Download Making Oceans and Continents

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Plate Tectonics 2
Making oceans and continents
Pangea*
seen at about
225 mya
Collision of Laurasia and Gondwana
Sir Francis Bacon 1620
Benjamin Franklin 1782
The crust of the earth must
be a shell floating on a fluid
interior. Thus the surface of
the globe would be broken
… by … movements of the
fluids….
Wegener 1912: evidence
* Breakup begins about 200 mya, floods about 190 mya
Alfred Wegener 1912
• Continental drift hypothesis
• Continents "drifted" to present
positions
• Evidence used in support of
continental drift hypothesis
• Fit of continents
• Fossil evidence
• Rock type and mountain belts
• Paleoclimatic evidence
Pangaea about 200 mya
Evidence:
Precise Matching of
Continental Shelves of
Circum-Atlantic
Continents
Ranges of Triassic Reptiles
Similar
Rocks on
opposite
shores
Example, NJ and Morocco
6
Why wasn’t Wegener’s idea accepted?
• Objections to drift hypothesis
• Inability to provide a
mechanism capable of moving
continents across globe
• Wegener suggested that
continents broke through the
ocean crust, much like ice
breakers cut through ice
Continental drift and paleomagnetism
In 1950’s there was renewed interest in
Wegener’s continental drift idea. New data came
from seafloor topography and paleomagnetics.
Magnetized minerals in rocks
• Show direction to Earth’s magnetic poles
• Provide a means of determining their original
latitude
• Horizontal Magnetite = at equator,
• vertical = at pole
• In between latitude can also be calculated
Identical fossils show proximity
The Ocean-Floor Topography discovered
Beginning
WWII
Sonar
revealed
Trenches,
Mid-Ocean
Ridges,
transform
faults,
sediments
The scientific revolution begins
Extensive mapping of the ocean floor
revealed the mid-ocean ridges in great
detail
Recall that Seafloor spreading hypothesis
was proposed by Harry Hess in the early
1960s
Geomagnetics tested Hess’ idea
• Geomagnetic reversals are recorded in the
ocean crust pillow lavas
• Data from towed magnetometers,
record North or South pointing minerals
• Hess’s concept of seafloor spreading predicts
matching bands of lava polarity on either side
of mid-ocean ridges.
• In early 60’s Fred Vine and D. Matthews
looked for symmetric magnetic stripes in the
ocean crust data near ridges.
Maps of Magnetic Stripes in Oceanic Crust
Paleomagnetic data were the most convincing
evidence to support the concept of seafloor spreading
Recall the tests
Geomagnetic reversals
• Magnetic North and South exchange
places at irregular intervals, average
~100K years but with large variance
• Dates when polarity of Earth’s magnetism
changed were determined from
radiometric dating of lava.
Magnetic Anomalies (again)
Example from the past 4 million years
Pattern is irregular so useful for corellation
Hess’ seafloor spreading in detail
Seafloor spreading occurs along relatively
narrow zones, called rift zones, located at
the crests of ocean ridges called Mid-Ocean
Ridges (MOR’s). These are above hot rising
mantle.
As plates pulled apart, cracks allow low
pressure and water to hit mantle. Causes
partial melting. Magma moves into
fractures and makes new oceanic
lithosphere
Hess’s Seafloor spreading (cont)
New lithosphere pulled from the ridge crest by
moving conveyor-belt. Conveyor belt formed
by convection currents in the asthenosphere
below
Newly created crust at the ridge is elevated
because it is heated and therefore occupies
more volume than the cooler rocks of the deepocean basin
Area also seems to be pushed up by mantle
upwelling
How fast do Plates Move?
Hot Spots are magmas from rising plumes from the
deep mantle, probably heated by the liquid outer
core. Their lavas are datable.
As plates move over them, new volcanic seamounts
and islands are formed. Eventually any subaerial
(exposed to the air) parts are eroded away, and as
they move away from the Hot Spot, they cool,
contract, and submerge. Called Guyots.
Hot spots form chains.
The Big Island of Hawaii
The big Island of
Hawaii is a
composite of five
volcanoes. Kohala
is the oldest.
Kilauea is very
active because it
is closest to the
hot spot, which is
to the southeast of
the big island.
Hot Spots and Hawaii
Worldwide, plate speeds vary from 1 to 10 centimeters per year
Flood Basalt was subducted
Hey look, the
direction changed!
Before satellites, we
measured plate
speeds as the distance
between two islands
divided by the age of
the youngest basalts
Hot Spots & Plate Motions
Average 5 centimeters/year
Determining plate speeds for continents
LAGEOS and
GPS satellites
determine that
plates move
1-10 cm per
year, avg 5
Just find position wrt distant stars, then watch fixed objects on earth move .
Latitude for ocean floor
• Orientation of magnetic minerals gives
latitude (north or south of equator)
• Radiometric dates of ocean floor basalts,
plus distance from ridge, gives
paleolongitude since 200 million years
ago, when Pangaea began to break apart.
http://www.odsn.de/odsn/services/paleomap/animation.html
150 mya Atlantic is already open
110 mya Displaced (Exotic) Terranes from
S. Am. hits W. N.Am.
60 mya another terrane forms Cuba, Hisp.
