Ocean Morphology and Bottom Relief

Introduction to the Hydrosphere

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• Total surface area of the globe: 509,950,000 km²
  • Hydrosphere: 361,060,000 km² (~71%)
  • Lithosphere: 148,890,000 km² (~29%)
• The hydrosphere is classified by size/location into oceans, inland seas, small enclosed seas, bays, etc.
• Major oceans (by area):
  • Pacific Ocean — 165,000,000 km²
  • Atlantic Ocean — 82,000,000 km²
  • Indian Ocean — 73,000,000 km²
• Notable seas: Arctic, Malay, Middle American, Mediterranean, Bering, Barents, Kara, East Siberian, Japan, East China, Okhotsk, Yellow, Andaman, South China, Caribbean, North, Celebes, Labrador, Beaufort, Arabian, Red Sea, etc.
• Like the lithosphere, the ocean floor also has relief: mid-oceanic ridges, trenches, deep sea plains, basins, submarine canyons.
• Average depth of oceans: 3,800 m vs average height of land: 840 m.
• The height/depth distribution of the earth’s surface is shown by the hypsographic (hypsometric) curve for land and the bathymetric curve for oceans.
• Four broad relief zones of ocean basins: continental shelves, continental slopes, deep sea plains, and oceanic trenches.

Ocean Morphology and Marine Provinces

• Ocean morphology = the configuration/shape of ocean basins in terms of the nature and dimensions of their relief features.
• Marine provinces = zones of the ocean basin grouped by shared/common relief characteristics.
• Historically, ocean floors were assumed to be flat and featureless, so early oceanographers showed little interest in studying them.
• Advances in exploration technology gradually revealed the true topographic complexity of the seafloor.

Key methods and instruments in the study of ocean floor relief

• Bathymetry: measurement/study of ocean depth zones using sounding techniques (bathos = depth, metry = measurement).
• Hypsometry (hypsography): measurement of land elevation above sea level.
  • Hypsometry → positive relief (above sea level)
  • Bathymetry → negative relief (below sea level)
• Historical milestones:
  • Depth measurement in the Mediterranean dates back to ~85 B.C., but the first scientific bathymetric survey was the HMS Challenger Expedition (1872).
  • Echosounder first used during the Meteor Expedition (1925), which discovered an undersea mountain range in the central South Atlantic.
  • Precision Depth Recorder (PDR) developed in the 1950s improved accuracy.
  • Modern tools: multibeam echosounders (e.g., SeaBeam), side-scan sonar systems such as Sea MARC (Sea Mapping and Remote Characterization) and GLORIA (Geological Long Range Inclined Acoustical instrument).

Classifications of Marine Provinces

Different scholars have classified ocean floor relief differently:

(A) Thurman & Trujillo (1999) — 3 major provinces (based on bathymetry):

1. Continental margins (shallow water areas close to continents: shelves + slopes)
2. Deep ocean basins (deep water away from continental margins)
3. Mid-ocean ridges (shallow water areas near the middle of the ocean)

(Limitation: this scheme doesn’t clearly accommodate deeps/trenches.)

(B) Traditional classification — 4 provinces:

1. Continental shelves
2. Continental slopes
3. Deep sea plains
4. Oceanic trenches

(C) Combined/Merged classification — 5 provinces:

1. Continental shelves
2. Continental slopes (including submarine canyons)
3. Deep sea plains
4. Mid-ocean ridges
5. Ocean trenches

Continental Margins

• Represent the boundary zone between land and ocean, and correspond to plate boundaries with shallow seawater.
• Thurman & Trujillo grouped continental shelf (+ shelf break), continental slope, and continental rise together as one province, but note their morphology differs considerably — however geologically they form one unit since all are built of continental rock (granite), as opposed to the basaltic ocean basins.
• Submarine canyons give the continental slope enough distinctiveness to be treated separately.
• Two types of continental margins:
  1. Active continental margins — associated with tectonic activity (faulting, folding, volcanism, seismicity)
     • (i) Transform active margins
     • (ii) Convergent active margins
  2. Passive continental margins — tectonically quiet, no major activity
• Continental margins comprise: continental shelf, shelf break, continental slope, and continental rise.

