PLATE TECTONICS by 3YMS37H8

VIEWS: 100 PAGES: 173

									Plate Tectonics
The Unifying Theory
    of Geology
Lecture notes start on page 151
– Plate Tectonics and Geocycles
are one lecture in two parts

A more or less photo-free version of this
PowerPoint will be linked from my webpage

Warning: The next 3 slides are not in your lecture notes
The 4 “Big Ideas” in Geology
  The   Rock Cycle (Friday’s lecture)
   – By the mid-1700s
  Antiquityof Earth - “Deep Time” (Fr. Stang’s
   W 11/10 Lecture)
   – By the early 1800s, millions of years
  Faunal   Succession (Fossil Record – Core 6)
   – By mid-1800s
  Plate   Tectonics
   – By late 1960s
  The Big Ideas 
       Earth is
  Old and Dynamic

Rather than Young and Static
•Recall: The Scientific Method
Observations – discovery, look for patterns
Hypothesis (a testable explanation)
  – Includes testing by prediction
More observations (testing) -> support
Eventually multiple hypotheses -> Theory
  – Not a “hunch” nor a “fact”, but the best
    available explanation using natural causes
  – A Theory is an explanatory framework of the
    highest level and includes many supported
    hypotheses
       Plate Tectonics

is a great example of discovery-driven
science that was eventually supported
by many hypothesis-driven predictions
A sequence of observations
  about Earth throughout
     human history…
      The Ancient Greeks
Knew    that Earth was round (not discovered
 by Columbus!) by 300 B.C.
In fact Eratosthenes (ca. 240 B.C.) measured
 its circumference




                         Photo Not Available
    St. Augustine, ca. 400 A.D.
 Not  only knew that Earth
  was a sphere…
 But worried about the
  theological issue of the
  Antipodes (the other side of
  the globe) Romans 1:8; 10:18;
  16:25-26; Col. 1:6, 23; I Tim. 3:16
  – So the physical issue of a
    spherical Earth was well
    established in Western thought
    by this time.
                 “Early” replaced with “Renaissance”




Renaissance Observations
FirstEuropean explorers started to gather
 knowledge on world geography
Early 1500s: E.g., Magellan & da Gama
Renaissance Observations
Francis  Bacon (1561-1626) first noted how
 coasts of Africa and South America fit.


                          Yikes!!!…
                           bacon!
Today it’s
  known:
 Fit is best
along their
continental
  shelves
            The 1800s
Bettermaps available
Some people proposed that all
 continents could fit together
In 1872, British Challenger
 began mapping ocean floor by
 soundings
Similarities in rocks of NW
 Europe and NE America were
 discovered
About 1910, the
Mid-Atlantic Ridge
was discovered
The Three “Paleos”
Paleontology
The study of fossils
   Evidence from
   Paleontology
    Similar fossils in
South America and Africa
 Glossopteris, a
seed fern whose
  seeds are too
   large to be
 carried far by
      wind
Mesosaurus, a fresh water reptile
 that couldn’t swim across the
           open sea
Paleoclimatology
 The study of ancient climates
Evidence of glaciers
      - Striations
Evidence of glaciers
in South America, Africa, India,
        and Australia...
   At same time the Northern
 Hemisphere had lush swamps...
and the pieces fit together like a puzzle...




            “Gondwana”
                Coal
Deposits found in
 Antarctica brrrrr….
Coal requires a warm,
 lush climate ===>
What’s Antarctica like
 today? ===>
     Paleomagnetism
Magnetic  minerals in molten rock align
 with Earth’s magnetic field
  Paleomagnetism
When  igneous rock cools, magnetism is
 “frozen in”, like little compasses...
“Polar
Wandering”
= apparent
change in
position of
poles over
time
             Polar Wandering
Different   continents
 indicate different position
 of North Pole - very
 strange...
Only logical explanation
 is that the
 CONTINENTS have
 moved since the rocks
 were formed
  WEGENER and
CONTINENTAL DRIFT
 In   the 1910s Alfred Wegener put
    all these pieces of evidence
    together and made the hypothesis
    of CONTINENTAL DRIFT...
   The Origin of Continents and Oceans (1915)
 Hard    to give up old ways of thought
    – Earth should be reliable, always there,
      and unmoving, “terra firma”
    – E pur si muove : Core 2
CONTINENTAL DRIFT
When    Wegener
 proposed to fit the
 continental shelves
 together, rather
 than the coastlines,
 we got a better fit
 for all modern
 continents...
Idea: All continents have moved to their present
  positions from one “supercontinent” he called
                 PANGAEA




