Geology lecture 9
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The document discusses metamorphic rocks and the rock cycle. It describes how rocks undergo metamorphism through solid-state processes involving changes in temperature, pressure, and fluids. This causes changes to mineral textures and compositions, resulting in new metamorphic rock types. Key metamorphic rock types include slate, phyllite, schist, gneiss, quartzite, and marble. The document also covers metamorphic classifications and different metamorphic environments associated with various geological settings and tectonic processes.
Geology lecture 9
- 1. Rock Cycle & Metamorphic Rocks Chapter 8
- 2. Outline • The rock cycle and metamorphic rocks -What is the cycle and basics of metamorphic rocks. • Metamorphism -A solid state process -Character of metamorphic rocks -Specific processes of metamorphism (5 types) -Causes of metamorphism (T, P, diff stress, hyd-th fluids) • Metamorphic Rocks – more details: -The foliated ones: slate, phyllite, schist, gneiss -The unfoliated ones: quartzite, marble -Type controlled by parent rock • Metamorphic classifications -Classes, intensity, grade, facies -Metamorphic environments Chapter Chapter 8 8
- 3. Metamorphic Rocks: Basics • Metamorphism change + form/shape • Change from original “parent” rock • Parent rocks are called “protoliths” • Any protolith can experience metamorphism Chapter 8
- 4. Metamorphic Rocks: Basics • Lots of change in physical or chemical conditions. • Burial • Tectonic stresses (compression/extension/shear) • Heating by magma Fluid alteration • Result: protolith changes… • Texture • Minerology Chapter 8
- 5. The Rock Cycle Chapter Chapter 8 8
- 6. Rock Cycle and Plate Tectonics Chapter Chapter 8 8
- 7. Rock Cycle and Plate Tectonics Chapter Chapter 8 8
- 8. Outline • The rock cycle and metamorphic rocks -What is the cycle and basics of metamorphic rocks. • Metamorphism -A solid state process -Character of metamorphic rocks -Specific processes of metamorphism (5 types) -Causes of metamorphism (T, P, diff stress, hyd-th fluids) • Metamorphic Rocks – more details: -The foliated ones: slate, phyllite, schist, gneiss -The unfoliated ones: quartzite, marble -Type controlled by parent rock • Metamorphic classifications -Classes, intensity, grade, facies -Metamorphic environments Chapter Chapter 8 8
- 9. Process of Metamorphism • Metamorphism occurs in the solid state. • It doesn’t include weathering, diagenesis, melting Chapter 8
- 10. Metamorphic Character • Metamorphic rocks have distinctive properties. • Texture – intergrown and interlocking grains • Minerals – some that are only metamorphic • Foliation – a planar fabric from aligned minerals Fossiliferous limestone Fossiliferous limestone Red mudstone Red mudstone Garnet gneiss Garnet gneiss Marble Marble Chapter 8
- 11. Metamorphic Processes • Metamorphic change is slow and in the solid state. • Several processes at work, simultaneously: 1. Recrystallization – minerals change size/shape 2. Phase change – new minerals form with… same chemical formula Kyanite different crystal structure Chapter 8
- 12. Metamorphic Processes 3. Neocrystallization – new minerals with P-T changes 1. Initial minerals become unstable; change to new minerals 2. E.g. in this way, a shale can transform into a garnet mica schist Chapter 8
- 13. Metamorphic Processes 4. Pressure solution – mineral grains partially dissolve 5. Plastic deformation – mineral grains soften and deform Chapter 8
- 14. Causes of Metamorphism 1. Heat (Temperature – T). 2. Pressure (P). 3. Differential stress. 4. Hydrothermal fluids. 5. Not all are required; they often do co-occur. 6. Rocks may be metamorphosed multiple times. Chapter 8
- 15. Differential Stress • Pressure that is greater in one orientation. • A common result of tectonic forces • 2 kinds of differential stress: Normal & shear. 1. Normal stress – perpendicular to a surface tension (pull-apart) compression (push-together) Chapter 8
- 16. Differential Stress • 2 kinds of differential stress: Normal & shear. 2. Shear stress – sideways across a surface causes material to be “smeared out” Chapter 8
- 17. Differential Stress • At high T & P, differential stress deforms rock. • Rocks change shape slowly without breaking Chapter 8
