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*Petrology > Chapter 01 - Fundamentals > Flashcards

Chapter 01 - Fundamentals Flashcards

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1
Q

Chilled margin (chill zone)

A

Contact effect of intrusive igneous rocks cross-cutting country rocks; exhibits narrow, fine-grained “chilled margin” within igneous body margin, or localized baking of country rock

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2
Q

Petrography

A

Branch of petrology; microscopic examination of thin sections

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3
Q

Interlocking texture

A

Specific texture associated with slow crystallization from a melt

As melt cools, more crystals form, eventually interfering with one another and inter grow, showing interpenetrating crystals.

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4
Q

Glassy texture

A

Rapid cooling and solidification of a melt; cools too fast for ordered crystal structures to form.

Result: non-crystalline solid, or glass.

Isotopic optical character inter the microscope.

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5
Q

Foliations

A

Rarely develop because liquids cannot sustain substantial directional stresses. Common textural distinction between igneous and high-grade metamorphic crystalline rock — igneous: based on isotopic texture (random orientation of elongated crystal).

NOTE: some igneous processes, e.g. crystal settling, magmatic flow, CAN produce mineral alignments and foliations.

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6
Q

Pyroclastic deposits

A

Result from explosive eruptions. Most difficult to recognize as igneous.

Magmatic portion solidified & cooled considerably before being deposited — along with much of pulverized pre-existing rocks caught in explosion.

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7
Q

Chemistry of rocks for identifying

A

Major elements, trace elements, isotopes, and some thermodynamics.

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8
Q

Earth’s interior - divided into three major units w/boundaries & discontinuities

A

Crust — oceanic & continental
Moho/M-discontinuity — boundary between crust & mantle
Mantle — contains: low velocity layer, 410-km discontinuity, 660-km discontinuity
Core — outer (liquid/molten) & inter (solid)

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9
Q

Oceanic crust

A

~10 km thick

Basaltic composition

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10
Q

Continental crust

A 
~36 km on average; up to 90 km
More heterogeneous
Too buoyant to subduction
Mantle-derived melts
Crude compositional average: granodiorite 
~1% of volume of Earth
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11
Q

Mantle

A

~83% of Earth’s volume
Nearly 3,000 km
Mainly Fe- and Mg-rich silicate minerals

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12
Q

Moho/M discontinuity

A

Between crust and mantle
Velocity of P-waves increases abruptly (from 7 to >8 km/sec)
Refraction & reflection of seismic waves

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13
Q

Low velocity layer

A

Seismic discontinuity within mantle
Physical difference, not chemical
Between 60-220 km
Seismic waves slow down slightly
Believed to be caused by 1-10% partial melting of mantle
Thin discontinuous film along mineral grain boundaries
Melt weakens mantle here —> makes mantle more ductile
Layer varies in thickness —> depends on local P, T, melting point, availability of water

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14
Q

410-km discontinuity

A

Seismic discontinuity

Believed to result from phase transition: olivine changes to spinel-type structure

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15
Q

660-km discontinuity

A

Coordination of Si in mantle silicates changes from IV-fold to VI-fold
Abrupt increase in density of mantle
Jump in seismic velocities
Below this discontinuity, wave velocities are fairly uniform until the core

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16
Q

Mantle/core boundary

A

Major chemical discontinuity

Silicates of mantle —> much denser Fe-rich metallic alloy with some Ni, S, Si, O, etc.

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17
Q

Outer core

A

Liquid/molten state
Fe-rich metallic alloy, with some Ni, S, Si, O, etc.
S-waves stop here; can’t travel through liquid (liquids cannot resist shear)
P-waves slow in liquid core, and refract downward: “shadow zone”

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18
Q

Inner core

A

Solid, due to increased P with depth

Same composition as outer core (Fe-rich metallic alloy, with some Ni, S, Si, O, etc.)

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19
Q

Rheological subdivisions of earth’s interior

A

Lithosphere
Asthenosphere
Mesosphere

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20
Q

Lithosphere

A

Crust & upper/rigid part of mantle (above low-velocity layer)
~70-80 km thick under ocean basins
~100-150 km thick under continents

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21
Q

Asthenosphere

A

More ductile portion of mantle

Thought to provide “zone of dislocation” that allows lithospheric plates to move

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22
Q

Mesosphere

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A

Mantle below the asthenosphere
Boundary at about ~700 km between asthenosphere & mesosphere, were transition from ductile to more rigid material occurs

23
Q

Oddo-Hawkins rule

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A

Atoms with even numbers are more stable & thus more abundant than odd-numbered neighbors

24
Q

Irons (meteorites)

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A

Mostly metallic Fe-Ni alloy
Believed to be fragments of the core of some terrestrial planets that have been differentiated
Contain siderophile (Fe-Ni alloy) & chalcophile (segregation’s of troilite: FeS) phases

Fe-Ni alloy: 2 phases: kamactite & taenite (exsolved from a single, homogenous phase as it cooled)
Commonly intergrown in a hatched pattern of exsolution lamellae called “Widmanstatten texture)

Considered “differentiated” meteorites; came from larger bodies that experienced chemical differentiation