                   - 200 Ma
 Scientific Consensus
 is that Earth is about
           (this and next two slides not in notes)


      4.5 billion years old (4.5 Ga),
so 200 million years ago isn’t so long ago
     – about 5 % of Earth’s history
                    Perspective, context
Breakup of
 Pangaea
  Pangaea at 200 Ma
   Is still just 200/4500 = 4.5% of
             Earth’s history!
There is evidence of plate movements
          well before this time
            1912 - 1945
A  few geologists looked
 for more evidence during
 this period...
Seismologists began
 studying Earth’s deeper
 layers and discovered a
 dense mantle and liquid
 outer core
   WWII and SONAR
Sonar, developed to find enemy subs, was
 used in the decade after WWII to map the
 deep sea floor...
            SONAR
Previously, most geologists thought the sea
 floor was rather flat and featureless
They were wrong...
A diverse topography was discovered
Topography of the Sea Floor
 Topography of the Sea Floor
Researchers  found an
 undersea mountain range
 40,000 miles long
And a trenches seven times
 deeper than the Grand
 Canyon
Even more striking were the
 geophysical findings...
Some Geophysics
Thousands of drilling
    samples were
       taken...
           Geophysics
Paleomagnetism   in the deep sea floor rocks
 indicated that many episodes of magnetic
 reversals had taken place...
 These reversals occur in parallel paired bands
    on opposite sides of a mid-ocean ridge.




Radiometric dating showed the rocks get
 older the farther you get from the ridge.
      Sea-Floor Spreading
In 1962, these data were collected into a
 theory called SEA-FLOOR SPREADING
 – New crust forms at mid-ocean ridges...
 – Oceanic crust pushes outwards from the
   ridge and (perhaps) takes the continents
   along
Earthquakes and Volcanoes
Seismology   is the study of earthquakes
Remember the 2004 Indian Ocean Tsunami?
Earthquakes and Volcanoes
Data  indicate that
 earthquakes and
 volcanoes do not
 occur at random
 locations, rather...
Earthquakes and volcanoes occur mostly
 along or near deep ocean trenches and
 mid-ocean ridges...
E.g., Pacific “Ring of Fire”
    Seismology studies



located a zone of weakness where seismic
 waves travel more slowly
It was called the asthenosphere.
The rigid lithosphere sits on top of this
Finally, a unifying theory...
In  1968, seismologists at Columbia put
 all the evidence together and came up
 with the theory of
 PLATE TECTONICS...
This combined the sub-theories of
 CONTINENTAL DRIFT and SEA-
 FLOOR SPREADING
     PLATE TECTONICS
The rigid upper 40 miles or so of Earth (lithosphere) is
 broken up into a dozen or so plates, which can slide
 around on the zone of weakness (asthenosphere).
         MECHANISM
Current idea: plates move as a result of
 mantle convection, driven by Earth’s
 internal heat
Why does Earth have
  internal heat?
 And our Moon doesn’t?
Why doesn’t the Moon have Plate Tectonics? –
or volcanoes, earthquakes, or a magnetic field?
Answer: Earth still has a lot
     of internal heat
    And our Moon doesn’t
            Why?
Moon is much smaller than Earth
 It’s cooled off – Deep Time
Consequences
       of
Plate Tectonics
      Earthquakes
       Volcanoes
    Mountain Ranges


     Why it’s a
  “Unifying Theory”
First a review of
  definitions...
      Types
        of
Plate Boundaries
   DIVERGENT - pulling apart
    – Mid-Atlantic Ridge
    – Great Rift Valley of Africa
Great
Rift Valley
of Africa
   CONVERGENT - coming together
    – Japan
    – Himalaya mountains
Himalaya
 mountains
 formed when
 India collided
 with Asia
   TRANSFORM - slide past each other
    –San Andreas Fault
San Andreas Fault
          Volcanoes
 Are associated
 with divergent
 and convergent
 plate
 boundaries ...
 Why ?
 Pacific rim
“Ring of Fire”
 Convergent
     and
  Divergent
  boundaries
                Earthquakes
 Are associated with ALL
 boundary types ...
 Why ?
   Movement!
Mountain Building
    “Tectonics”
          “Tectonics”
Comes from the Greek word for “building”
- For example, architect “master builder” or
   technology “the study of built things”
Some ages

