Chapter 15: Ocean Basins

1

• Chapter 15 Ocean Basins

2

• Tubeworms, tiny crabs and other sea life use
  heat, minerals and chemical energy near hot water
  vents on the deep sea floor to survive without
  sunlight.

3

• Bolide impacts may have brought volatiles to
  the inner planets, which eventually formed our
  atmosphere, oceans and foundation for life on
  Earth.
• See page 376 for an explanation of this drawing.

Fig. 15-1, p.354
4
The Earths Oceans

• Oceans cover about 71 of the Earths surface.
  The seafloor is about 5 km deep in the central
  part of ocean basins. Oceans basins are
  continually changing. What is happening with the
  Pacific and the Atlantic ocean basins? (one is
  closing while the other is enlarging)Why does
  water eventually end up in the oceans? (read on
  the density of oceanic crust, Page 377).
• Oceans affect global climate and the biosphere in
  many ways
• They reflect and store solar heat different than
  rocks/soil (oceans are generally warmer in the
  winter and cooler in the summer than adjacent
  land).
• Most precipitation is from evaporation from
  oceans.
• Ocean currents transport heat toward poles.
• Plate tectonics alters basins which alters
  currents and affect climate.

5

• Halleys Comet. The early Solar System was
  crowded with comets, meteoroids and asteroids.
  Bolide impacts may have imported volatiles such
  as CO2, water vapor, ammonia, simple organic
  molecules and other volatiles.

Fig. 15-2, p.354
6

• Schematic cross section of the continents and
  ocean basins. Vertical axis shows elevation
  relative to sea level. Horizontal axis shows the
  relative areas of the types of topography (e.g.,
  mountains and ocean floor).

Fig. 15-3, p.355
7

• Studying the seafloor Oceanographers extract
  sediment from a core retrieved from the seafloor.

Fig. 15-4, p.356
8

• Alvin is a research submarine capable of diving
  to the sea floor. Scientists on board control
  robot arms to collect sea-floor rocks and
  sediments.

Fig. 15-5, p.357
9

• Mapping the topography of the sea floor with an
  echo sounder. A sound wave bounces from the sea
  floor and back up to the ship, where its travel
  time is recorded.

10
Fig. 15-6a, p.357
11

• A seismic profiler records both the sea floor
  topography and the layering of sea floor
  sediments and rocks.

Fig. 15-6b, p.357
12

• Features of the Sea Floor

Fig. 15-7, p.358
13

• The Mid-Oceanic Ridge System (MORS) and other
  features of the sea floor show there is as much
  topography here as on the continents. MORS is a
  continuous submarine mountain chain that
  encircles the globe it rises 2-3 km above the
  sea floor, and is Earths largest mtn chain
  (covering 20 of its surface).

14

• Divergent plate boundaries, or spreading centers,
  coincide exactly with the MORS in the worlds
  oceans.

Fig. 15-7b, p.358
15

• The sea floor sinks as it grows older. At the
  MORS, new lithosphere is buoyant because it is
  hot and of low density. It ages, cools, thickens
  and becomes denser as it moves away from the
  ridge and sinks. The central part of the sea
  floor lies at a depth of about 5 km.

Fig. 15-8, p.360
16

• A cross-section view of the central rift valley
  of the MORS. As the plates separate, blocks of
  rock drop down along the fractures to form the
  rift valley. The moving blocks cause earthquakes
  along normal faults.

Fig. 15-9, p.360
17

• Transform faults offset segments of the MOR.
  Adjacent segments of the ridge may be separated
  by steep cliffs 3 km high. Note the flat abyssal
  plain far from the ridge.

Fig. 15-10, p.361
18

• The MORS can cause a rise and fall in global sea
  level (if they didnt exist, sea level would fall
  400 meters). Slow spreading (above) creates a
  narrow, low-volume ridge that displaces less sea
  water and lowers SL.
• Rapid sea floor spreading (right) creates
  high-volume ridge, displacing more sea water and
  raises SL.

Fig. 15-11, p.361
19

• Life on the Mid-Oceanic Ridge.
• Black smoker to right (see page 384).

Fig. 15-12, p.362
20
Fig. 15-13, p.362
21

• Oceanic Trenches and Island Arcs An oceanic
  trench forms at a convergent boundary between two
  oceanic plates. One plate sinks, generating
  magma that rises to form a chain of volcanic
  islands called an island arc.