About 50 mya Southern Ocean forms
20 mya Himalayas forms
About 5-3.5 mya Central America forms
Origin of Pangaea
Origin of Pangaea
Active Rifting of A Continental Plate
Note 3-D Triple Junction
Discussion: eggshells
Active Rifting of A Continental Plate
Inactive Branch: Aulocogen;Subsided Passive Margins
East African Rift Zone
Active: Red Sea and Gulf of Aden Failed Arm: Great Rift Valley (aulocogen)
Discussion: Fault Block Mountains, HA normal
fault, rain shadows, divergent margin. global cooling
& grasslands
Humans as tall savannah specialists, voice
Story: The drunk and the lamp post
Mid-Ocean Ridge dimensions
Total 65000 kilometers (40,000 miles)
long
As wide as 1500 km (900 miles)
Some more than 3 km high above ocean
floor.
Mid-Ocean Ridge System Motion
Fracture Zones and
Transform Faults
Shallow weak earthquakes
Subduction-Zone Features
Note sequence from
land to trench
Note: over here are
some ocean plate rocks
that don’t get
subducted in a collision
We will see some on
the field trip, as well as
the volcanic arc
Reverse faults at
convergent margin
If a continent converges from
the left, what rocks will fold
in the collision?
Rocks in the Himalayas
Mélange from California Coast
Sea-floor and
land-derived
sediments,
+ some volcanics.
When stuffed down
trench into
Low TemperatureHigh Pressure
zone, result is
Blueschist Facies
Source: Betty Crowell/Faraway Places
Shield + Platform = Craton
High Angle Normal
faults of Rift Escarpment
Active and unstable continental margin
Craton : the stable portion of the continental
crust versus regions that are more
geologically active and unstable
Anatomy of a Continent
Canadian Shield,
North America’s
Crystalline core
exposed by glaciers
Exotic (Displaced) Terrains
Collisions with Volcanic Island Arcs and microcontinents
Continental
Crust buoyant
hard to subduct.
Erosion resistant
parts
Suture Zone
Pieces are volcanic
island arcs, and
microcontinents
Moved along
transform faults,
then accreted.
Anecdote Western
California
Ideas:Earth's Convection Cells
Aesthenosphere shallow convection model
Ideas: Earth's Convection Cells
Deep mantle/core convection model – Plumes cause MOR’s – Morgan
Ideas: Earth's Convection Cells
Combination
Mapping the ocean floor
Three major topographic units of the
ocean floor
•Continental margins
•Deep-ocean basins
•Mid-ocean ridges
Continental margins
Passive continental margins
• Found along coastal areas that
surround oceans w central MOR
• Not near active plate boundaries
because MOR is far offshore
• Little volcanism and few earthquakes
• East Coast of US an example
A passive continental margin
Active continental margins
• Continental slope descends abruptly
into a deep-oceanic trench
• Located primarily around the Pacific
Ocean
• sediment and oceanic crust scraped off
ocean crust to form accretionary
wedges
An active continental margin
The world’s trenches and ridges
Trench an entrance to Subduction Zone, Ridges and Rises are Mid-Ocean Ridges
CONTINENT
Back Arc Basin
Volcanic Island
Arc (Japan)
Trench
Abyssal Plain
FAB
Accretionary Wedge
Seamounts
Features of the deep-ocean basin
Abyssal plains
• Can be sites of thick
accumulations of sediment
• Found in all oceans
• Studded by old cold
seamounts and ridges
See previous slide
Seafloor sediment
Ocean floor is mantled with sediment
Sources
• Turbidity currents on continent margins
• Sediment that slowly settles to the bottom
from above – fine mud and plankton
Thickness varies
• Thickest in trenches – accumulations may
exceed 9 kilometers there
Types of sediment
• Biogenous sediment
–Shells and skeletons of marine
animals and plants
–Calcareous oozes from
microscopic organisms (only in
shallow water)
–Siliceous oozes composed of
opaline skeletons of diatoms and
radiolarians (only in deep water)
– Carbonate compensation depth - 4km
Foraminifera (a.k.a. Forams)
http://www.geomar.de/zd/labs/stab-iso/forams.jpg
Form deepwater carbonate oozes, depths less than 4 km
Chert
sample
below carbonate line
>4 km
Diatoms
(siliceous
ooze)
Mid-ocean ridges
Characterized by
• Heating => elevated ridge w/ radial cracks
• Closely spaced normal faulting: HW down
• Mantle flow below pulls the crust apart –
High Angle Normal Faults steeper than cartoon
• Newly formed basalt ocean floor fills in
cracks
http://rblewis.net/technology/EDU506/WebQuests/quake/normalfault.gif
Bathymetry of the Atlantic Ocean
Abyssal Plain
Passive Margin
Abyssal Plain
MOR
Passive Margin
The structure of oceanic crust
Hydrothermal Metamorphism
Recall …
Black Smokers
http://collections.ic.gc.ca/geoscience/images/detail/F92S0220.jpg
Circulation of hot water in cracks at mid-ocean ridge dissolves metals (Copper, Iron, Zinc, Lead, Barium)
which are re-precipitated as (for example) sulphide ores. Hydrothermal waters are capable of metamorphism.
Ocean Floor layers:Ophiolite Suite
Structure of oceanic crust
• Three layers in crust
– Upper layer – consists of sediments over pillow lavas
– Middle layer – numerous interconnected dikes called
sheeted dikes
– Lower layer – gabbro formed in basaltic magma
chambers
• Layer in mantle also part of the Ophiolite complex
- Magma that creates new ocean floor originates from
partially melted mantle rock (peridotite) in the
asthenosphere
Ophiolite Suite
Some Serpentine is formed
due to hot water
(called Hydrothermal)
circulation
Outcrop of
pillow basalt
End Plate Tectonics 2