Plate Tectonics 2 Ocean crust forms at mid-ocean ridges (with magnetic stripes )! some more evidence for plate tectonics: (1)! magnetic stripes (conclusion) and (2) seeing it live with high-precision GPS! what makes these plates move around! anyway? Figure 2.22 1950"s : first towed magnetometer survey NOAA Geophysical evidence for the global tectonics (1960 s): magnetic stripes on the seafloor magnetic survey data from the 1950"s NOAA The US Coast and Geodetic Survey ship Pioneer (1955) Figures 2.20 and 2.21
Stripes and timing of magnetic field reversals tell us how fast oceanic plates are moving Vine-Matthews or Morley-Vine-Matthews hypothesis First ocean floor magnetic stripes map ever made: seafloor off BC and northwest USA, 1961 Canadian geophysicist submits papers to two science journals explaining the anomalies, both are rejected (L. Morley) His idea is an interesting one - I suppose - but it seems most appropriate over martinis, say, [rather] than the Journal of Geophysical Research Others get published first: Vine and Matthews, 1963 GPS satellite and receiver about 24 of these GPS and Plate Tectonics antenna solar panel repeated position measurements over time give velocity to within less than 1 mm/year!
Plate motions from geology and GPS match almost everywhere Mantle hot spots, which do not seem to move much www.unavco.ucar.edu Figure 2.58 geological estimates of plate motion velocities match GPS observations! Yellow - NNR NUVEL 1A plate motion model(geology) Purple - GPS site velocities relative to the International Terrestrial Reference Frame (ITRF) Figure 2.62 as plate is dragged across the hot spot, a chain of volcanoes may form Hawaii mantle hot spot records plate motion direction for millions of years - and this matches the current GPS velocity! The Antarctic Plate - rotating clockwise... from volcanoes Figure 2.63 from GPS
Edge of North America in SW BC is deforming elastically -- and will bounce back in a big quake (more in a future lecture!) So what happened last week (and the week before) here in SW BC? GPS sites moved backward Backward motion at the surface means slip deep on the Cascadia subduction zone Slow earthquakes happen on the CSZ every 400 days or so (equivalent to M6.5 to M7!) GPS receiver at Albert Head, Vancouver Island
The latest slow earthquake ended last week These slow earthquakes do increase the chance of a big earthquake on the CSZ - but VERY slightly Mantle Convection (model) PLATE TECTONICS: Why do plates move? Ch. 2 pages 60-61 Plummer, Canada First Edition http://www.ipgp.jussieu.fr/~labrosse/movies.html S. Labrosse
Forces that move plates now: slab pull hot mantle is gooey, buoyant, and tends to rise to the surface where it cools cold mantle is stiffer, dense, and tends to sink pulling the rest of the cold plate along with it early Earth: plates formed by cooling, and were passively dragged along by convecting mantle... heavy subducted slab of ocean plate pulls the rest of the plate along Figure 2.55 Forces that move plates now: ridge push Other forces that act on plates: plate s downhill from the ridge toward the subduction zone (ridge is high standing because it is hot and heat makes solids expand) Figure 2.54 (19.38) Trench suction (subduction zone) pulls continents to subduction zone Lateral drag along plate edges (like the San Andreas Fault): one plate pulls the other along Drag on the bottom of plates usually resists plate movement Figure 2.56 continental plates move slower than ocean plates. why?
What causes plate tectonics? Next: Earthquakes. now: plates are mostly self-propelled by pull from sinking slabs at subduction zones this is why ocean plates move much faster than continents hot, flowing mantle and weak plate boundaries make this possible: as Earth cools further plate tectonics will stop This is a very active area of research in geophysics! Centennial Earthquake Catalog (1900-1999) 60 N 60 N 30 N 30 N 0 0 30 S 30 S 60 S 60 S 0 30 E 60 E 90 E 120 E 150 E 180 150 W 120 W 90 W 60 W 30 W