Download Plate Tectonics 2

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 new 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
Figure 2.58
www.unavco.ucar.edu
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)
Hawaii mantle hot spot records plate motion
direction for millions of years - and this matches the
current GPS velocity!
fro
m
vo
lca
Figure 2.63
fro
m
GP
S
no
es
Figure 2.62
as plate is dragged across
the hot spot, a chain of
volcanoes may form
The Antarctic Plate - rotating clockwise...
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
new
slide
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!)
new
slide
GPS receiver at Albert Head,
Vancouver Island
new
slide
The latest slow earthquake ended last week
These slow earthquakes do increase the
chance of a big earthquake on the CSZ - but VERY
slightly
new
slide
new
slide
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
heavy subducted slab of
ocean plate pulls the rest
of the plate along
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...
Forces that move plates now: ridge push
Figure 2.55
Other forces that act on plates:
Trench suction (subduction zone) pulls continents to
subduction zone
plate slides 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)
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