Continental Shelf

Definition (Thurman & Trujillo, 1999): the shelf-like zone from the shore to the point where slope angle increases sharply (the shelf break); beyond this point lies the continental slope.

• Alternative definition (Pinet, 2000): the nearly flat plains/terraces at the top of the sedimentary wedge beneath the submerged edge of continents.
• Average water depth: ~100 fathoms (1 fathom = 6 ft/1.8 m) or ~180 m.
• Slope angle: gentle, about 1°–3°.
• Shelf break occurs at an average depth of 130 m, sometimes >200 m; slope angle at the break averages 1°–4°.

Width of Continental Shelves

• Ranges from 60 km to over 1,500 km, depending on the relief of adjoining coastal land.
  • Narrow where high mountains lie close and parallel to the coast — e.g., the Pacific shelf off western South America is only ~16 km wide (due to the Andes).
  • Wide where coastal plains are extensive.
  • Generally wider near river mouths, but the Mississippi River mouth shelf is an exception (narrow).
• Average width ≈ 48 km, though Sheppard’s estimate is 67 km (42 miles).
• Examples:
  • Narrow shelf: Pacific coast of South America (16 km)
  • Medium shelf: Atlantic coast of North America (96–120 km)
  • Very wide shelves: off East Indies, Arctic Sea, China Sea, Adriatic Sea, Arafura Sea (few hundred km)
• Continental shelves occupy 8.6% of total ocean basin area.
  • Regional share: Atlantic 13.3%, Pacific 5.7%, Indian Ocean 4.2%

Other Notes

• Passive margins (e.g., east coasts of North & South America) have relatively wider shelves than active margins (e.g., west coasts of the Americas).
• Average shelf break depth ≈ 135 m, but ~350 m around Antarctica.
• Broadest shelves: northern Siberia, Arctic coast of North America, Alaska.
• Wider, shallower shelves reduce the destructive force of tsunamis.

Ecological Importance

• Provide ideal habitats for marine plants, animals, and micro-organisms; support rich fishing grounds.
• Coral reefs act as natural frontline buffers against storm surges and tsunamis by absorbing disruptive wave energy.
  • Example: Coral reefs on the Maldives’ shelves are credited with limiting deaths to only 98 during the December 26, 2004 Sumatra tsunami.
• Mangrove forests on shallow shelves near coasts provide habitat for marine and land species (e.g., Bengal tiger in the Sundarbans).
  • India examples: Pichhavaram (Tamil Nadu), Bhitarkanika (Odisha) — mangroves that buffered the 2004 tsunami’s impact on India’s east coast.

Continental Shelves of India

• Seaward limit demarcated by the 100-fathom contour.
• Width: ~50 km (east coast), ~150 km (west coast).
• Narrow (30–35 km) off the mouths of the Ganga, Mahanadi, Godavari, Krishna, and Cauvery rivers.
• Wider off the estuaries of the Narmada, Tapi, and Mahi rivers.
• Average slope: ~21° off the eastern coast; ~10° near Cape Comorin; ~1° near the Gulf of Khambhat (Cambay).
• Formation differs regionally:
  • Ganga, Godavari, Krishna, Cauvery mouths — formed via delta formation.
  • Midnapur to Madurai shelves — sedimentation and subsidence.
  • Andaman-Nicobar, Lakshadweep, Gulf of Mannar shelves — coral reef origin.
  • Western coast shelves — faulting and subsidence.

Theories on the Origin of Continental Shelves

1. Marine erosion + fluvial deposition — waves/currents erode margins, forming platforms; rivers deposit sediment that consolidates into shelves.
2. Terrigenous fluvial deposition alone — prolonged sediment deposition in calm seas causes continuous subsidence and sedimentation (constructional, most extensive type).
3. Subsidence of continental margins — convective currents beneath continents/ocean basins cause compressive subsidence at the continent-ocean boundary.
4. Faulting and subsidence — parallel faults form in continental margins, causing land subsidence and submergence (tectonically-formed shelves).
5. Glacial control + marine erosion — sea level fall during ice ages (Daly estimated a ~38-fathom fall in the Pleistocene) exposed continental margins to glacial erosion, forming platforms later re-submerged as sea level rose.
6. Cliff erosion and submergence of wave-cut platforms — wave-driven cliff recession creates wave-cut platforms later submerged to form shelves.
7. Tilting — land tilting toward the sea submerges margins, forming/extending shelves.