And other facts…
 Mountain Ranges

Occur in linear patterns that
 parallel current or former
     plate boundaries
Appalachians
Started forming 400
  million years ago (400
  Ma)
Folded sediments and
  hard rock, but
  evidently worn down
  by erosion
Highest elevation = Mt.
  Mitchell - 6,684 ft


Cumberland Gap
 Rockies
Started
 forming
 60 Ma
Highest
  elevation =
  14,440 ft
  (Mt. Elbert)

Maroon bells, CO
Himalayas

Started forming
 30 Ma
Highest elevation
 = 29,035 ft
 (Mt. Everest)
     Laurentians (in Quebec)
  Canadian
  “Shield”
 > 500 Ma
 Hard rock
  “roots” of an
  ancient
  mountain range
 Highest
  elevation =
  3,825 feet
Folded
Strata
Rockies 




Appalachians 
MOUNTAIN BUILDING
        and
 PLATE TECTONICS
CONVERGENT BOUNDARY
        - the Real Deal for mountain building




Complex mountain system develops
     ---> Continent Grows
Example: The Andes
The Andes
 Geocycles




Rocks, Water, Air
       GEOCYCLES
Recycling   of matter at or near Earth’s
 surface
Rock cycle
  –Lithosphere
Water cycle
  –Hydrosphere
Weather and Climate
  –Atmosphere
   First, the
ROCK CYCLE
Nice Factoids to know – How big
is our planet?
Earth  is about 8,000 miles in diameter
and 25,000 miles in circumference
So, it will take you about 20 hours in a
 jetliner at 600 mph to get from Spain to
 New Zealand
     4 concentric layers
Inner core (solid)
Outer core (liquid)
Mantle (gooey in
 places)
Crust (solid)
 3-40 miles thick
What is the Lithosphere and what
        are the “Plates”?
The  crust, plus the upper 40 miles of
 the mantle, form the lithosphere,
 broken into plates which slowly slide
 around on the asthenosphere (zone of
 weakness)… via energy from Earth’s
 internal heat
       IGNEOUS RXS
            “Fire-formed”
 Liquid rock is called magma or lava,
 depending on amount of dissolved gas
 INTRUSIVE - cooled under surface
  – Examples: Granite, Gabbro
 VOLCANIC - cooled at surface
  – Examples: Basalt, Obsidian, Scoria
     SEDIMENTARY RX
                   “Settling”
Clastic - formed from pieces “clasts”,
 which come from the weathering and
 erosion of other rock.
  – Examples: Shale, Sandstone, Conglomerate
Bio-chemical      and Organic
  – microskeletons, e.g. in coral reefs: Limestone
  – Salts in stagnant pools, e.g., Halite
  – Organic - decayed remains of plants: coal
   METAMORPHIC RX
           “Changed form”
Depends   on original rock type and
 amount of heat and pressure
 Shale ----> Slate or Schist (more heat)
Granite or “dirty” sandstone ---> Gneiss
Sandstone ----> Quartzite
Limestone ----> Marble
    IGNEOUS RXS and
    ABSOLUTE DATING
 Some igneous rocks can be
 radiometrically “dated” – i.e., have
an ABSOLUTE AGE assigned (+/-)
On the other hand…

SEDIMENTARY
   ROCKS

Are most likely to
  contain fossils
But, SEDIMENTARY
      ROCKS
Can only be relatively dated…
RELATIVE DATING
RELATIVE DATING
       DEEP TIME

Oceans vs. Puddles
 AIR
 and
WATER
          Water Facts...
70  % of Earth’s surface is covered by H2O
70 % of human body’s weight is H2O
Water can exist in 3 states on Earth’s surface
  – exists only as solid and vapor on Mars
  – only vapor on Venus
Mars is too cold...
Venus is too hot...
Earth is just right...
  More Water Facts...
Water  freezes at 0 degrees Celsius and
 boils at 100 degrees Celsius
 –This is higher than similar substances
   because of hydrogen bonding
 –Water is most dense at 4 oC. This
   explains why ice floats, which prevents
   lakes from freezing solid in Winter
Hydrogen Bonding
  ........O          H ........
  /  \         /
 H    H ---- O
  |    |     \
  O   O          H .........
 / \ / \
H H H H .........
| | | |
               Water...
Itsheat capacity is higher than similar
 substances
  – “A watched pot never boils.”
Moderates   Earth’s temperatures
  – Compare San Francisco and St. Louis
               AIR
Main  gases in present atmosphere:
 Nitrogen (78%), Oxygen (21%), and
 Argon (0.9%), variable amounts of
 water vapor
Average sea level atmospheric
 pressure is 14.7 pounds per square
 inch and decreases with altitude
DISTRIBUTION
      of
   EARTH’S
WATER and AIR
Origin
             Origin
How:   Impact of Icy Comets and
 Degassing of volatiles ... bottle



