Plate tectonics 2 making oceans and continents
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http://www.ucmp.berkeley.edu/geology/tectonics.html. 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

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Plate tectonics 2 making oceans and continents

http://www.ucmp.berkeley.edu/geology/tectonics.html

Plate Tectonics 2Making oceans and continents


Pangea seen at about 225 mya

Pangea* seen at about225 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


Plate tectonics 2 making oceans and continents

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


  • Evidence precise matching of continental shelves of circum atlantic continents

    Evidence:Precise Matching of Continental Shelves of Circum-Atlantic Continents

    Pangaea about 200 mya


    Ranges of triassic reptiles

    Ranges of Triassic Reptiles


    Similar rocks on opposite shores

    Similar Rocks on opposite shores

    Example, NJ and Morocco


    Why wasn t wegener s idea accepted

    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

    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

    The Ocean-Floor Topography discovered

    Beginning WWII

    Sonar revealed Trenches,

    Mid-Ocean Ridges,

    transform faults,

    sediments


    The scientific revolution begins

    The scientific revolution begins

    • Extensive mapping of the ocean floor revealed the mid-ocean ridges in great detail

    • Recall that Seafloor spreading hypothesiswas proposed by Harry Hess in the early 1960s


    Plate tectonics 2 making oceans and continents

    • 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.


    Plate tectonics 2 making oceans and continents

    Maps of Magnetic Stripes in Oceanic Crust

    • Paleomagnetic data were the most convincing evidence to support the concept of seafloor spreading


    Recall the tests

    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

    Magnetic Anomalies (again)


    Plate tectonics 2 making oceans and continents

    Example from the past 4 million years

    Pattern is irregular so useful for corellation


    Hess seafloor spreading in detail

    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

    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

    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

    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

    Hot Spots and Hawaii

    Worldwide, plate speeds vary from 1 to 10 centimeters per year

    Before satellites, we measured plate speeds as the distance between two islands divided by the age of the youngest basalts

    Flood Basalt was subducted

    Hey look, the direction changed!


    Hot spots plate motions

    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

    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

    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.


    Plate tectonics 2 making oceans and continents

    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


    Origin of pangaea1

    Origin of Pangaea


    Active rifting of a continental plate

    Note 3-D Triple Junction

    Active Rifting of A Continental Plate

    Discussion: eggshells


    Active rifting of a continental plate1

    Inactive Branch: Aulocogen;Subsided Passive Margins

    Active Rifting of A Continental Plate


    East african rift zone

    Active: Red Sea and Gulf of Aden Failed Arm: Great Rift Valley (aulocogen)

    East African Rift Zone

    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

    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

    Mid-Ocean Ridge System Motion

    Fracture Zones and

    Transform Faults

    Shallow weak earthquakes


    Subduction zone features

    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

    If a continent converges from

    the left, what rocks will fold

    in the collision?

    Rocks in the Himalayas

    Reverse faults at convergent margin


    M lange from california coast

    Mélange from California Coast

    Sea-floor and

    land-derived

    sediments,

    + some volcanics.

    When stuffed down trench into

    Low Temperature-

    High Pressure

    zone, result is

    Blueschist Facies

    Source:Betty Crowell/Faraway Places


    Shield platform craton

    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

    Anatomy of a Continent

    Canadian Shield,

    North America’s

    Crystalline core

    exposed by glaciers


    Exotic displaced terrains

    Collisions with Volcanic Island Arcs and microcontinents

    Exotic (Displaced) Terrains

    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

    Ideas:Earth's Convection Cells

    Aesthenosphere shallow convection model


    Ideas earth s convection cells1

    Ideas: Earth's Convection Cells

    Deep mantle/core convection model – Plumes cause MOR’s – Morgan


    Ideas earth s convection cells2

    Ideas: Earth's Convection Cells

    Combination


    Mapping the ocean floor

    Mapping the ocean floor

    • Three major topographic units of the ocean floor

      • Continental margins

      • Deep-ocean basins

      • Mid-ocean ridges


    Continental margins

    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 USan example


    A passive continental margin

    A passive continental margin


    Active continental margins

    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

    An active continental margin


    The world s trenches and ridges

    The world’s trenches and ridges

    Trench an entrance to Subduction Zone, Ridges and Rises are Mid-Ocean Ridges


    Plate tectonics 2 making oceans and continents

    CONTINENT

    Back Arc Basin

    Volcanic Island Arc (Japan)

    Trench

    Abyssal Plain

    FAB

    Accretionary Wedge

    Seamounts


    Features of the deep ocean basin

    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

    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


  • Plate tectonics 2 making oceans and continents

    • 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

    Foraminifera (a.k.a. Forams)

    http://www.geomar.de/zd/labs/stab-iso/forams.jpg

    Form deepwater carbonate oozes, depths less than 4 km


    Plate tectonics 2 making oceans and continents

    Chert

    sample

    below carbonate line

    >4 km

    Diatoms

    (siliceous

    ooze)


    Mid ocean ridges

    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

    Bathymetry of the Atlantic Ocean

    Abyssal Plain

    Abyssal Plain

    Passive Margin MOR Passive Margin


    The structure of oceanic crust

    The structure of oceanic crust


    Hydrothermal metamorphism

    Hydrothermal Metamorphism

    Recall …


    Black smokers

    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.


    Plate tectonics 2 making oceans and continents

    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

    Ophiolite Suite

    Some Serpentine is formed

    due to hot water (called Hydrothermal)

    circulation


    Plate tectonics 2 making oceans and continents

    End Plate Tectonics 2

    Outcrop of

    pillow basalt


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