Structure of Ocean Basins Chapter 4

Focus now on structure  Problem of the ocean being remote  First “soundings” were made with a weight attached to a rope (piano wire reduced the bulk) and this took a long time  1910’s & 20’s acoustic methods – echo sounding, sonar –Submarine tracking –Shipwrecks (the Titanic in 1912) –Icebergs  Based on sound and the time it takes for sound to travel from a surface vessel to the bottom and back –Distance = speed of sound x time/2 –Speed of sound is ~ 1500 meters/second  First hydrophones (WWI) were passive listening devices  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 more beams – sends down a “swath” And you get more data and better detail. As a result of continued improvements in technology, we now have lots of data! Echo-sounding is how we learn about the large and small scale morphology of the oceans This is useful for things like identifying shipping lanes!

Echo sounders have transducers that both transmit and receive sounds

Satellite altimetry measures sea surface height and use radar energy (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 Shelf  Part of the continent covered by water  Average width is about 75 km (range is 0 – 600 km); widest in regions with large coastal plains (passive margins) and narrowest near mountain ranges (active margins)  Shelf covers 7-8% of the ocean floor  Ends at a depth of about 130 m where the slope changes (steepens)  Greatly 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!

Glaciation  During glacial periods, sea level is low  During interglacial periods (today) sea level is high.  Sea level depends on net effect of water/ice balance and isostatic adjustment of continents to the weight of the ice.  Ice ages caused by Milankovitch cycles involving changes in orbit and precession of 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 shelfs  Part of the continents  Find drowned river valleys on shelves  Wave-cut terraces (old beaches)  Fossil remnants of shallow water life forms in deeper water further out on shelf  Amount of water tied up in ice (glaciers) largely determines sea level  If all present ice were melted, sea level would rise 600 m. Because of isostacy, actual rise would be only 60 m. –What would happen to Norfolk?

Features of Continental shelves  Submarine canyons –Extend shelf to slope  Origins are uncertain –Carved by Rivers when sea level was lower- many canyons are associated with present-day rivers (Hudson and Monterey canyons) –Formed in place by turbidity currents – fast-moving water avalanches carrying big particles; sediments dislodged by seismic or other disturbance and cascades down cutting a canyon.

Submarine Canyon – La Jolla

Take home points  How we know ocean structure (acoustic methods such as echo sounding and sonar; satellite images of sea surface)  Dominant structures  Continental shelves are part of continents and are sometimes submerged and sometimes exposed – analogies on land  Continental shelves and canyons  Relationship between sea level and glaciation  How do canyons form  Passive versus active margins