- 18. Results of Differential Stress • Deformation acts on minerals with specific shapes. • Equant – equal in all directions • Inequant –unequal dimensions • Platy (pancake-like)- 1 dimension shorter • Elongate (cigar-shaped)- 1 dimension longer • Differential stress causes minerals to align • Aligned fabric records stress orientation Chapter 8
- 19. Results of Differential Stress • Mineral alignment called foliation. • Banded appearance • Develops perpendicular to compression. • Minerals flatten, recrystallize, and rotate • Inequant grains align by rotation and new growth Chapter 8
- 20. Hydrothermal Fluids • Hot water with dissolved ions and volatiles • Hydrothermal fluids facilitate metamorphism by… • Accelerating chemical reactions • Alternating rocks by adding/subtracting elements • Hydrothermal alteration is called metasomatism. Chapter 8
- 21. Outline • The rock cycle and metamorphic rocks -What is the cycle and basics of metamorphic rocks. • Metamorphism -A solid state process -Character of metamorphic rocks -Specific processes of metamorphism (5 types) -Causes of metamorphism (T, P, diff stress, hyd-th fluids) • Metamorphic Rocks – more details: -The foliated ones: slate, phyllite, schist, gneiss, migmatite -The unfoliated ones: amphibolite, hornfels, quartzite, marble -Type controlled by parent rock • Metamorphic classifications -Classes, intensity, grade, facies -Metamorphic environments Chapter Chapter 8 8
- 22. Metamorphic Rock Types • 2 major subdivisions of metamorphic rocks. 1. Foliated – has a through-going planar fabric 1. Due to differential stress 2. Have platy minerals 3. Classified by composition, grain size, and foliation type Chapter 8
- 23. Metamorphic Rock Types • 2 major subdivisions of metamorphic rocks. 2. Non-foliated – no planar fabric 1. Crystallized without differential stress 2. Comprised of equant minerals 3. Classified by mineral composition Chapter 8
- 24. Foliated Metamorphic Rocks • Compositional banding develops in several ways: • Original layering in the protolith • Extensive, high T shearing Chapter 8
- 25. Foliated Metamorphic Rocks • Development of compositional banding during formation Chapter 8
- 26. Foliated Metamorphic Rocks • Slate – clay protolith, low-grade metamorphic shale. • Distinct foliation called slaty cleavage • Alignment of platy clay minerals • Cleavage perpendicular to compression • Slate breaks along foliation as flat sheets Chapter 8
- 27. Foliated Metamorphic Rocks • Phyllite - Fine mica-rich rock. • Formed by low-medium grade alternation of slate • Clay minerals neocrystallize into mica (shiny luster) • Phyllite is between slate and schist Chapter 8
- 28. Foliated Metamorphic Rocks • Schist – rock with larger micas. • Medium-to-high-grade metamorphism • Distinct foliation called schistosity • Parallel alignment of mica crystals micas visible becase they grew at higher T • Schist often has other minerals due to neocrystallization: • Quartz • Feldspars • Kyanite • Garnet • Staurolite • Sillimanite Large non-mica minerals are called porphyroblasts Chapter 8
- 29. Foliated Metamorphic Rocks • Gneiss – distinct banded foliation (high metam. Grade) • Light bands of felsic minerals (quartz and feldspars) • Dark bands of mafic minerals (biotite or amphibole) Chapter 8
- 30. Non-foliated Metamorphic Rocks • Non-foliated rocks lack planar fabric. • No foliation due to: • Rock not subjected to differential stress • Dominance of equant minerals • Lack of platy minerals Chapter 8
- 31. Non-foliated Metamorphic Rocks • Quartzite – Almost pure quartz in composition. • Forms by alternation of sandstone • Sand grans in the protolith recrystallize and fuse • Cant see gran boundaries anymore Metamorphic Alteration Chapter 8
- 32. Non-foliated Metamorphic Rocks • Marble – coarse crystalline carbonate. • Forms from a carbonate (i.e. limestone) protolith • Recrystallization occurs • Origional textures/fossils in parent are destroyed Metamorphic Alteration Chapter 8
- 33. Metamorphic Rocks • Protolith controls type. • Minerals contribute elements. • Some protoliths yield specific rocks. • Broad compositional classes: 1. Pelitic. 2. Basic (or Mafic). 3. Calcareous. 4. Quartzo-feldspathic. Chapter 8
