Geography of the Oceans

• Hydrosphere (water) and cryosphere (ice)• Major oceans – Pacific, Atlantic, Indian• Minor oceans- Mediterranean, Arctic, Caribbean,

Gulf of Mexico, Black Sea, Sea of Japan, Bering Sea, Okhotsk N&S China Sea, Gulf of Aden, Baltic Sea, Philippine Sea

• Sea level changes – eustatic & tectonic• Topography of sea floor- ocean ridges,

trenches, guyots, islands, seamounts, strand line, continental margin, abyssal plain

• Land/sea ratio, average depths of oceans

Pacific Ocean• Deepest, oldest, largest, coldest, most

tectonically active, ave. depth ~3 miles• Covers ~1/3 of Earth with >½ the water yet small

river and sediment discharge, “sediment starved”, remnant of proto-ocean “Panthalassa”

• >100K seamounts, ~25K islands; guyots to NW• 3 volcanic systems- hot spots, arc-trench

systems, East Pacific Rise (EPR)• EPR (spreading center) continuous from Sea of

Cortez, SE below Australia into Indian Ocean• Shrinking by ~5”/yr – narrow continental shelves• Ringed by arc-trench systems (volcanic

archipelagos & 4-5 mi deep trenches) “Ring of Fire” shaped like curtained stage

Atlantic Ocean• >¼ Earth’s water• “S” shaped• Volcanics nearly confined to central MAR-passive

margins• Arc-trench system on E. margin of Caribbean• Highest temp & salinity of the 3 major oceans due to

adjacent Mediterranean Sea• Wide continental shelves receive copious fluvial runoff

and tons of sediment annually “sediment gorged” – huge deltas on margins

• Only 200 million years old • Average depth – 2.2 mi• “Limestone ocean” due to warm sea temperatures• Significantly fewer islands, seamounts &guyots than

Pacific

Arctic Ocean

• A land-locked northern arm of Atlantic

• Wide shelves, passive margins, mid-ocean ridge connected to Mid Atlantic Ridge

• Covered by cryosphere (ice) ½ of the year

• Probable oil/gas beneath wide shelves

Indian Ocean• Half/ocean- transition between Pacific & Atlantic models• Shaped like collapsing teepee, India at top• <1/4 Earth’s water• Tectonically similar to Pacific in East- Atlantic in West

(both active & passive margins)• Has huge chunk of Africa (Madagascar) and another on

the way (Horn of Africa)• Narrow shelves & mid-ocean “inverted Y” ridge

connecting to EPR and MAR• East 90° ridge is N/S chain of volcanoes due to hot spot• Huge runoff from Indus, Ganges & Bramaputra rivers

from Himalayas with large deltas• “Sediment gorged” Enormous submarine fans extend

1200 to 1500 miles out from deltas onto abyssal plains• ~ 50 million yrs old- youngest of 3 major oceans• Expanding ~ 2”/yr

Marginal Oceans I

• Smaller oceans on periphery of major oceans but important politically and economically!

• Closely bounded by land so all lie within EEZ of nations

• Close to runoff from land so abundant nutrients; foster good fisheries

• Greater variation than major oceans in temp & salinity due to small size

• All are near tectonic active sites-cultures endure earthquakes, tsunamis, volcanoes regularly

Marginal Oceans II

• Four types– Back arc basins- situated between island arc and

mainland (sea of Okhotsk, Philippine Sea, Bering Sea, Sea of Japan, S & E China Seas, Caribbean Sea)

– Basins trapped between continental blocks (Mediterranean, Black Sea, Gulf of Mexico)

– Shallow basins on top of continent (Hudson Bay, North Sea, Baltic Sea)

– Linear seaways where mid-ocean ridge intersects continent (Red Sea, Gulf of Aden, Sea of Cortez)

Wegener’s Evidence for Drift

• Perfect fit of American/European Atlantic coasts• Matching features as faults & geology across the

vast expanse of the Atlantic (3000 miles!)• Karoo system of S. Africa- matching strata,

fossils, glaciation of old Gondwanaland (DuToit)• Weakness of Wegener’s thesis- mechanism?

sea anchor effect of mountain roots?• Idea disregarded for ~ 40 yrs until exploration of

ocean floors provided the answer to a stunned geologic community in the mid to late 1960’s

Drift & PlatesThe solution to moving continents had to wait for ocean

research/technology

• Very early drifting continents was contemplated mainly due to excellent fit of Atlantic margins in: – 1620 (Sir Francis Bacon) – 1782 (Ben Franklin)– 1908 (FB Taylor)– 1910 (H. Baker)– 1912 & 1929 (Wegener)– 1921 (AL Du Toit, S. Africa)

• 1905- magnetic polarity reversals in rocks discovered

• 1930’s Meteor (Ger.) discovers MAR in Atlantic with echo sounding between WW I & WW II

Drift & PlatesThe solution to moving continents had to wait for ocean

research/technology

• WWII – Harry Hess discovers “guyots” in NW Pacific

• 1950’s - Heezen & Tharp make map of oceans connecting all ridges to 50k mi. long chain

• 1961 – magnetic striping and bilateral symmetry on the MAR SW of Iceland

• 1962 – Harry Hess suggests “sea floor spreading” by convection cells

• 1963 – Vine & Matthews propose solution to magnetic striping (Morley shafted!)

