Transcript The Origin of Ocean Basins

The Origin of Ocean Basins
Ocean basin is defined as that part of the sea floor
deeper than 2000 m (6000 ft).
Plate Tectonic Theory:
states that the Earth's
outermost layer (i.e.,
the lithosphere) is
fragmented
comprised of a dozen or
more large and small
plates;
plates move relative to
one another and 'glide'
atop the hot, mobile
mantle;
relatively new scientific
concept, accepted only
about ~35 years ago
• Historical Recognition of
Plate Dynamics:
• many believed
continents were
fragmented pieces of
preexisting larger
landmasses
• 1596 – Dutch map
maker Abraham Ortelius
– suggested that the
Americas were "torn
away" from Europe and
Africa
• 1858 – Italian
geographer Antonio
Pellegrini
– constructed plate
reconstruction maps
– attempted to
demonstrate the fit of
the American and
African continents
1912 – meteorologist Alfred
Wegener introduced theory of
continental drift
noted the remarkable fit of the
South American and African
continents
identified geologic 'occurrences'
on matching coastlines of South
America, Africa
continuance of geologic
structures (mountain belts,
mineral deposits)
common plant and animal fossils
Wegener contended that the
modern landmasses had formed a
'supercontinent'
• Continental Drift Evidence:
• Wegener noted tropical
plant fossils (in the form of
coal deposits) in Antarctica
• suggested that Antarctica
had been situated closer to
equator (temperate
climate)
• Wegener identified ancient
(fossil) coral reefs at high
latitudes
• noted that modern coral
reefs do not occur where
temperatures fall below
18°C
• also noted the occurrence
of glacial deposits,
striations in present-day
arid regions
•What was the fatal flaw in
Wegener's theory?
•Wegener's theory failed to explain
the nature of the forces propelling
the plates!
•Wegener incorrectly suggested
that continents simply plowed
through the ocean floor!
• Development of the 'Modern' Theory of Plate
Tectonics:
• early 1950s – new evidence emerged to revive
Wegener's theory
– identification of the global mid-ocean ridge system
– demonstration of the youth of the ocean floor
– magnetic 'stripes' found on the seafloor
– pattern of earthquake hypocenters in well-defined
belts
• emergence of the seafloor-spreading hypothesis
by Dietz, Hess, and Vine
• suggested that mantle convection drives plate
motion
• demonstrated that oceanic crust formed at midocean ridges and spread laterally
Sea floor spreading3-2 Sea-Floor Spreading
demonstrates that the sea
floor moves apart at the
oceanic ridges and new
oceanic crust is added to
the edges.
• Rift valleys along oceanic
ridge crests indicate tension,
are bounded by normal
faults and are floored by
recently-erupted basaltic
lava flows.
• Axis of the oceanic ridge is
offset by transform (strikeslip) faults which produce
lateral displacement.
• Whereas oceanic ridges
indicate tension, continental
mountains indicate
compressional forces are
squeezing the land together.
Source of the Earth's
magnetic field is the
liquid core
magnetic field
structure similar to
that of a bar magnet
(i.e., two
distinct poles)
magnetic field
periodically reverses
•Magnetometers
detect and
measure Earth’s
magnetic field.
3-2
• Moving across the
ocean floor
perpendicularly to the
oceanic ridges,
magneometers
alternately record
stronger (positive) and
weaker (negative)
magnetic fields (called
magnetic anomalies) in
response to the
influence of the sea
floor rocks.
• Magnetic anomalies and
the rocks causing them
form parallel bands
arranged symmetrically
about the axis of the
oceanic ridge.
Sea-Floor Spreading
• As basaltic rocks
crystallize, some
minerals align
themselves with
Earth’s magnetic
field, as it exists
at that time,
imparting a
permanent
magnetic field,
called
paleomagnetism,
to the rock.
• Periodically
Earth’s magnetic
field polarity
(direction)
reverses poles.
3-2
Sea-Floor Spreading
Because of their
3-2of Sea-Floor Spreading
paleomagnetism, rocks
the sea floor influence the
magnetic field recorded
by magnetometers.
• Rocks on the sea floor with
normal polarity
paleomagnetism locally
reinforce Earth’s magnetic
field making it stronger and
producing a positive
anomaly.
• Rocks on the sea floor with
reverse paleomagnetism
locally weaken Earth’s
magnetic field, producing a
negative anomaly.
• Rocks forming at the ridge
crest record the magnetism
existing at the time they
solidify.
• Sea floor increases in
age away from the
ridge and is more
deeply buried by
sediment because
sediments have had a
longer time to collect.
• Rates of sea-floor
spreading vary from 1
to 10 cm per year for
each side of the ridge
and can be determined
by dating the sea floor
and measuring its
distance from the
ridge crest.
• Continents are moved
by the expanding sea
floor.
3-2
Sea-Floor Spreading
3-3
Because Earth’s size is
constant, expansion of
the crust in one area
requires destruction of
the crust elsewhere.
• Currently, the Pacific Ocean
basin is shrinking as other
ocean basins expand.
• Destruction of sea floor
occurs in subduction zones.
• Seismicity is the frequency,
magnitude and distribution
of earthquakes.
Earthquakes are
concentrated along oceanic
ridges, transform faults,
trenches and island arcs.
• Tectonism refers to the
deformation of Earth’s
crust.
Global Plate Tectonics
Seven Major
Plates
African Plate
Antarctic Plate
Eurasian Plate
Indian-Australian Plate
North American Plate
Pacific Plate
South American Plate
Seven Minor
Plates
Arabian Plate
Caribbean Plate
Cocos Plate
Juan de Fuca Plate
Nazca Plate
Philippines Plate
Scotia Plate
3-3
• Mantle plumes
originate deep
within the
asthenosphere
as molten rock
which rises and
melts through
the lithospheric
plate forming a
large volcanic
mass at a “hot
spot”.
