plate tectonics as convection plate tectonics is surface manifestation of convection cells in...
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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
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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
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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
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Increase of ocean depth away from ridges shows thermal evolution
Davidson 10.7
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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
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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
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CHAINS OF ISLANDS & SEAMOUNTS
ACTIVE VOLCANISM NOT ASSOCIATED WITH SPREADING RIDGES
WHAT FORMS THESE “HOTSPOTS”?
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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 ?
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HAWAIIAN-EMPEROR BEND - WHAT WE BELIEVED
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YELLOWSTONE- NORTH AMERICAN HOTSPOTACTIVE VOLCANIC, SEISMIC, AND GEOTHERMAL REGION
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YELLOWSTONE ERUPTIONS FORM TRACE ACROSS SNAKE RIVER PLAININ EXPECTED PLATE MOTION DIRECTION
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BUT, YELLOWSTONE SEISMIC TOMOGRAPHY PROBLEM
LOW VELOCITY ANOMALY (PRESUMABLY ASSOCIATED WITH HOT UPWELLING)
ONLY IN UPPERMOST MANTLE
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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?
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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
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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
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APPALACHIAN MOUNTAINS
FORMED BY CONTINENTAL COLLISION
CLOSED PREVIOUS ATLANTIC OCEAN
INCLUDE OLD VOLCANIC ISLAND ARCS & CONTINENTAL FRAGMENTS FROM AFRICA
Press & Siever
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BECAUSE CONTINENTS AREN'T SUBDUCTED, THEY CAN BE BILLIONS OF YEARS OLD.
CONTINENTS ARE MUCH OLDER THAN SEA FLOOR
APPALACHIAN MOUNTAINS
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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
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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)
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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"