Fig. 15-14, p.362
22

• Onekotan is one of many volcanic islands in the
  Kuril Island arc that formed along the Kuril
  trench in the western Pacific. The deepest place
  on Earth is the Mariana trench of the sw Pacific,
  where the ocean floor sinks to about 11 km below
  SL.

23

• Island arcs eventually migrate toward a continent
  and becomes part of it (to buoyant to sink).
  This is a way continents can grow by accreted
  terranes.

Fig. 15-16, p.363
24
Fig. 15-16a, p.363
25
Fig. 15-16b, p.363
26
Fig. 15-16c, p.363
27

• The accreted terranes of western North America
  are micro-continents and island arcs from the
  Pacific Ocean that were added to the continent.

Fig. 15-17, p.364
28
Seamounts, Oceanic Islands and Atolls

• Seamount submarine mtn that rises 1 km or more
  above the ocean floor.
• Oceanic Island is a seamount that rises above
  sea level.
• -both are volcanoes commonly made of basalt
  formed at a hot spot above a mantle plume. As
  the plate overrides the hot spot, the seamount
  becomes inactive. The Hawaiian Island-Emperor
  Seamount Chain is an example (15.18). As the
  seamounts move away they erode into a flat-topped
  guyot and sink.

29

• The Hawaiian Island-Emperor Seamount Chain
  becomes older in a direction going away from the
  island of Hawaii. In 10-15 million years the
  island of Hawaii may sink and become eroded.

Fig. 15-18, p.365
30

• The Hawaiian Islands and Emperor Seamounts sink
  as they move away from the mantle plume.

Fig. 15-19, p.365
31

• Formation of a guyot.

Fig. 15-20, p.366
32
Fig. 15-20a, p.366
33
Fig. 15-20b, p.365
34

• A fringing reef grows along the shore of a young
  volcanic island. As the island sinks, the reef
  continues to grow upward to form a barrier reef
  that encircles the island. Finally, the island
  sinks below sea level and the reef forms a
  circular atoll.

35
Fig. 15-22a, p.367
36
Fig. 15-22b, p.367
37
Fig. 15-22c, p.367
38

• The Tetiaroa Atoll in French Polynesia formed by
  the process described in Figure 15.22. Over
  time, storm waves wash coral sands on top of the
  reef and vegetation grows on the sand.

Fig. 15-21, p.366
39

• Sediments and rocks of the sea floor.
• The three layers of oceanic crust are layer 1
  sediment (Terrigenous and Pelagic) layer 2
  pillow basalt and layer 3 upper mantle (basalt
  dikes and gabbro).
• The oceanic crust is 4-7 km thick (1-2 km of
  pillow basalts and 3-5 km of dikes/gabbro).

Fig. 15-23, p.368
40

• Terrigenous sediment is sand, silt and clay
  eroded from the continents and carried to the
  deep sea floor by gravity (rivers, landslides)
  and submarine currents.
• Pelagic sediment collects even on the deep sea
  floor far from continents (clay and remains of
  tiny plants and animals). It accumulates at a
  rate of 2-10 mm/1000 yrs. Near the MOR there is
  virtually none (why?).

Fig. 15-24, p.368
41

• Pillow lavashow do they form?

Fig. 15-25, p.368
42

• Continental margins.

Fig. 15-26, p.370
43

• Continental crust fractured as Pangea began to
  rift.

Fig. 15-26a, p.370
44

• Faulting and erosion thinned the crust as it
  separated. Rising basaltic magma formed new
  oceanic crust in the rift zone.

Fig. 15-26b, p.370
45

• Sediment eroded from the continents formed broad
  continental shelves on the passive margins of
  North America and Africa.

Fig. 15-26c, p.370
46

• A passive continental margin consists of a broad
  continental shelf, slope and rise formed by the
  accumulation of sediment eroded from the
  continent.
• Submarine canyons are deep valleys from the edge
  of a continent to the rise (where abyssal fans
  may form) and occur where large rivers enter the
  sea. Sediments from rivers create turbidity
  currents that can travel as speed greater than
  100 km/hr for up to 700 km.

Fig. 15-27, p.371
47
Fig. 15-28, p.371
48

• At an active continental margin an oceanic plate
  sinks beneath a continent, forming an oceanic
  trench. The continental shelf is narrow, the
  slope is steep and no rise exists.

Fig. 15-29, p.372
49
p.374