25
Stones (meteorites)
Mostly silicate minerals | Include significant portion of silicate (lithophile) segregation mixed in
26
Stony-irons (meteorites)
Subequal amounts of Fe-Ni alloy and silicate minerals “Differentiated” meteorites, came from larger bodies that experienced chemical differentiation
27
Lithophile
“Stone-loving”; elements form a light silicate phase Most common in early earth: probably olivine, orthopyroxene, clinopyroxene
28
Chalcophile
“Copper-loving”; elements form an intermediate sulfide phase
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Siderophile
“Iron-loving”; elements form a dense metallic phase
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Atmophile
Separate phase; elements also have formed in the early Earth as a very minor ocean & atmosphere; light gaseous elements.
31
Subdivision of stones (meteorites)
Based upon presence of chondrules 1. Chondrites - contain chondrules 2. Achondrites - do not contain chondrules
32
Chondrules
Nearly spherical silicate inclusions between 0.1-3.0 mm in diameter Some appear to have formed as droplets of glass, subsequently crystallized to silicate minerals
33
Chondrites
Stones with chondrules Undifferentiated; as heat required to melt & differentiate would’ve destroyed the chondrules Small size of chondrules indicate rapid cooling (< 1 hr) Probably formed after condensation, but before formation of planetesimals Considered to be most “primitive” type of meteorites —> have compositions closest to the original solar nebula Suggested that: all inner terrestrial planets formed from a material of average chondritic composition —> thus the Chondritic Earth Model (CEM)
34
Achondrites
Stones without chondrules | Considered differentiated meteorites
35
Chondritic Earth Model (CEM)
Provides close fit to composition of Earth for most elements However: Earth = much denser —> must have higher Fe/Si ratio than chondrites
36
Pressure gradient
``` P=pgh Pressure = P Density = p Gravity acceleration = g Height of the column of material above = h ```
37
Hydrostatic pressure
Water = capable of flow —> pressure is equalized —> pressure is the same in all directions Horizontal pressure = vertical pressure
38
Pressure near surface and rock behavior
Rocks behave in more brittle fashion They thus can support unequal pressures If horizontal pressure > vertical ones —> rocks fault or fold
39
Pressure and behavior of rocks at depth
Rocks become ductile, capable of flow, like water
40
Lithostatic pressure
Just as with hydrostatic pressure, when rocks become ductile and can flow, pressure is equal in all directions
41
Average density of continental crust
2.8 g/cm^3
42
Average density for upper mantle
3.35 g/cm^3
43
Average pressure gradients of crust and upper mantle
Crust: 30 MPa/km | Upper mantle: 35 MPa/km
44
Geothermal gradient
Temperature variation with depth | No simple physical model analogous to pressure equation
45
Primary sources of heat in Earth (2)
1. Cooling: heat from accretion and gravitational differentiation gradually escaping; possibly some continued gravitational partitioning of iron in inner core also 2. Decay of radioactive isotopes: heat generated here also; most radioactive elements are concentrated in continental crust; decay produces 30-50% of the heat that reaches the surface
46
Processes of heat transfer (4)
1. Radiation: if material is transparent or translucent; movement of particles/waves move through a medium 2. Conduction: if material is opaque and rigid; involves transfer of kinetic energy (mostly vibrational) from hotter atoms to cooler; fairly efficient for metals 3. Convection: if material is ductile; movement of material due to density differences caused by thermal or compositional variations 4. Advection: similar to convection, but involves heat transfer with rocks that are in motion (e.g. hot region at depth is uplifted, heat rises physically/passively with the rocks)
47
Petrogenesis
Generation of magma and the various methods of diversification of such magmas to produce igneous rocks
48
Mid-ocean ridges
Most common/voluminous igneous activity Divergent plate boundary Shallow mantle undergoes partial melting Basaltic magma rises, crystallizes Plates move laterally, eventually subducted beneath continental or another oceanic plate
49
Continental rift
Divergent plate boundary Commonly alkaline, typically shows evidence of contamination by the thick continental crust If rifting continues, will become more like mid-ocean ridge
50
Oceanic-oceanic subduction
Volcanic island arc forms
51
Oceanic-continental subduction
Continental arc forms along active continental margin More silica rich than oceanic arc Plutons are more common in continental arcs (a) because melts rise to surface less efficiently through lighter continental crust or (b) because uplift & erosion is greater in continents and exposes deeper material
52
Back-arc extension
Plate divergence behind volcanic arc/subduction zone Due to frictional drag associated with subduction get plate Drag pulls down part of overlying mantle, so replenishment from behind and below is needed Back-arc magmatic is similar to MOR volcanism Slower spreading than MOR, volcanism is more irregular/less voluminous; crust is commonly thinner
53
Mantle plumes
Hot spots Occurs within toe plates (both oceanic & continental) Ocean: basaltic, but more commonly more alkaline than ridge basalts Deep, well into asthenosphere Intraplate: much more variable than within oceans; usually alkaline
54
Igneous-tectonic association
Broad types of igneous occurrence, e.g. MOR, island arc, intra-continental alkaline systems Ex: kimberlites and carbonitites —> occur within continental provinces

*Petrology (3 decks)

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