GEOL: CHAPTER 9The Seafloor

Learning OutcomesLO1: Examine the history and methods of oceanic explorationLO2: Describe the structure and composition of the oceanic crustLO3: Identify the continental marginsLO4: Discuss the features found in the deep-ocean basins

Learning Outcomes, cont.LO5: Discuss sedimentation and sediments on the deep seafloorLO6: Explore coral reefsLO7: Recognize the types of natural resources found in the oceans

IntroductionOceanic crust is thinner and denser than continental crustGabbro at depth and basalt at surfaceOceanic crust produced continuallyLargest part of Earths surfaceVaried topography of seafloor

Exploring the OceansInterconnected body of saltwaterOceans and seas cover 71% Earths surfaceOceans are large; seas are smaller1400s and 1500s: voyages of exploration:Columbus 1492

Exploring the Oceans, cont.Scientific voyages later:Captain Cook in 1768, 1772, 1777HMS Beagle 1831-1836 with Charles Darwin evolution of organisms and coral reefsHMS Challenger 1872: voyage to sample seawater, determine depths, collect seafloor sediments, and classify organisms

Exploring the Oceans TodayEcho sounder: sound waves used to determine ocean depthsSeismic profiling: seismic waves penetrate seafloor and reflect from layers; helps determine structure of oceanic crustOcean ships drill into seafloor R/V ChikyuSubmersibles: Alvin

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

Oceanic Crust Structure and CompositionOphiolites: sections of oceanic crust and upper mantle emplaced by subduction zones and thrust faults in mountain rangesTop to bottom:Pillow lava and sheet lava flowsSheeted dike complex, basalticGabbro

Continental MarginsContinental margin: area separating continent portion above water from the deep seafloorContinental shelfGently sloping; 1 degree or lessBetween shore and steeper continental slopeShelf-slope break averages 135 m deepPleistocene (1.8 million years ago to 10,000 years ago): much of shelf above sea level

Continental Margins, cont.Continental slope: begins at shelf-slope breakContinental rise: gently sloping area between continental slope and abyssal plainAbsent in Pacific (oceanic trenches)Present in most of Atlantic

Continental Margins, cont.Shelf-slope break:Landward: sediments are affected by waves and tidal currentsSeaward: gravity transports and deposits sedimentsMuch of land-derived sediment is seaward of shelf-slope break and covers the continental slope and continental rise

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.

Turbidity CurrentsUnderwater flows of sediment/water mix; denser than seawater aloneReaches relatively flat seafloorDeposits sediments in graded beds, with largest particles first and smallest particles lastForms overlapping submarine fansEvidence: 1929 event breaks North Atlantic cables

Submarine CanyonsBest developed on continental slopes, but also found on continental shelvesSome connect to land rivers, but most dontTurbidity currents move through submarine canyons, and likely are the primary agents of their formation

Active Continental MarginsAt leading edge of continental plate where oceanic lithosphere is subductedNarrow continental shelfContinental slope descends to trench, so no continental riseSouth AmericaPacific NorthwestEarthquakes and volcanoes

Passive Continental MarginsBroad continental shelvesWell-developed continental slopes and risesAbyssal plains extend from continental riseWithin a plateOverlapping submarine fans at continental rise

Features of the Deep-Ocean BasinsAverage 3.8 km deep; dark, coldAbyssal plainsOceanic trenchesOceanic ridgesHydrothermal ventsSeafloor fracturesSeamounts, guyots, aseismic ridges

Abyssal PlainsBeyond continental rises of passive continental marginsFlat and cover large areas A few peaks up to 1 kmFlatness from sediment deposition that covers topographyNot found near active margins: sediments are trapped in oceanic trenches

Oceanic TrenchesLong steep-sided depressionsNear convergent boundariesSubduction: cool, dense oceanic lithosphere is consumedCommon in Pacific Ocean basinEarthquakes along Benioff zonesVolcano chain on overriding plate

Oceanic RidgesMostly submarine mountain system composed of basalt and gabbroFound in all ocean basinsDivergent boundary: new crust formationMay have rift along crestMid-Atlantic RidgeEast Pacific Rise

Submarine Hydrothermal VentsAt oceanic ridgesCold seawater seeps below crust, is heated at depth, discharges as plumes up to 400CBlack smoker: from dissolved mineralsCommunity of organisms: bacteria, crabs, mussels, starfish, tube wormsChemosynthesis: bacteria oxidize sulfur compounds

