 SiO2 SiO2 Quartz Sandstone Quartzite

METAMORPHIC GRADE Refers to the intensity of heat to which a rock was subjected. Low Grade Temp: 200-4000C Depth: < 6 miles P: 3-4 Kbars Medium Grade Temp: 400-6000C Depth: 6-25 miles P: 4-12 Kbars High Grade Temp: 600-8000C Depth: 25-35 miles P: 12-15 Kbars L M H

RISING MAGMA AS A SOURCE HEAT When magma gets in contact with rocks it causes the temperature of the rock to rise and result in metamorphism.

200-3000C 400-6000C 300-4000C 600-8000C

METAMORPHIC TEXTURES 1. Foliated Metamorphic Rocks SLATE Refers to whether metamorphic rocks possess a layered appearance or not. 1. Foliated Metamorphic Rocks Presence of minerals such as mica and chlorite give the rock a layered appearance SLATE 2. Non-foliated Metamorphic Rocks Lack layered appearance, thus are massive Amphibolite

1. Foliated Metamorphic Rocks A. SLATE Clay minerals in original rock (shale) break down & form very fine grained (microscopic) crystals of biotite, muscovite & chlorite. Shale is a very common sedimentary rock. Consequently metamorphic rocks derived from shale are also very common. As shale is subjected to higher and higher grades of metamorphism, different metamorphic rocks form. Protolith: Original, pre-metamorphosed rock V) Low-grade metamorphic rock Clay minerals in original rock (shale) break down & reform into very fine grained (microscopic) crystals of biotite, muscovite & chlorite. The microscopic minerals give the rock a dull surface & also define a thin foliation along which the rock breaks readily  Slaty cleavage shale SLATE

slate walkway Slate roof Slate roof Slate chalkboard

B. PHYLLITE Made up of very small crystals of muscovite & chlorite that give the rock a sheen. Commonly shows a wavy, wrinkled surface

C. Schist Made up of crystals of mica (muscovite & biotite) that are larger than in slate & phyllite and are easily visible  Glittery appearance.

D. Gneiss Temperatures are above the stability range of most micas, so the micas in schist break down to form large crystals of feldspars & other minerals It is a layered rock, with alternating light-colored minerals and dark-colored minerals

2. Non-foliated Metamorphic Rocks QUARTZITE MARBLE Hardness test: Qtz in Qzt has H7; calcite in LST has H3; scratch them with a H5 mineral, the one that scratches is calcite  marble! Apply few drops of HCl, the one that effervesces is calcite  Marble Parent rock? Sandstone Parent rock? Limestone

Some Uses of Metamorphic Rocks Washington Monument

Marble, because of its workability, is a widely used building stone. Lincoln Memorial in Washington, D.C. Taj Mahal, India Marble, because of its workability, is a widely used building stone.

Increased heat & pressure COAL Peat Unconsolidated plant remains Lignite Increased heat & pressure Bituminous coal Anthracite coal T > 2000C (Metamorphic ) Peat Lignite Bituminous coal Anthracite coal

TEST YOUR KNOWLEDGE For the metamorphic rock shown answer the following questions: Is the rock foliated or non-foliated? If foliated, determine whether it is slate, phyllite, schist or gneiss At what temperature range could the rock have formed? If the temperature on the rock in the picture was increased by 3000C, the rock that would form would be (choose one): a) Sedimentary b) Igneous c) Metamorphic 5) What was the parent rock for the rock in this picture?