GEOLOGICAL TIME SCALES
Fundamental to any historical study is a time frame or time line;

Geological time can be reckoned in two different ways

Relative Geologic Time Scale puts events of strata or life history into an ordered succession A>B>C>D etc.

Absolute Geologic Time Scale  attempts to assigned magnitudes or durations  of time elapsed between events and each other or with the present; generally assessed in millions of years, it is a quantification of geologic time; much harder to attain than relative time.(MORE SOON)

DEVELOPMENT OF THE RELATIVE GEOLOGIC TIME SCALE

Early attempts at ordering geologic time were based crudely upon superposition but tended to follow the incorrect assumption that rock type could be equated with rock age; hence Lehmann (1756) recognized a scale later adopted and promoted in the Neptunist philosophy of A. Werner  Primary (or primitive: igneous and metamorphic rocks) , Transitional (weakly metamorphosed sediments), Secondary (consolidated sedimentary rocks), and Tertiary (unconsolidated sediments or alluvium); basalt was included as a special category ought to be produced by subterranean burning of coal;

later a more rational scheme developed based on superposition plus crosscutting relationships and faunal succession.

The large entitites-eras were subdivided based upon faunal crises:
by Adam Sedgewick (1838) who recognized the Paleozoic Era bounded below by (putatively) unfossiliferous rocks and above by the great Permo-Triassic extinction; John Phillips (1841) divided the rest of earth's history into Mesozoic (middle life) and Cenozoic (modern life) split by the great Cretaceous-Tertiary extinctions

Most of the Paleozoic periods were founded in England; the impetus for this spate of naming and subdivision came from William Strata Smith's mapping of England and use of biostratigraphy to trace units; his geologic traverse from London to North Wales became the focus of much study:

Cambrian: Adam Sedgewick 1834 proposed Cambrian for Cambria (Wales) ; for the base of Smith's "Transition Series"

Ordovician: [not originally recognized) compromise system; named by Chas Lapworth (1879) to resolve a conflict

Silurian: Roderick Murchison 1834 proposed Silurian for fossil-rich rocks near the old territory of the Silures an ancient Welsh border tribe (some overlap with Sedgewick's Cambrian-both wanted to name period with the "oldest fossils";

This led to great controversy and generally falling out of S and M:
resolved by Lapworth 1879 who proposed to take out the interval of overlap between Sedgewick's Cambrian and Murchison's Silurian and call it  Ordovician: for the Ordovices, yet another old Welsh warlike tribe

Devonian: Sedgewick and Murchison jointly named Devonian for rocks below the Carboniferous in county Devon (Devonshire) England; they fought against De Le Beche who wanted all slaty rocks to be put in the same system: Cambrian: this was the clash of litho- vs. biostratigraphy

Carboniferous: already named in 1822 for the interval that included the Coal Measures; in North America this was split into

Mississippian (lower Carb) by Alexander Winchell 1870 for limestone exposures on the Mississippi River; and the term:

Pennsylvanian: was coined by Henry Shaler Williams in 1891 for exposures of coal-bearing strata in the state of Pennsylvania
But these are treated as sub-periods in most of the world where Carboniferous is still the primary term

Permian was named by Murchison in 1841 for Perm (later called Molotov) in Russia for rocks above Carboniferous but below Triassic

Triassic was named for the three-fold Bunter-Muschelkalk-Keuper red-gray-red succession of Germany by von Alberti (1834)

Jurassic an early named system was coined by the great explorer von-Humboldt 200 years ago (1799) for massive limestone cliffs of the Jura Mountains, France/Switzerland

Cretaceous: for the chalk cliffs on either side of the English channel but espec. near Normandy, France by D'Halloy (1822) (same year Carboniferous was named in England-lithology was in vogue)

Tertiary earliest named by Arduino (1760) as a part of the succession of Primary, Secondary, Tertiary (unconsolidated sediments), this period was/is used for the bulk of the Cenozoic Era, all but the last 1.8 million years which were later assigned to:  :

Quaternary by Desnoyers (1829), a Neptunist "holdout as the fourth stage of Earth history-for the great "dilluvial "sediments (glacial) of Europe

A fairer split for the Cenozoic periods is:

Paleogene Period(Paleocene, Eocene, Oligocene epochs) by Naumann (1866),

Neogene Period (Miocene and Pliocene epochs) devised by Hoernes (1853)

Quaternary Period (Pleistocene, Holocene epochs)

