1. Metamorphic Rocks

2. Metamorphic Rocks "Changed form" rocks Produced from ___________
Igneous rocks
Sedimentary rocks
Other metamorphic rocks

3. Metamorphism
Takes place where preexisting rock is subjected to ____________ and _________ unlike those in which it formed
Degrees of metamorphism
Exhibited by rock texture and mineralogy
Low-grade (e.g., shale becomes slate)
High-grade (obliteration of original features)

4. Force and Stress
Force is push or pull, expressed as amount of acceleration experienced by a mass
What happens if same amount of force is applied to two wooden pillars?
EXPLANATION
Force from a stone weight is distributed evenly across the top of a broad wooden pillar
Stress is greater if same amount of weight (force) distributed on thinner pillar
Stress = force per area
08.01.a

5. How Rocks Respond to Force and Stress
Small amount of stress: block remains unchanged
08.01.b
Strain
Displacement
Rotation
EXPLANATION
Stress shown in blue arrows
If small amount of stress imposed, rock may compress a little but may otherwise be unchanged
Displacement: moved from one place to another
Rotation: can tilt or spin horizontally
Strain: can deform internally—changing size or shape
NOTES
Left photograph: white layers displaced by movement along fractures (faults)
Middle photograph: layers deposited horizontally, but then rotated (tilted); Dakota Hogback, Colorado
Right photograph: pebbles in a conglomerate strained in response to stress pushing on rock

6. At shallow depths, most rocks _____
Consider the structural behavior of rocks at shallow versus deep conditions
At shallow depths, most rocks _____
08.01.c
Rocks ____ in deep conditions
INSTRUCTIONS TO STUDENTS
Consider the structural behavior of rocks at shallow versus deep conditions
EXPLANATION
Graph shows that temperature and pressure increase with depth within Earth
Temperature and pressure increase with depth

7. Consider how minerals respond at shallow versus deep conditions
Shallow: minerals may be _________
Deep: minerals may __________
INSTRUCTIONS TO STUDENTS
Consider how minerals respond at shallow versus deep conditions
08.01.c

8. Different Kinds of Stress
________ pressure
________ stress
MEDIA
0802a3_fluid_pressure.mov
EXPLANATION
Confining pressure: same amount of stress from each direction
Differential stress: amount of combined stress is greater in some directions than in others
Fluid pressure counteracts stress: fluids push outward in all directions and reduce the amount of overall pressure on rock
Fluid pressure counteracts stress
08.02.a

9. Strength of Rock Small stress Increase stress
EXPLANATION
Small stress may cause rock to contract but remain undeformed
Increasing stress may leave rock undeformed as long as strength of rock is greater than differential stress
Too much stress will make a rock fail structurally by flowing or fracturing
Shallow levels, the crust deforms by fracturing or other brittle deformation
At increasing depth rocks may begin to flow ductilely
A greater depths, rocks flow more easily and strength of crust decreases
Too much stress = failure
Strength of continental crust
08.02.b

10. Observe three kinds of stress and resulting structures
___________
_______
_____
Shallow levels: rocks ________
INSTRUCTIONS TO STUDENTS
Observe three kinds of stress and resulting structures
EXPLANATION
Compression
At shallow levels deforms brittlely (fracturing or faulting)
At deeper levels, hot enough to flow, ductile folds can form
Tension
Tension can form fractures at shallow levels
Tension hard to achieve at depth, but differential stresses can stretch a rock
Shear
Shearing at shallow levels forms faults
In deeper levels, shearing distributed across a wide zone, called a shear zone
Deeper levels: rocks ____
08.02.c

11. Metamorphic agents _____ __________ (stress) Chemically active ______
From burial (confining pressure)
From differential stress during mountain building
Chemically active ______
Mainly _____ and other volatiles
Promote recrystallization by enhancing ion migration

12. Processes that Cause Metamorphism
Burial
Subduction
Heating
EXPLANATION
Burial: rocks taken to higher pressure-temperature conditions
Sedimentary or volcanic burial
Tectonic burial
Subduction: rocks move downward in subduction zones
Heating
By magma: heat transported into the wall rock can cause metamorphism
Crustal heating: flux of heat from depth
Hot fluids: hot fluids carry dissolved chemical and may hydrothermally alter rocks
Change in stress: tectonics can cause a rocks to deform and form new metamorphic fabrics
Shearing: one rock mass moves past another can deform rocks close to the zone of shearing
(Not shown) Shock metamorphism: meteoroids that strike earth can send shock waves through rocks
Hot fluids
Change in stress
Shearing
08.09.a