Submarine Hydrothermal Vents, cont.Economic potential:Heated seawater reacts with crustWhen discharged into ocean it cools, and iron, copper, and zinc sulfides and other minerals precipitate

Seafloor FracturesOceanic ridges terminate at fractures that run at right angles to the ridgesSeveral hundred kilometers longShallow-focus earthquakesTransform faults (active)Fracture zone (inactive)

Seamounts and GuyotsAll are volcanic in originSeamount: at least 1 km heightGuyot: Volcano originally above sealevelPlate carries it away from ridge and into deeper watersWaves eroded top and made it flatAbyssal hills: 250 m high

Aseismic RidgesRidge or broad area rising up to 2-3 km above seafloor; lacks seismic activitySome are microcontinentsForm as linear succession of hot-spot volcanoes near oceanic ridgesCan also form in the interior of platesHawaiian Islands/Emperor Seamount chain

Deep Seafloor SedimentsSilt- and clay-sized particlesSources:Windblown dust and volcanic ashShells of microscopic plants and animals from near-surface watersParticles from chemical reactions in seawaterCosmic dust

Deep Seafloor Sediments, cont.Pelagic clay: particles from continents and islandsCalcareous ooze: calcium carbonate skeletons of marine organismsSiliceous ooze: silica skeletons of some marine organisms

ReefsSkeletons of marine organisms: corals, mollusksShallow tropical seas with clear water and water temperature above 20CCorals with symbiotic photosynthetic algae: 50 m deep maximum

Reefs, cont.Fringing reefsAttached to island or continentRough tablelike surfaceSlope steeply to seafloorBarrier reefslagoon separates reef from shoreAtollOval/round reef surrounds lagoonForm around subsiding volcanic islands

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.

Resources from OceansEvaporation of seawater: sodium chlorideSeafloor depositsOwnership questionsU.S. Exclusive Economic Zone 200 nautical milesOil productionMethane hydrateManganese nodulesSulfide deposits at hydrothermal vents

Underwater view of a reef in Hawaii. Coral reefs, as they are commonly called, are actually composed of the skeletons of corals, various mollusks (such as clams), as well as encrusting organisms including sponges and algae.

Figure 9.1 The Seafloor This map shows the four oceans and many of the seas, which are marginal parts of oceans. The seafloor constitutes the largest part of Earths surface.

Figure 9.2 Oceanographic Research Vessels The R/V Chikyu (Earth), a research ship in the Integrated Ocean Drilling Program.

Figure 9.3 Composition of Oceanic Crust New oceanic crust made up of the layers shown here forms as magma rises beneath oceanic ridges. The composition of oceanic crust was known from ophiolites, sequences of rock on land consisting of deep-sea sediments, oceanic crust, and upper mantle, before scientists observed it when they descended in submersibles to seafloor fractures.

Figure 9.4 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.

Figure 9.5 Submarine Fans and Graded Bedding

Figure 9.5 Submarine Fans and Graded Bedding

Figure 9.6 Passive and Active Continental Margins

Figure 9.6 Passive and Active Continental Margins

Figure 9.7 Deep Seafloor Fractures Features found on the deep seafloor include oceanic trenches (brown), abyssal plains (green), the oceanic ridge system (yellow), rift valleys (red), and some aseismic ridges (blue). Other features such as seamounts and guyots are shown in Figure 9.10.

Figure 9.8 Central Rift and Submarine Hydrothermal Vents

Figure 9.8 Central Rift and Submarine Hydrothermal Vents

Figure 9.8 Central Rift and Submarine Hydrothermal VentsFigure 9.9 Seafloor Fractures Diagrammatic view of an oceanic ridge offset along a fracture. That part of the fracture between displaced segments of the ridge is a transform fault. Recall from Chapter 2 that transform faults are one type of plate boundary.

Figure 9.10 The Origin of Seamounts and Guyots As the plate on which a volcano rests moves into greater water depths, the submerged volcanic island is called a seamount. Those that are flat-topped are called guyots.

Figure 9.11 Sediments on the Deep Seafloor The particles making up the calcareous ooze are skeletons of (b) foraminifera (floating single-celled animals) and (c) coccolithophores (floating single-celled plants), whereas siliceous ooze is composed of skeletons of (d) radiolarians (single-celled floating animals) and (e) diatoms (single-celled floating plants).

Figure 9.12 Reefs and Their Origin

Figure 9.12 Reefs and Their Origin

Figure 9.13 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.