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PLATE TECTONICS AS CONVECTION

Plate tectonics is surface manifestation of convection cells in earth’s mantle .

Ridges (spreading centers) mark upwelling, hot limbs.

Trenches (subduction zones) mark cold downwelling limbs.

Oceanic plates form at ridges, cool as they spread and move across the ocean basins, and heat up as they subduct.

More complicated than simple pot of water, but idea’s the same

Davidson 2.25

ISOTHERMS - LINES OF CONSTANT TEMPERATURE - DEEPEN AS THE SQUARE ROOT OF PLATE AGE

LITHOSPHERE DEFINED AS REGION BELOW (COOLER THAN) ISOTHERM ABOUT 1300 DEG

THERMAL EVOLUTION OF OCEANIC LITHOSPHERE

COOLS & THICKENS MOVING AWAY FROM RIDGE

SIMILAR BEHAVIOR FOR COOLING OF A LAVA POND

HEAT FLOW THROUGH SEA FLOOR DECREASES WITH SQUARE ROOT OF AGE

OCEAN DEPTH INCREASES WITH SQUARE ROOT OF AGE

COOLING PLATE BECOMING DENSER WITH AGE CAUSES "RIDGE PUSH" FORCE - ONE OF FORCES DRIVING PLATE MOTIONS

DEPTH & HEAT FLOW VS AGE

COOLING OF LITHOSPHERE AWAY FROM RIDGE MAKES ROCK CONTRACT & BECOME DENSER

Davidson 9.7

Increase of ocean depth away from ridges shows thermal evolution

Davidson 10.7

SEISMICTOMOGRAPHY SHOWS THERMAL STRUCTURE OF DOWNGOING SLAB

COLD (HIGH SEISMIC VELOCITY) OCEANIC PLATE SUBDUCTS INTOWARMER (LOWER SEISMIC VELOCITY) MANTLE

HEATS UP SLOWLY (MILLIONS OF YEARS)

COLD SLAB DENSER THAN SURROUNDING MANTLE

PROVIDES "SLAB PULL" PLATE DRIVING FORCE

Davidson 10.3

THERMAL BUOYANCY FORCES

RIDGE PUSH AND SLAB PULL DRIVING FORCES-BOUYANCY FORCES FROM THERMAL DENSITY CHANGES

OCEANIC LITHOSPHERE COOLS AFTER IT FORMS &REHEATS UPON SUBDUCTION

Davidson 2.25

CHAINS OF ISLANDS & SEAMOUNTS

ACTIVE VOLCANISM NOT ASSOCIATED WITH SPREADING RIDGES

WHAT FORMS THESE “HOTSPOTS”?

HOTSPOT / PLUME HYPOTHESISAssume hotspots result from plumes of hot material rising from great depth, perhaps core-mantle boundary

Plumes would be secondary convection mode, ~ 5% of heat transfer

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LINEAR VOLCANIC CHAINS THOUGHT TO BE DUE TO PLATE MOTION

OVER A FIXED OR SLOWLY MOVING

HOTSPOT

ISLANDS GET OLDER ALONG CHAIN

EVENTUALLY SUBSIDE BELOW SEA

Davidson 7.14

BEND ?BEND ?

HAWAIIAN-EMPEROR BEND - WHAT WE BELIEVED

YELLOWSTONE- NORTH AMERICAN HOTSPOTACTIVE VOLCANIC, SEISMIC, AND GEOTHERMAL REGION

YELLOWSTONE ERUPTIONS FORM TRACE ACROSS SNAKE RIVER PLAININ EXPECTED PLATE MOTION DIRECTION

BUT, YELLOWSTONE SEISMIC TOMOGRAPHY PROBLEM

LOW VELOCITY ANOMALY (PRESUMABLY ASSOCIATED WITH HOT UPWELLING)

ONLY IN UPPERMOST MANTLE

MANTLE PLUMES?

NEAT IDEA BUT MANY PROBLEMSUNCLEAR WHETHER HOTSPOTS REFLECT

- HOT PLUMES FROM GREAT DEPTH (CORE-MANTLE BOUNDARY)- LOCALIZED UPPER MANTLE INTRAPLATE VOLCANISM

- OR SOME ARE ONE AND SOME THE OTHER?

