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Plate Tectonics 2 Making oceans and continents

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Plate Tectonics 2Making oceans and continents

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


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

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

    Pangaea about 200 mya

    Ranges of Triassic Reptiles

    Similar Rocks on opposite shores

    Example, NJ and Morocco

    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,


    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

    • 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

    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

    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

    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.

    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

    Note 3-D Triple Junction

    Active Rifting of A Continental Plate

    Discussion: eggshells

    Inactive Branch: Aulocogen;Subsided Passive Margins

    Active Rifting of A Continental Plate

    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

    • 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

    If a continent converges from

    the left, what rocks will fold

    in the collision?

    Rocks in the Himalayas

    Reverse faults at convergent margin

    Mélange from California Coast

    Sea-floor and



    + some volcanics.

    When stuffed down trench into

    Low Temperature-

    High 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

    Collisions with Volcanic Island Arcs and microcontinents

    Exotic (Displaced) Terrains


    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


    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 USan 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


    Back Arc Basin

    Volcanic Island Arc (Japan)


    Abyssal Plain


    Accretionary Wedge


    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)

    Form deepwater carbonate oozes, depths less than 4 km



    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

    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.

    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)


    End Plate Tectonics 2

    Outcrop of

    pillow basalt

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