The Floors of the Oceans:The Floors of the Oceans:Concealed Revealers of Concealed Revealers of

ChangeChange

Earth2Class Workshops for Earth2Class Workshops for TeachersTeachers

Originally presented 20 Sep Originally presented 20 Sep 20032003

with Guest Scientistswith Guest Scientists

Dr. Gerard BondDr. Gerard Bond

Doherty Senior Scholar, Biology and Doherty Senior Scholar, Biology and

Paleoenvironment DivisionPaleoenvironment Division

Ramona (Rusty) Lotti Ramona (Rusty) Lotti

Curator, L-DEO Deep Sea Sample Curator, L-DEO Deep Sea Sample

RepositoryRepository

World Ocean FactsWorld Ocean Facts

• Oceans cover 71% of Oceans cover 71% of Earth’s surfaceEarth’s surface

• Northern Hemisphere Northern Hemisphere is 39% land, 61% seais 39% land, 61% sea

• Southern Hemisphere Southern Hemisphere is 19% land, 81% seais 19% land, 81% sea

• Pacific is largest Pacific is largest basin, followed by basin, followed by Atlantic, Indian, Atlantic, Indian, Southern, and ArcticSouthern, and Arctic

• Southern is defined Southern is defined as waters between as waters between 6060o o S and S and Antarctica, where Antarctica, where the Antarctic the Antarctic Circumpolar Circumpolar Current flowsCurrent flows

• Numerous seas Numerous seas and gulfs lie and gulfs lie adjacent to the adjacent to the major ocean basinsmajor ocean basins

Earth’s CrustEarth’s CrustContinental crustContinental crust

• graniticgranitic

• thickerthicker

• less denseless dense

• Average elevation 838 m (2,750 ft.)Average elevation 838 m (2,750 ft.)

Oceanic CrustOceanic Crust

• basalticbasaltic

• thinnerthinner

• DenserDenser

• Average depth 3800 m (12,500 ft)Average depth 3800 m (12,500 ft)

Continental MarginsContinental Margins• Continental shelfContinental shelf

gentle slope out to average depth of gentle slope out to average depth of 130 m (430 ft.)130 m (430 ft.)

variations in width, materials, origins, variations in width, materials, origins, etc.etc.

coral reefscoral reefs

• Continental slopeContinental slope

more steeply sloping edge of continentmore steeply sloping edge of continent

• Continental riseContinental rise

apron of sedimentsapron of sediments

Submarine CanyonsSubmarine Canyons

• Cut through continental slope and shelfCut through continental slope and shelf

• Probably created through erosion by Probably created through erosion by turbidity currentsturbidity currents

• Also connected to Pleistocene river Also connected to Pleistocene river drainage; e.g., Hudson Canyondrainage; e.g., Hudson Canyon

• Differences between East and West Differences between East and West Coast canyonsCoast canyons

• Submarine fans at base of many canyonsSubmarine fans at base of many canyons

Ocean BasinsOcean Basins

Abyssal PlainsAbyssal Plains• vast, flat basinsvast, flat basins• abyssal hills generally less than 100 mabyssal hills generally less than 100 m• sediment blankets from terrestrial and sediment blankets from terrestrial and

pelagic sourcespelagic sources

Ocean RidgesOcean Ridges• volcanic mountain range with rift valleyvolcanic mountain range with rift valley• about 23% of sea floorabout 23% of sea floor

Other Ocean Basin FeaturesOther Ocean Basin Features

• Trench and island arc systemsTrench and island arc systems

• Volcano and seamount chainsVolcano and seamount chains

• AtollsAtolls

• GuyotsGuyots

• Hydrothermal ventsHydrothermal vents

““Heezen-Tharp Maps”Heezen-Tharp Maps”

Perhaps the most famous version of what the floors of the oceans look like were the series of maps created beginning in the 1950s by Bruce Heezen and Marie Tharp of Lamont-Doherty. Using thousands of sonar tracks, they produced physiographic charts of the ocean basins. These were later published in enhanced versions by the National Geographic Society, and are still widely used.

