Transcript Document
Earth’s major plates Note: Boundaries rarely correspond to the contact between oceans and continents! Implied rates of plate divergence, convergence and strike-slip motion Rayleigh number = Buoyancy Viscous drag acceleration Momentum diffusivity X Thermal diffusivity Thermal expansion Temperature contrast Length scale Kinematic viscosity Thermal diffusivity A numerical model of whole-mantle convection in a 2-D earth Types of plate boundaries Relation between igneous activity and plate boundaries Starting point: Continents and oceans are the pieces of an Eden broken up by the great flood: Alfred Wegener proposes continental drift ca. 1912 1: The earth is divided into continents and oceans Alfred Wegener, ca. 1920, in a weird hat. 2: The continents look like pieces of a puzzle, and share elements of their geology. 3: The distribution of fossil animals and plants re-enforces the tie-points between the continents 4: The edges of the continents are locations of faulting and seismicity 5: when continents rift apart, you find an ocean between them Horst Rift Graben Put it all together, and see that the continents were once connected but have drifted apart by ‘sliding’ over or ‘plowing’ through the ocean floor Driving force: mystery, or centrifugal force, or both Counter-arguments This is just stupid Any force strong enough to ‘push’ a continent over a bed of ocean floor would internally deform the continent instead Counter-arguments Not all pieces of the continental puzzle really match Counter-arguments Many continental margins don’t even fit geometrically Counter-arguments Even if they fit, so what! Lots of ‘fits’ are possible just by chance Breakthrough: Harry Hess and the exploration of the ocean floor A ridge • Led to discovery that the ocean floor is the active part of the plate system, not the passive medium through which continents move. A ridge and its transforms seen in plan view Ahh, trenches The real story with seismicity Wagener was only partially correct. The ring of fire is a locus of faulting, but there are other loci not expected if continents drift Benioff-Wadati zones And, the seismicity at the edges of the ring of fire don’t represent continents sliding over oceans; they are places where ocean floor plunges into the deep earth interior. Active Volcanism Centered on most great belts of seismicity, and is rare elsewhere 1600: Recognition that earth has a magnetic field; supposed to be like a ‘lodestone’ (magnetite with a permanent magnetic field) 1919: Larmor proposes it is actually a dynamo 1946: Elsasser explains field as consequence of convection of induced electric currents in liquid outer core Geodynamo • Ohmic decay would dissipate earth’s field, if static, in ~20,000 years Therefore, must be sustained by some dissipative process (convection) • Fundamental cause of magnetism is induction in a convecting, conducting medium – electrically conductive medium – convection in response to density gradients in that medium – maintenance of density gradients by heat generation/exchange and/or chemical differentiation • ~80% dipole; rest other modes • Varies in intensity and orientation and relative strength of various harmonic components with a wide range of t constants Numerical model of ‘tangled’ magnetic field lines in core Terrestrial Magnetism Inclination… …and declination Map of inclination Intensity (in mysterious units — 10,000 ’s) Preservation in rocks of the orientation of the magnetic field Ferrimagnetism in magnetite Fe+3 only Fe+3 and Fe+2 spin of unpaired electrons in the Fe+2 and Fe+3 sub-lattices can order at low temperature. These oppose each other, but are unequal, leading to a net magnetic moment Variation through time of the apparent location of the north magnetic pole, based on records from North American rocks Phanerozoic records of magnetic polar wander from Europe and North America disagree…unless they have moved relative to each other (or, the shape of the Earth’s magnetic field has varied) Interesting, but all it really does is support and ‘flesh out’ Wegener’s view of ‘continental drift’. Much more important is an incredibly subtle detail to the fine structure of the modern magnetic field… A closer look… Anomaly Measured (I.e., relative to long-wavelength field) The anomalies represent positive and negative interference from magnetic rocks in the crust A magnetic ‘reversal’ occurs between these two times How do we know? We’ve seen them appear after volcanic eruptions on the sea floor. Some real examples Calibrating the ocean floor’s ‘strip-chart recorder’ 1: Collecting samples with the Glomar Challenger K-Ar dating 0.01167 % of natural K e- emission; = 4.982x10-10 yr-1 40Ca 88.8 % 40K e- capture; e = 0.581x10-10 yr-11 40Ar 11.2 % = e + = 5.543x10-10 yr-1 40Ar = e/40K(et-1) + 40Ar0 Some ‘closure temperatures’ w/r to K/Ar dating: Amphibole: 500 to 700 ˚C Biotite: 300 to 400 ˚C K-feldspar: 200-250 ˚C Use in dating magnetic reversals Maps of magnetic ‘stripes’ Sometimes it’s fun to pretend that our record of the seafloor’s magnetic stripes is complete: