Transcript ch20_Oceans_online_notes

Frank Press, Raymond Siever,
John Grotzinger, and Thomas H. Jordan
Understanding Earth
Fourth Edition
Chapter 17:
Earth Beneath the Oceans
Copyright © 2004 by W. H. Freeman & Company
Marine Geology
•Most of the world is under water -- 71%
•Plate Tectonics came about mainly from the
study of marine geology
Major Differences between the Ocean
Floor and the Surface of Continents
Continental surfaces are a source of
sediment, that is they are regions where
erosion and weathering prevail. Continents
are ~4 by old
Ocean floors are regions where net
sedimentation and volcanism prevail. Oceans
are no more that 200 Myr old
About Oceanic Geology
•Volcanism dominates near oceanic ridges
and hot spots.
•Sedimentation dominates elsewhere
•Older volcanic crust usually has a thicker
pile of overlying sediments
How have earth scientists learned
about the depths of the world’s
oceans?
HMS Challenger, 1872 throughout a 4year voyage, used rope lines to estimate
ocean depths.
Today we use echo sounders, devices that
emit a pinging sound and record its return
later in time. Knowing the speed of sound
and the time of flight of these echoes
can be used to calculate water depths.
How have earth scientists learned
about the geology of the world’s
oceans?
•Submersibles-- e.g. Johnson SeaLink,
Alvin -- biology, petrology
•Academic Drilling- DSDP, ODP, IODP
•Commercial drilling and seismic
exploration for oil and gas
•Swath Mapping--Beams of sound (many
pings in many directions received at many
stations on the underside of a vessel) -bathymetry
•seismometer stations around the world
How have earth scientists
learned about the geology of
the world’s oceans?
Satellites data can be used to make
bathymetric maps
•Satellites can access the most remote
parts of the world
•Satellites can detect ocean surface
elevation.
•Ocean height depends on waves,currents,
and the underlying topography
(bathymetry)
A “typical” oceanic bathymetric profile
across two plates that are diverging …..
The Atlantic Ocean
•is 5000 km across
•is shallowest in the center (2500 m deep) and
deeper along its flanks in the abyssal plain
Key Features across the Atlantic
Seafloor
•a continental shelf ( < 200 m water depth)
that is often cut by submarine canyons
•A continental slope ( > 200 m, 4 degrees dip,
i.e. 70 m drop per 1 km run)
•A continental rise (2000-3000 m)
•An abyssal plain (4000-6000 m)
•Seamounts - extinct underwater volcanoes
•Mid-oceanic Ridges - active underwater
mountain chain
Key Features across the Pacific
Seafloor
•Trenches
•seamounts
•guyots - flat-topped extinct marine
volcanoes, flat-topped because they were
once above sea-level and were eroded
A Great Discovery
•Life does not only derive from
sunlight energy!
Sources of Deep-Sea
Sediments
Derive from
(1) the dead skeletons of foraminifera
(single-cell animals) that live in the
(2) wind-borne or underwater-borne clays
Carbonate Compensation Depth (CCD)
Below a certain depth (CCD), the skeletons
dissolve back into sea water and do not
make it into the sediment!
Wave Action
•Waves in general, are created by wind
•Wind velocity, area affected by winds
and length of time that the wind blows
are all factors that help to increase the
height of a wave
•Wave particle motion is circular at the
surface to elliptical at the bottom
•Waves that hit the coast at an angle
created a longshore drift of sediments
along the coast
Wave Action
•Waves that hit the coast at an angle
created a longshore drift of sediments
along the coast
Tides
•Tides can leave you high and dry,
twice a day and can be intensified
by local bathymetry
•The largest tides occur when the
sun and moon are aligned (spring
tides) and the smallest are when
the sun and moon are aligned at
right angles to each other.
“The Beginning for a few is
the End for Many”