13. Causes of Metamorphism
Response of Rock
Tectonic stress
Pressure
Grow larger minerals
Deform
EXPLANATION
Causes of metamorphism
Pressure: higher pressures compress rocks and cause some minerals to be unstable and others to be stable
Temperature: increase in temperature weakens rocks and causes new minerals to grow
Tectonic stress causes rocks to move and deform internally
Fluids interact chemically with certain minerals, and cause new minerals to replace existing ones
Response of rock
Deform
Grow larger minerals
Grow new minerals
Cut by veins
Cut by Veins
Grow new minerals
08.08.a
Fluids
Temperature

14. Physical Processes During Metamorphism
Deformation of objects
Rotation
08.08.b
EXPLANATION
Deformation of objects: deformation can flatten grains in pancake shapes or long and thin like skateboards
Rotation: elongated and platy minerals can rotate so they become almost parallel to one another
Shearing: aligns or stretch out unoriented crystals
Small-scale folding: if layered rocks compressed, they generally will fold
Fracturing: parts of rocks can fracture before or during metamorphism
Folding
Shearing
Fracturing

15. Chemical Processes During Metamorphism
Pressure solution
Recrystallization
EXPLANATION
Pressure solution: material is dissolved from highly stressed edges of grains and precipitates out elsewhere in the rock; rock can actually lose volume
Recrystallization: existing minerals can recrystallize or new minerals can grow during metamorphism
Remobilization: chemical constituents in the rock can diffuse, dissolve, or partially melt; produces light and darks bands in gneiss
Remobilization
08.08.c

16. Types of Metamorphism _______, or thermal, metamorphism
Occurs near a body of magma
Changes are driven by a rise in temperature
________ metamorphism
Directed pressures and high temperatures during mountain building
Produces the ________ volume of metamorphic rock

18. Observe how metamorphic conditions vary with depth
Heating by magma (contact metamorphism)
Regional metamorphism
Subduction-zone metamorphism
INSTRUCTIONS TO STUDENTS
Observe how metamorphic conditions vary with depth
EXPLANATION
Colors show where different metamorphic equivalents of shale form
08.09.b1

22. Metamorphic Textures ________ texture ___________ texture
Minerals are in a parallel alignment
Minerals are perpendicular to the compressional force
___________ texture
Contain equidimensional crystals
Resembles a coarse-grained igneous rock

24. Foliation of Metamorphic Rock
To view this animation, click “View” and then “Slide Show” on the top navigation bar. 24

25. Foliation Processes
To view this animation, click “View” and then “Slide Show” on the top navigation bar. 25

26. Foliated Rocks Slate Fine-grained Splits easily Schist
Strongly foliated
"Platy"
Types based on composition (e.g., mica schist)

29. Foliated Texture Gneiss Strong segregation of silicate minerals
"Banded" texture

31. Non-Foliated Marble Parent rock is limestone
Large, interlocking calcite crystals
Used as a building stone
Variety of colors
Quartzite
Parent rock – quartz sandstone
Quartz grains are fused

34. Why should I care about rocks?
Metallic mineral resources
Gold, silver, copper, mercury, lead, etc.
Concentrations of desirable materials are produced by
Igneous processes
Metamorphic processes

35. Why should I care about rocks?
Most important ore deposits are generated from hydrothermal (hot-water) solutions
Hot
Contain metal-rich fluids
Associated with cooling magma bodies
Types of deposits include
_____ deposits in fractures or bedding planes, and
___________ deposits which are distributed throughout the rock

37. Why should I care about rocks?
Nonmetallic mineral resources
Make use of the material’s
Nonmetallic elements
Physical or chemical properties
Two broad groups
Building materials (e.g., limestone, gypsum)
Industrial minerals (e.g., fluorite, corundum, sylvite)

40. You use rocks and minerals fromTo
Death
Birth