Earth’s atmosphere was forming 4.4 to
 4.0 Ga
It then (probably) consisted mainly of
 water vapor, carbon dioxide, ammonia,
 methane (Some controversy)
Mt. Erebus, Antarctica
        Brief History
Icy Comet impacts ---> add H2O to
 atmosphere and down to about 70km
Earth cools ---> Water condenses
Lots of rain; Volcanoes add CO2
Most of CO2 dissolved in oceans
Photosynthesizing organisms evolve
  – CO2 + H2O ------> C6H12O6 + O2
  The Water
    Cycle
How Earth’s water and air
          interact ...
Natural “recycling” process
The Water Cycle
  ZONES OF
CONCENTRATION
 Of natural waters
        ZONES OF
      CONCENTRATION
Marine  (saltwater)- The oceans contain
 over 97 % of earth’s water -- salt water
Brackish (mixture of salt and fresh):
Found in estuaries, where fresh and salt
 water meet
  – Important to aquatic life
  – 3rd most productive ecosystem
  – Example: Chesapeake Bay
        Fresh Water
Less  than 3 % of Earth’s water is fresh
Glaciers contain over 75 % of Earth’s
 fresh water, but this source is not usable
 by life
       Fresh Water
Lakes   and rivers contain less than 1 %
 Groundwater (including soil moisture)
 accounts for the remaining 24 %
If all Earth’s water fit in a gallon jug,
 there would be one tablespoon of
 available fresh water
  Structure
    of the
 Atmosphere
 Layers classified by
temperature changes ...
  The Atmosphere
    All weather occurs in the
troposphere = “churning sphere”
Convection causes the
“churning”


Temperature
 decreases
with altitude
Convection in Humid Air
Thunderhead
       Stratosphere
 The ozone layer is contained in
 the stratosphere ...
 No “churning” because of a
 temperature inversion, i.e.,
 Temperature increases with
 altitude...
                              A
                              l
                              t
                              i
                              t
Stratosphere --->             u
                              d
                              e
Troposphere --->


                    Temperature
 Good Ozone and Bad Ozone
The same molecule, O3
     The Ozone Layer
Ozone,   O3 , is an air pollutant in the
 troposphere, but in the stratosphere
 protects us from UV radiation :
    O3 + UV --------> O2 + O
 Chlorine radicals (like from CFCs)
 interfere with this process:
    Cl + O --------> ClO
WEATHER
  and
CLIMATE
- Atmosphere
  Geocycles
  Coriolis Effect
   Caused by an unattached
atmosphere over a rotating Earth
Which way does Earth
      rotate?
     East to West
          Or
     West to East?
Coriolis Effect
Coriolis Effect on Winds
Coriolis Effect on Winds
NON-ROTATING   ACTUAL
PREVAILING WINDS
 The direction the winds blow most of
 the time ...
 Wind direction changes temporarily due
 to weather systems ...
                            o    o
In the middle latitudes (30 - 60 ), the
 prevailing winds are from the west.
 These affect climate as we shall see in a
 moment ...
PREVAILING WINDS
     OCEAN CURRENTS
 (Surface) currents caused by prevailing
 winds and Coriolis Effect ...
 They play a major role in redistributing
 Earth’s heat ...
 They are called warm or cold, compared
 with other water at that latitude ...
  –Example: The Gulf Stream is a warm
   current.
OCEAN CURRENTS
    Effects on Climate
 Many factors affect climate
 Prevailing winds and ocean currents are
 only two of these
 Example: Why is the weather in Paris
 more pleasant than in Quebec City ?
 Gulf Stream + Prevailing westerlies
Which is farther North?
Minneapolis or Venice
 Minneapolis: 44.58o N
 Venice: 45.27o N
         Who’d ‘a thunk it?

								
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