Plate Tectonics

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Plate Tectonics

• The Unifying Theory
• of Geology

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The 4 Big Ideas in Geology

• The Rock Cycle (Fridays lecture)
• By the mid-1700s
• Antiquity of Earth - Deep Time (Wednesdays
  Lecture)
• By the early 1800s, millions of years
• Faunal Succession (Fossil Record Core 6)
• By mid-1800s
• Plate Tectonics
• By late 1960s

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The Big Ideas ? Earth is Old and Dynamic

• Rather than Young and Static

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Recall The Scientific Method

• Observations
• Hypothesis (a testable explanation)
• Includes testing by prediction
• More observations (testing)
• If ALL observations fit ---gt Theory

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The Ancient Greeks

• Knew that Earth was round (not discovered by
  Columbus) by 300 B.C.
• In fact Eratosthenes (ca. 250 B.C.) measured its
  circumference

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Early Observations

• First European explorers started to gather
  knowledge on world geography
• Early 1500s E.g., Magellan da Gama

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Early Observations

• Francis Bacon (1561-1626) first noted how coasts
  of Africa and South America fit.

Yikes!!! bacon!
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Today its known Fit best along their
continental shelves
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The 1800s

• Better maps 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

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About 1910, the Mid-Atlantic Ridge was discovered
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Paleontology

• The study of fossils

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Evidence from Paleontology

• Similar fossils in
• South America and Africa

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Glossopteris, a seed fern whose seeds are too
large to be carried far by wind
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Mesosaurus, a fresh water reptile that couldnt
swim across the open sea
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Paleoclimatology
The study of ancient climates
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Evidence of glaciers
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Evidence of glaciers
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in South America, Africa, India, and
Australia...At same time the Northern Hemisphere
had lush swamps...
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and the pieces fit together like a puzzle...

Gondwanaland
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Coal

• Deposits found in Antarctica brrrrr.
• Coal requires a warm, lush climate gt
• Whats Antarctica like today? gt

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Paleomagnetism

• Magnetic minerals in molten rock align with
  Earths magnetic field

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Paleomagnetism

• When igneous rock cools, magnetism is frozen
  in, like little compasses...

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Polar Wandering apparent change in position of
poles over time
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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

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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)

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CONTINENTAL DRIFT

• When Wegener proposed to fit the continental
  shelves together, rather than the coastlines, we
  got a better fit for all modern continents...

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• All continents have moved to their present
  positions from one supercontinent he called
  
  PANGAEA

- 200 Ma
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Scientific Consensus is that Earth is about

• 4.5 billion years old
• (4.5 Ga)

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Breakup of Pangaea
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1912 - 1945

• A few geologists looked for more evidence during
  this period...
• Seismologists began studying the deeper layers of
  the earth and discovered a dense mantle and
  liquid outer core.

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WWII and SONAR

• Sonar, developed to find enemy subs, was used in
  the decade after WWII to map the deep sea floor...

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SONAR

• Previously, most geologists thought the sea floor
  was rather flat and featureless
• They were wrong...
• A diverse topography was discovered

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Topography of the Sea Floor
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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...

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Some Geophysics

• Thousands of drilling samples were taken...

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Geophysics

• Paleomagnetism in the deep sea floor rocks
  indicated that many episodes of magnetic
  reversals had taken place...

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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.

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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

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Earthquakes and Volcanoes

• Seismology is the study of earthquakes

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Remember the Tsunami?
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Earthquakes and Volcanoes

• Data indicate that earthquakes and volcanoes do
  not occur at random locations, rather...

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• Earthquakes and volcanoes occur mostly along or
  near trenches and mid-ocean ridges...

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E.g., Pacific Ring of Fire
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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

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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

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PLATE TECTONICS

• The rigid upper 35 miles or so of the earth
  (lithosphere) is broken up into a dozen or so
  plates, which can slide around on the zone of
  weakness.

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MECHANISM

• Current idea plates move as a result of mantle
  convection, driven by earths internal heat

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Consequences of Plate TectonicsEarthquakesVolc
anoesMountain Ranges

• Why its a
• Unifying Theory

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FIRST, A FEW DEFINITIONS...
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Types of Plate Boundaries

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• DIVERGENT - pulling apart
• Mid-Atlantic Ridge
• Great Rift Valley of Africa
  

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• Great
• Rift Valley
• of Africa
  

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• CONVERGENT - coming together
• Japan
• Himalaya mountains

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• Himalaya mountains formed when India collided
  with Asia

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• TRANSFORM - slide past each other
• San Andreas Fault
  

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San Andreas Fault

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Volcanoes

• Are associated with divergent and convergent
  plate boundaries ...
• Why ?

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• Pacific rim
• Ring of Fire
• Convergent
• and
• Divergent boundaries

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Earthquakes

• Are associated with ALL boundary types ...
• Why ?

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Mountain Building

• Tectonics

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Appalachians

• Started forming 400 million years ago (400 Ma)
• Cumberland Gap

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Rockies

• Started forming 60 Ma
• Maroon bells, CO

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Himalayas

• Started forming 30 Ma

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Folded Strata

• Rockies ?
• Appalachians ?