Continental Slope

• The steep zone connecting the continental shelf to the deep sea plains.
• Slope angle: ranges from 5° to over 60°
  • ~40° near St. Helena
  • ~30° off the Spanish coast
  • ~62° near St. Paul
  • 5°–15° near Calicut, India
• Water depth range: 200 m to 2,000 m.
• Occupies only ~8.5% of total ocean basin area, but this varies by ocean:
  • Atlantic: 12.4%
  • Pacific: 7%
  • Indian Ocean: 6.5%
• Most extensive continental slopes are found between 20°N–50°N and at 80°N and 70°S latitudes.
• The slope’s steep gradient generally prevents sediment accumulation (materials are quickly transported downward), though a thin veneer of deposits can occur in some cases.
• Most significant relief features on the slope: submarine canyons and trenches, generally transverse to the shelves/coast.

Theories on the Origin of Continental Slopes

• Erosional theory: slopes formed by marine erosion (mainly wave action); supported by the presence of submarine canyons.
• Tectonic theory: faulting responsible for slope formation.
• Aggradational theory: slopes formed by bending/warping of shelves followed by sedimentation.

Submarine Canyons

Characteristics

• Long, narrow, very deep valleys/trenches on continental shelves and slopes, with steep/vertical walls resembling terrestrial canyons — hence the name.
• Classified (by origin) into:
  • (i) Glacially eroded canyons
  • (ii) Non-glacial canyons (far more numerous and widespread)
• Mostly transverse to the coast, located in front of major river mouths (with some exceptions).
• Comparable in profile to terrestrial river valleys, but deeper; some show dendritic tributary patterns.
• Longitudinal course: usually sinuous (vs. straight for terrestrial canyons).
• Gradient: steeper than land canyons.
• Typically several km wide at the head; average length ≈ 16 km.
• Average longitudinal gradient ≈ 1.7%; canyons facing river mouths (e.g., Congo Canyon) tend to be long with gentle gradient, while canyons near islands are steeper (up to ~13.8%).
• Based on a study of 102 canyons (Sheppard & Beard): average gradients were 11.62% (upper), 6.63% (middle), 4.76% (lower segment).
• Depth: typically 610–915 m, occasionally up to 3,048 m.
• Floor deposits: sands, clays, silt, gravel, pebbles — coarser than adjacent shelf sediments; valley sides largely lack loose material.
• Notable comparison: the Monterey Canyon (California, USA) is comparable in scale to the Grand Canyon of the Colorado River (Arizona, USA).

Global Distribution

• No clear latitudinal control on distribution.
• Sheppard and Beard identified 102 submarine canyons worldwide via soundings.
• More common along straight coasts than highly indented/crenulated coastlines; found on both stable and unstable coasts.
• Prominent regions:
  • East coast of USA (Canada to Cape Hatteras)
  • Californian and Mexican coasts
  • North Mediterranean, Philippines, Japan, Aleutian Islands
  • West coast of Africa
  • East coast of India

Regional examples

• Atlantic: Hudson Canyon (827 m deep, off the Hudson River mouth), Chesapeake Canyon, Mississippi Trough, Fosse de Cap Breton (Bay of Biscay), Nazare Canyon (Portugal, 4,000 m deep), Congo Canyon.
• Pacific: Columbia Canyon; Monterey Canyon (with tributaries — Ascension, Soquel, Carmel canyons); Mugu, Scripps, Dume canyons (California); Panama Canyon (off Burica Peninsula); Pusan Chang Canyon (Korea); Philippine Canyon (Luzon); Saganin, Fizi canyons.
• Indian Ocean: canyons along India’s east coast, in front of the Indus River, along Sri Lanka’s northeastern coast, along Africa’s eastern coast.

