Basics: Three Types of Plate Boundaries:
* Divergent - plates moving apart.
* Convergent - plates moving together.
* Transform – plates slip laterally past.
Divergent - New lithosphere created
* Many small volcanoes
* Mid-Ocean Ridges
* Continental Rift
* Analogous to a conveyor belt carrying material to the sides
* Youngest at the ridge, older away from it.
Convergent - “Destruction” of old lithosphere
* Mountains
* Volcanoes
* Earthquakes
* Subduction zones are where one plate is pulled beneath another
because it is heavier.
* Mostly ocean lithosphere (denser) goes down
Transform - plates slip past each other laterally.
* San Andreas fault.
* Earthquakes.
* No volcanoes
* Many segments associated with mid-ocean ridges.
* No production or destruction of old lithosphere.
Continental Drift - continents are not fixed in one place, but move thru time....
- 1858 Antonio Snider-Pellagrini (France) - Creation and It's Mysteries Revealed - continents were together until the Great Flood separated them.
- 1885 Edward Suess - (Austria) - linked much of the geological evidence for a southern Supercontinent consisting of India, Antarctica, Australia, South Africa and South America (Gondwana).
- 1915 Alfred Wegener (German meteorologist, explorer) - published The
Origin of the Continents and the Oceans
- established theory of continental drift
- named Pangaea as the supercontinent
Other Supercontinent Names:
2 smaller supercontinents just before Pangaea (~225Ma)
- Laurentia: North America & Europe (~400 Ma)
- Laurasia: add Russia, China... (~300 Ma)
- Gondwana: India, Antarctica, Australia, South Africa and South America
(~600-225 Ma)
1 large supercontinent about 1.0 Ga: Rodinia
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Evidence supporting Continental Drift
1. fit of the continents
2. distinctive rock types on Gondwanan continents
3. similarity of paleoclimatic conditions over much of the world,
indicating past supercontinents. Examples:
- distribution of coals containing Glossopteris (large seeds not transportable
by wind across oceans)
- distribution of Mesosaurus, a fresh-water aquatic reptile
- distribution of warm water limestones (now at high latitudes, formerly
where? Tropical!)
- distribution of Permian evaporites (evidence of low latitude aridity…
20-30 degrees from equator)
- distribution of glacial striations and tillites from ancient glaciations
- Cenozoic mammalian faunas differ among the continents
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** Geophysicists said No! to Continental Drift. It can't be
done. Period. No mechanism. Phooey!
Key Support for Drift: Paleomagnetism
What is Paleomagnetism? record of earth's magnetic field in the past.
- magnetism of earth is due to convection in the molten iron-nickel
core – liquid iron moving around generates an electromagnetic field
- igneous rocks freeze in a Remanent Magnetic Field when they cool
– record of the earth’s field at the time of cooling.
- sedimentary rocks freeze in a very low strength magnetic field, especially
in shales - a record of the earth’s field at the time of deposition.
- take rocks, measure this remanent field in a sensitive room (magnetically
shielded room: no field) to get the:
- Declination (direction to the pole)
- Inclination (dip of the field: low dips = low latitudes,
high dip = near the poles)
Together, these values point toward old paleopole locations, and also
represent the paleolatitude of the sample.
Paleomagicians, as we call them, practice….
PALEOMAGNETISM = "NOT A SIMPLE GAME".
In the 1950's, geophysicists found that paleomagnetism posed FOUR problems:
1) Rock magnetism didn't usually reflect the present earth's
pole position
- looks like the magnetic poles must have moved!
2) Rock magnetism was consistent within a continent, but not between
continents
- looks more like the continents have moved!
3) Magnetic reversals - flip flops in the earth's magnetic field
direction
- gives us a time-history of field polarity
4) Stripes of magnetic intensity on the sea floor recognized in
1950s, thought to be folding of the sea floor at first. 1960’s found stripes
to be symmetrical on both sides of ocean ridges.
- hmmm… how in the world?
Numbers 1 and 2 provided first geophysical support for Continental
Drift.
- either Poles had moved (#1),
- or Continents had moved (#2).
By reconstructing a Pangaea supercontinent, the paleopole positions
all lined up, indicating that:
1) the magnetic poles stay aligned with earth’s rotational poles, and
2) the continents have drifted and rotated through time.
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Sea Floor Spreading - Harry Hess, 1962.
- New ocean floor created at mid-ocean ridges (MOR)
Draw:
- continents move apart as on a conveyor belt away from the ridges
- amount of ocean floor is conserved :
=> produced at MOR - consumed at deep sea trenches.
