•! 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?

Plate Tectonics 2

Ocean crust forms at mid-ocean ridges (with magnetic ‘stripes’)

Figure 2.22

1950"s : first towed magnetometer survey

The US Coast and Geodetic

Survey ship Pioneer (1955)

magnetic survey data

from the 1950"s

NOAA

NOAA

Geophysical evidence for “the new global tectonics” (1960’s): magnetic stripes on the seafloor

Figures 2.20 and 2.21

Stripes and timing of magnetic field reversals tell us how fast oceanic plates are moving

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)

Others get published first: Vine and Matthews, 1963

‘Vine-Matthews’ or ‘Morley-Vine-Matthews’ hypothesis

“His idea is an interesting one - I suppose - but it seems most appropriate over martinis, say, [rather] than the Journal of Geophysical Research”

GPS and Plate Tectonics

GPS satellite and receiver

antenna

about 24 of these

solar panel

repeated position measurements over time give velocity to within less than 1 mm/year!

Yellow - NNR NUVEL 1A plate motion model

Purple - GPS site velocities relative to the International

Terrestrial Reference Frame (ITRF)

www.unavco.ucar.edu

Plate motions from geology and GPS match almost everywhere

geological estimates of plate motion velocities match GPS observations!

(geology)

Figure 2.58

Mantle ‘hot spots’, which do not seem to move much

as plate is dragged acrossthe hot spot, a chain of

volcanoes may formFigure 2.62

Hawaii mantle hot spot records plate motion direction for millions of years - and this matches the

current GPS velocity!

Figure 2.63

from volcanoesfrom GPS

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

newslide

newslide

Backward motion at the surface means slip deep on the Cascadia subduction zone

GPS receiver at Albert Head, Vancouver Island

“Slow earthquakes” happen on the CSZ every 400 days or so (equivalent to M6.5 to M7!)

newslide

newslide

The latest slow earthquake ended last week These slow earthquakes do increase the chance of a big earthquake on the CSZ - but VERY

slightly

newslide

PLATE TECTONICS: Why do

plates move?

Ch. 2 pages 60-61 Plummer,

Canada First Edition

Mantle Convection (model)

http://www.ipgp.jussieu.fr/~labrosse/movies.html S. Labrosse

early Earth: plates formed by cooling, and were passively dragged along by convecting mantle...

hot mantle is gooey, buoyant, and tends to rise to the surface where it coolscold mantle is stiffer, dense, and tends to sink pulling the rest of the cold plate along with it

Forces that move plates now: slab pull

heavy subducted slab of ocean plate pulls the rest of the plate along

Figure 2.55

Forces that move plates now: ridge push

Figure 2.54 (19.38)

plate slides downhill from the ridge toward the subduction zone

(ridge is high standing because it is hot and heat makes solids expand)

Other forces that act on plates:

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

continental plates move slower than ocean plates. why?Figure 2.56

now: plates are mostly ‘self-propelled’ by pull from sinking slabs at subduction zones

this is why ocean plates move much faster than continents

What causes plate tectonics?

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!

Next: Earthquakes.

0˚ 30˚E 60˚E 90˚E 120˚E 150˚E 180˚ 150˚W 120˚W 90˚W 60˚W 30˚W

60˚S 60˚S

30˚S 30˚S

0˚ 0˚

30˚N 30˚N

60˚N 60˚N

Centennial Earthquake Catalog (1900-1999)