Convergent Plate Boundaries: ocean-ocean and ocean-continent subduction, continent-continent collision (orogenic belts);
why no subduction at continent-continent convergence?
Subduction Zones: trench (and bulge), volcanic arc, Benioff zone earthquakes;
Wadati-Benioff Zone: plane of descending EQs down to ~670 km, double plane imaged at some slabs, dip of slab/subductio angle vs. age of subducting lithosphere, seismic tomography shows high velocity plane descending deep into lower mantle at some subduction zones, cause of deep focus EQs (Green: sudden transformation of metastable minerals);
Volcanic Arcs: parallels deep ocean trench, lie over line where Benioff EQs are ~100 km below surface, is arc magma produced by melting of the subducting slab/crust via shear heating as originally hypothesised??? No, shear heating is insufficient to melt the slab/crust, chemistry of most arc magmas inconsistent with melting of subducted ocean crust, chemistry consistent with partial melting of mantle peridotite with incorporation of (contamination by) some material from overlying crust as magma rises, Caveat: a few localized examples have chemistry indicating probable melting of subducting ocean crust plus partial melt of mantle peridotite (Adakites) where subducting crust is young and hot; so how are arc magmas produced?  apparently by flux melting - metamorphic dewatering of subducting crust releases water into overlying mantle peridotite, lowering its melting temp and producing basaltic (mafic) magma via partial melting of mantle peridotite (ultramafic);
Subduction Complex - accretionary wedge, outer bulge, forearc basins

<article approvals due>  <reviewed midterm exam>   
subduction zones: accretionary wedge, forearc basins, backarc basins (marginal basins), remnant arcs (eg., Mariana subduction zone, Caribbean & Lesser Antiles)
continental collisions and orogenic belts: suture zones (trapped ocean crust), thrust belts, foreland basins, packets of eroded sediments (molasse and flysch), India-Asia collision, Siwalik redbeds (molasse), Tibetan plateau, does the lower crust flow & prevent crust from getting any thicker and smooth out the surface features?, zones of strike-slip and even normal faulting (extension) in continental collisions (e.g., India-Asia) "escape tectonics," Southeast Asia is being "extruded from between India and Siberia, Turkey being squeezed westward by the collision of Arabia with Asia, with movement on many strike-slip faults

more continental mountain building: accreted terranes in western North America (slivers of island arcs, oceanic plateaus), what causes orogenic events when no continental collision or major arc collision occurs? example: the North American Western Cordillera - decreasing subduction angle - flat subduction;
are tectonic plates really rigid within and only deform on the edges? Mostly, but... examples from Asia, difuse motion in middle of Indo-Australian plate, N. Amer-S. Amer difuse plate boundary east of the Caribbean
driving forces of plate tectonics (Chap. 11):  does mantle convection drive plate tectonics? No! (well probably not for the most part)
is the Earth expanding and pulling the continents apart?  No! (paleomagnetic test)
most important forces:  gravity = "ridge push" and "slab pull"  (Forsyth and Uyeda, 1975)
mantle flow may be important locally with respect to the motion of continents with deep keels, but not for ocean crust
is tectonics just a theory? Theory refers to all of the science associated with a concept. To a scientist theory does not imply that we don't really know if it is true.  We can measure the movement of tectonic plates using satellite GPS and other kinds of measurements (Chap. 5, p. 90).  We know it is true.  The plates and continents are moving, and in the direction and speed that we thought!
hotspot tracks and mantle plumes (Chap. 5): Hawaii-Emperor age progressive chain of volcanic islands and seamounts, guyots, Darwin's explanation of coral atols; mantle plumes and basaltic magma produced by decompression melting