WILSON CYCLE DESCRIBES OPENING & CLOSING OF OCEANS

CONTINENTS RIFT

FORM NEW OCEAN BASINS

OCEAN BASINS OPEN & WIDEN

EVENTUALLY THEY CLOSE BYSUBDUCTION, CONTINENTALCOLLISION & MOUNTAIN BUILDING

CONTINENTS LATER RIFTAPART AGAIN

OCEANS BORN, LIVE, & DIE WHEREASCONTINENTS NEVER SUBDUCTBUT ARE REARRANGED

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ATLANTIC RIFTING

OPENING OF PRESENT ATLANTIC OCEAN PART OF PANGAEA BREAKUP ABOUT 200 MYR AGO

CONTINENTAL RIFTING RECORDED IN RIFT VALLEYS LIKE CONNECTICUT RIVER VALLEY

VALLEYS FORMED BASINS THAT TRAPPED SEDIMENTS & INTRUDED BY BASALTIC LAVA FLOWS (PALISADES, HUDSON RIVER)

Press & Siever

NOT ACTIVE PLATE BOUNDARIES SO "PASSIVE" CONTINENTAL MARGINS

THICK (> 5 KM) SEDIMENTS ON CONTINENTAL SHELF

SOME CONTINENTAL MARGIN SEDIMENTARY BASINS ARE MAJOR SOURCES OF OIL & NATURAL GAS

PASSIVE CONTINENTAL MARGIN SEDIMENTS- AS NORTH ATLANTIC OCEAN WIDENED, CONTINENTAL MARGIN COOLED & SUBSIDED

Press & Siever

APPALACHIAN MOUNTAINS

FORMED BY CONTINENTAL COLLISION

CLOSED PREVIOUS ATLANTIC OCEAN

INCLUDE OLD VOLCANIC ISLAND ARCS & CONTINENTAL FRAGMENTS FROM AFRICA

Press & Siever

BECAUSE CONTINENTS AREN'T SUBDUCTED, THEY CAN BE BILLIONS OF YEARS OLD.

CONTINENTS ARE MUCH OLDER THAN SEA FLOOR

APPALACHIAN MOUNTAINS

CONTINENTS GROW WITH TIME AS MATERIAL ACCRETES TO THEIR EDGES

CONTINENTAL INTERIORS CONTAIN CRATONS, LARGE FLAT AREAS >500 MY OLD

ERODED REMAINS OF OLDER EPISODES OF MOUNTAIN BUILDING, COLLISIONS, ETC ...

BORDERED BY PROGRESSIVELY YOUNGER BELTS OF ROCKS

CONTINENTAL PLATE INTERIORS ARE NOT

TOTALLYINACTIVE

GEOLOGICALLY

SMALL MOTIONS WITHIN PLATE INTERIOR CAUSE MOTION ON FOSSIL FAULTS

INTRAPLATE EARTHQUAKES RARE, COMPARED TO INTERPLATE ONES FROM RAPID MOTION AT PLATE BOUNDARIES, BUT CAN BE LARGE(MAG 7 EARTHQUAKES IN 1811 & 1812, NEW MADRID)

PLANET EARTH IS A DYNAMIC EVOLVING SYSTEM - FROM 4.6 BILLION YEARS AGO TO NOW

STRUCTURE & COMPOSITION REFLECT ONGOING EVOLUTION

EVOLUTION DEPENDS ON HOW HEAT- "THE GEOLOGICAL LIFEBLOOD OF PLANETS" - TRANSFERRED OUT OF COOLING EARTH BY THERMAL

CONVECTION (Hot stuff rises)

THERMAL CONVECTION CAUSES PLATE TECTONICS: PLATES OF EARTH'S SURFACE MOVE RELATIVE TO EACH OTHER AT A FEW mm/yr

(about speed fingernails grow)

PLATE MOTIONS CAUSE EARTHQUAKES, VOLCANOS, MOUNTAIN BUILDING AT PLATE BOUNDARIES

PLATE TECTONICS MAKES EARTH WHAT IT IS - DIFFERENT FROM NEIGHBOR PLANETS (Mars, Venus, & Earth: 3 bears principle)

PLATE MOTIONS ARE CRUCIAL FOR THE ORIGIN OF LIFE, ITS SURVIVAL, AND OUR CLIMATE

PLATE MOTIONS PROVIDE RESOURCES AS WELL AS HAZARDS TO SOCIETY

"CIVILIZATION EXISTS BY GEOLOGICAL CONSENT"