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.
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More
complicated
than simple
pot of water,
but idea’s
the same
Davidson 2.25
THERMAL EVOLUTION OF
OCEANIC LITHOSPHERE
COOLS & THICKENS MOVING
AWAY FROM RIDGE
ISOTHERMS - LINES OF
CONSTANT TEMPERATURE DEEPEN AS THE SQUARE
ROOT OF PLATE AGE
LITHOSPHERE DEFINED AS
REGION BELOW (COOLER
THAN) ISOTHERM ABOUT 1300
DEG
SIMILAR BEHAVIOR FOR
COOLING OF A LAVA POND
DEPTH & HEAT FLOW VS AGE
COOLING OF LITHOSPHERE
AWAY FROM RIDGE MAKES ROCK
CONTRACT & BECOME DENSER
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
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
THERMAL BUOYANCY FORCES
RIDGE PUSH AND SLAB PULL DRIVING FORCESBOUYANCY 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”?
Assume hotspots
result from plumes of
hot material rising
from great depth,
perhaps core-mantle
boundary
HOTSPOT / PLUME HYPOTHESIS
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
BEND ?
ISLANDS GET OLDER
ALONG CHAIN
EVENTUALLY SUBSIDE
BELOW SEA
Davidson 7.14
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
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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
PASSIVE CONTINENTAL MARGIN SEDIMENTS- AS NORTH ATLANTIC
OCEAN WIDENED, CONTINENTAL MARGIN COOLED & SUBSIDED
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
Press & Siever
APPALACHIAN
MOUNTAINS
FORMED BY
CONTINENTAL
COLLISION
CLOSED PREVIOUS
ATLANTIC OCEAN
INCLUDE OLD VOLCANIC
ISLAND ARCS &
CONTINENTAL
FRAGMENTS FROM
AFRICA
Press & Siever
CONTINENTS ARE MUCH OLDER THAN SEA FLOOR
BECAUSE CONTINENTS AREN'T SUBDUCTED, THEY CAN BE
BILLIONS OF YEARS OLD.
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"