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*Petrology > Chapter 01 - Fundamentals > Flashcards

Chapter 01 - Fundamentals Flashcards

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

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

A

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

24
Q

Irons (meteorites)

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
Q

Stones (meteorites)

A

Mostly silicate minerals

Include significant portion of silicate (lithophile) segregation mixed in

26
Q

Stony-irons (meteorites)

A

Subequal amounts of Fe-Ni alloy and silicate minerals

“Differentiated” meteorites, came from larger bodies that experienced chemical differentiation

27
Q

Lithophile

A

“Stone-loving”; elements form a light silicate phase

Most common in early earth: probably olivine, orthopyroxene, clinopyroxene

28
Q

Chalcophile

A

“Copper-loving”; elements form an intermediate sulfide phase

29
Q

Siderophile

A

“Iron-loving”; elements form a dense metallic phase

30
Q

Atmophile

A

Separate phase; elements also have formed in the early Earth as a very minor ocean & atmosphere; light gaseous elements.

31
Q

Subdivision of stones (meteorites)

A

Based upon presence of chondrules

  1. Chondrites - contain chondrules
  2. Achondrites - do not contain chondrules
32
Q

Chondrules

A

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
Q

Chondrites

A

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
Q

Achondrites

A

Stones without chondrules

Considered differentiated meteorites

35
Q

Chondritic Earth Model (CEM)

A

Provides close fit to composition of Earth for most elements

However:
Earth = much denser —> must have higher Fe/Si ratio than chondrites

36
Q

Pressure gradient

A 
P=pgh 
Pressure = P
Density = p
Gravity acceleration = g
Height of the column of material above = h
37
Q

Hydrostatic pressure

A

Water = capable of flow —> pressure is equalized —> pressure is the same in all directions
Horizontal pressure = vertical pressure

38
Q

Pressure near surface and rock behavior

A

Rocks behave in more brittle fashion
They thus can support unequal pressures
If horizontal pressure > vertical ones —> rocks fault or fold

39
Q

Pressure and behavior of rocks at depth

A

Rocks become ductile, capable of flow, like water

40
Q

Lithostatic pressure

A

Just as with hydrostatic pressure, when rocks become ductile and can flow, pressure is equal in all directions

41
Q

Average density of continental crust

A

2.8 g/cm^3

42
Q

Average density for upper mantle

A

3.35 g/cm^3

43
Q

Average pressure gradients of crust and upper mantle

A

Crust: 30 MPa/km

Upper mantle: 35 MPa/km

44
Q

Geothermal gradient

A

Temperature variation with depth

No simple physical model analogous to pressure equation

45
Q

Primary sources of heat in Earth (2)

A
  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
Q

Processes of heat transfer (4)

A
  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
Q

Petrogenesis

A

Generation of magma and the various methods of diversification of such magmas to produce igneous rocks

48
Q

Mid-ocean ridges

A

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
Q

Continental rift

A

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
Q

Oceanic-oceanic subduction

A

Volcanic island arc forms

51
Q

Oceanic-continental subduction

A

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
Q

Back-arc extension

A

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
Q

Mantle plumes

A

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
Q

Igneous-tectonic association

A

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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