Geology Lecture Outline -
Divergent
Plate Tectonism and Ocean Floor
Geology – (Ch 13)
I. Lecture Content
Introduction –
Earth's
Hidden Surface - The Seafloor
Ocean
Exploration - Earth's Last Frontier
Seafloor Topography
- Undersea Bottom Relief
Seafloor
Physiology - Major Features of the Sea bottom
Continental Margins -
Deep-Sea
Floors - Basaltic Crust Conveyor Systems
Islands, Reefs and Atolls - Interesting Places
Life and Death of an
Sea Bottom Resources - Natural Treasure
II. Introduction
A. The Earth’s Seafloor is Topographically Rugged and Symmetrical
1.
Seafloors cover nearly 3/4's of Earth's surface.
2.
A highly variable topography, which includes, rugged
mountain ranges,
very deep
trenches, amazing island chains, ocean plateaus, seamounts,
and deep canyons
that rival anything seen on land.
3.
Symmetrical cross-section – very rugged basaltic mid ocean
ridge mountain
chain near
center; 4-km deep hilly to flat abyssal plains on each side of MOR;
either
continental rise or deep sea trench between abyssal plains and
continental slope
and shelf.
4. The
seafloor is the final receptacle for vast amounts of terriginous
and organic sediment.
B. The Earth’s
Seafloor is Geologically Young and Made of Basaltic
Material
1. Generated
at Oceanic Divergent Plate Boundaries by Seafloor Spreading.
·
Seafloor Spreading Process creates new seafloor
·
Mid-Ocean Ridge systems are the site of seafloor spreading
· Basaltic magmas generated by
decompression melting of rising asthenospheric
mantle beneath
MOR
III. Seafloor Topography
- Underwater Bottom Relief
A. Earth has Two Distinctive
Topographic Regions
·
·
Continental
Highlands - Continents
· · Oceanic
B. The Earth's Seafloors are Rugged in
Appearance and
Have Considerable Topographic
Relief.
·
· See
Figures
·
· Much more
topographic relief than the continents
·
· Seafloors have distinctive topographic
features
·
· Seafloors look
much different than dry continents
C. Earth's Seafloor is Divided
into Two Major Provinces
1. Continental Margins
·
· Submerged
shallow platforms
·
· Floored
mostly by granitic rock
·
· Varies
greatly width, depth, and topographic relief
·
·
Vast
majority of marine life concentrated there
2. Deep-ocean basins
are rugged with variable relief, and
have a wide variety of distinctive physiological
features
Ø Ø See Figure - Seafloor Topographic Map
1.
Mid-ocean ridges
2. Mid-ocean ridge fractures
3.
Hydrothermal vents
4.
Abyssal plains and Abyssal hills
5. Seamounts
and Guyouts
6. Oceanic
island chains
7. Oceanic
plateaus
8. Trenches
and
3. The two-province division is based upon the
major
inherent
differences between continental and oceanic
crust.
·
· Composition
(density)
·
· Thickness
·
· Isostatic equilibrium
IV.
Continental Margins - Shallow Marine
A. Shallow Seafloor Rims of Ocean Basins
1. Continental margins - the submerged edges of continents
2. Continental margins are underlain by
faulted blocks of
granitic crust
, overlying sediment piles, and possible
accreted subduction zone material
B. Continental Margins are
Classified into Two Types
1. Passive
Margins =
§ § Situated
within a plate
§ § Develops after
continental rifting and opening of a new
ocean basin
opening
§ § Typically
broad (avg. 100's km) with a very thick pile of
accumulating
sediments
§ § Lacks much
seismic or volcanic activity
2. Active Margins
=
· · Situated at the leading edge of a
continental plate
· · Develops after initiation of subduction
· · Typically narrow with rugged
topography
· · Outer edge typically forms inner
wall of ocean trench
· · Regionally unstable with much seismicity & volcanism
3. See Figures )
C. Physiological Features of a
Continental Margin
·
·
Continental
Shelf
·
·
Continental
Slope
·
· Submarine Canyons
·
·
Continental
Rise
D. The Continental Shelf (See Figs )
· · Shallow, submerged edge of
continent between the
shoreline
and continental slope (shelf-slope break)
· · Has a very low sloping angle
(<< 1degree)
· · Typically shallow water depths
(avg. = 75 m = 250 ft)
· · Greatly influenced by
fluctuations in sea level
· · Shelf sediments are mainly
influenced by waves and
tidal currents
· · Site of abundant mineral
resources and sea life
E. Continental Slope and Rise (See Figs )
· · Deeper, steeper, outermost edge
of continent between the
continental shelf
and the deep ocean floor
· · A continental
rise may separate the continental slope
from the
deep ocean basin along passive margins
ü ü A continental rise forms a thick pile of
sediments
that have
accumulated at the base of the
continental slope
· · The shelf-slope
break marks the abrupt transition between
the slope and
the shelf
· · Location of Earth's greatest
depository of sediments
ü ü Roughly 70% of Earth's sediments
· · Slope and Rise sediments are
mainly influenced by
gravity, and
are transported down-slope via strong
turbidity
currents and deposit as submarine fans.
·
·
Submarine canyons and fan deposits are present
on all
continental
slopes and rises, and on some continental
shelves
·
·
Submarine
fan deposits grade into deep-ocean deposits
V. Deep-Ocean Basins -
True Oceanic Seafloor
A.
