Download Ch 3 Seafloor - San Diego Mesa College

Chapter 3: The Sea Floor
Prior to the development of plate tectonic theory, geologists thought of the sea floor as a
topographically monotonous place akin to a gigantic bathtub.
Marine geologist, Bill Menard summarized the prevailing view of the sea floor in the decades
leading up to the revolution (Menard 1969). Except for the discovery of a few deep trenches
and ridge segments, remarked Menard,
In 1940, it appeared that the sea floor was a relatively quiet place with minimal relief and that all
mountain building and other important geological processes occurred on continents or on their margins.
During the 1950s, everything changed as researchers began exploring the sea floor with new
technologies, including echo sounders and magnetometers, supported by military funding. In
fact, new discoveries made at sea in the 1950s and 1960s initiated the plate tectonic revolution.
In the same paper cited above, Menard wrote that,
Shattered beliefs soon became a commonplace as it developed that almost everything supposed about
ocean basins was wrong. The crust under ocean basins is not as thick as continental crust but in fact is
much thinner and also astonishingly uniform. The sediment in the basins is not a mile or more thick as
anticipated from the known rate of erosion of continents. Instead it is only a thousand feet thick, which is
the amount of sediment that would accumulate in only a few hundred million years rather than in the total
age of the earth. Moreover, no really ancient rocks can be found in the basins comparable to the threebillion-year-old rocks of the continents.
Amazingly, all of these new discoveries can be nicely explained in terms of plate tectonic theory:
Oceanic crust is thinner than continental crust because it originates not by compressional
mountain building but by sea floor spreading. Oceanic sediments are thinner than expected,
and sea floor rocks are younger than expected due to plate subduction, which prevents sea floor
sediments and rocks from getting too old before they’re pulled into Earth’s interior.
We now understand that while the world ocean itself is very old, the sea floor rock beneath the
modern ocean basins is quite young, because it’s constantly created at the mid-ocean ridges
and recycled into Earth’s interior at subduction zones.
Here’s an online link to a seafloor age map. You’ll notice that sea floor age patterns can be
nicely explained in terms of seafloor spreading. The further away one moves from the midocean ridges, the older the sea floor becomes. This is exactly what we’d expect to see as a
consequence of seafloor spreading.
Feel free to skim the section entitled, “Methods of Studying the Sea Floor.”
Although we can collect isolated sediment and rock cores from the sea floor, most of what we
know about the sea floor comes to us through indirect observations such as echo sounding
(Figure 3.4). The reason for this is that ocean water is quite effective in filtering out sunlight. In
fact, the sea floor is completely dark below a depth of several hundred meters. It’s often said
that we know more about the surfaces of the Moon and Mars than about our own sea floor. The
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“meat” of this chapter begins with the section entitled, “Features of the Sea Floor.” Although
your text doesn’t make this distinction, I find it helpful to lump sea floor features into two broad
categories, including (1) continental margins, and (2) ocean basins.
Continental Margins
As the name implies, continental margins represent seafloor regions that occur on the margins
of the continents. Think of them as the submerged edges of the continents. For the most part,
you’ll find continental crust beneath the sea floor sediments and sedimentary rocks found on the
continental margins. Continental margins themselves can be further subdivided into passive
and active margins (Figure 3.5). The basic distinction between passive and active margins is
quite simple—passive margins aren’t marked by a plate boundary, whereas active margins are.
Passive Continental Margins
Figures 3.7 and 3.8 are sketches of passive continental margins. Here’s a simplified geologic
cross section through a typical passive margin:
Shelf - Slope break
Shelf
Slope
Rise
Not a plate
boundary
Simplified geologic cross section through a passive continental margin.
(modified from Wikipedia; http://en.wikipedia.org/wiki/Passive_margin)
Along passive margins we see a broad, gently sloping continental shelf (which in some places
exceeds 500 km in width), a steeper continental slope, and a continental rise beneath the
continental slope. Notice that there’s a change, or break, in slope between the continental shelf
and the continental slope, which is called the shelf-slope break. The shelf-slope break is not
labeled on Figures 3.7 and 3.8 but it’s easily recognized. Typically, the shelf-slope break occurs
is at a depth of 100-200 meters.
Much of the sedimentary cover beneath the shelf, slope, and rise is derived from the continents
as rivers flush continental sediments into the marine environment, where they build up and
ultimately bury the jagged, faulted continental crust beneath the continental margin.
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In the above cross section, notice that the transition from continental to oceanic crust isn’t
marked by a plate boundary. Certainly, it’s a geologic transition (from granitic to basaltic rock);
however. The lack of a plate boundary is what makes this a passive continental margin.
