Plate Boundaries

1
Plate Boundaries
Lect. 04

• There are 4 types of plate boundaries
• Divergent boundaries where new crust is
  generated as the plates pull away from each other
• Convergent boundaries where crust is destroyed
  as one plate dives under another
• Transform boundaries where crust is neither
  produced nor destroyed as the plates slide
  horizontally past each other
• Plate boundary zones broad belts in which
  boundaries are not well defined and the effects
  of plate interaction are unclear

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Plate Tectonics
Lect. 04
3
Plate Boundaries
Lect. 04
Divergent
Convergent
Transform
4
Plate Boundaries
Lect. 04
5
Divergent Plate Boundaries
Lect. 05

• Perhaps the best known of the divergent
  boundaries is the Mid-Atlantic Ridge.
• This submerged mountain range, which extends
  from the Arctic Ocean to beyond the southern tip
  of Africa, is but one segment of the global
  mid-ocean ridge system that encircles the Earth.
• The rate of spreading along the Mid-Atlantic
  Ridge averages about 2.5 centimeters per year
  (cm/yr), or 25 km in a million years.
• This rate may seem slow by human standards, but
  because this process has been going on for
  millions of years, it has resulted in plate
  movement of thousands of kilometers.
• Seafloor spreading over the past 100 to 200
  million years has caused the Atlantic Ocean to
  grow from a tiny inlet of water between the
  continents of Europe, Africa, and the Americas
  into the vast ocean that exists today.

6
Divergent Plate Boundaries
Lect. 05

• Constructive plate boundaries are sites where
  new lithosphere is created - by sea floor
  spreading.
• The relationship between the age of the sea
  floor and its bathymetry - increasing away from
  ridges - can be seen in a graph.

7
Divergent Plate Boundaries
Lect. 05
The patterns of magnetic anomalies on the sea
floor can be seen on the Juan de Fuca ridge for
example.
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Divergent Plate Boundaries
Lect. 05
9
Divergent Plate Boundaries
Lect. 05
10
Divergent Plate Boundaries
Lect. 05

• The formation of oceanic crust involves melting
  of underlying uppermantle.
• The main way this occurs is by decompression -
  through passive up-welling.

11
Plate Tectonics
Lect. 05
Any other ways of melting the mantle ?
12
Plate Tectonics
Lect. 05

• Adding heat by frictional heating (shear heating)

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Plate Tectonics
Lect. 05

• Adding fluids (along subduction zones, shear
  zones, ...)

14
Divergent Plate Boundary
Lect. 05
15
Plate Tectonics
Lect. 05

• Seismic tomography

16
Convection in the Upper Mantle
Lect. 05

• Different models (EPSL 127, 123-139)

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Plate Tectonics
Lect. 05

• The volcanic country of Iceland, which straddles
  the Mid-Atlantic Ridge, offers scientists a
  natural laboratory for studying on land the
  processes also occurring along the submerged
  parts of a spreading ridge. Iceland is splitting
  along the spreading center between the North
  American and Eurasian Plates, as North America
  moves westward relative to Eurasia.

Krafla volcano lava fountain
18
Iceland
Lect. 05

• Aerial view of the area around Thingvellir,
  Iceland, showing a fissure zone that is an
  on-land exposure of the Mid-Atlantic Ridge.
• Left of the fissure, the North American Plate is
  pulling westward away from the Eurasian Plate
  (right of fissure).

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Iceland geotourism
Lect. 05
20
Iceland geotourism
Lect. 05
21
Continental Spreading Centers (rifts)
Lect. 05

• East Africa shows some historically active
  volcanoes (red triangles) and the Afar Triangle,
  a so-called triple junction (or triple point),
  where three plates are pulling away from one
  another the Arabian Plate, and the two parts of
  the African Plate (the Nubian and the Somalian)
  splitting along the East African Rift Zone.

22
Continental Spreading Centers (rifts)
Lect. 05

• Active lava lake within the summit crater of
  'Erta 'Ale (Ethiopia), one of the active
  volcanoes in the East African Rift Zone.

• Two helmeted, red-suited volcanologists,
  observing the activity from the crater rim
  provide scale.
• 150 m diameter
• 80 m deep
• Red color within the crater shows where molten
  lava is breaking through the lava lake's
  solidified, black crust.

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Continental Spreading Centers (rifts)
Lect. 05
'Erta 'Ale (Ethiopia),
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Continental Spreading Centers (rifts)
Lect. 05
'Erta 'Ale (Ethiopia),
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Continental Spreading Centers (rifts)
Lect. 05
'Erta 'Ale (Ethiopia),
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Continental Spreading Centers (rifts)
Lect. 05
'Erta 'Ale (Ethiopia),
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Continental Spreading Centers (rifts)
Lect. 05
'Erta 'Ale (Ethiopia),
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Continental Spreading Centers (rifts)
Lect. 05
Oldoinyo (Tanzania)
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Continental Spreading Centers (rifts)
Lect. 05
Oldoinyo (Tanzania)
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Continental Spreading Centers (rifts)
Lect. 05
Oldoinyo (Tanzania)
31
Spreading Ridge movie
Lect. 05