GEOL: CHAPTER 9GEOL: CHAPTER 9

The SeafloorThe Seafloor

LO1: Examine the history and methods LO1: Examine the history and methods of oceanic explorationof oceanic exploration

LO2: Describe the structure and LO2: Describe the structure and composition of the oceanic crustcomposition of the oceanic crust

LO3: Identify the continental marginsLO3: Identify the continental margins

LO4: Discuss the features found in the LO4: Discuss the features found in the deep-ocean basinsdeep-ocean basins

Learning OutcomesLearning Outcomes

• LO5: Discuss sedimentation and LO5: Discuss sedimentation and sediments on the deep seafloorsediments on the deep seafloor

• LO6: Explore coral reefsLO6: Explore coral reefs

• LO7: Recognize the types of natural LO7: Recognize the types of natural resources found in the oceansresources found in the oceans

Learning Outcomes, cont.Learning Outcomes, cont.

• Oceanic crust is thinner and denser Oceanic crust is thinner and denser than continental crustthan continental crust

• Gabbro at depth and basalt at surfaceGabbro at depth and basalt at surface

• Oceanic crust produced continuallyOceanic crust produced continually

• Largest part of Earth’s surfaceLargest part of Earth’s surface

• Varied topography of seafloor Varied topography of seafloor

IntroductionIntroduction

• Interconnected body of saltwaterInterconnected body of saltwater

• Oceans and seas cover 71% Earth’s Oceans and seas cover 71% Earth’s surfacesurface

• Oceans are large; seas are smallerOceans are large; seas are smaller

• 1400s and 1500s: voyages of 1400s and 1500s: voyages of exploration:exploration:– Columbus 1492Columbus 1492

Exploring the OceansExploring the Oceans

• Scientific voyages later:Scientific voyages later:– Captain Cook in 1768, 1772, 1777Captain Cook in 1768, 1772, 1777– HMS HMS BeagleBeagle 1831-1836 with Charles 1831-1836 with Charles

Darwin – evolution of organisms and Darwin – evolution of organisms and coral reefscoral reefs

– HMS HMS ChallengerChallenger 1872: voyage to sample 1872: voyage to sample seawater, determine depths, collect seawater, determine depths, collect seafloor sediments, and classify seafloor sediments, and classify organismsorganisms

Exploring the Oceans, cont.Exploring the Oceans, cont.

• Echo sounder: sound waves used to Echo sounder: sound waves used to determine ocean depthsdetermine ocean depths

• Seismic profiling: seismic waves penetrate Seismic profiling: seismic waves penetrate seafloor and reflect from layers; helps seafloor and reflect from layers; helps determine structure of oceanic crustdetermine structure of oceanic crust

• Ocean ships drill into seafloor Ocean ships drill into seafloor

• R/V R/V ChikyuChikyu

• Submersibles: Submersibles: AlvinAlvin

Exploring the Oceans TodayExploring the Oceans Today

Oceanographic Research Vessels The R/V Chikyu (“Earth”), a research ship in the Integrated Ocean Drilling Program

• Ophiolites: sections of oceanic crust Ophiolites: sections of oceanic crust and upper mantle emplaced by and upper mantle emplaced by subduction zones and thrust faults in subduction zones and thrust faults in mountain rangesmountain ranges

• Top to bottom:Top to bottom:– Pillow lava and sheet lava flowsPillow lava and sheet lava flows– Sheeted dike complex, basalticSheeted dike complex, basaltic– Gabbro Gabbro

Oceanic Crust StructureOceanic Crust Structure and Composition and Composition

• Continental margin: area separating Continental margin: area separating continent portion above water from the continent portion above water from the deep seafloordeep seafloor

• Continental shelfContinental shelf– Gently sloping; 1 degree or lessGently sloping; 1 degree or less– Between shore and steeper continental Between shore and steeper continental

slopeslope– Shelf-slope break averages 135 m deepShelf-slope break averages 135 m deep– Pleistocene (1.8 million years ago to 10,000 Pleistocene (1.8 million years ago to 10,000

years ago): much of shelf above sea levelyears ago): much of shelf above sea level

Continental MarginsContinental Margins

• Continental slope: begins at shelf-Continental slope: begins at shelf-slope breakslope break

• Continental rise: gently sloping area Continental rise: gently sloping area between continental slope and between continental slope and abyssal plainabyssal plain– Absent in Pacific (oceanic trenches)Absent in Pacific (oceanic trenches)– Present in most of AtlanticPresent in most of Atlantic

Continental Margins, cont.Continental Margins, cont.

