Re: The Hudson River appears to have cut a trench deep into the ocean bed

Hi Donald,

the opinions are divided as to the origin of these submarine canyons.

Virtually all continents are surrounded by a gently sloping submerged
plain called the continental shelf, which is an underwater extension of
the coastal plain. Changes in sea level have alternatingly exposed and 
inundated portions of the continental shelf. 

The continental slopes begin at the shelf break and plunge downward
to the great depths of the ocean basin proper. Deep submarine
canyons, some comparable in size to the Grand Canyon of the Colorado
River, are sometimes found cutting across the shelf and slope, often
extending from the mouths of terrestrial rivers. The Congo, Amazon,
Ganges, and Hudson rivers all have submarine canyon extensions. It is
generally assumed that submarine canyons on the continental shelf were
initially carved during periods of lower sea level in the course of the ice 
ages. 
Water, once contained in the ocean basins, fell as snow onto the 
continents,
where it was stored as glacier ice. Worldwide sea level fell as glaciers
expanded. 

However, there are alternative theories that state turbidity currents, 
which can be thought of as similar to an undersea avalanche, to be 
the sole cause for submarine canyons. Turbidity currents are mixtures
of sediment and sea water that are denser than pure sea water and
therefore will flow down slope underwater. 
Evidence favouring this theory is the fact that submarine canyons 
have been found to depths of 3,000m below sea level; and sea level 
has not been that low during the ice ages.  At the full extent of the
last glaciation, sea level was approximately 100 m lower than at 
present. As consequence, subaerial river erosion cannot be responsible
for carving canyons at great depths. 

Turbidity currents can flow down the continental slope and on to the
ocean basin at a rate of tens of kilometers per hour and have strong 
erosional power; they can be slow, steady flows or large catastrophic
events triggered by an earthquake.  This was first hypothesized in 1929 
when a series of Western Union trans-Atlantic cables broke in a sequence 
from north to south after an earthquake near the northern most cable. 

A third theory advocates a combination of both theories (I think this is 
the most likely one myself): The submarine canyons of the Congo,
Amazon, Ganges, and Hudson were carved during times of lower sea
levels and more recently modified by turbidity currents -subsea 
“landslides” of a dense slurry of  water and sediment.

 
Reference Websites: http://w
ww.msstate.edu/dept/geosciences/geologymb/ocean.htm
         http://pubs.usgs.gov/factsh
eet/fs102-98/

Numerous, deep submarine canyons cut by subsea avalanches occur off New 
Jersey, 
Delaware and Maryland.  See picture A at: http://geomorphology.geo.arizona.edu//geos450/LECTURE22/lecture22.html

 http://www.factmons
ter.com/ce6/sci/A0860098.html