- accounts for Benioff zones - dipping earthquake trends at deep sea
trenches
- accounts for Ophiolite suites in mountain belts
Benioff zones – (draw) a dipping zone of seismic activity that starts
at deep sea trenches and extends downward to 680km in the mantle.
Ophiolites - suites of ultramafic & mafic igneous rocks & deep sea sediments – ophiolites are found in many mountain belts of the world.
Simple form: (~7 km thick from bottom to top)
Ultramafics => Mafics => Deep-sea sediments
Where do Ophiolites Form? Mid-ocean ridges!
Draw (inside of a mid-ocean ridge):
How do ophiolites end up in mountain belts? Orogeny!
Draw (continent-continent collision):
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Orogeny – Mountain Building. An orogenic belt is another name
for a mountain belt. “Orogeny” is the group of plate tectonic
processes that build mountains, or the mountain building process itself,
or even a specific mountain building event (ex: the "Appalachian" Orogeny
was the late Paleozoic orogeny that created the Appalachian mountains.)
Old saying: "Subduction Leads To Orogeny"
Five general types of orogenies:
1) Divergent boundaries
2) Ocean-Ocean subduction
3) Oceanic subduction beneath Continent
4) Continent-continent collision
5) Terrane Accretion - or Microplate tectonics
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1) Divergent boundaries - most start in continents (weaker
rocks) then evolve into oceanic divergence. Continental rifting evolves
into seafloor spreading.
A. Continental divergence (Rift zones) - Extension
- Early stage: normal faults, basaltic volcanism, Basin & Range
topography - Rio Grande Rift
- Middle stage: oceanic crust begins to be produced, continents separate
- Red Sea
- Late stage: continental margins subside (thermal contraction) - Atlantic
passive margins
B. Oceanic divergence: Mid-Ocean Ridges (r) with arrows on them
showing direction of plate spreading ( <= or => ). Ridge
segments are offset by transform faults (–––––––––) between the ridge segments.
Outside the ridge segments, there is no transform fault motion, but there
remains a "scar" in the seafloor called a "Fracture Zone", represented
here by a dashed line (- - - - - - ). Fracture zones may extend for
hundreds of kilometers from the ridge/transform zone.
Here’s a sketch:
(sketch)
<= |r| =>
<= |r| =>
<= |r| =>
<= |r| =>
- - - - - - - - - - - –––––––––––––––––––––––––– - - -
- - - - - - - - - -
<= |r| =>
<= |r| =>
<= |r| =>
<= |r| =>
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2) Ocean-Ocean subduction - creates volcanic Island Arcs
- Japan, Aleutians
- volcanic island arc - andesitic and basaltic volcanism and plutonism
- accretionary wedge - (or subduction complex)
- composed of mélange: metamorphosed deep sea sediments
plus ophiolite fragments
Draw:
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3) Oceanic subduction beneath Continent - ex. Andes, Cascades.
- continental arc - andesitic to felsic volcanism and plutonism
- accretionary wedge – mélange again
- Fold-Thrust Belt and Foreland basin (FTB-FB)
- thrust faults and folds in sedimentary rocks on the overriding
continent. Detrital sediment sourced from FTB and continental arc fills
the FB. Foreland basin (FB) forms due to loading of FTB depressing the
lithosphere.
Draw:
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4) Continent-continent collision - ex. Himalayas, Appalachians
(Late Paleozoic).
- FTB-FB: deformation between continents
- Metamorphic and Plutonic Belt
- int to high grade, regional metamorphism
- includes Suture Zone - ophiolites marking boundary between
two continents.
- FTB-FB: deformation on the overriding continent
Draw:
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5) Terrane Accretion - "Microplate tectonics" - jigsaw
puzzle model for building orogenic belts. Tiny pieces of continental
or thickened ocean crust ride on ocean plates and accrete to continental
margins.
- Terrane - a 3-D block of crust containing a distinctive assemblage
of rocks.
- Displaced Terrane is a terrane on oceanic lithosphere that travels
the world until colliding with a continent at a subduction zone, where
it slides around until it gets stuck (accretion).
- Suspect Terrane is a terrane, located within an orogenic belt, bounded
by strike slip faults.
- examples of Displaced Terranes:
- continental - Madagascar, Seychelles, Britain
- oceanic - Hawaii, Iceland, 90 East Ridge
- examples of Suspect Terranes:
- western U.S. and Canada
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The Grand Reconstruction – also called the Plate Tectonics Revolution.
