Learning Goal: Understand how metamorphic rocks are formed and classified. S6E5. Students will investigate the scientific view of how the earth’s surface is formed c. Classify (group) rocks (metamorphic) by their process of formation (formed) Rocks that have changed (metamorphism) due to HEAT and PRESSURE. These rocks form DEEP underground! Magma heats up surrounding rock. Pressure from plate movement folds and buckles the crust. Metamorphic rocks are formed by the effect of heat and pressure on existing rocks. This can greatly affect the hardness, texture or layer patterns of the rocks. Pressure from surface rocks metamorphic rock forming here Magma heat Metamorphic rocks are classified by texture in 2 ways… 1.Foliated 2.Non-Foliated The processes of compaction and recrystallization change the texture of rocks during metamorphism. Compaction The grains move closer together. The rock becomes more dense. Porosity is reduced. Example: clay to shale to slate Recrystallization Growth of new crystals. No changes in overall chemistry. New crystals grow from the minerals already present. A preferred orientation of minerals commonly develops under applied pressure. Platy or sheet-like minerals such as muscovite and biotite become oriented perpendicular to the direction of force. This preferred orientation is called foliation. Mineral grains flatten and line up in parallel bands OR layers. That are formed under directed Slate: Foliated (with layers) pressure Gneiss: Foliated (with banding) Foliated Texture Foliated Foliated Textures Textures Slatey •• Slatey Schistose •• Schistose -- looks looks like like blackboard blackboard dull surface surface >>dull -- smooth, smooth, thin thin layering layering -- breaks breaks into into flat flat slabs slabs referred to to as as slatey slatey cleavage cleavage >>referred -- no no mineral mineral grains grains visible visible Phyllitic •• Phyllitic garnets, staurolites staurolites >>garnets, -- may may have have shiny shiny appearance due to to mica mica minerals minerals >>due Gneissic •• Gneissic -- looks looks like like waxed waxed surface surface has aa "sheen" "sheen" to to itit >>has -- may may have have little little "waves" "waves" on on surface surface referred to to as as >>referred -- distinct bands of minerals -- visible visible grains mineral grains crenulations crenulations -- some some small small grains grains visible visible -- larger larger grains grains -- may may look look like like igneous igneous rock rock -- may may have have crude crude banding banding intensely distorted distorted >>intensely -- different different minerals minerals than than schistose schistose Foliated MM Rocks slate phyllite QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. schist gneiss MM MM Rocks Rocks that that could could form form as as aa shale shale (sedimentary) (sedimentary) parent parent rock rock is is exposed to increasing directed pressure and temperature Mineral grains change, grow, and rearrange, but don’t form bands or layers. Formed under uniform Quartzite pressures Marble Non-foliated Rocks Examples • Marble: - metamorphosed limestone Quartzite forms in two different ways. pressures • • and --temperatures of deep burial, In the first way, under the high sandstone Quartzite: metamorphosed sandstone Metamorphic Zones Metamorphic Zones • Metamorphism is common along most plate boundaries like this. The Types of Metamorphism Read this slide Then use the next two sides to complete the chart Regional Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins • Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress) • Foliated rocks are a characteristic product Types of metamorphism Regional Shock metamorphism metamorphism Depth, km 0 Regional high-pressure metamorphism Contact metamorphism Regional metamorphism Oceanic crust 35 Oceanic lithosphere 75 Water Seafloor metamorphism Types of metamorphism Shock metamorphism Regional metamorphism Regional high-pressure metamorphism Contact metamorphism Regional metamorphism Depth, km 0 Oceanic crust 35 Oceanic lithosphere 75 Burial metamorphism Heat Pressure Pressure Sedimentary Sedimentary Igneous IgneousPressure or Metamorphic or Metamorphic Rock Metamorphism Effect G r a i Classification based on Texture s n Foliated Nonfoliated Heat & Pressure NO ↑ Flatten Grains ↑ Layers Bands Layers Slate Phyllite Schist Gneiss HEAT Bands PRESSURE Slate Gneiss Quartzite Marble Learning Goal: Understand how metamorphic rocks are formed and classified. Do Now Q: What features do foliate metamorphic rocks show?