Chapter 13 – The Ocean Floor: Comprehensive Bullet-Point Notes

13.1 The Vast World Ocean

• Earth as the “Blue/Water Planet”

  • \approx71\% of Earth’s surface (\approx360\text{ million km}^2) is covered by the ocean.

  • Remaining \approx29\% (\approx150\text{ million km}^2) is continental & island landmass.

  • Disproportionate land–water distribution:

  • Northern Hemisphere = Land Hemisphere: 61\% water vs. 39\% land.

  • Southern Hemisphere = Water Hemisphere: 81\% water vs. 19\% land.

    

• Four Principal Ocean Basins

  • Pacific Ocean: Largest geographical feature on the planet

    • occupies > 1/3 of total surface, > 1/2 of oceanic area.

    • Average depth \approx3940\text{ m} \,(\sim2.5\text{ mi}); could hold all continents with space left.

  • Atlantic Ocean:\approx1/2 the area of the Pacific; slightly shallower

    • bounded by near-parallel continental margins → comparatively narrow.

  • Indian Ocean: Slightly smaller than the Atlantic; comparable depth; largely a Southern-Hemisphere basin.

  • Arctic Ocean

    • Smallest ( \approx7\% of Pacific’s size )

    • shallowest ( \approx1/4 average depth of other oceans ).

  • Southern / Antarctic Ocean (informal)

    • Waters south of 50^\circ S where Antarctic currents converge; composed of southern portions of Pacific, Atlantic & Indian.

• Oceans vs Continents

  • Mean continental elevation \approx840\text{ m} above sea level.

  • Mean ocean depth \approx3729\text{ m} → oceans are \sim4.5× deeper than continents are high.

  • Hypothetical perfectly smooth Earth would be submerged by >2000\text{ m} of seawater.

13.2 An Emerging Picture of the Ocean Floor

• Bathymetry

  • Definition: measurement & mapping of ocean depths/topography (bathos = depth, metry = measure).

• Historic Beginnings – HMS Challenger (1872-1876)

  • First global survey; 127,500-km voyage; hand-line soundings recorded deepest point (later named Challenger Deep).

• Modern Mapping Technologies

  • Echo Sounder / Single-beam Sonar

  • Emits acoustic "ping"; depth = \tfrac12(1500\,\text{m s}^{-1}\times\text{travel-time})

  • Sidescan Sonar

  • Tow-fish produces fan-shaped swath images; good for texture but originally lacked accurate depth.

  • Multibeam Sonar

  • Hull-mounted array; obtains tens-km-wide depth swaths; cm–m vertical resolution; vessels "mow the lawn" at \sim10$–$20\,\text{km h}^{-1}.

  • Only \sim5\% of seafloor mapped at high resolution—entire mapping would require >100 ships for centuries.

  • Satellite Radar Altimetry

  • Measures sea-surface height anomalies (cm-scale) caused by gravitational attraction of sub-seafloor masses; converts to bathymetric maps.

  • Key discovery: ocean surface mimics hidden topography (ridges → bulges; trenches → depressions).

• Three Major Physiographic Provinces

  • Continental Margins

  • Deep-Ocean Basins

  • Oceanic (Mid-Ocean) Ridges

13.3 Continental Margins

• Passive Continental Margins (Atlantic-type)

  • Tectonically inactive; far from plate boundaries; develop after continental rifting → seafloor spreading.

  • Broad sediment-rich shelves, slopes & rises.

  • Continental Shelf

    • Submerged extension of continent; avg. slope \approx0.1^{\circ}; width from <1 km to >1500 km.

    • Economically valuable: petroleum reservoirs, fisheries.

  • Shelf Break: abrupt change to steeper slope.

  • Continental Slope

    • Avg. slope \approx5^{\circ} (locally >25^{\circ}); marks transition to oceanic crust.

  • Continental Rise

    • Wedge of merged deep-sea fans at base of slope; built by turbidity currents.

  • Submarine Canyons

    • Deep valleys incising shelf & slope; carved mainly by episodic turbidity currents (dense, sediment-laden flows); extend beyond rise.

    • Produce graded-bedded turbidites.

• Active Continental Margins (Pacific-type)

  • Coincident with convergent boundaries (subduction zones) or transform systems; characterized by:

  • Narrow shelf, steep slope, offshore deep-ocean trench.

  • Accretionary Wedge: sediments scraped from subducting plate plastered against overriding plate (common in low-angle subduction).

  • Subduction Erosion: opposite process—sediment & rock peeled from overriding plate, carried downward (steep-angle zones).

  • Associated phenomena: strong earthquakes, continental volcanic arcs or volcanic island arcs.

13.4 Features of Deep-Ocean Basins

• Deep-Ocean Trenches

  • Long, narrow depressions—the deepest parts of oceans; sites of descending slabs.

  • Examples: Mariana Trench (Challenger Deep \approx10{,}994\text{ m}), Peru-Chile, Aleutian, Tonga.

  • Parallel volcanic arcs: island arcs (ocean-ocean) or continental arcs (ocean-continent).

• Abyssal Plains

  • Flattest places on Earth; relief < 3\text{ m} over >1000\text{ km} (
    e.g., Argentine Basin).

  • Formed when irregular oceanic crust is buried by thick (>1\text{ km}) blankets of fine terrigenous, biogenous & hydrogenous sediment.