The great Charles Lyell tried to quantify the subdivision of Cenozoic epochs by using percentages of the contained mollusk faunas that were still extant: thus he defined Eocene (3.5-5%); Miocene (~15%); Pliocene (50%)
(later others were added)

Thus: all of the periods and most epochs of the geologic time scale had been designated by late 1800s; we still utilize this scheme of names to rapidly place events into a framework of time (somewhat like constellations in astronomy) ; the relative time scale continues to be refined at the level of series, stages and biostratigraphic zones

PRECAMBRIAN TIME FRAMES

Ironically, most of geologic time, about 8/9,  is not encompassed in these well known periods; and it can not be so easily subdivided because of a lack of distinct index fossils: it was tucked into Precambrian. Present usage:

Phanerozoic Eon             540 Ma- O Ma
      Hadrynian (Neoproterozoic)  1000 Ma = 1.0 Ga
Proterozoic Eon   Helikian (Mesoproterozoic)   1600 Ma =  1.6 Ga
      Aphebian (Paleoproterozoic)  2500 Ma = 2.5 Ga

Archean Eon               3800 Ma = 3.8 Ga
Hadean Eon               4600 Ma = 4.6 Ga
                                                                                                           ABSOLUTE GEOLOGIC TIME SCALES

All of the relative geologic time scale could be put together with tools existing in 1800s (superposition, cross-cutting relationships, faunal/floral succession) BUT it was not possible to give reliable absolute dates until discovery of radioactivity;

What was needed was an  a) irreversible, time governed process of b) known and c) constant rate, that could be d) recorded in rocks

Early attempts to establish age of Earth:

Bishop Ussher 1658 : Used Biblical chronology: Earth created October 23, 4004 BC at 9:00 AM (most precise date ever!; not very accurate)

Compte de Buffon  late 1700s  based on cooling of sphere suggested 75,000 years; incorrect presumption about source of Earth's heat

Halley (1795) and Joly (1800s) : based on salinity of  rivers vs. salinity of oceans argued that at present rates of delivery oceans would have  attained their present salinity in ~ 90 million years ; BUT salinity increase not irreversible, salinity builds up salts precip out; probably oceans attained present salinity in first few 10s of Ma

Many scientists tried to compile rates of sedimentation for different environments and then compile total thickness of various facies: generally came up with a maximum of a few hundred million years:
BUT: sed rates not constant, not well known, AND sedimentation is reversed by erosion

Lyell's estimates based on a guessed assumption as to how long it would take for complete evolutionary turn-over of mollusk species ( 20  Ma) argued that about  240 million years had passed since bivalves first appeared (Ordovician): evolution is irreversible, but does not occur at known or constant rate

Kelvin's estimates: in 1860s the great physicist Lord Kelvin held sway; precise mathematical calculations were based on: a) how long Sun could "burn" at its presently known rate; b) how long Earth would take to cool from molten mass based on present heat loss measurements; both seemed to give similar results:  20 to 40 million years for age of the solar system.
BUT Kelvin could  not take into account the generation of immense amounts of energy by nuclear fusion in sun and the constant (though decreasing) generation of heat in Earth's crust by radioactive decay

Radiometric Dating:  Discovery of radioactivity had synergistic impact on geochronology:
a) providing a mechanism for Sun's energy production that could keep going for billions rather than millions of years (eliminates Kelvin-a)

b) explained heat flow from Earth as largely due to heat generated by decay of radioactive isotopes in the crust (eliminates Kelvin b)

c) provided a realistic mechanism for dating:

Radioactive decay involves spontaneous nuclear change of unstable parent isotopes to daughter isotopes (atoms of an element with varying atomic mass due to different numbers of neutrons; NOTE: some isotopes are stable and do not decay, others unstable))

alpha decay: loss of an alpha particle (2 protons, 2 neutrons = He nucleus);  atomic number decreases by 2, atomic mass decreases by 4)

beta decay: nucleus loses a high speed electron neutron changes to a proton:  atomic number increases by 1; atomic mass unchanged

electron capture: nucleus picks up a high speed electron, converts a proton                 to a neutron; atomic number decreases by 1, atomic mass unchanged

Radioactive decay involving one or more of these processes occurs in such a way that it is a non-linear process; probability of decay for a given atom is constant; this means that as amount of parent isotope decreases the number of decays/time decreases such that it is a decreasing log function; we define

 Half life : as the time required for half of a given amount of parent isotope to decay to daughter; this is constant and under most circumstances, irreversible; radioactive decay goes on regardless of temperature, pressure (up to a point) or chemical environment; perfect system for dating; many different radiometric clocks can be checked against one another:
 
