Plate Tectonics 2 Making oceans and continents
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Transcript of Plate Tectonics 2 Making oceans and continents
Plate Tectonics 2Plate Tectonics 2Making oceans and continentsMaking oceans and continents
http://www.ucmp.berkeley.edu/geology/tectonics.html
Pangea* seen at about225 mya
Sir Francis Bacon 1620
Benjamin Franklin 1782 The crust of the earth mustbe a shell floating on a fluidinterior. 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
Collision of Laurasia and Gondwana
• Continental drift hypothesisContinental drift hypothesis• Continents "drifted" to present Continents "drifted" to present
positions positions
• Evidence used in support of Evidence used in support of continental drift hypothesis continental drift hypothesis
• Fit of continents Fit of continents
• Fossil evidenceFossil evidence
• Rock type and mountain belts Rock type and mountain belts
• Paleoclimatic evidencePaleoclimatic evidence
Alfred Wegener 1912
Evidence: Precise Matching of Continental Shelves of Circum-Atlantic Continents
Pangaea about 200 mya
Ranges of Triassic Reptiles
6
Similar Rocks on opposite shores
Example, NJ and Morocco
Why wasn’t Wegener’s idea Why wasn’t Wegener’s idea accepted?accepted?
• Objections to drift hypothesisObjections to drift hypothesis
• Inability to provide a mechanism Inability to provide a mechanism capable of moving continents capable of moving continents across globeacross globe
•Wegener suggested that Wegener suggested that
continents broke through the continents broke through the ocean crust, much like ice ocean crust, much like ice breakers cut through icebreakers cut through ice
Continental drift and paleomagnetismIn 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 WWIISonar 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 deep-ocean 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 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.
The Big Island of Hawaii
Hot Spots and HawaiiWorldwide, plate speeds vary from 1 to 10 centimeters per year
Hey look, the direction changed!
Flood Basalt was subducted
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
LAGEOS and GPS satellites determine that plates move 1-10 cm per year, avg 5
Determining plate speeds for continents
Just find position wrt distant stars, then watch fixed objects on earth move .
Latitude for ocean floorLatitude 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 open110 mya Displaced (Exotic) Terranes from S. Am. hits W. N.Am. 60 mya another terrane forms Cuba, Hisp.About 50 mya Southern Ocean forms20 mya Himalayas formsAbout 5-3.5 mya Central America forms
Origin of PangaeaOrigin of Pangaea
Origin of PangaeaOrigin of Pangaea
Active Rifting of A Continental PlateNote 3-D Triple Junction
Discussion: eggshells
Active Rifting of A Continental PlateInactive Branch: Aulocogen;Subsided Passive Margins
East African Rift ZoneActive: 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 & grasslandsHumans as tall savannah specialists, voiceStory: 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
If a continent converges from the left, what rocks will fold in the collision? Rocks in the Himalayas
Note: over here are some ocean plate rocks that don’t get subducted in a collisionWe will see some on the field trip, as well as the volcanic arc
Reverse faults at convergent margin
Mélange from California Coast
Source: Betty Crowell/Faraway Places
Sea-floor andland-derived sediments, + some volcanics.When stuffed down trench into Low Temperature-High Pressurezone, result is Blueschist Facies
Shield + Platform = Craton
High Angle Normal faults of Rift Escarpment
Craton : the stable portion of the continental crust versus regions that are more geologically active and unstable
Active and unstable continental margin
Anatomy of a Continent
Canadian Shield,North America’sCrystalline coreexposed by glaciers
Exotic (Displaced) Terrains
ContinentalCrust buoyanthard to subduct. Erosion resistant parts
Collisions with Volcanic Island Arcs and microcontinents
Pieces are volcanic island arcs, and microcontinents
Moved along transform faults, then accreted.
Anecdote Western California
Suture Zone
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 RidgesTrench an entrance to Subduction Zone, Ridges and Rises are Mid-Ocean Ridges
Abyssal Plain
Trench
Accretionary Wedge
Seamounts
Volcanic Island Arc (Japan)
FAB
Back Arc Basin
CONTINENT
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 sedimentOcean 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
Diatoms(siliceousooze)
below carbonate line>4 km
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 Abyssal Plain
Passive Margin MOR Passive Margin
The structure of oceanic crust
Hydrothermal Metamorphism
Recall …
Black Smokers
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.
http://collections.ic.gc.ca/geoscience/images/detail/F92S0220.jpg
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
Ocean Floor layers:Ophiolite Suite
Ophiolite SuiteSome Serpentine is formeddue to hot water (called Hydrothermal) circulation
Outcrop of pillow basalt
End Plate Tectonics 2