  • More extensive in Atlantic (few trenches to intercept sediment) than Pacific.

• Volcanic Structures

  • Seamounts: submarine volcanoes; >1\times10^{6} estimated; may form linear chains (e.g., Hawaiian-Emperor).

  • Volcanic Islands: seamounts that breach sea level (e.g., Tahiti, Galápagos).

  • Guyots: flat-topped, submerged volcanoes—eroded former islands that subsided with plate motion.

  • Oceanic Plateaus: massive, thick accumulations of flood basalts from mantle-plume heads (e.g., Ontong Java, Kerguelen); >30\text{ km} thick.

13.5 The Oceanic Ridge System

• Global Morphology

  • World-encircling, >70{,}000\text{ km} long, 2$–$3\text{ km} high swell; width 1000$–$4000\text{ km}.

  • Segmented (offset by transform faults) & named: Mid-Atlantic Ridge, East Pacific Rise, Mid-Indian Ridge, etc.

• Rift Valleys

  • Deep, fault-bounded troughs (30–50 km wide; walls up to 2500\text{ m} high) along some ridge axes → sites of crustal stretching & magma injection.

• Elevated Topography – Thermal Buoyancy

  • Newly formed lithosphere is hot & less dense; as it moves away it cools, contracts & subsides.

  • Thickness of oceanic lithosphere increases with age, stabilizing at \sim80$–$100\text{ km} after \sim80 Myr.

• Processes & Heat Flow

  • High heat flow, frequent basaltic volcanism, abundant normal/strike-slip faulting, hydrothermal circulation (black smokers → metal sulfides).

13.6 Seafloor Sediments

• Three Genetic Categories

  • Terrigenous (land-derived): mineral grains weathered from continents; delivered by rivers, wind, glaciers, turbidity currents.

  • Blanket entire ocean but thin (
    abyssal clay accumulation rate \sim1\text{ cm}/10{,}000\text{ yr}).

  • Biogenous (organism-derived): shells/skeletons of planktonic organisms.

  • Calcareous ooze (CaCO_3) from foraminifera, coccolithophores; dissolves below CCD \sim4500\text{ m}.

  • Siliceous ooze (SiO_2) from diatoms, radiolarians.

  • Phosphatic remains from fish bones/teeth.

  • Hydrogenous (precipitated from seawater):

  • Manganese nodules: concentric layers of Mn, Fe + Cu, Ni, Co.

  • Calcium carbonates: direct precipitates in warm, shallow seas → oolitic limestone.

  • Metal sulfides: precipitated around hydrothermal vents; rich in Fe, Cu, Zn, Ag, Au.

  • Evaporites: halite (NaCl), gypsum, anhydrite from restricted basins with high evaporation.

• Sediments as Climate Archives

  • Micro-fossil assemblages & isotopic composition reflect surface-water conditions → reconstruct temperature, ice volume, productivity.

  • Drill cores (e.g., JOIDES/Chikyu/IODP) provide multi-Myr records – crucial for Quaternary Ice-Age studies.

13.7 Resources from the Seafloor

• Energy Resources (>$95\%$ of economic value)

  • Oil & Natural Gas

  • Origin: buried marine microorganisms in sedimentary basins on continental margins.

  • Offshore now provides >30\% of global oil; major provinces: Persian Gulf, Gulf of Mexico, North Sea, Brazil, West Africa.

  • Environmental risks: spills, blowouts (e.g., Deepwater Horizon 2010).

  • Gas Hydrates

  • Ice-like clathrates of water + mainly methane; stable beneath >525\text{ m} water or permafrost.

  • Global volume est. \sim20 quadrillion \text{m}^3 ((\sim2\times) carbon of all other fossil fuels).

  • Challenges: dissociate at surface P–T; extraction & seafloor stability.

• Non-energy Mineral Resources

  • Sand & Gravel

  • Used in construction, land reclamation, beach nourishment; second only to petroleum in marine value.

  • May host placer deposits (gold, tin, titanium, diamonds).

  • Evaporative Salts

  • \sim30\% of world’s NaCl from seawater (solar ponds); also Mg, bromine.

  • Manganese Nodules & CoCrNi-rich Crusts

  • Potential source of strategic metals; technological & legal hurdles (UNCLOS mining codes) + environmental concerns.

Integrative Connections & Broader Significance

• Plate-Tectonic Context

  • Continental margins, trenches & ridges are surface expressions of divergent, convergent & transform boundaries.

  • Processes operating underwater mirror those shaping continents (rift valleys vs. East African Rift; volcanic arcs vs. Andes).

• Climate–Ocean Feedbacks

  • Distribution of land & water influences atmospheric circulation; Southern Ocean’s uninterrupted westerlies affect global heat balance.

  • Sedimentary records supply long-term context for modern climate change debates.

• Biosphere Interactions

  • Hydrothermal vents support chemosynthetic ecosystems independent of sunlight → origin-of-life implications.

  • Coral-reef development (Darwin’s atoll model) illustrates interplay of volcanism, plate motion & biology.

• Human & Ethical Dimensions

  • Resource extraction vs. environmental stewardship (oil spills, nodule mining, gas-hydrate destabilization).

  • Mapping & understanding oceans underpin hazard mitigation (tsunamis, submarine landslides) & conservation (marine protected areas).