 
 

RADIOMETRIC DATING

With development of scintillometers it was possible to measure rates of decay and extrapolate radioactive half lives from lab samples; highly variable:

e.g.  235U--207 Pb      HL:  710 million years
 238U--206 Pb       H.L  4510 million years

Mass spectrometry: beam of ionized gas passed through huge magnets the stream of ions deflected by magnetic field to differing extents based on mass; impacts of various ions detected on ion collectors
With the invention of mass spectrometry it became possible to monitor the relative amounts of parent and daughter isotopes in a sample and determine parent daughter ratios;  if half life is known can then determine age of crystallization of the sample;

Caveats:
a)  there are errors in measurement with even the most precise mass spectrometry leads to +-  error factors

b) date gives time of closure of a system; i.e. age of crystallization in minerals; death of organisms for 14C

c) must assume closure: if daughter product has leaked from the system then will not give age of crystallization, may give an age of metamorphism;  e.g., in K/Ar dating inert gas Argon is readily driven from minerals by heating; will give apparent age too young

d) there may be some daughter isotope sealed into the system when it crystallizes; would give apparent ages too old; in some cases we can correct for this by subtracting a proportionality constant off using a third, non-radiogenic isotope of the element and recognizing that for non-radiogenic (common) isotope in solar system proportions is relatively constant

e) Most rocks can not be dated; different materials can be used for different systems but there are very few radioactive minerals in sedimentary rocks; so most dating is indirect; bentonites are one of the best  tools; otherwise, need to use cross cutting relationships with datable dike or sills that are injected into  fossil-bearing sedimentary rocks ;

recently some new isotopic methods are being developed that may permit direct dating of skeletal phosphates, etc.

GEOCHRONOLOGIC SYSTEMS

Some of the most commonly used radiometric systems are:

1)  U/Pb (with two potential systems: U-238; U-235; each decays by series of alpha and beta steps)
used for dating zircons; long half-life of U-238 limits use for very recent samples

2) K/Ar: used for micas, volcanic glass, feldspars, glauconite (sedimentary mineral); problems with leakage

3) Rb/Sr; usable for ancient samples, can correct for common Sr-87; mainly metamorphic rocks; long half life

4) C-14; a bit different; used for organic materials; very short half-life(so limited to just about 80,000 years or younger) do NOT measure parent/ daughter ratio but ratio of C-14 to C-12;  C-14 constantly being created by cosmic radiation bombardment of N-14 in upper atmosphere; unstable, will decay back to N-14; C-14 taken in during life activities, incorporated into wood bone or carbonate shell; in equilibrium with atmosphere while organism is alive, but once dead decays away (assuming no contamination )

5) fission track dating: relies on spontaneous fission of U-238 which produces defects or fission tracks in lattices of crystals;

a) these tracks can be enlarged by etching in HF and
b) then counted under a microscope to give number of fissions that have taken place to date;
c) then sample is bombarded with neutrons in a reactor which causes all remaining U-238 to undergo spontaneous fission;
d) etching and counting is repeated" ratio of tracks before/tracks after gives age in conjunction with known half-life for U-238

There are thus many radiometric systems that can be cross-checked and these yield similar results

Also, new methods are being developed that may give greater precision, e.g. Milankovitch cyclostratigraphy may permit bracket durations of geologic units to within a few thousand years

....................................
PLATE TECTONICS

Review of Earth Structure

Earth is oblate spheroid slightly flattened at poles; average radius: 6370 km

Earth  is divisible into several internal layers defined by seismology

Core 3470 km radius comprises:

Solid inner core; dense Fe Ni, 1250 km radius; temp. ~ 4000 C

Liquid outer core : 2220 km radius; molten Fe, Ni
Boundary of core is major discontinuity where S waves are stopped: Gutenburg discontinuity

Mantle:  about 2825-2850 km radius;
 lower mantle high density silicates rich in Mg, Fe; atoms packed together under pressure; has zones; in particular:

upper mantle is more rigid composed of
ultramafic rocks, such as  peridotite (olivine, plag feldspar):

Another key discontinuity: seismic low velocity zone; 60 to 150 km below Earth's surface: drop in earthquake wave velocity, separates plastic asthenosphere (~ 100 km thick)from rigid lithosphere it is thought that lithospheric plates ride along the asthenosphere
Lithosphere equal rigid upper mantle plus crust;