• 1965-69 – DSDP proof of Plate Tectonics

Seven Major Tectonic Plates

• Pacific- (largest) moving ~3”/yr toward NW (no continents-horse without rider)

• Nazca- ~3”/yr toward east (no continents)• American- ~1.5”/yr west to WSW• Indo-Australian ~2”/yr due north• Antarctic- ~2”/yr north into S. Pacific• Eurasian- ~1.5”/yr toward SE• African ~ stationary today, came from SW

Plate Tectonics

• Plates outlined by quakes and volcanoes

• Spreading ridges and transform faults

• Subduction zones

• Continental arcs & island arcs

• Mantle plumes (hot spots) [J. Tuzo Wilson]

Age of Oceans I

• Embryonic (fetal) – along rift zones, on continents, great faults border deep long valleys (E. African Rift)

• Newborn – long, straight expanding inland seaways: when fracture is wide enough oceans invade bringing salt water (Red Sea/Gulf of Aden, Gulf of California)

Age of Oceans II

• Youthful (adolescent) – margins far apart but sides still match (like a puzzle), expanding, wide shelves, passive margins, volcanism in center (Atlantic)

• Mature (well established) – very large but

shrinking ocean, active margins, narrow shelves, shorelines no longer match (Pacific)

Age of Oceans III

• Old Age (elderly/retired) – sick, collapsing, narrow opening to other oceans, silled basins, poor circulation, may be anoxic in deep parts (Mediterranean, Black Sea)

• Dead (or on life support) – still have water but landlocked (Aral sea, Caspian sea)

Age of Oceans IV

• Fossil (extinct) – mountain ranges with compressed marine rocks, raised out of oceans as seaway was crushed between continents [Appalachians, Himalayan range, Alps, Caledonians, Atlas, Urals, Cordillera (Rockies)]

Ancient Oceans

• All oceans today are either expanding or contracting

• Oceans recycle themselves very quickly -although the Earth is ~ 4.5 billion yrs old, oldest ocean sediments are Jurassic ~ 150 mya (only 5% of Earth’s age)

Ancient Oceans I

• Panthalassa “all oceans”- forerunner to present Pacific; once covered >70% Earth’s surface

• Iapetus “father of Atlas” - forerunner to Atlantic; destroyed before modern Atlantic formed 200 mya

• Tethys “sea of the dead”; “wife of Oceanus”

Ancient Oceans II Tethys

• Tethys remnants are widespread limestones and chalk in:US Gulf Coast, S. Europe, N. Africa, Himalayas, E.

China

– Tethys seaway was destroyed as Indian Ocean formed 50 mya when India merged with Asia

– East-west seaway allowed tropical ocean organisms to freely spread to ALL other oceans

Spreading Margins

• Ridge/Rise, rifts

• Volcanics – basalt, pillows, black smokers

• Iceland (for look at ocean floor)

• Ophiolites (slab of sea floor)

• Economics of ophiolites (Au, Ag, Pt, Cu)

• Transform faults – mechanism for ridges turning and bending

Plate Collision Boundaries

• Subduction – Benioff zone, partial melting

• Volcanic archipelagos (island arc & continental arc)

• Trench & arc system

• Accretionary wedge and mélange

• Fore-arc basin and fore-arc high

• Back-arc basin

Plate Collision Boundaries

• Subduction – Benioff zone, partial melting

• Volcanic archipelagos (island arc & continental arc)

• Trench & arc system

• Accretionary wedge and mélange

• Fore-arc basin and fore-arc high

• Back-arc basin

Plate Tectonics

• Plates outlined by quakes and volcanoes

• Spreading ridges and transform faults

• Subduction zones

• Continental arcs & island arcs

• Mantle plumes (hot spots) [J. Tuzo Wilson]

The Messinian Event

• Near-death of the Mediterranean Sea ~ 6 mybp • Gibraltar closed ~ 6mya due to pressure of Spain against

Morocco• Because of rapid evaporation and limited fluvial input the

sea dried up & remained like Death Valley for ~ 1m.y. (mts on north, desert to south)

• Original evidence unraveled by Russians building Aswan Dam in Egypt- deep canyons into Med. Filled with gravel/sand

• D.S.D.P. found thick evaporite sediments (salt!) beneath Med. Sea floor that confirmed the event

• ~ 5 mybp the “Gibraltar waterfall” refilled the Med. from Atlantic as tectonics destroyed the sill