Dynamic Earth

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Dynamic Earth

• Class 5
• 24 January 2006

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Any Questions?
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Interacting Earth SystemsVolcanoes contribute
gases to the atmosphere. This is how the oceans
were formed 3-4 Billion years ago
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Lithosphere Hydrosphere Atmosphere Biosphere
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The Deep The beginnings of Plate
Tectonics(Chapter 2)
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Plate Tectonics

• Fundamental Concept and Unifying Theory in Earth
  Science
• Idea is gt 100 yrs old
• Acceptance only within the past 30 years

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Plate Tectonics

• How and why did it come about?
• During the 1800s geologists recognized many
  strange things that seemed to imply that the
  continents had once been together.

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Early geologists sawsimilarities between the
coasts of Africa and South America.
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This map from 1858 shows that Africa and South
America can be fit together very well.
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The fit of the continents

• 1915 Alfred Wegener Origin of the Continents and
  Oceans

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The Worlds Continental Shelves
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Fit of the continents

• Alexander du Toit
• Continuity of rocks between Africa and South
  America

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If the northern continents are fit together, rock
units match very well.
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The same kinds of fossils appear on both sides of
the Atlantic(Mesosaurs lived in fresh water)
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Again, a nice match if the continents are fit
together
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There is evidence that huge glaciers occupied
large parts of the southern continents about 250
million years ago.
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This distribution is very difficult to explain if
the continents were in this position during the
time of glaciation.
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But, if we move the continents together, it is
easy to explain the glacial evidence.
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During the Permian Period (225 Million Years
Ago), there was one super continent called Pangea
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The Theory of Continental Drift
proposed that Pangea began to break upin the
Triassic (200 Million Years Ago)
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Gondwanaland (southern continent) began to break
up in the Jurassic Period(135 Million Years Ago)
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The south Atlantic opened in the Cretaceous (65
Million Years Ago)
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And the continents continued to drift to their
Present Day positions
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Wegeners reconstruction
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Acceptance of Continental Drift?

• Evidence for continental drift purely descriptive
• Description is not an explanation
• Theory must explain how continents move
• No plausible explanation for driving force
• Wegener thought that continents floated like
  boats across the solid ocean crust, dragged along
  by the tidal forces of the Sun and Moon

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Holmes explanation of continental drift (1928)

• Convection deep in the Earth could push and pull
  the continents apart, creating new oceanic crust
  through the process of seafloor spreading.

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Convection
Water boiling in a pan on your stove is an
example of convection
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Convection
Hot water rises Cold water sinks
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Holmes explanation of continental drift (1928)
Holmes proposed convection currents dragged the
two halves of the original continents apart, with
consequent mountain building in the front where
the currents are descending, and the ocean floor
development in the site of the gaps, where the
currents are ascending.

• Holmes theory was almost universally ignored
  because of the influence of Sir Harold Jeffreys

Jeffreys thought the idea crazy! His
observations of earthquakes suggested that
Earths interior far too rigid as rigid as
steel.
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Holmes explanation of continental drift (1928)

• Holmes conceded that purely speculative ideas of
  this kind, specifically invented to match the
  requirements, can have no scientific value until
  they acquire support from independent evidence.

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Why was Continental Drift not accepted?

• Because it was difficult to under-stand how
  continents could move.
• What did we learn to make us more willing to
  accept the idea that the continents have moved?

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Jump ahead to the 1960s
Maps and paintings by Marie Tharp allowed
geologists to visualize the newly available
bathymetry
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Mid-ocean Ridges
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Mid-Atlantic Ridge
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Deep-sea Trenches
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Drilling into the seafloor provided more evidence
supporting sea-floor spreading

• Micropaleontology of sediments
• Dating of the underlying lavas

Drilling ship Glomar Challenger
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Age of Seafloor Crust
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Age of Seafloor Crust
But, it is not possible to drill enough holes in
the seafloor to produce this map. How was it made?
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Earths Magnetic Field
Another key to understanding the puzzle.
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Earths Magnetic Field
The Earth behaves like a magnet whose poles are
nearly coincident with the spin axis (i.e., the
geographic poles).
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Earths Magnetic Field
Magnetic lines of force emanate from the magnetic
poles. A suspended magnet is inclined due to the
magnetic field.
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A compass points North because theneedle is made
of iron. It becomes aligned in the Earths
magnetic field.
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Magma formed at spreading centers contains minute
pieces of iron that become aligned in the
direction of the Earths magnetic field.
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When the magma cools, the iron pieces remain
frozen in this alignment, pointing toward the
Earths magnetic North Pole.
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Magnetic reversals

• A special property of the Earths magnetic field
  is that it has reversed its direction many times
  in the past.
• When this happens, rocks formed at that time are
  magnetized pointing toward the South Pole.

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Magnetic reversals

• The last reversal was about 700,000 years ago.
• These reversals appear to be abrupt (probably
  only take about 1000 years or so).

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Magnetic reversals

• A period of time in which magnetism is dominantly
  of one polarity is called a magnetic epoch.
• We call north polarity normal and south polarity
  reversed.

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Formation of Magnetic Anomalies
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Magnetic Anomalies in the Atlantic
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The GeomagneticTime Scale
Based on determining the magnetic characteristics
of rocks of known age (from both the oceans and
the continents). We have a good record of
geomagnetic reversals back to about 60 Million
years ago.
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Formation of Magnetic Anomalies
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Thursday

• Continue with Sea Floor Spreading and Plate
  Tectonics
• Please finish reading Chapter 2

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Homework 2Due January 31st

• Find it on the Web Site