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