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.
The LITHOSPHERE plates are ~100 km thick, but rigid.
The AESTHENOSPHERE is plastic (soft and deformable) and
generally 200-400 km thick.
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
also native only in the
Western Hemisphere, while
the shrubs and trees of
Eucalyptus
are
found
naturally only in Australia -->
<------- Monkeys with
prehensile tails are
found only in the
New World…. while
the Great Apes --->
are strictly 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.
[ 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!
Soooo, how does this work?
This is driven by CONVECTION in the mantle,
produced by heat from the Earth's deep interior.
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 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 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 in
southern Alaska
show this
pattern,
including one
on last Friday,
marked with
the brown star.
THIS is the typical end product. These are the Andes, from the
south flank of Pichincha Volcano in Ecuador, at an elevation of
about 4500 m (15,000 feet).
(The high peak at the left is Atacazo, another volcano.)
Imbabura
Cotopaxi
Tungurahua
Imbabura,
Cotopaxi
and
Tungurahua are just three
of many volcanoes in
Ecuador that have been
produced by this process.
The same process is going on off the North American coast
in Oregon and Washington State:
(Andesite, Dacite)
Mt. Rainier
Mt. Hood, Mt. Lassen, Mt. Shasta
and Mount St. Helens are all
products of the same process.
Mt. St. Helens,
May 18, 1980
Mt. Lassen
Mt. Shasta
Mt. Hood
Type II: oceanic lithosphere is subducted beneath other
oceanic lithosphere - creating an island arc system.
TYPICAL Island-arc systems include….
The Aleutian
Islands with an
earthquake
from last
Friday.
……. or the
Philippines, where
this earthquake
occurred this
morning ...
... or Japan, where
they also had one
yesterday......
... or Puerto rico,
where THIS
earthquake
occurred the day
before yesterday.
(The purple lines
are the subduction
zones – note that
Puerto Rico lies
between TWO of
them!)
So, island arc systems include ...
The Kuril Islands
The Aleutian Islands
The Japanese
archipelago
The
Lesser
Antilles
The
Philippines
Indonesia
Fiji (and many other islands of the SW Pacific)
Subduction of oceanic lithosphere beneath continental
margins is taking place along .........
The Kuril Islands
The Aleutian Islands
Kamchatka
NW North America
Central America
The Japanese
archipelago
The Lesser
Antilles
The
Philippines
Indonesia
The West
Coast of
South
America
Type III. Continent-continent collisions
This is responsible for many of the major mountain
ranges in the modern world:
The European Alps
The Zagros Mtns. of Iran
The Himalaya
The Appalachians
The Urals
The Brooks Range of N. Alaska
The Verkhoyansk & Cherskiy Ranges of Siberia
Case 1: the Alps of Europe
Relative to Europe, Africa is moving northwards, at least in the
northern part of the continent. This is slowly closing off the
Mediterranean Sea - the largest remnant of what used to be the
Tethys Sea. (The Black, Caspian and Aral Seas are the other large
pieces still in existence, all salt water.)
Case 2: The Zagros Mountains of Iran
Case 3: The Grand-daddy of 'em all, the Himalaya!
By some estimates, the highest mountains on Earth in over 500,000,000 years!
So, what's going on here?
A geologic map - even a simple one (!) - gives
just a small hint of the complexity that
dominates this region!
There are two opposing camps in the argument over the rise
of the Himalaya. This is the scenario supported by some
people:
That is, that the Indian subcontinent itself is being subducted
beneath the southern margin of the Asian mainland block.
The opposing view is that the subduction ceased when the
intervening oceanic crust disappeared, and that the
following is a more accurate cartoon image of what is taking
place:
The APPALACHIANS in this area owe their origin
to a two-part sequence:
The Catskill Mountains of New York are part of
the sedimentary apron that was created to the west
of the Acadian Mountains.
The NORUMBEGA
FAULT marks the
boundary between the
North American block
and the sliver of
Avalonia, which extends
southwards to at least
Maryland and
Virginia.
Farther south in the Appalachians, you can see the
same kinds of folds as those being created today
in the Zagros Mountains of Iran:
Appalachian Mountains in central
Pennsylvania -->