Transcript Slide 1 - Linn-Benton Community College

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 & Plates
The 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 & Plates
The 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