- 34. Outline • The rock cycle and metamorphic rocks -What is the cycle and basics of metamorphic rocks. • Metamorphism -A solid state process -Character of metamorphic rocks -Specific processes of metamorphism (5 types) -Causes of metamorphism (T, P, diff stress, hyd-th fluids) • Metamorphic Rocks – more details: -The foliated ones: slate, phyllite, schist, gneiss -The unfoliated ones: quartzite, marble -Type controlled by parent rock • Metamorphic classifications -Classes, intensity, grade, facies -Metamorphic environments Chapter Chapter 8 8
- 35. Metamorphic Classes 1. Pelitic – Shale protoliths. • Al-rich clay minerals yield micas • Rock type depends on grade (degree of metamorphism). • Slate • Phyllite • Schist • Gneiss Chapter 8
- 36. Metamorphic Classes 2. Mafic – basalt or gabbro protolith. • Turn into biotite and amphibole-dominated rocks Chapter 8
- 37. Metamorphic Classes 3. Calcareous – carbonate protolith. • Recrystallize into marbles Chapter 8
- 38. Metamorphic Classes 4. Quartzo-feldspathic – Granitic protolith. • Recrystallize and become foliated gneisses Chapter 8
- 39. Metamorphic Intensity • Grade is a measure of metamorphic intensity. • Low grade- slight • High grade- intense Chapter 8
- 40. Metamorphic Intensity • Specific minerals typify particular grades. Chapter 8
- 41. Metamorphic Grade • Certain minerals have a limited P-T range. • These “index minerals” indicate grade • Index mineral maps • Define metamorphic zones Chapter 8
- 42. Metamorphic Facies • Metamorphic facies – mineral assemblage from a specific protolith made at specific P-T conditions • Named for dominant mineral Chapter 8
- 43. Metamorphic Environments • Different settings yield different effects via… • P & T gradients • Differential stresses • Hydrothermal fluids These characteristics are governed by tectonics. Chapter 8
- 44. Metamorphic Environments • Types (and settings) of metamorphism are... • Thermal – heating by magma intrusion (”contct” metamorph) • Burial – increases in P and T • Regional – P and T change due to mountain building • Hydrothermal – alteration by hot water • Subduction – high P- low T alteration • Shock – very high P due to impact • Mantle – very high P causes mineral phase changes Chapter 8
- 45. Contact Metamorphism • Heat from magma intrusion. • Creates zoned bands of alteration in country rock. • Called a contact aureole • Aureole surrounds the intrusion • Zoned form high to low grade Chapter 8
- 46. Burial Metamorphism • As sediments are buried… • P increases due to weight above • T increases due to geothermal gradient • Requires burial below diagenetic effects • E.g. >5-15km depth Chapter 8
- 47. Regional Metamorphism • Tectonic collisions deform rocks. • Creates mountains. • Rocks are… • Heated by geothermal gradient and intrusions • Squeezed and heated by burial • Smashed and sheared by differential stresses Chapter 8
- 48. Subduction Metamorphism • Trenches & accretionary prisms have… • Low temperature (lowh7yuuuuuy geothermal gradient) • High pressures (collision) • High P/low T formation of blueschist • Rock with a blue mineral called glaucophane Chapter 8
Editor's Notes
- #11: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #12: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #13: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #14: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #15: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #23: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #24: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #42: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #44: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #45: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #46: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #47: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.
- #49: Analogy to firing of potter ’s clay The scientific value of metamorphic rocks is in what it tells you about ancient plate boundaries and history of mountain building Metamorphosis of limestones (a process similar to the production of cement and concrete) produces CO2 that eventually comes out volcanoes and can impact our climate. TRANSPARENCY: Shields composed of metamorphic rocks.