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

SEISMIC TOMOGRAPHY SHOWS THERMAL STRUCTURE OF DOWNGOING SLAB COLD (HIGH SEISMIC VELOCITY) OCEANIC PLATE SUBDUCTS INTO WARMER (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

COLD SLAB IS DENSER THAN SURROUNDING MANTLE PROVIDES "SLAB PULL" PLATE DRIVING FORCE

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 HYPOTHESIS Assume 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

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 ?

HAWAIIAN-EMPEROR BEND - WHAT WE BELIEVED

YELLOWSTONE- NORTH AMERICAN HOTSPOT ACTIVE VOLCANIC, SEISMIC, AND GEOTHERMAL REGION

YELLOWSTONE ERUPTIONS FORM TRACE ACROSS SNAKE RIVER PLAIN IN 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 PROBLEMS UNCLEAR 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 BY SUBDUCTION, CONTINENTAL COLLISION & MOUNTAIN BUILDING CONTINENTS LATER RIFT APART AGAIN OCEANS BORN, LIVE, & DIE WHEREAS CONTINENTS NEVER SUBDUCT BUT ARE REARRANGED

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 TOTALLY INACTIVE 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"