India’s east coast submarine canyons (selected, from the source table):

Theories on the Origin of Submarine Canyons

1. Diastrophic (tectonic) theory — proposed by Andrade, Lawson, De la Roche Ponie, J.W. Gregory, Yanasaki, Jensen, Bourcart, and others. Attributes canyon origin to earth movements: faulting, folding, warping, sinking of the sea floor, creating grabens/synclinal troughs that become canyons.
   • Criticisms: most canyons run transverse to the coast while faulting is usually parallel to coasts; many canyons show dendritic tributary patterns inconsistent with pure faulting; not all shelves/slopes show fault evidence. The theory works reasonably for Pacific coasts and the Mediterranean (active tectonics) but less so for the relatively stable margins of the Atlantic.
2. Subaerial erosion theory — proposed by J.D. Dana, F.P. Sheppard, Hull, and others, based on the resemblance between submarine and terrestrial canyons. Canyons were cut by rivers when sea level was much lower (land emergence), then drowned as sea level rose (submergence) or the margins subsided.
   • W.M. Davis critiqued this, arguing it requires implausible vertical oscillation of land over long geological time, and that such canyons should extend onto land (but they are typically found offshore, away from river mouths).
   • Emery & Sheppard responded that a ~1,000 m sea-level fall during Pleistocene glaciation provided platforms for river entrenchment, later drowned by post-glacial sea-level rise — though this leaves canyons deeper than 2,000 m unexplained.
3. Submarine density current theory — proposed by Holmann (1883), Adolf Von Salis (1884), and Florel. Density differences (from temperature/salinity variation) generate currents that erode shelves and form trenches, with sediment/dyke (levee) formation on either side. Criticized because such currents are mainly confined to enclosed seas, reservoirs, and lakes, and rarely occur over open, shallow continental shelves.
4. Turbidity current theory — proposed by W.M. Davis, W.E. Rither, Tangier Smith, P.D. Trask, Lawson, Daly, Buchanan, and others. Onshore winds pile up water near the coast, generating dense, sediment-laden undercurrents (“turbidity currents”) that flow seaward and erode the shelf, cutting canyons and valleys. Daly linked increased turbidity to sea-level fall during glacial periods.
   • Critics (Zeppelin, Heim, Bucher) doubted these currents were powerful enough to erode hard continental rock; Bucher argued that currents triggered by earthquakes/volcanic eruptions are more capable of such erosion.
5. Kuenen’s view — canyons in different regions, with varying lithology/structure, likely form through different mechanisms: canyons in stable, compact/tenacious rock areas form via drowning of old subaerial valleys, while those in unconsolidated material may form through landslides, turbidity currents, etc.

Deep Sea Fans and Continental Rise

• Deep sea fans (submarine fans): fan-, lobate-, or apron-shaped depositional features at the base of the continental slope and at the mouths of submarine canyons — analogous to continental alluvial fans.
• Formed by sediment carried by submarine turbidity currents flowing down submarine canyons toward the deep sea plains.
• When multiple deep sea fans merge, the resulting larger depositional feature is called the continental rise.
• Continental rise:
  • Abundant in the Atlantic and Indian Oceans; scarce in the Pacific.
  • Average width < 300 km; relief amplitude < 40 m.
  • Water depth range: 1.5–5.0 km.
• Turbidity currents are considered the primary agent behind rise formation: as they cross canyon mouths, reduced gradient slows them, causing suspended sediment to settle in a graded sequence (coarser near the canyon mouth, finer toward the deep plains).
• The deposition shows graded bedding — vertical fining upward within each depositional unit — building up fan/lobe shapes over time as successive turbidity flows deposit new graded layers atop older ones.
• These accumulated graded sediment sequences are called turbidite deposits; growth is horizontal and vertical, typically reaching heights below ~40 m.

Deep Ocean Basins and Associated Features

Deep ocean basins contain both elevated features (abyssal hills) and depressions (trenches/deeps). Main features:

• Abyssal plains
• Abyssal hills
• Seamounts (guyots) and seatablemounts
• Ocean deeps and trenches