Crust: less critical discontinuity is crust mantle boundary Mohorovicics (Moho) discontinuity; 5 to 75 km down; thinnest near MOR oceanic crust

Two types of crustal rocks:
continental crustal : sialic or felsic rocks  Al rich silicates; granitic

oceanic crustal: simatic or mafic; Mg Fe rich silicates; basaltic

continental crust slightly lower density 2.6 vs. 2.8-2.9 g/cc; "floats" high on denser crust;
isostacy
 
 

PLATE TECTONICS

Tectonic Elements of Continents and Ocean Basins

Continents: low density felsic crust, high standing;

 stable platform (craton); some tectonic features such as domes  and arches

 marginal (mobile) orogenic belts  (most mountain belts occur  near  edges of continents or former margins;  e.g. Urals
 (older idea equated these with "geosynclines"

Ocean Basins: higher density, thin, mafic crust

 abyssal plains

 mid-ocean ridges: the really major mountain belts of Earth, ~
 46,000 km long, with relief of 3 km; best known segment is Mid  Atlantic Ridge; jagged with fault scarps: transform faults

 oceanic trenches; deepest parts of oceans; long narrow troughs;  very deep

 island arcs: chains of volcanoes associated with trenches

 oceanic island chains (nemataths) like Hawaiian  islands

These various pieces fit together in a plate tectonic model

First, consider some geophysical features of the Earth:  gravity, heat flow, seismology, magnetism

A) gravity:  gravitation proportional to m1 x m2/ d2;
:
force of gravity differs slightly over Earth’s surface

low gravity anomalies over mountain belts (implies deep "roots" of low density; like "icebergs" floating on water, sialic crust (d= 2.8-2.9 g/cc) floats on more mafic crust (d= 2.8-2.9 g/cc)  Isostacy

high gravity anomalies over MORs dense, mafic/ultramafic rock near the surface:

B) heat flow: geothermal gradient reflects dissipation of heat from Earth’s interior  (regular increase in temperature going down in the Earth  (ave for continents is 30 C/km) But heat flow varies from one tectonic element to another
low hf  in stable cratonic areas
high hf over mid ocean ridges: hot magmas close to surface; also in rift zones

C) Seismology:  earthquake waves produced by faulting are an important source of data about Earth’s interior (noted)
another key piece of data comes from distribution of earthquake epicenters; compiled from global network of seismic stations

noted shallow focus earthquakes over MORs;
dipping planes of earthquake distribution associated with trenches

distribution of earthquakes over time outlines major tectonic plate boundaries: MORs, transform faults (with earthquakes only between opposing ridge segments)l and, especially trenches

D) Magnetism:  Earth's core serves as a giant bar magnet with north and south polarity and magnetic force lines radiating from pole to pole; exact source of this magnetic field still incompletely known but may relate to circulation of liquid outer core around solid Fe, Ni rich core; self inducing dynamo

some facts:
 a) there is a definite polarity to the magnetic axis (N vs S)

 b) axis roughly corresponds on average with Earth’s rotational pole (11 50 off pole now)

 c) force lines radiate from pole to pole and dip angle of force lines varies from about  90 at poles to 0 near equator; inclination varies

 d) polarity of the magnetic field reverses erratically of relatively short periods, gives rise to magnetic epochs and events
 

e) from experiments in labs we know that when rocks form they lock in "fossil" or remanant magnetism of the time they form:
TRM thermoremanent magnetism for rocks the cool through Currie temperature from igneous melts (575 C for magnetite)

DRM for settling sediments that act like tiny compass needles

CRM chemical remanent magnetism for precipitated Fe oxides

(also may need to remove effects of later partial remagnetization) VRM (viscous remanent magnetism)

Remanent magnetism has three components :

Magnetic Declination: angle formed between apparent pole position and the modern north pole :

Magnetic Inclination: dip of force lines within sample (near zero for samples formed near the equator, 90 near poles

Magnetic Polarity: north or south

So what?  if Earth’s surface had stayed fixed should be no difference between remanent magnetism and present field
BUT:
this is not the case; rocks show declinations of many tens of degrees, inclinations do not match with latitudes
Paleomagnetism has had two critical impacts on plate tectonic theory:
1) in reconstructing past positions of plates and continents
2) pattern of magnetic polarity forms stripes on ocean floors that reveal ocean floor spreading patterns; led to the discovery of SFS

PLATE TECTONICS I : CONTINENTAL DRIFT

Earth’s surface subdivided into mobile lithospheric plates; created at mid-ocean ridges and spreading apart;  slide past one another along transform faults;  plates consumed at trenches (subduction zones)