hotspots (chap 5):
- are they caused by mantle plumes or hot lines along fractures in ocean lithosphere? 
- laboratory and numerical modelling of plumes (plume head & tail)
     plume heads spread out at base of mantle and should produce melt (decompression melting) over perhaps 2000 km diameter
     steady state hotspot follows
- mantle plumes, LIPs (large igneous provinces - flood basalts) and hotspot tracks
     the real world is just like the models
- how deep is the source region for mantle plumes? 
    the old convention model (Don Anderson):
        separate upper and lower mantle convection; so plumes (if they exist) must originate in upper mantle
    but a number of observations support the model of plumes from the lower mantle
        incompatible elements: the upper mantle is depleted in certain elements, hotpots are enriched as lower mantle is believed to be
        Os isotope ratios of hotspot volcanic rocks are more like the core (or core-mantle boundary, CMB) than midocean ridge basalt
        shear wave anisotropy at CMB beneath Hawaii is interpreted as showing lateral flow converging to vertical flow beneath Hawaii
        mantle tomography beneath Icelandic plume shows low velocity root extending beneath Iceland deep into lower mantle
- are mantle plumes stationary thereby forming a reference frame for determining "absolute" plate motions? 
     Indo-Atlantic hotspots maintain their spacing (within analytical error)
     but comparison of Indo-Atlantic hotpsot tracks with Pacific hotspot tracks (es. Hawaii) yields a large disagreement
     deformation in Antarctica has been posed as a reason for the above disagreement
     paleomagnetic study in Antarctica shows there has been movement between East and West Antarctica in the past 100 m.y.
     so could that account for the apparent disagreement of hotspots
     no, it doesn't - therefore, hotspots, as a general rule, are not fixed

introductory review of the origin of the Earth
solar nebula hypothesis
early Earth was hot
      impact heating, gravitational compression, 3-5 X radioactive heat
hot, partly/largely molten Earth segregated into iron core and silicate mantle
mantle silcates are ultramafic
how do you make the crust (mafic, intermed., felsic) from mantle rocks (ultramafic)
      answer: partial melting once Earth cooled-> yields mafic magma
      mafic magma can "fractionate" into intermediate and felsic magmas as they rise
Precambrian crustal growth and the origin of the continents (chapter 12)
Archean (4.6 - 2.5 Ga) and Proterozoic (2.5 Ga to 543 Ma) Eons
shields, platforms, cratons
oldest surviving crust: 4 b.y. old
oldest surviving minerals: 4.1, 4.2, 4.3, (4.4) Ga zircons found in younger rocks
oldest rocks: 4.57 Ga moon rocks and meteorites
Archean crust: greenstone belts and granite/tonalite/gneiss belts
      model for formation: back-arc basins and volcanic arcs/subduction zones
Kenoran/Algoman Orogenies (~2.5 Ga) microcontinents collide to form minicontinents
Hudsonian Orogeny (~1.7 Ga) minicontinents collide to form Laurentia (ancestral N. Amer.)
Grenville Orogeny (~1 Ga) continental collision (e.g., Fordham gneiss)
      formation of "Rodinia"
late Proterozoic - early Paleozoic (beginning ~700 Ma) passive margin sequence (continental rift)
      breakup of Rodinia
      Gondwana forms from remnants
3 Paleozoic orogenies form the Appalachians and Pangea
Ordovician (~475-450 Ma) Taconic Orogeny (accretion of island arc)
     metamorphic belt (e.g., Manhattan Schist)
     clastic wedge
Devonian (~380-350 Ma) Acadian Orogeny  (collision w/NW Eur & an exotic terrane)
     metamorphic belt
     granite intrusions (New Hampshire: the Granite State, Maine coast granites)
     clastic wedge (Catskill red beds)
     ---Silurian-Devonian Caledonian Orogeny of NW Europe is same event
     collision of Laurentia w/ NW Europe forms "Laurussia"
Penn-Perm Appalachian Orogeny (collision of Laurussia and Gondwana)
      metamorphism, folding & thrusting of sedimentary strata, clastic wedge
Late Paleozoic widespread mountain building
     Appalachian Orogeny & Ouachita Orogenies in N. Amer
     Mauritanide Orogeny in NW Afr
     Variscan/Hercynian Orogeny in S. Eur
     Uralian Orogeny between Eur & Asia
     caused by amalgamation of Laurussia + Gondwana + N. Asia
     to form Pangea