• Shelf-slope break:Shelf-slope break:– Landward: sediments are affected by Landward: sediments are affected by

waves and tidal currentswaves and tidal currents– Seaward: gravity transports and Seaward: gravity transports and

deposits sedimentsdeposits sediments– Much of land-derived sediment is Much of land-derived sediment is

seaward of shelf-slope break and covers seaward of shelf-slope break and covers the continental slope and continental the continental slope and continental riserise

Continental Margins, cont.Continental Margins, cont.

Features of Continental Margins A generalized profile showing features of the continental margins. The vertical dimensions of the features in this profile are greatly exaggerated, because the vertical and horizontal scales differ.

• Underwater flows of sediment/water Underwater flows of sediment/water mix; denser than seawater alonemix; denser than seawater alone

• Reaches relatively flat seafloorReaches relatively flat seafloor• Deposits sediments in graded beds, Deposits sediments in graded beds,

with largest particles first and smallest with largest particles first and smallest particles lastparticles last

• Forms overlapping submarine fansForms overlapping submarine fans• Evidence: 1929 event breaks North Evidence: 1929 event breaks North

Atlantic cablesAtlantic cables

Turbidity CurrentsTurbidity Currents

• Best developed on continental Best developed on continental slopes, but also found on continental slopes, but also found on continental shelvesshelves

• Some connect to land rivers, but Some connect to land rivers, but most don’tmost don’t

• Turbidity currents move through Turbidity currents move through submarine canyons, and likely are submarine canyons, and likely are the primary agents of their formationthe primary agents of their formation

Submarine CanyonsSubmarine Canyons

• At leading edge of continental plate At leading edge of continental plate where oceanic lithosphere is subductedwhere oceanic lithosphere is subducted

• Narrow continental shelfNarrow continental shelf

• Continental slope descends to trench, so Continental slope descends to trench, so no continental riseno continental rise

• South AmericaSouth America

• Pacific NorthwestPacific Northwest

• Earthquakes and volcanoesEarthquakes and volcanoes

Active Continental MarginsActive Continental Margins

• Broad continental shelvesBroad continental shelves

• Well-developed continental slopes and Well-developed continental slopes and risesrises

• Abyssal plains extend from continental Abyssal plains extend from continental riserise

• Within a plateWithin a plate

• Overlapping submarine fans at Overlapping submarine fans at continental risecontinental rise

Passive Continental MarginsPassive Continental Margins

• Average 3.8 km deep; dark, coldAverage 3.8 km deep; dark, cold

• Abyssal plainsAbyssal plains

• Oceanic trenchesOceanic trenches

• Oceanic ridgesOceanic ridges

• Hydrothermal ventsHydrothermal vents

• Seafloor fracturesSeafloor fractures

• Seamounts, guyots, aseismic ridgesSeamounts, guyots, aseismic ridges

Features of the Features of the Deep-Ocean BasinsDeep-Ocean Basins

• Beyond continental rises of passive Beyond continental rises of passive continental marginscontinental margins

• Flat and cover large areas Flat and cover large areas

• A few peaks up to 1 kmA few peaks up to 1 km

• Flatness from sediment deposition that Flatness from sediment deposition that covers topographycovers topography

• Not found near active margins: sediments Not found near active margins: sediments are trapped in oceanic trenchesare trapped in oceanic trenches

Abyssal PlainsAbyssal Plains

• Long steep-sided depressionsLong steep-sided depressions

• Near convergent boundariesNear convergent boundaries

• Subduction: cool, dense oceanic Subduction: cool, dense oceanic lithosphere is consumedlithosphere is consumed

• Common in Pacific Ocean basinCommon in Pacific Ocean basin

• Earthquakes along Benioff zonesEarthquakes along Benioff zones

• Volcano chain on overriding plateVolcano chain on overriding plate

Oceanic TrenchesOceanic Trenches

• Mostly submarine mountain system Mostly submarine mountain system composed of basalt and gabbrocomposed of basalt and gabbro

• Found in all ocean basinsFound in all ocean basins

• Divergent boundary: new crust Divergent boundary: new crust formationformation

• May have rift along crestMay have rift along crest

• Mid-Atlantic RidgeMid-Atlantic Ridge

• East Pacific RiseEast Pacific Rise

Oceanic RidgesOceanic Ridges

• At oceanic ridgesAt oceanic ridges• Cold seawater seeps below crust, is Cold seawater seeps below crust, is

heated at depth, discharges as plumes heated at depth, discharges as plumes up to 400up to 400ººCC

• Black smoker: from dissolved mineralsBlack smoker: from dissolved minerals• Community of organisms: bacteria, Community of organisms: bacteria,

crabs, mussels, starfish, tube wormscrabs, mussels, starfish, tube worms– Chemosynthesis: bacteria oxidize sulfur Chemosynthesis: bacteria oxidize sulfur

compoundscompounds

Submarine Hydrothermal Submarine Hydrothermal VentsVents

• Economic potential:Economic potential:– Heated seawater reacts with crustHeated seawater reacts with crust– When discharged into ocean it cools, When discharged into ocean it cools,

and iron, copper, and zinc sulfides and and iron, copper, and zinc sulfides and other minerals precipitateother minerals precipitate

Submarine Hydrothermal Submarine Hydrothermal Vents, cont.Vents, cont.

