Transcript Fourth lecture - 16 September, 2015

2015 Colby Geology T-shirts are
in. Designed by June Li '16,
they're $10 each in the Geo
Dept. office
This Friday's Seminar Speaker:
Dr. Timothy McCoy
Curator-in-Charge of the Meteorite Collection
Smithsonian Institution
Four Cores, 5 Missions and
10,000 Meteorites Ago: What is
Inside Planets?
1:00 p.m.
Keyes 105
Geology in the news ...
Japan's Mount Aso erupted
on Monday, Sept. 14th,
sending clouds of
volcanic ash kilometers
into the sky and forcing
the evacuation of dozens
of hikers and tourists.
Also on Monday, flash flooding
near the Utah-Arizona border
has left sixteen people dead and
at least five more missing.
What does the winter hold in store for us?
PLATE TECTONICS and
CONTINENTAL DRIFT - Part 1
This has REVOLUTIONIZED the Earth Sciences
much the way Darwin and Natural Selection did to
biology in the 19th century.
The earliest
mapmakers noticed
the apparent
close fit of continental
margins.
As map making improved, the possible fits of the
continents were noted by Francis Bacon (15611626), George Buffon (1707-1788), and
Alexander von Humboldt (1769-1859)! This isn't
new!
BUT ….
With better,
modern mapping the fit got even
better yet! - about
96% perfect!
Alfred Wegener
proposed in 1912 that the
continents were not only
moving, but actually plowing
through the sea floor.
The leading physicists of his day, however, were
able to show that this was physically not
possible. The proposed mechanism was thus
discredited, so the entire hypothesis was set
aside (by most!) as yet more wishful thinking.
Now, however, plate tectonics is accepted by
most of the scientific community as
beyond significant doubt.
BASIC PREMISE:
The Earth's surface is comprised of 7
major plates, and about 20 minor plates,
that behave as relatively brittle slabs,
called LITHOSPHERE.
They move
over slow flow in an underlying plastic
layer, the AESTHENOSPHERE.
NOTE that these lithospheric plates are mostly
mantle, with the crust carried along on top.
The plates of the LITHOSPHERE are ~100-150 km thick, but rigid.
The AESTHENOSPHERE is plastic (soft and deformable) and generally
200-400 km thick.
Note that Iceland is split between TWO plates!
SO, what's the EVIDENCE ????
1. Matching geology
across areas of
apparent fit
This even includes matching mountain belts!
2. Matching fossils distributed across areas of proposed fit.
Lystrosaurus, a primitive Triassic reptile
specimen from Africa
specimen from Australia
Glossopteris, a primitive seed fern, ancestor of flowering plants
This is significant because …. the MODERN natural
biotas are completely different!
 For example, the
plant family Cactaceae
(the cacti) is native
only in the Western
Hemisphere, while the
shrubs and trees of
Eucalyptus are found
naturally
only
in
Australia 
 Monkeys that have
prehensile tails are found
only
in
the
New
World….
while
the
Great Apes  are only
found in the Old World.
3. Glacial geologic matching in ancient rocks - of glacial patterns
that make absolutely no sense unless the continents WERE
connected in the past, as matching margins suggest they were.
[ Oorlogskloof Nature Reserve,
South Africa ]
4. Paleomagnetic signatures in rocks of the sea floor...
... supplemented by demonstrably
greater sea-floor ages the farther
one is from the center.
5. "Polar wandering" curves - those from North America
and Europe are very different. BUT there's only one north
pole in the world!
This apparent discrepancy
ONLY makes sense if you
allow the continents to
move.
6. We now even have very accurate GPS measurements that show
how fast the plates are actually moving - DIRECTLY!
http://xenon.colorado.edu/spotlight/index.php?action=kb&page=3
Soooo, how does this work?
This is driven by CONVECTION in the mantle,
produced by heat from deep within the Earth.
If we look at the ocean floor, we see:
This mountain range is called the
Mid-Oceanic Rise and Ridge System (MORRS)
If we look closely at the MORRS, we see this topography
The increased age of rocks from the center outwards
implies the ridge in the center is expanding outwards,
pushing the continents apart as it grows.
This is called a Zone of Divergence of lithospheric plates.
BUT, if this is happening, either ...
… the Earth must be getting larger and
larger,
OR …….
Something has to be destroyed somewhere else!
This occurs in Zones of Plate Convergence
Type I: oceanic lithosphere is subducted beneath a continental
margin (an Andean-type subduction zone)
"Subduction leads to orogeny."
This subduction process creates earthquakes,
defining a plane called a Benioff-Wadati Zone.
This process also generates a great deal of heat, which
melts rocks, producing magma. We'll come back to this.
This subduction is
evident in the
increasing depth of
earthquake origins
(foci) inland which is how this
hypothesis
originated in the
first place.
Earthquake origins off the
coast of Kamchatka show
this pattern, including one on
February 10th, 2004, marked
by the green star.
FRIDAY:
The drama continues:
WHY? HOW?
mountain building,
earthquakes, &
volcanoes - oh my!