<article summaries due>
reviewed Grenville Orogeny through formation of Pangea
Pangea breakup
     rifting, but not separation, began ~ 225 Ma (Newark rift basin etc.)
     ocean basin opened (separation began) about 170 Ma, mid-Jurassic, Central Atlantic between N.Amer & Afr
          this was the separation of Gondwana from "Laurasia"
     Gondwana unzipped sequentialy from around Antarctica
          S.Amer+Afr separated from Ant ~160-155 Ma
               S.Amer separated from Afr ~125-120 Ma (Parana-Etendeka flood basalt), spreading in S. Atlantic
          India separated from Ant ~125 Ma (Rajmahal traps)
          Australia separated from Ant ~100
          New Zealand separated from Ant ~85 Ma
     N.Amer separated from Eurasia (North Atlantic Igneous Province), spreading in N.Atlantic by ~65 Ma
consequences of Pangea breakup
     Africa/Arabia and India drifted northward away from Antarctica to ultimately collide with southern Eurasia
          forming the Alpine-Himalayan mountain belt
     westward movement of N.Amer caused it to approach midocean ridge and subduct v. young, buoyant ocean crust
          resulting in the Laramide Orogeny and uplift and deformation of the western cordillera (Rocky Mts., etc.)
tectonics and sea level
     N.Amer (and other continents) flooded by high sea level about 5X in last ~500 m.y.
     increasing rate of seafloor spreading means greater area of ocean basins with young, buoyant ocean crust
          making ocean basins shallower and displacing water onto the continents
primer on the carbon cycle and climate
     the greenhouse effect
     photosynthesis-respiration
           burial of organic matter in marine sediments removes carbon from atmosphere
     weathering of rocks
           carbon is removed from the atmosphere to weather rocks, and is stored in carbonate rocks
           CO2 > carbonic acid > weathering > bicarbonate & calcium ions > limestone
     CO2 may be returned to the atmosphere by metamorphism of carbonate rocks or burning fossil fuels
     CO2 is continuously added to the atmosphere by outgassing from the mantle via volcanic activity
     rate effects: changing the rate of one part of the carbon cycle will change atmospheric CO2 and the climate
     negative feedbacks in the carbon cycle: changes in the carbon cycle and climate produce effects that counter the
          change, thereby buffering the climate from too great a change
     positive feedbacks in the climate system (ice-albedo effect):  glaciation (ice) increases the albedo (reflectivity)
          of the Earth causing climate to cool further and produce more ice
          melting of glaciers decreases the Earth's albedo causing climate to warm and more melting
tectonics and climate
     "Snowball Earth" ~750-650 Ma (late Proterozoic)
          obs: glacial tillites formed at low latitude, carbonate rocks (limestone) interlayered with glacial tillites
          tectonic cause: barren continents at low latitude, increased Earth's albedo resulting in glacial period
          model: freeze-fry cycles (runaway positive feedbacks)
     late Paleozoic Gondwana glaciation
          obs: glaciation of Gondwana continents
          tectonic causes: 1) Gondwana drifted over the south pole
               2) collisions and formation of Pangea produced many mountain ranges - increased weathering - decreased CO2
     Cretaceous Greenhouse Climate
          obs: warm climate and no ice caps mid Jurassic - early Cenozoic  (also time of high sea level)
          tectonic cause: breakup of Pangea - lots of mantle plumes & rapid seafloor spreading - increased CO2
     Descent to Cenozoic Icehouse Climate
          obs: gradual cooling from late Eocene through the Cenozoic to recent geologic times
          tectonic causes: 1) gradual reduction in seafloor spreading and CO2 outgassing
               2) Alpine-Himalayan and western cordilleran mountain building
          obs: abrupt cooling near Eocene-Oligocene boundary (~37-35 Ma) and onset of Antarctic glaciation
          tectonic cause: final separation of Antarctica from South America allowed circum-Antarctic current to form,
               thermally isolating antarctica, growing icecap, increasing albedo, cooling global climate
          obs: sudden cooling 2-4 Ma and onset of northern hemisphere glaciation
          tectonic cause: closure of Isthmus of Panama caused change in oceanic circulation, sending more equatorial water
               into the far north Atlantic and possibly either:
                    increasing the salinity and density so it sinks before bringing warmth into Arctic Ocean, or
                    warming the water and air in the far north causing increased precipitation through increased humidity