Chapter 01 - Fundamentals Flashcards
Chilled margin (chill zone)
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
Petrography
Branch of petrology; microscopic examination of thin sections
Interlocking texture
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.
Glassy texture
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.
Foliations
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.
Pyroclastic deposits
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.
Chemistry of rocks for identifying
Major elements, trace elements, isotopes, and some thermodynamics.
Earth’s interior - divided into three major units w/boundaries & discontinuities
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)
Oceanic crust
~10 km thick
Basaltic composition
Continental crust
~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
Mantle
~83% of Earth’s volume
Nearly 3,000 km
Mainly Fe- and Mg-rich silicate minerals
Moho/M discontinuity
Between crust and mantle
Velocity of P-waves increases abruptly (from 7 to >8 km/sec)
Refraction & reflection of seismic waves
Low velocity layer
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
410-km discontinuity
Seismic discontinuity
Believed to result from phase transition: olivine changes to spinel-type structure
660-km discontinuity
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
Mantle/core boundary
Major chemical discontinuity
Silicates of mantle —> much denser Fe-rich metallic alloy with some Ni, S, Si, O, etc.
Outer core
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”
Inner core
Solid, due to increased P with depth
Same composition as outer core (Fe-rich metallic alloy, with some Ni, S, Si, O, etc.)
Rheological subdivisions of earth’s interior
Lithosphere
Asthenosphere
Mesosphere
Lithosphere
Crust & upper/rigid part of mantle (above low-velocity layer)
~70-80 km thick under ocean basins
~100-150 km thick under continents
Asthenosphere
More ductile portion of mantle
Thought to provide “zone of dislocation” that allows lithospheric plates to move
Mesosphere
Mantle below the asthenosphere
Boundary at about ~700 km between asthenosphere & mesosphere, were transition from ductile to more rigid material occurs
Oddo-Hawkins rule
Atoms with even numbers are more stable & thus more abundant than odd-numbered neighbors
Irons (meteorites)
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
Stones (meteorites)
Mostly silicate minerals
Include significant portion of silicate (lithophile) segregation mixed in
Stony-irons (meteorites)
Subequal amounts of Fe-Ni alloy and silicate minerals
“Differentiated” meteorites, came from larger bodies that experienced chemical differentiation
Lithophile
“Stone-loving”; elements form a light silicate phase
Most common in early earth: probably olivine, orthopyroxene, clinopyroxene
Chalcophile
“Copper-loving”; elements form an intermediate sulfide phase
Siderophile
“Iron-loving”; elements form a dense metallic phase
Atmophile
Separate phase; elements also have formed in the early Earth as a very minor ocean & atmosphere; light gaseous elements.
Subdivision of stones (meteorites)
Based upon presence of chondrules
- Chondrites - contain chondrules
- Achondrites - do not contain chondrules
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
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)
Achondrites
Stones without chondrules
Considered differentiated meteorites
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
Pressure gradient
P=pgh Pressure = P Density = p Gravity acceleration = g Height of the column of material above = h
Hydrostatic pressure
Water = capable of flow —> pressure is equalized —> pressure is the same in all directions
Horizontal pressure = vertical pressure
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
Pressure and behavior of rocks at depth
Rocks become ductile, capable of flow, like water
Lithostatic pressure
Just as with hydrostatic pressure, when rocks become ductile and can flow, pressure is equal in all directions
Average density of continental crust
2.8 g/cm^3
Average density for upper mantle
3.35 g/cm^3
Average pressure gradients of crust and upper mantle
Crust: 30 MPa/km
Upper mantle: 35 MPa/km
Geothermal gradient
Temperature variation with depth
No simple physical model analogous to pressure equation
Primary sources of heat in Earth (2)
- Cooling: heat from accretion and gravitational differentiation gradually escaping; possibly some continued gravitational partitioning of iron in inner core also
- 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
Processes of heat transfer (4)
- Radiation: if material is transparent or translucent; movement of particles/waves move through a medium
- Conduction: if material is opaque and rigid; involves transfer of kinetic energy (mostly vibrational) from hotter atoms to cooler; fairly efficient for metals
- Convection: if material is ductile; movement of material due to density differences caused by thermal or compositional variations
- 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)
Petrogenesis
Generation of magma and the various methods of diversification of such magmas to produce igneous rocks
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
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
Oceanic-oceanic subduction
Volcanic island arc forms
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
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
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
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