Online Bathymetry MapsOnline Bathymetry Maps

http://www.ngdc.noaa.gov/mgg/image/2minrelief.htmlhttp://www.ngdc.noaa.gov/mgg/image/2minrelief.html

Ocean Basins and Plate Ocean Basins and Plate TectonicsTectonics

• Francis Bacon (1561 – 1626) noted Francis Bacon (1561 – 1626) noted “fit” of South America and Africa“fit” of South America and Africa

• Alfred Wegener (1880 – 1930) and Alfred Wegener (1880 – 1930) and Frank Taylor (1860 – 1939) Frank Taylor (1860 – 1939) independently proposed continental independently proposed continental driftdrift

• Challenged existing theories, could Challenged existing theories, could not provide satisfactory mechanismnot provide satisfactory mechanism

Supporting EvidenceSupporting Evidence

• Earthquake and volcano patternsEarthquake and volcano patterns• SONAR and other observation techniquesSONAR and other observation techniques• Geomagnetic patternsGeomagnetic patterns

-- “normal” and “reverse” polarity-- “normal” and “reverse” polarity -- “mirror” images on opposite side of MOR-- “mirror” images on opposite side of MOR

-- 170 reversals over past 76 million years-- 170 reversals over past 76 million years• Increasing age away from MOR rift valleyIncreasing age away from MOR rift valley• Greatest heat flow along MORGreatest heat flow along MOR• Increasing thickness of sediments away from Increasing thickness of sediments away from

MORMOR

Modern Exploration Modern Exploration TechnologiesTechnologies

Acoustic Monitoring Acoustic Monitoring of Ocean of Ocean SeismicitySeismicityThe hydroacoustic method

allows detection of low-magnitude seismicity and volcanic activity and provides more accurate source locations than land seismic networks.

http://www.pmel.noaa.gov/vents/acoustics/seismicity/seismicity.html

Evolution of a theoryEvolution of a theory

During the past century, During the past century, understanding of the ocean floors understanding of the ocean floors changed dramatically.changed dramatically.

• ““Continental Drift”Continental Drift”

• ““Convection” in the upper mantleConvection” in the upper mantle

• ““Sea-floor Spreading”Sea-floor Spreading”

• Plate TectonicsPlate Tectonics

3 Types of Plate Boundaries3 Types of Plate Boundaries

• DivergentDivergent

• ConvergentConvergent

• TransformTransform

http://www.cotf.edu/ete/modules/msese/earthsysflr/plates4.html

Spreading and Closing Spreading and Closing CyclesCyclesJ. Tuzo Wilson (U. Toronto) proposed that oceans open J. Tuzo Wilson (U. Toronto) proposed that oceans open and close over cycles lasting 500 – 600 million years.and close over cycles lasting 500 – 600 million years.

An excellent explanation has been created by Prof. Lynn Fichter of James Madison University

http://csmres.jmu.edu/geollab/Fichter/Wilson/Wilson.html

The cycle starts with a stable craton (core of a continent.)

““Embryonic” stageEmbryonic” stage• Supercontinents remain in one spot for Supercontinents remain in one spot for

hundred of millions of years, acting like a hundred of millions of years, acting like a blanketblanket

• Mantle convedction warms the overlying Mantle convedction warms the overlying supercontinent, causing expansion and supercontinent, causing expansion and stretchingstretching

• Fracturing forms a rift valley Fracturing forms a rift valley

• Examples: East African Rift ValleyExamples: East African Rift Valley

““Juvenile” stageJuvenile” stage

• Rift valleys gradually widen and connect Rift valleys gradually widen and connect to oceanto ocean

• Examples: Red Sea, Gulf of CaliforniaExamples: Red Sea, Gulf of California

““Mature” stageMature” stage• Lateral spreading of rift valley leads to Lateral spreading of rift valley leads to

development of a divergent plate development of a divergent plate boundary and new oceanic crustboundary and new oceanic crust

• Atlantic is a mature ocean with passive Atlantic is a mature ocean with passive edgesedges

““Declining” stageDeclining” stage

• Subduction becomes more widespreadSubduction becomes more widespread

• Convection in mantle drags lithosphere Convection in mantle drags lithosphere plates into subduction zonesplates into subduction zones

• Examples: Scotia Arc in South Atlantic, Examples: Scotia Arc in South Atlantic, West Indies near BarbadosWest Indies near Barbados

• Explains why there is little oceanic crust Explains why there is little oceanic crust more than 200 mymore than 200 my

• Ocean basin begins to close as subduction Ocean basin begins to close as subduction rates exceed spreading ratesrates exceed spreading rates

• Sediment deposits are deformed and Sediment deposits are deformed and uplifted, creating mountain ranges in new uplifted, creating mountain ranges in new supercontinent on site of former oceansupercontinent on site of former ocean