(a) Abyssal Plains

• The most extensive but flattest terrain on Earth’s surface (flatter even than continental plains).
• Average slope gradient ≈ 0.5°.
• Underlying basaltic (volcanic) crust is overlain by unconsolidated, layered terrigenous sediment transported by turbidity currents, along with occasional direct volcanic deposits.
• Also characterized by pelagic deposits — remains of plants, marine animals, and siliceous material, mixed with terrigenous sediment.
• Depth range: 3,000–6,000 m.
• Cover 75.9% of total ocean basin area, but this varies:
  • Pacific: 80.3%
  • Indian Ocean: 80.1%
  • Atlantic: 54.9% (comparatively low, attributed to the Atlantic’s more extensive continental shelves)
• Though generally featureless, some long/narrow/elongated ridges and guyots occur; some ridges even reach/project above sea level as islands (e.g., Mid-Atlantic Ridge, East Pacific Rise, mid-Indian Ocean Ridge).
• Why abyssal plains differ by ocean:
  • Passive plate margins (Atlantic, Indian Oceans) → extensive abyssal plains, since there are no trenches to trap land-derived sediment before it reaches the deep basin.
  • Active plate margins (Pacific) → ocean trenches intercept sediments at convergent zones, so less sediment reaches the deep basins → limited abyssal plains.

(b) Abyssal Hills

• Volcanic hills projecting above abyssal plains, various forms:
  • Volcanic islands — hills that break the sea surface
  • Abyssal hills / seaknolls — smaller volcanic hills (~1,000 m high, 0.1–100 km wide), below sea level
  • Seamounts — conical volcanic hills, always submerged, relict extinct volcanic mountains, average height ~1,000 m above the ocean floor (some are active volcanic peaks); steep-sided; isolated or in groups
  • Tablemounts / guyots — flat-topped submerged volcanic hills
• Clusters of abyssal hills are termed abyssal hill provinces; common across the deep plains of the Atlantic and Indian Oceans.
• Most originate from divergent plate boundaries and associated seafloor spreading-driven volcanism.

(c) Ocean Deeps and Trenches

• The deepest zones of ocean basins, generally parallel to coasts with mountains, and near island arcs.
• Classified by size:
  • Deeps — very deep but less extensive depressions
  • Trenches — long, narrow, linear depressions
• Both feature very steep (sometimes near-vertical) slopes.
• Usually named after explorers or geographic locations (e.g., Murray Deep after J. Murray; Japan and Sunda Trenches after their locations).
• Of 57 surveyed deeps: Pacific 32, Atlantic 19, Indian Ocean 6.
• Mariana Trench (west of the Philippines, North Pacific) is the deepest at 11.02 km.
• Average trench depth: 3–5 km below the surrounding ocean floor.
• Trenches form near coastlands and island arcs, at active/convergent plate margins where a heavier plate subducts beneath a lighter one — hence their concentration in the eastern/western Pacific and near the Japan–Philippines island arcs. Rare in mid-ocean regions.

Major Ocean Deeps/Trenches (selected):

(d) Mid-Ocean Ridges

• Volcanic in origin; the most extensive relief feature on Earth, not just in ocean basins.
• Not all ridges are centrally located within their basin:
  • Mid-Atlantic Ridge and mid-Indian Ocean Ridge — roughly central
  • East Pacific Rise — distinctly off-center (far from the Pacific basin’s central axis)
• Mid-Atlantic Ridge and East Pacific Rise are the most extensively studied ridges.
• Key characteristics:
  • Longest mountain chains on Earth: run 60,000–65,000 km, covering about one-third of the ocean floor.
  • All are volcanic in origin, made of basaltic pillow lava.
  • Always associated with divergent plate margins and seafloor spreading.
  • Crests are either dome-shaped with rounded tops, or feature rift valleys (from spreading + faulting).
  • Average width ≈ 1,000 km; average height above the deep sea plains ≈ 2,500 m.
  • Zones of active volcanism and seismicity.
• Crestal rift valley features:
  1. Hydrothermal vents — hot springs formed as seawater seeps through fractures in faulted ridge crests, gets heated, and erupts:
     • White smokers: temperature 30°C–350°C
     • Black smokers: temperature > 350°C
  2. Oceanic ridges — sections with steep, irregular slopes.
  3. Oceanic rises — sections with gentler slopes.
  4. Transform faults — break the ridge’s continuity; caused by divergence of plates and associated seafloor spreading; oriented perpendicular to the spreading axis.

Mechanism of mid-ocean ridge formation: As oceanic plates diverge, faults form, reducing the load pressure on the upper mantle, causing melting. Magma rises through fractures, is expelled with gases/steam, and cools/solidifies on contact with seawater, forming new basaltic crust along the constructive (divergent) plate boundary. Repeated volcanism piles up basalt lava over time, building the massive ridge structure along the spreading zone.