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MOUNTAIN BUILDING and PLATE TECTONICS
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CONVERGENT BOUNDARY
Complex mountain system develops---gt Continent
Grows
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http//www.pbs.org/wnet/savageearth/animations/rif
t-subduction.html

• http//www.cnn.com/2006/WORLD/asiapcf/11/09/new.is
  land.ap/index.html

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Example The Andes
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The Andes
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Geocycles

• Rocks, Water, Air

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GEOCYCLES

• Recycling of matter at or near earths surface
• Rock cycle
• Lithosphere
• Water cycle
• Hydrosphere
• Weather and Climate
• Atmosphere

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First, the ROCK CYCLE
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4 concentric layers

• Inner core (solid)
• Outer core (liquid)
• Mantle (gooey in places)
• Crust (solid) 3-40 miles thick

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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 zone of weakness

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IGNEOUS RXSFire-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

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SEDIMENTARY RXSettling

• 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

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METAMORPHIC RXChanged form

• Depends on original rock type and amount of heat
  and pressure
• Shale ----gt Slate or Schist (more heat)
• Granite or dirty sandstone ---gt Gneiss
• Sandstone ----gt Quartzite
• Limestone ----gt Marble

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IGNEOUS RXS andABSOLUTE DATING

• Some igneous rocks can be radiometrically
  dated i.e., have an ABSOLUTE AGE assigned...

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On the other handSEDIMENTARY ROCKS

• Are most likely to contain fossils

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But, SEDIMENTARY ROCKS

• Can only be relatively dated

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RELATIVE DATING
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RELATIVE DATING
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DEEP TIME

• Oceans vs. Puddles

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AIR and WATER
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Water Facts...

• 70 of Earths surface is covered by H2O
• 70 of human bodys weight is H2O
• Water can exist in 3 states earths surface
• exists only as solid and vapor on Mars
• only vapor on Venus

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Mars is too cold...
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Venus is too hot...
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Earth is just right...
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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

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Hydrogen Bonding

• ........O H ........
• / \ /
• H H ---- O
• \
• O O H .........
• / \ / \
• H H H H .........

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Water...

• Its heat capacity is higher than similar
  substances
• A watched pot never boils.
• Moderates Earths temperatures
• Compare San Francisco and St. Louis

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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

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DISTRIBUTION of EARTHSWATER and AIR
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Origin
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Origin

• How Impact of Icy Comets and Degassing of
  volatiles ... bottle
• Earths atmosphere was forming 4.4 to 4.0 Ga
• It then (probably) consisted mainly of water
  vapor, carbon dioxide, ammonia, methane (Some
  controversy)

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Mt. Erebus, Antarctica
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Brief History

• Icy Comet impacts ---gt add H2O to atmosphere and
  down to about 70km
• Earth cools ---gt Water condenses
• Lots of rain Volcanoes add CO2
• Most of CO2 dissolved in oceans
• Photosynthesizing organisms evolve
• CO2 H2O ------gt C6H12O6 O2

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The Water Cycle

• How Earths water and air interact ...
• Natural recycling process

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The Water Cycle
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ZONES OF CONCENTRATION

• Of natural waters

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ZONES OF CONCENTRATION

• Marine (saltwater)- The oceans contain over 97
  of earths 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

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Fresh Water

• Less than 3 of earths water is fresh
• Glaciers contain over 75 of Earths fresh
  water, but this source is not usable by biota

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Fresh Water

• Lakes and rivers contain less than 1
• Groundwater (including soil moisture) accounts
  for the remaining 24
• If all Earths water fit in a gallon jug, there
  would be one tablespoon of available fresh water

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Structure of the Atmosphere

• Layers classified by temperature changes ...

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The Atmosphere

• All weather occurs in the troposphere
  churning sphere

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Convection causes the churning
Temperature decreases with altitude
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Convection in Humid Air
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Thunderhead
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Stratosphere

• The ozone layer is contained in the stratosphere
  ...
• No churning because of a temperature
  inversion, i.e.,
• Temperature increases with altitude...

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A l t i t u d e
Stratosphere ---gt Troposphere ---gt
Temperature
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The Ozone Layer

• Ozone, O3 , is an air pollutant in the
  troposphere, but in the stratosphere protects
  from UV radiation
• O3 UV --------gt O2 O
• Chlorine radicals (like from CFCs) interfere
  with this process
• Cl O --------gt ClO

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WEATHER and CLIMATE - Atmosphere Geocycles
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Coriolis Effect

• Caused by an unattached atmosphere over a
  rotating Earth

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Coriolis Effect
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Coriolis Effect on Winds
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Coriolis Effect on Winds
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ACTUAL
NON-ROTATING
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PREVAILING WINDS

• The direction the winds blow most of the time
  ...
• Changes temporarily due to weather systems ...
• In the middle latitudes (30o- 60o), the
  prevailing winds are from the west.
• These affect climate as we shall see in a moment
  ...

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PREVAILING WINDS
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OCEAN CURRENTS

• (Surface) currents caused by prevailing winds
  and Coriolis Effect ...
• They play a major role in redistributing earths
  heat ...
• They are called warm or cold, compared with
  other water at that latitude ...
• Example The Gulf Stream is a warm current.

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OCEAN CURRENTS
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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

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