Introduction and Tectonic Plates

Introduction and Tectonic Plates  What is a plate ?  Why do plates move ?  What is the source of tectonic plate motion ? Introduction and Tectonic Plates Outline Today Plate Tectonics Introduction  Heat Energy in the Earth  Heat Conduction  Lithospheric plates  Theories on tectonic plate motion   Dynamic Theory - forces which give rise to deformation or change - cause of magmatism, metamorphism which affect the Earth's crust Kinematic Theory - describes motions but does not consider forces that move them We know it's true because it's “scientifically proven” What does “scientifically proven” mean ?  Geodynamicists develop models to study problems - build models using computers of labs - test models with observed data - improve and update models - results may show same observations but may be nonunique, others may give similar results - so models may indicate and explore possible phenomena but don't necessarily prove this result  Newton's theory of gravity: - did well at explaining gravity in the vacinity of the Earth  However, Einstein's theory on the subject did better on a larger spacial scale  Time Scales of Geology and Tectonic Motion The idea that continents move or shift on the Earth's surface is only believable when you can appreciate geologic time.  What is geologic time ? How long is this ? How is it different from time that we think of ordinarily ?  Since the theory of plate tectonics was first introduced, how far has the tectonic plate we're standing on moved ? Where did it go ?  (Frank Taylor first proposed this idea in 1910, Alexander du Toit later in 1937, Harry Hess explored the geomorphology of the seafloor by ship in 1960's)  Time Scales of Geology and Tectonic Motion Hutton and Hall studied geologic processes in Siccar Point, Britain where a subhorizontal Devonian sandstone rests on near-vertical Silurian slates.  “We felt ourselves necessarily carried back to the time when the shistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited....An epoch still more remote presented itself, when even the most ancient of these rocks, instead of standing upright in vertical beds, lay in horizontal planes at the bottom of the sea, and was not yet distrubed by that immeasurable force which has burst asunder the solid pavement of the globe...” - John Playfair, 1788 They had no quantitative estimate of the exact time intervals, but knew these periods were much greater than thousands of years.  Time Scales of Geology and Tectonic Motion Lyell in the 1850's provided a better estimate of geologic time with his theory of uniformitarianism.  He stated that catastrophies played no part in the past and all observable processes occurred by slow action.  “ Stupendous as is the aggregate result, there is no escape from the necessity of assuming a lapse of time sufficiently enormous to allow of so tedious an operation.” Time Scales of Geology and Tectonic Motion Charles Darwin in his “Origin of the Species” gave one of the most quantitative estimates of geologic time.  Time Scales of Geology and Tectonic Motion Charles Darwin in his “Origin of the Species” gave one of the most quantitative estimates of geologic time.  As an illustration, he estimated the time to erode a formation in England was 300 million years.  Upon his arrival in south America on the vessel, Beagle, he noticed the elevation of strata in the Andes mountains correlated with declining proportion of fossil species. He inferred progressive uplift of strata and the mountain itself.  Darwin's experience of a powerful earthquake in the Andes influenced his observation that catastrophes may play a role in geologic processes. They observed uplift of 1-2 meters from the quake, suggesting that at 100 year intervals, this could produce the mountain range in roughly 100 million years!  Time Scales for the Age of the Earth Estimates for the age of the Earth produced long standing arguments, but these values were surprisingly similar....  Lord Kelvin calculated the time for the Earth to cool after formation if most of the planet's surface was molten. Using known values for thermal conductivity of mantle rocks and their melting temperature, he come up with something < 100 My.  Other geologists estimated the time to accumulate sedimentary strata (from the Phanerozoic) was a few hundred million years.  Both of these estimates are within the same order of magnitude, and only off by a factor of 2 or 3.  Even the time estimate from scripture is close if you consider the time since the written record and civilization.  Time Scales for the Age of the Earth We still can't directly measure the age of the Earth  The oldest rocks are 4 Ga (crustal rocks) to 4.27 Ga (zircons)   Meteorites by comparison are 4.57 Ga. The Earth's mean lead isotope composition with some assumptions for lead sources suggests that the Earth is about 50-120 million years younger than meteorites (~4.5 Ga).  But the lead isotope event assumed may actually be an “event” like the separation of metal from silicates.  Heat Energy in the Earth  Heat energy drives the Earth's interior mantle system If the Earth was hotter in the past and is cooling with time, then we strive to understand what processes act to cool the Earth down ?  Subducting cold plates  Volcanic eruptions at convergent boundaries  Volcanic eruptions at spreading centers  Others ?  Heat conduction  Rising plumes  Heat Energy in the Earth Lord Kelvin claimed in his calculations that there were “no unknown physical processes at work”.   Was this true ? Radioactivity (discovered by Marie Curie in late 1800's)  The Earth was not totally solid and rigid as assumed Later understanding of crustal rebound showed that mantle rocks “creep” slowly over time and behave “plastically” .  This style of slow deformation allows for formation and transport of convective structure like plumes and convection cells.  Heat Energy in the Earth's Crust  They did know the thermal conductivity of crustal rocks. They could estimate the temperature gradient with depth to the base of the crust which suggested it should be 1200o C. Is this normal ?   No! Crustal Moho temperatures are actually 400 – 700o C! What is the problem ? Maybe conductivity is not the only active heating process ? What else ?  Radiogenic material was later found to be highly concentrated in the continental crust compared with other tectonic structures.  Heat Energy in the Earth's Reviewing: What are the 3 heat transport processes in the Earth that we have discussed so far ?  Conductivity  Radioactivity  Convection   Which are the most important in the Earth ? Why ? Heat Conduction Homer, “dough!”    Let's start with conductivity of heat. Heat, as everyone knows, flows from a hot body to a cold body (not in the reverse). The rate heat is transferred depends on..... what ? - the temperature gradient - thermal conductivity of material cold L hot Heat Conduction The rate heat is transferred depends on..... what ?  - the temperature gradient (T2 - T1) - thermal conductivity of material (k) cold L hot T T + T T (temp) The rate of heat flow across the plate is described by Q, T -T Q = -k 2 1 L Heat Conduction cold L hot T T + T T (temp) The rate of heat flow across the plate is described by Q, Q = -k T L  Thermal conductivity, -k, is negative because heat flows from hot to cold regions. Units of k are watts per meter per degree C (Wm-1oC-1)  The thickness of the plate is L.  Q is measured in units of watts per square meter (Wm-2) Heat Conduction cold hot T T + T T (temp) Q = -k T L  To describe this in terms of a differential equation, let's replace T with (Thot – Tcold).  Distance, L, can be written as x. Q = -k (T+ T ) - T x Heat Conduction cold hot Q = -k T + T -T T T + T T (temp) x Q = -k T x So if a block or plate is thicker, how will heat flow Q vary ?  What if the block is made of granite and has low conductivity ?  What if it is made of basalt ? 