Ocean Basins are Classified by Size and Extent
·
·
Oceans - broad, large, and globally extensive
Examples: Pacific,
·
·
Seas - narrow, smaller, and regionally limited
Example:
Mediterranean,
B. Deep-ocean basins are
underlain by basaltic crust
1. Ocean Crust - A typical cross section (See Figs.)
· · Layered basaltic crust covered by
sediments
· · Rugged volcanic surface covered
by layers upon
layers of very
fine pelagic sediment
ü ü Pelagic clays
ü ü Silica and carbonates Oozes
· · Oceanic igneous crustal column is also layered
Ø Ø Pillow lava basalt
Ø Ø Sheeted gabbroic dikes
Ø Ø Massive gabbro
(intrusions)
Ø Ø Layered gabbro
(intrusions)
Ø Ø Layered Peridotite
· · Oceanic crustal
sections found on land are
termed an ophiolite suite
C. Ocean Basins are Relatively
Young Earth Features
·
·
Oldest
part of ocean basins is 180 million years old
·
·
Average
age of deep ocean seafloor is 60 million y.o.
·
·
Age
distribution pattern of deep-ocean crust is striking
·
·
See
Figure - Ocean crust age map
E. Most Deep Ocean Features are
the Result of Seafloor
Spreading
Processes Occurring at Mid Ocean Ridges
1. Seafloor
spreading processes create:
ü ü Mid-ocean rift valleys and ridge
flanks
ü ü Vast expanses of ocean crust
(abyssal plains)
ü ü Chains of volcanoes (seamounts
and islands)
ü ü Transform fracture systems
ü ü Hydrothermal systems (black
smokers)
2. See Figures
VI. Deep Sea
Sedimentation - Pelagic Clays and Oozes
A. Nearly All Deep
Ocean Sediment is Very Fine Grained
·
· No mechanisms to
transport coarse-grained material
ü ü Exception
is icebergs
·
· Vast majority of deep
seafloor sediment is deposited via
vertical
settling of suspended material
·
· Sediment deposited
from suspension is termed pelagic
B.
1. Windblown dust and volcanic ash
·
· Continentally derived
·
·
Deep-sea
clays are termed pelagic clays
2.
Microscopic marine organism skeletons
·
· Carbonate hard parts
= calcareous ooze
·
· Silica hard parts = siliceous
ooze
C.
Distribution of Deep-Sea Sediments - A Global Pattern
1.
Pelagic clays carpet the deepest parts of the oceans
ü ü Cover about 38% of world's
deep-sea bottoms
ü ü Deposition rate is roughly 2
mm/1000 years
2.
Pelagic calcareous oozes cover a good portion of the
Atlantic, Indian and
ü ü Cover about 48% of world's
deep-sea bottoms
ü ü Accumulation rate is 1 to 6 cm
per 1000 years
3. Pelagic siliceous oozes cover
equatorial Pacific and
ü ü Cover about 14% of world's deep-sea bottoms
ü ü Accumulation rate is 1 to 6 cm
per 1000 years
4. See Figure - Global deep-sea sediment
map
VII. Origin of
A.
1.
Formed on or near mid-ocean ridges
2.
Basaltic shield volcanoes
3.
Migrate away from mid-ocean ridges over time
B. Atolls and Guyouts
are Modified Oceanic Islands
1.
Circular coral reef systems develop around islands.
2. Oceanic
crust cools and subsides with increasing age,
causing the
attached islands to also subside over time.
3.
4.
Upwards reef growth keep ups with sinking island.
5.
only
the growing reef able to maintain at sea level.
ü
ü
This stage of an island
is termed an atoll.
6. Eventually reef growth lags behind rate of
atoll
subsidence, and
entire atoll structure becomes
permanently submerged -
this is termed a guyout.
VIII. Birth, Growth and
Death of an
---- The
A. Initiation of New
1. Initial stages of plate divergence
2. Rift valley floored by new basaltic (oceanic) crust.
3. Further widening of rift, marine waters begin filling valley
B. Young Ocean Basin is Born - A True Sea
1. Continued plate divergence now
in full swing
2.True seafloor spreading in operation = Mini ocean basin
3. Matching set of opposing coastlines frame
the sea
C. Full
Maturation of
1. Divergence begins to stall - spreading rate
slows
2. Continental margins, abyssal seafloors, and
mid-ocean ridge
3. Fully-developed ocean has emerged with an
age 200-400 Ma
D. Mature Ocean
Basin Starts to Collapse near Its Margins
1. Old, dense ocean lithosphere
becomes isostatically unstable
2. Subduction
initiated; ocean basin lithosphere dives into
upper mantle
forming ocean trenches and island arcs.
3. Beginning of plate convergence
of sides of ocean basin
E. Collapsing
1.
Plate convergence in full swing
2. Subduction zones
established along continental margins
3. Extensive volcanic and uplifted mountain
chains result
from continued subduction and
intense collision forces
F. Total Collapse
of
1.
Plate convergence reaches an apex - subduction
wanes
2. Last of oceanic lithosphere subducted - Ocean basin gone
3. Massive thrusted and
uplifted mountain ranges form a
complex continental suture zone marking the site of the
now totally
collapsed ocean basin
A. Continental
Margins
1. Oil
and Gas
2. Sand
and Gravel
3. Plankton,
Fish and Shellfish
B. Deep-Sea
Bottom
1. Manganese nodules
2. Massive sulphide deposits
3.
Migratory fish
X. Ocean floor
Vocabulary - Chapter 13
Abyssal plain
Active
continental margin
Atoll
Black Smoker
Continental
margin
Continental rise
Continental
slope
Guyout
Isostatic
equilibrium
Hydrothermal
Vent
Mid-oceanic
ridge
Oceanic trench
Ooze
Ophiolite
Passive
continental margin
Pelagic clay
Reef
Ridge fracture
zones
Seafloor
Spreading
Seamount
Submarine canyon
Submarine hydrothermal
vent
Submarine fan
Turbidity
current