Includes two major components and a (largely) apparent effect:

A) Continental Drift: notion n of continents moving relative to one another;  are passive "riders" on larger lithospheric plates

B) Seafloor (ocean floor) spreading:; plates of oceanic lithosphere move relative to one another and to continents

C) (apparent) Polar Wander migration of ancient pole positions with respect to present pole and continents (there may be some true polar wander but also much of this is an artifact of continental drift

CONTINENTAL DRIFT
Idea of continental drift go back to Wegener and DuToit in early 1900s; their ideas were based on empirical observations not theory; amassed an enormous amount of data but it was largely ignored due to the lack of a plausible mechanism
postulated continental drift and the past existence of supercontinents: Gondwanaland, Laurasia, and Pangea
Evidence for Continental Drift
1) Matching of coastlines: especially Africa-South America
Bullard fit used 1000 m isobath on continental  drift:; computerized fit is excellent except for minor erosional gaps and prograded deltas

2) Tectonic Evidence: match of mountain belts : when continents reconstructed,  Samfrau Belt of Dutoit (Paleoandes-Cape Fold Belt-Transantarctic Mountains-Tasman belts all align

Caledonian-Appalachian belt
Match of pieces of Precambrian tectonic and geochronologic provinces

3)  Paleoclimatic evidence:  climatic indicators out of place:

paleowind evidence: latitude indicator; westerlies in 40s, easterlies 20-30
bauxites and coals:should be tropical
reefs: subtropical
evaporites should be in 20-30 belt
glacial evidence; tillites on southern continents pathways of glacial striations do not make sense in modern geography (ice coming onto South America out of ocean) ; glaciiers appear to come from equator but make good sense when plotted on reconstructed continent
4) Stratigraphic evidence:: e.g. the similarity of the four-part Gondwanaland succession on all southern continents
A) Jurassic flood basalts (associated with rifting of Gondwanaland
B) Triassic redbeds with mammal-like reptiles; include Lystrosaurus
C) Permian: coals and fresh-brackish water deposits with  Mesosaurus, Glossopteris seed ferns
D) Permo-Carboniferous tillites, varvites and other glacial evidence (Talchir, Dwyka, Buckeye tillites)

5)  Paleontological and paleobiogeographic evidence: fossil distributions do not make sense in terms of modern continental distributions;  e.g. the fresh water Mesosaurus in Africa and South America; Lystrasaurus and a high percentage of other reptiles in Triassic common to several southern continents
Glossopteris flora found on all southern continents

anomalous distributions of fossils in "suspect terranes"; e.g. Paradoxides in Boston and England

anomalies  in distribution of living earthworms, lungfish , etc.;  vicariance

6)  Paleomagnetic evidence:  polar wander paths (plot apparent pole positions [declinations] through time for different continents; Polar Wandering Curves); converge toward modern poles; are non-superimposable (continents have moved relative to each other anomalies of inclination suggest changes in latitude (near 90 in Ordovician of Sahara; near 0 for Devonian in Arctic etc.

IN SUM: many independent lines of evidence pointed same way: Consilience

Wegener, DuToit postulated  Gondwanaland, Pangea in early 1900s

But:  continental drift was not widely accepted in northern hemisphere; geophysicists argued that continents can not plow through oceanic crust: they don’t; ironically, geophysics provided key evidence that led to Plate Tectonics revolution
 
 

SEAFLOOR SPREADING
 

(also anticipated by A. Wegener)

background - several seemingly unrelated phenomena were well known by 1960's (Harry Hess seminal paper 1962)

1.      seismicity of mid ocean ridges and trenches
2.      high heat flow in ridges and gravity anomalies
3.      a) deep sea drilling (Glomar Challenger) began collecting a lot of cores from deep sea floor; all age dates from deep sea floor samplesl eventually geologists concluded no datable seafloor basalts older than Jurassic.  Furthermore...

b) age of oldest rocks becomes progressively younger toward the ridge crests
4.      Sediment cover thickness varies considerably with relatively thick oozes of planktonic radiolarian and red clay near cont. Margins - thinner toward the ridges
5.      Magnetic anomalies: magnetic surveys of ocean floor by towed magnetometer across mid ocean ridges showed a pattern of high & low values - corresponding to areas where earth's present field partly canceled or reinforced.  Geologists called them magnetic anomalies.  Most striking observation was the marked symmetry of the anomalies on either side of the ridge axis ridge center with more extensive survey found anomalies formed continuous strips on either side of ridge crest.  This was puzzling for a long time - proved to be the key piece that caused whole series of puzzles to come to a common solution;
        Fred Vine & Mathews of Cambridge looked at this data and compared them with the profiles of land based geomagnetic time scale (this scale had been worked out in detail)