When plate theory hit in the 1960’s, it caused a complete turnaround
in how geologists viewed the world.
* Pangaea: Suddenly, Wegener’s ideas became very popular.
So much geology fit neatly together.
* Mountain belts: Previously thought of in terms of up and down movements,
suddenly they were reinterpreted in terms of lateral plate movements.
* Paleoclimates: Tropical limestones in polar regions are just
about impossible without plates that move.
* Economic geology: Exploration for petroleum and many mineral deposits
has been greatly aided by the ability to predict what the paleoclimates
and lithologies of various regions might be BEFORE millions of dollars
are spent drilling wells.
* Paleontology: Much of our understanding of the evolution of various
groups of fossil organisms is improved by our understanding of plate motions
and drift of continents.
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Driving Mechanisms of Plate Motion
Some processes causing plates to move:
1. Slab pull. Cold, ocean lithosphere slabs are more dense than the underlying asthenosphere, and hence sink. Slabs pull the ocean plate behind it, sometimes with continents attached.
2. Ridge push. Ocean ridges are elevated, and provide some gravitational push on the rest of the lithospheric plate, toward a subduction zone.
3. Plate drag from whole mantle. The lithosphere is connected to the underlying asthenosphere – it is a continuous transition from cooler mantle rock to hotter mantle rock. Therefore, as the asthenosphere moves, it drags the overlying plate along.
4. Plate drag from upper mantle. Same as #3, except that some geophysicists see evidence that most of the convection in the mantle occurs in the upper 300-700 km of the mantle instead of throughout the entire mantle. Not as popular an idea today as it was in the 1980s.
5. Plate drag from plume spreading . Hotspot plumes rise, and their spreading beneath the lithosphere causes drag. Not too likely. There’s little evidence that hot spot plumes rising under the lithosphere today cause any significant spreading and drag. May have worked with especially large plumes (superplumes) in the past (none around today, so it’s a bit speculative), and especially beneath continents, but doubtful that this is a major contributor.
Summary: All these mechanisms play some role, and some other
mechanisms as well, in moving plates around. Basically, plates move
because the mantle is hot, plastic, dynamic and moving due to convection,
while the surface (lithosphere) is cold, elastic and brittle, and responds
to the underlying dynamo.
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Review: Images of color topographic maps from around
the world, showing plate boundary examples:
1. Oceanic Rifting - Mid-ocean ridge - A Divergent margin, with
Ridge segments separated by transform faults
“Fracture zones” are not boundaries
but “scars” extending into adjacent plates
2. Continental rifting - A Divergent margin, with Narrow rift
valleys, faulting, volcanoes
Narrow ocean basin between continental blocks
after it gets a little older
3. Mid-ocean ridge (close-up)
4. Oceanic transform boundaries - A Transform margin
Parallel to plate spreading direction,
Perpendicular to ridge orientation
5. Oceanic transform boundaries
6. Continental transform boundaries - A Transform margin
Not as perfectly straight as the oceanic
transforms
7. Continental transform boundaries
8. Ocean-Continent Subduction - A Convergent margin
Oceanic plate subducts beneath a continental
plate
Volcanic range develops on the overriding
continental plate
Deep-sea trench at the boundary
9. Ocean-Ocean Subduction - A Convergent margin
Oceanic plate subducts beneath another
oceanic plate
"Island arc" volcanic range develops
on the overriding plate
10. Continental collision - A Convergent margin
Two continents or plates with
continental crust colliding
One attempts to subduct beneath the
other
Faulted, folded mountain belt forms
on the overriding plate
11. Displaced Terranes - Madagascar & Seychelles Is.
Small continental fragments.
Formerly between India/Africa.
Both left behind in ocean as India moved
away.
Seychelles are only granitic island
chain in an ocean
Website where you can find all these images (look at the pictures
yourself):
National
Geophysical Data Center (http://www.ngdc.noaa.gov/mgg/image/2minrelief.html)
See them in Appendix B of your Lab Manual, too.
Locations for all these images:
1. South Atlantic Ocean, near Brazil.
2 Red Sea, Africa and Arabian Peninsula
3. South Atlantic Ocean, near Brazil.
4. South Atlantic Ocean, near Brazil.
5. Indian Ocean, near the Seychelles.
6. Red Sea, Africa and Arabian Peninsula
7. West coast North America, San Andreas fault
8. South America, Andes and Peru Trench
9. Australian plate subducting beneath Indonesian island arc
10. India-Asia collision along Himalayas, Tibetan plateau behind