Structure of Ocean BasinsStructure of Ocean Basins
Chapter 4
Focus now on structureFocus now on structure Problem of the ocean being remoteProblem of the ocean being remote First “soundings” were made with a weight attached to a rope First “soundings” were made with a weight attached to a rope
(piano wire reduced the bulk) and this took a long time(piano wire reduced the bulk) and this took a long time 1910’s & 20’s acoustic methods – echo sounding, sonar1910’s & 20’s acoustic methods – echo sounding, sonar
– Submarine trackingSubmarine tracking– Shipwrecks (the Titanic in 1912)Shipwrecks (the Titanic in 1912)– IcebergsIcebergs
Based on sound and the time it takes for sound to travel from a Based on sound and the time it takes for sound to travel from a surface vessel to the bottom and backsurface vessel to the bottom and back– Distance = speed of sound x time/2Distance = speed of sound x time/2– Speed of sound is ~ 1500 meters/secondSpeed of sound is ~ 1500 meters/second
First hydrophones (WWI) were passive listening devicesFirst hydrophones (WWI) were passive listening devices Echo sounders provide easy, continuous depth information.Echo sounders provide easy, continuous depth information.
Echo Sounding
Seismic Reflection
Ocean Floor
Sub-bottom sedimentary layers
Depth charges/guns
Hydrophone is a sensor that detects acoustic energy
Sound energy spreads out in a narrow cone from the ship•The deeper the water, the broader the cone
Multibeam systems have morebeams – sends down a “swath”And you get more data and better detail.
As a result of continuedimprovements in technology,we now have lots of data!
Echo-sounding is how we learnabout the large and small scalemorphology of the oceans
This is useful for things like identifying shipping lanes!
Echo sounders havetransducers that both transmit and receive sounds
Satellite altimetry measures sea surface height and use radarenergy (1 inch accuracy)
Differences in sea surface height from gravitational pull of submarine “topographic” features.
Satellite produced map shows features accurately!
Compare cross-sections – similar features topographically and bathymetrically.
Hypsographic curve again
Shows distribution of land relative to sea level
More than half of the earth is below sea level
Average depth of the ocean is greater than the average height of the continents.
Continental margins (transition between continent and ocean)most impacted by the land. This is the area most exploited and impacted by human activities.
Continental ShelfContinental Shelf
Part of the continent covered by waterPart of the continent covered by water Average width is about 75 km (range is 0 – Average width is about 75 km (range is 0 –
600 km); widest in regions with large coastal 600 km); widest in regions with large coastal plains (passive margins) and narrowest near plains (passive margins) and narrowest near mountain ranges (active margins)mountain ranges (active margins)
Shelf covers 7-8% of the ocean floorShelf covers 7-8% of the ocean floor Ends at a depth of about 130 m where the Ends at a depth of about 130 m where the
slope changes (steepens)slope changes (steepens) Greatly affected by changes in sea levelGreatly affected by changes in sea level
Right now, sea level is pretty high so wider continental shelves.Changes in sea level largely due to glaciation (amount of water in ice)
Impacts coastal appearance and habitat!
GlaciationGlaciation
During glacial periods, sea level is lowDuring glacial periods, sea level is low During interglacial periods (today) sea level During interglacial periods (today) sea level
is high.is high. Sea level depends on net effect of water/ice Sea level depends on net effect of water/ice
balance and isostatic adjustment of balance and isostatic adjustment of continents to the weight of the ice.continents to the weight of the ice.
Ice ages caused by Milankovitch cycles Ice ages caused by Milankovitch cycles involving changes in orbit and precession of involving changes in orbit and precession of the earth.the earth.
•The Earth's axis rotates (precesses) just as a spinning top does. The period of precession is about 26,000 years.
•Therefore, the North Celestial Pole will not always be point towards the same starfield.
•Precession is caused by the gravitational pull of the Sun and the Moon on the Earth.
Active versus passive margins
Origin of shelfsOrigin of shelfs
Part of the continentsPart of the continents Find drowned river valleys on shelvesFind drowned river valleys on shelves Wave-cut terraces (old beaches)Wave-cut terraces (old beaches) Fossil remnants of shallow water life forms in deeper Fossil remnants of shallow water life forms in deeper
water further out on shelfwater further out on shelf Amount of water tied up in ice (glaciers) largely Amount of water tied up in ice (glaciers) largely
determines sea leveldetermines sea level If all present ice were melted, sea level would rise If all present ice were melted, sea level would rise
600 m. Because of isostacy, actual rise would be 600 m. Because of isostacy, actual rise would be only 60 m.only 60 m.– What would happen to Norfolk?What would happen to Norfolk?
Features of Continental shelvesFeatures of Continental shelves
Submarine canyonsSubmarine canyons– Extend shelf to slopeExtend shelf to slope
Origins are uncertainOrigins are uncertain– Carved by Rivers when sea level was lower- many Carved by Rivers when sea level was lower- many
canyons are associated with present-day rivers (Hudson canyons are associated with present-day rivers (Hudson and Monterey canyons)and Monterey canyons)
– Formed in place by turbidity currents – fast-moving Formed in place by turbidity currents – fast-moving water avalanches carrying big particles; sediments water avalanches carrying big particles; sediments dislodged by seismic or other disturbance and cascades dislodged by seismic or other disturbance and cascades down cutting a canyon.down cutting a canyon.
Submarine Canyon – La JollaSubmarine Canyon – La Jolla
Take home pointsTake home points
How we know ocean structure (acoustic methods such How we know ocean structure (acoustic methods such as echo sounding and sonar; satellite images of sea as echo sounding and sonar; satellite images of sea surface)surface)
Dominant structuresDominant structures Continental shelves are part of continents and are Continental shelves are part of continents and are
sometimes submerged and sometimes exposed – sometimes submerged and sometimes exposed – analogies on landanalogies on land
Continental shelves and canyons Continental shelves and canyons Relationship between sea level and glaciationRelationship between sea level and glaciation How do canyons formHow do canyons form Passive versus active marginsPassive versus active margins
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