Transcript Fourth lecture - 8 February, 2017

This Friday's Geology Seminar speaker:
"High latitude marine
climate proxies: Oxygen
isotopes, biogenic
silica, and diagenesis"
Friday, 1:00 p.m.
Mudd 218
Where we left off .....
What we believe about them is based on:
1. Meteorites
There's a whole display case of real meteorites
down the hall from the lab, outside Mudd 212!
2. inclusions in volcanic rocks called xenoliths
3. rare rocks (fragments of uppermost mantle) exposed in
mountains where upper parts of the oceanic crust and mantle
are folded, buckled & pushed up ( called ophiolites).
Part of an ophiolite
block in
Newfoundland 
Olivine-rich
peridotite 
4. behavior of seismic waves from earthquakes & large
explosions (e.g., nuclear tests)
5. theoretical studies
BUT for MOST of the semester, we're going
to be focusing on the part we know the best the crust.
(Be it ever so humble, there's no place like home!)
Plate tectonics ~
A revolution hits
the Earth Sciences
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 (1561- 1626), 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
virtually all 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.
LITHOSPHERIC
mantle is rigid. The
AESTHENOSPHERIC
mantle is plastic, and
deformable – like
warm cheese.
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
MADAGASCAR
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.
Earthquake origins off the
coast of Kamchatka also
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!