• Oceanic ridges terminate at fractures Oceanic ridges terminate at fractures that run at right angles to the ridgesthat run at right angles to the ridges

• Several hundred kilometers longSeveral hundred kilometers long

• Shallow-focus earthquakesShallow-focus earthquakes– Transform faults (active)Transform faults (active)

• Fracture zone (inactive)Fracture zone (inactive)

Seafloor FracturesSeafloor Fractures

• All are volcanic in originAll are volcanic in origin

• Seamount: at least 1 km heightSeamount: at least 1 km height

• Guyot: Guyot: – Volcano originally above sealevelVolcano originally above sealevel– Plate carries it away from ridge and into Plate carries it away from ridge and into

deeper watersdeeper waters– Waves eroded top and made it flatWaves eroded top and made it flat

• Abyssal hills: 250 m highAbyssal hills: 250 m high

Seamounts and GuyotsSeamounts and Guyots

• Ridge or broad area rising up to 2-3 km Ridge or broad area rising up to 2-3 km above seafloor; lacks seismic activityabove seafloor; lacks seismic activity

• Some are microcontinentsSome are microcontinents

• Form as linear succession of hot-spot Form as linear succession of hot-spot volcanoes near oceanic ridgesvolcanoes near oceanic ridges

• Can also form in the interior of platesCan also form in the interior of plates– Hawaiian Islands/Emperor Seamount chainHawaiian Islands/Emperor Seamount chain

Aseismic RidgesAseismic Ridges

• Silt- and clay-sized particlesSilt- and clay-sized particles

• Sources:Sources:1.1. Windblown dust and volcanic ashWindblown dust and volcanic ash

2.2. Shells of microscopic plants and Shells of microscopic plants and animals from near-surface watersanimals from near-surface waters

3.3. Particles from chemical reactions in Particles from chemical reactions in seawaterseawater

4.4. Cosmic dustCosmic dust

Deep Seafloor SedimentsDeep Seafloor Sediments

• Pelagic clay: particles from Pelagic clay: particles from continents and islandscontinents and islands

• Calcareous ooze: calcium carbonate Calcareous ooze: calcium carbonate skeletons of marine organismsskeletons of marine organisms

• Siliceous ooze: silica skeletons of Siliceous ooze: silica skeletons of some marine organismssome marine organisms

Deep Seafloor Sediments, Deep Seafloor Sediments, cont.cont.

• Skeletons of marine organisms: Skeletons of marine organisms: corals, molluskscorals, mollusks

• Shallow tropical seas with clear water Shallow tropical seas with clear water and water temperature above 20and water temperature above 20ººCC

• Corals with symbiotic photosynthetic Corals with symbiotic photosynthetic algae: 50 m deep maximumalgae: 50 m deep maximum

ReefsReefs

• Fringing reefsFringing reefs– Attached to island or continentAttached to island or continent– Rough tablelike surfaceRough tablelike surface– Slope steeply to seafloorSlope steeply to seafloor

• Barrier reefsBarrier reefs– lagoon separates reef from shorelagoon separates reef from shore

• AtollAtoll– Oval/round reef surrounds lagoonOval/round reef surrounds lagoon– Form around subsiding volcanic islands Form around subsiding volcanic islands

Reefs, cont.Reefs, cont.

Barrier Reef The white line of breaking waves marks the site of a barrier reef around Rarotonga in the Cook Islands in the Pacific Ocean. The island is only about 12 km long.

• Evaporation of seawater: sodium chlorideEvaporation of seawater: sodium chloride• Seafloor depositsSeafloor deposits

– Ownership questionsOwnership questions– U.S. Exclusive Economic Zone – 200 nautical U.S. Exclusive Economic Zone – 200 nautical

milesmiles– Oil productionOil production– Methane hydrateMethane hydrate– Manganese nodulesManganese nodules– Sulfide deposits at hydrothermal ventsSulfide deposits at hydrothermal vents

Resources from OceansResources from Oceans