• Example: Appalachians (formed at end of Example: Appalachians (formed at end of previous Wilson cycle)previous Wilson cycle)

• Wilson cycles influence sea level relative Wilson cycles influence sea level relative to continents of 10to continents of 105 5 yearsyears

• When supercontinents dominate, land When supercontinents dominate, land stands higher as mantle warms themstands higher as mantle warms them

• As spreading begins, land is lowered and As spreading begins, land is lowered and seas encroach, flooding low-lying seas encroach, flooding low-lying continental areascontinental areas

• As basin widens and crust ages, it deepens As basin widens and crust ages, it deepens and sea level fallsand sea level falls

• During periods of higher sea level, oceans During periods of higher sea level, oceans covered about 80% of surfacecovered about 80% of surface

• During periods of lower sea level, oceans During periods of lower sea level, oceans covered only about 65% of surfacecovered only about 65% of surface

Sea Floor SedimentsSea Floor Sediments

• Terrestrial and pelagic originsTerrestrial and pelagic origins

(important types in next slide)(important types in next slide)

• Provide evidence of processes and Provide evidence of processes and environmental conditions at time of originenvironmental conditions at time of origin

• Relatively protected from erosion, so more Relatively protected from erosion, so more complete “record” than continental complete “record” than continental depositsdeposits

• Connection with rocks now forming Connection with rocks now forming continents (ex., limestones, shales)continents (ex., limestones, shales)

Types of Deep Sea SedimentsTypes of Deep Sea Sediments

• Terrigenous mudsTerrigenous muds

• TurbiditesTurbidites

• Glacial (“ice-rafted”)Glacial (“ice-rafted”)

• Brown/red claysBrown/red clays

• Authigenic (hydrogenous) depositsAuthigenic (hydrogenous) deposits

• Manganese nodulesManganese nodules

• Pelagic oozes—diatoms, “forams,” “rads”Pelagic oozes—diatoms, “forams,” “rads”

• Volcanic Volcanic

Ocean Sediment DistributionOcean Sediment Distribution

Sediment thickness is influenced by:Sediment thickness is influenced by:

• Age of the underlying crustAge of the underlying crust

• Tectonic history of the underlying crustTectonic history of the underlying crust

• Structural trends in the basement rocksStructural trends in the basement rocks

• Location and nature of sediment sourcesLocation and nature of sediment sources

• Nature of depositional processes delivering Nature of depositional processes delivering the sedimentsthe sediments

You can see a digital image of worldwide sediment distribution through the DataStreme Ocean web page:www.ametsoc.org/amsedu/DS-Ocean/home.html

LDEO Deep Sea Sample LDEO Deep Sea Sample RepositoryRepository

• Largest collection in Largest collection in the worldthe world

• E2C has visited E2C has visited many times – see many times – see archived series for archived series for related PowerPointsrelated PowerPoints

• Web site provides Web site provides many useful starting many useful starting points for educationpoints for education

http://www.ldeo.columbia.edu/CORE_REPOSITORY/RHP1.html

Sediments and Climate ChangeSediments and Climate Change

Dr. Gerard Bond will discuss some of his Dr. Gerard Bond will discuss some of his research about climate change based research about climate change based on interpretation of deep-sea on interpretation of deep-sea sediments. Of particular interest for sediments. Of particular interest for much of his study are ice-rafted much of his study are ice-rafted materials collected in the North materials collected in the North Atlantic. You can see versions of some Atlantic. You can see versions of some of his work in the Core Repository of his work in the Core Repository pages.pages.

Resources for Educational Resources for Educational ProjectsProjects

In addition to what is available through In addition to what is available through the L-DEO and other university-the L-DEO and other university-related web sites, one of the best related web sites, one of the best sources for information about this sources for information about this topic may be found in the USGS Coast topic may be found in the USGS Coast & Marine Geology Program web pages& Marine Geology Program web pages

http://http://walrus.wr.usgs.gov/infobankwalrus.wr.usgs.gov/infobank//

Special Reference Special Reference

“ “DataStreme Ocean” Preview PackageDataStreme Ocean” Preview Package

Ch. 2: Ocean Basins and Plate TectonicsCh. 2: Ocean Basins and Plate Tectonics

Authors: M. Grant Gross and Elizabeth Authors: M. Grant Gross and Elizabeth GrossGross

Sponsor: AMS/NOAA Cooperative Sponsor: AMS/NOAA Cooperative Program for Earth System Education Program for Earth System Education