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