State was set:

Vine-Mathews made important hypothesis that parallel strips of mag. Anomalies occurred over strips of rock that had either normal or reversed paleomagnetic polarity and each half of the symmetrical strip pattern has same sequence as found in land-laid lava dikes - but with youngest (Bruhnes normal polarity) at ridge crests and getting successively older away from crest on either side

This could be testable by dating of basalt cores!

Suggested, in effect that the ridge crests represent a sort of paleomagnetic "tape recorder" new basalt added (accreted) at crest due to upwelling of magma; this rock cooled to its Currie Temperature - locked in earth's polarity in magnetite (TRM) then solidified rock was split in two by injection of new magma and moved away on either side

This is the first tenet of sea floor spreading model - new s.f. Basalt (sima) is added at ridge crests and spreads away on either side - links a series of unexplained phenomena together:
1.      High heat flow at ridge is uprising plumes of mantle    material
2.      High frequency of shallow focus earthquakes -   splitting of newly formed seafloor and movement apart
3.      Magnetic anomaly pattern now makes sense
4.      Explains increasing age of rocks away from ridge     crests
5.      Thickening of sediments away from ridge crests

Seafloor spreading empirically confirmed by accurate sighting between two islands on either side of ridge crests

Rates calculated by determining distance of equivalent anomalies of know ages on either side of ridge crest found rates to be about 2-6 cm/yr

But the rate of spreading can not be equal all along the mid-ocean ridge because fracture is on a spherical surface.  In order to spread evenly, vectors of spreading velocity must decrease toward poles (of spreading) pole of spreading

Problem because of this the mid ocean rifts - one affect along a series of permanent built-in faults: Transform Faults: allows separation of ocean ridge to occur at different rates in different segments.

What remains to be explained is what happens to sea floor plates once created and why no ocean floor rocks are very old.

W. Carey - expanding earth;  BUT

We know earth's crust isn't expanding so something must give

Second basic tenet of sea floor spreading:
1.      Seafloor is returned to the mantle (consumed)-subduction zones - slabs of seafloor are recycled into the mantle

This subduction is believed to occur in the deep oceanic trenches this also explains several problems
        1.      Explains why there is no very old sea floor - it eventually all gets destroyed
        2.      Explains the Benioff Zones-dipping plains defined by earthquake epicenters
        3.      Gravity anomalies over the ocean trenches and low heat flow in trenches
        4.      Island arcs - explained volcanism
 

because of light density, we think that continents are rarely subducted - they "ride high" - there is no evidence that they slide along the simatic lower crusts - apparently they move along passively as parts of plates which include the seafloor (sima) as well as the under layers of the sima

They may be bounded on one side by a subduction zone that "takes up the slack" - this being the case we can obviously see that a continent has a leading edge and a trailing edge (leading vs. Trailing edges)
 

Thus seafloor spreading is clearly linked with the way and to how continental drift takes place.

But is not a mechanism in itself.

The mechanism that powers this system is however, still a matter of debate
1.      Convection cells in upper mantle?
2.      Upwelling plumes of mantle material
3.      Downward pull of descending lithosphere slabs
 

The entire synthesis plate tectonics:  actually it is not just the sea floor crust that moves but the entire plates of the lithosphere - I.e. Both the crust and the rigid upper mantle - all move together as a unit and this "slides" on the asthenosphere

Thus the lithosphere as divided into a series of rigid but moveable plates that are added to at the fractures along the mid ocean ridges and that are consumed in subduction zones represented by oceanic trenches.

How do we recognize and define the boundaries of these plates? - Back to seismology - the plot of earthquakes shows where the activity is.
        A.      On the ridge crests
        B.      On the trenches (Benioff Zones)
        C.      On the faults that offset the ridge creswts - transform faults
Thus each plate may have these types of boundaries:
        1.      Spreading ridge - divergent boundary
        2.      Transform faults - transform boundary
        3.      Subduction (trenches) - convergent boundary

As indicated by earthquakes & volcanoes - these are the areas where the action is - it is at these places where the large plates interact that most major mountain building action takes place and other actions, e.g. Along the San Andreas

Next time - mountain building and how it is related to the interactions of plates.