Chapter 13 Origin of the Ocean Floor

1
Chapter 13 Origin of the Ocean Floor

• Oceans cover 71 of Earth. Continental rocks
  include old sedimentary and meta-sedimentary
  rocks.
• Sonar other seafloor mapping methods discussed
  on pp. 382 384.
• Ocean Floor Provinces
• Continental Margins
• Passive Cont. shelf, slope, rise
• Active Accretionary wedge
• Deep Ocean Basins
• Mid-Ocean Ridges

2
2
Passive continental margins Modern day
Atlantic Coast, Paleozoic Iapetus Ocean.
Continental shelf is part of the conti-nent, as
it is underlain by normal-faulted continental
crust.
3
3

• Continental Shelves range from almost
  non-existent to 1500 km (930 mi) wide.
• The average continental shelf is 80 km (50 mi.)
  wide and 130 m (425 ft) deep at its outer edge.
  The average inclination is about 10 feet per
  mile.
• Continental shelves comprise less than 10 of
  ocean area, but hold 90 of ocean species.
  Shallow depth is largely within the photic zone
  light needed by aquatic plants.
• Remember the facies progression from shore
  sand, silt, clay.

4
4

• The outer edges of some parts of the
  conti-nental shelf have been dissected by river
  erosion during the Pleistocene Ice Age (when sea
  level was lower), producing submarine canyons.
  Landslides produce submarine fan/turbidite
  sequences.
• Beyond the edge of the shelf is the conti-nental
  slope, which averages 5 degrees (but may reach 25
  degrees). The slope is generally about 12 miles
  wide.
• The continental rise is a gradual transition to
  the flat Abyssal Plain. The continental rise
  gradient may only be 1/3 degree and it may be
  hundreds of miles wide.

5
The most notable sedimentary settings
associated with Active Margins (Island Arc or
Continental Arc Systems) are the Subduction
Complexes or Accretionary Wedge or Melange,
which is scraped off the descending plate.
5
6
6

• Deep Ocean Basin features
• Subduction zone-related Deep Ocean Trenches. The
  Marianas Trench has been measured at 36,163 ft
  below sea level. Only two occur in the Atlantic
  Ocean (Fig. 13.9).
• Abyssal Plains the flattest portion of the deep
  ocean bottom.
• Seamounts, Guyots, and Oceanic Plateaus
• Seamounts conical. Guyots flat-topped.
• Oceanic Plateaus oceanic floor flood basalts.

7

• Oceanic Ridges Mid-Ocean Ridges/Rift zones
  inter-connected system 43,000 miles long. The
  crest of the ridge stands 2 to 3 km (1.2 to 1.8
  mi) above the adjacent abyssal plains. The width
  ranges from 1000 to 4000 km (600 to 2400 mi).
• The crest of some ridges exhibit a central rift
  valley graben or series of grabens. The rift
  valley may be as wide as 50 km (30 mi). Rift
  valleys may include submarine volcanoes.

7
8
8

• Darwins Hypothesis on Coral Atolls pp. 392
  393.
• Underwater portions of active volcanic island
  become sites for colonization by floating coral
  larvae. The attached corals grow, attract other
  organisms and form fringing coral reefs. As
  volcano becomes inactive and sinks, the corals
  grow upward to stay within the photic zone,
  initially becoming a barrier reef and later an
  atoll, with an exposed, circular reef and a
  central lagoon.

9
9

• Oceanic Ridges are elevated above the
  abyssal plain because
• The rising mantle plumes bulge the overlying
  area.
• The newly-formed crust is hotter and more bouyant
  (less dense).
• Away from the active part of the rift, the
  oceanic crust cools, contracts, and sinks.
• Ridge topography as related to spreading rates.
  Slow-moving rifts exhibit more vertical faulting
  and a well-defined central rift valley (p. 397,
  Fig. 13.13)

10
10
The sequence of rocks that compost the
oceanic crust are called Ophiolite Suites or
Complexes. Old ophiolites in Finland, Cyprus,
Cali-fornia, Newfoundland
11
11

• The seafloor basalt flows erupted in the rift
  zone form pillow lavas (see Fig. 13.17, p. 399).
  Each pillow has a rind (rim) of chilled
  obsidian and a core of basalt.
• In some areas of submarine volcanic activity, hot
  seawater circulates through fractures in the
  basalt, dissolving some of the minerals and
  metals. When the seawater becomes sufficiently
  hot, it spews upward through the fractures and
  onto the ocean bottom through chimney-like
  structures. The dark, mineral-rich emissions
  from black smokers are erupted at 350 degrees
  Celsius.

12
12

• The minerals precipitate upon reaction with
  the cold seawater, enriching the rocks with Cu,
  Au, Zn, and other metals. Copper deposits of
  Copperhill, GA/Ducktown, TN are thought to have
  formed in this manner.
• There are also oddball ecosystems associated
  with these black smokers. The base of the
  foodweb is composed of chemosynthetic bacteria,
  as there are no plants. Other organisms, such as
  tube-worms (Fig. 13.E, p. 400) subsist by
  consuming these bacteria.

13
13
The formation of new Ocean Basins begin with
continental rift zones, e.g., Rio Grande Rift,
East Africa Rift
Initiation begins with stretching and
splitting of continental crust. Extension
results in formation of graben basins.
14
Tensional strain Brittle deformation
continental rift crustal stretching
14
Graben basin
High angle Normal fault (gravity fault)
Rio Grande Valley from El Paso northward into S.
Colorado consists of a series of graben basins,
resulting from stretching of crust. Basins
in-fill with sediments basalts.
15
15
The basaltic flows and cinder cones of the Rio
Grande Rift are typical. Mafic composition is
due to thinned crust.
16
16
How oceans close the end of subduction.
Older, colder oceanic crust subducts at a
steeper angle.
Younger, hotter oceanic crust subducts at a lower
angle.