Global Plate Tectonics
• Benioff Zone is an
area of
increasingly
deeper seismic
activity, inclined
from the trench
downward in the
direction of the
island arc.
• Subduction is the
process at a
trench whereby
one part of the
sea floor plunges
below another
and down into the
asthenosphere.
3-3
Global Plate Tectonics
As the Pacific Plate slides under New
Zealand's North Island, it melts. Lighter
rocks float up as magma, to create the
mighty volcanoes of the active volcanic
zone. This another Benioff zone.
• Here the plate junction is
very complex since:
1) along the alpine fault
(South Island) the plates are
sliding past each other.
2) North, the Australian
plate rides over the Pacific
plate. (Kermadec trench).
3) South, the Pacific plate
rides over the Australian
plate. (Macquarie ridge).
4) In the volcanic triangle
of the North Island, the
country is tearing apart,
creating the jutting East Cape
in the process.
The deep split in the earth's
crust, which is the plate
boundary, twists over at New
Zealand. It slopes down to
the west, as the Pacific Plate.
It then flips over to slope
down to the east south of the
country, where the Australian
Plate does the sinking
Kilauea Eruptions
3-3
Global Plate Tectonics
Wilson Cycle refers to the sequence of
events leading to the formation,
expansion, contracting and eventual
elimination of ocean basins.
Stages in basin history
are:
Embryonic - rift valley
forms as continent
begins to split.
Juvenile - sea floor
basalts begin forming
as continental sections
diverge.
Mature - broad ocean
basin widens, trenches
develop and
subduction begins.
Declining - subduction
eliminates much of sea
floor and oceanic
ridge.
Terminal - last of the
sea floor is eliminated
and continents collide
forming a continental
• Chronology of Modern Ocean Basin Development:
• ~200 million years ago
•
– formation of the supercontinent Pangaea
– presence of the Panthalassan 'super ocean'
– Atlantic and Indian Oceans not present
~180 million years ago
– initial break up of Pangaea and formation of Laurasia and Gondwanaland
– presence of an east-west trending basin (the Tethys Sea)
• ~140 million years ago
– separation of Eurasian and North American plates
– onset of mid-Atlantic ridge development
– early formation of the North Atlantic
– South Atlantic still closed
• ~80 million years ago
– separation of the African and South American plates
– early formation of the South Atlantic
• ~60 million years ago
– Indian plate reaches equator after separation from Australia and Antarctica
– North and South Atlantic continue to widen ~40 million years ago
– onset of Indian and Asian plate collision, initial formation of the Himalayas
– North and South Atlantic achieve modern appearance
• ~20 million years ago
– major uplift of the Himalayas and formation of the Tibetan Plateau
– expansion of the Southern Ocean
• ~5 million years ago
•
– closure of mid-American seaway as North and South American plates collide
– isolation of the Pacific and Atlantic Oceans
Ocean Floor from 2 MYBP to present
•
•
Major Ocean Basins:
• Pacific Ocean
–
–
–
–
largest (180,000,000 km2) and deepest (averaging 3,940 m) basin
extensive marginal seas and volcanic island systems and trenches
considerable mountain building and earthquake activity along boundaries
little freshwater input
–
–
–
–
second largest basin (107,000,000 km2)
average depth 3,310 m
large freshwater input (Amazon, Congo, Mississippi, Niger, Orinoco Rivers)
small number of marginal seas (Gulf of Mexico, Caribbean, Mediterranean)
–
–
–
–
smallest basin (74,000,000 km2)
average depth 3,840 m
large sediment input (Indus and Ganges River deltas)
small number of marginal seas (Arabian Sea, Persian Gulf, and Red Sea)
–
–
–
–
centered on the north pole
shallow and land-locked
covered by sea ice
large sediment input from active glaciers
–
–
–
–
–
continuous ring of water around the Antarctic continent (south pole)
coldest of all oceans (near freezing)
most biologically productive ocean (high nutrient concentrations)
extensive winter sea ice coverage
small number of marginal seas (Weddell and Ross Seas)
• Atlantic Ocean
• Indian Ocean
• Arctic Ocean
• Southern Ocean
• Passive (Atlantic-Type)
Margins:
• found on continentbearing plates
• continental margin moving
away from the mid-ocean
spreading center
• not characterized by
mountain building
• zone of low seismicity and
no volcanism – essentially
stable
• characterized by thick
sediment deposits and old
oceanic crust
• comprised of shelf, slope,
and rise
• examples include the
eastern coasts of North
and South America
• Active (Pacific-Type) Margins:
• considered 'destructive'
• continental margin moving
toward a subduction zone
• characterized by volcanism,
many earthquakes, and active
mountain building
– friction of subducting plate causes
earthquake activity and heat
generation
– ocean crust is heated to melting
point
– molten rock (magma) rises to the
surface to create island arcs and
volcanoes
• dense oceanic crust is
subducted beneath thicker, less
dense continental crust
• Chilean (e.g., Peru, Chile) shallow trench, accretionary
prism, volcanic mountains
• Marianas (e.g., Japan,
Marianas) - deep trench,
volcanic island arc, back-arc
basin
San Andreas fault 100
miles north of L.A
Basalt injections
in narrow
trough resulting
is separation
from Africa
New crust
forming in axial
trough via seafloor spreading
Hot salty brine
high in metal
concentrations
from basalt
leaching. High
enough metal
concentration to
be exploited
commercially.