If you look closely at the continental crust beneath the passive margin shown in Figure 3.7 and
also in the above cross section, you’ll notice that the edge of the continent has broken up into a
series of faulted blocks that have broken away from the continent and slid seaward along curved
dip-slip faults.
Can you identify the headwall and footwall along one of these fault-bounded blocks? Notice that
the headwall moves down relative to the footwall, which makes these faults normal faults. This
is an indication that the fault blocks originated in response to tensile stress. In fact, passive
margins owe their origin to plate divergence, which is also associated with tensile stress.
The passive continental margin along the east coast of North America originated during the
breakup of Pangaea approximately 200 million years ago. As the North American plate tore
away from Africa and Europe, a passive continental margin was created and seafloor spreading
was initiated. So even though we don’t currently find a plate boundary along the east coast of
North America, we can understand the origin of the passive margin along the east coast in
terms of plate boundary processes. Here’s a map showing the worldwide distribution of passive
margins:
Distribution of passive margins.
(courtesy of Wikipedia; http://en.wikipedia.org/wiki/Passive_margin)
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Another prominent feature of continental margins in general, including passive margins, is a
submarine canyon--a V-shaped valley that cuts through the continental shelf and runs down the
continental slope (Figure 3.8).
Here’s a link to a webpage where you can see a multibeam bathymetry image of a submarine
canyon off of the San Diego coastline.
As your text points out, several erosional processes contribute to the origin of submarine
canyons, including turbidity currents—large masses of sediment-laden water that are pulled
downslope by gravity.
In some cases, turbidity currents can be so strong and forceful that they can erode the
underlying submarine canyon through which they flow. Although large turbidity currents have
not been directly photographed at sea, small turbidity currents can be recreated in the
laboratory.
Here’s a link to a website where you can watch video clips of turbidity currents (they take
awhile to download but they’re worth the wait). You’ll need Apple Quicktime to watch these
(download Quicktime here).
As shown in Figure 3.8, great fan-shaped deposits called abyssal (submarine) fans are found at
the base of many submarine canyons. Check out this link showing two massive abyssal fans
off the coast of India.
Active Continental Margins
Active continental margins are “active” because they occur at plate boundaries—regions of
active seismicity and often active volcanism as well.
Your book only mentions one type of active margin—what we’ll call a subduction-type active
margin (Figure 3.15). Think of this type as an ocean-continent subduction zone.
Subduction-type continental margins are characterized by earthquakes, a young mountain belt,
and volcanoes on land.
Typically, a continental shelf and slope are present, but the shelf along an active margin is
typically much narrower than along a passive margin.
Active margins typically lack a continental rise.
For some reason, the authors of your textbook consider the continental margin off of southern
California to be a passive margin (p. 64); however, there’s nothing passive about it!
Although you won’t find an active subduction zone off the southern California coast, you will find
a wide continental shelf broken up into a series of northwest trending, tectonically down-dropped
basins and uplifted ridges.
Both the basins and the ridges are bordered by active, strike-slip faults, all of which are related
to the San Andreas Fault east of San Diego, as shown in the image below:
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Continental borderland offshore of southern California.
(image courtesy of USGS; http://woodshole.er.usgs.gov/operations/obs/rmobs_pub/html/borderland.html)
In the above image, the continental shelf includes the offshore area between the continental
slope (Patton Escarpment) and the southern California coast (shaded). This region is known to
marine geologists at the Continental Borderland. Offshore faults include the Coronado Bank
Fault (CBF), San Diego Trough Fault (SDT), San Clemente Island Fault (SCIF), and others.
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The numerous faults offshore of southern California are mostly right-lateral strike-slip faults and
represent the localized expression of the transform plate boundary between the Pacific plate
(west of the San Andreas Fault) and the North American plate (east of the San Andreas Fault).
These faults (some just a few kilometers offshore of San Diego and Los Angeles) are capable of
generating large, damaging earthquakes (perhaps as large as Magnitude 6.5 -7.5) and even
tsunamis!
Here’s a link that summarizes the geology of the southern California continental margin, locally
referred to as the Continental Borderland.
So, let’s distinguish between “subduction-type” active margins (characterized by a trench, a
narrow shelf, active subduction, active seismicity, and volcanism on land) and “transform-type”
active margins (characterized by a wide continental shelf broken up into a series of basins and
ridges bordered by active strike-slip faults).
In any case, both subduction-type and transform-type active margins occur along active plate
boundaries and are characterized by active faulting, seismicity, and mountain building.
In the summary sketches below, see if you can recognize the major differences between these
two types of active margin:
Trench
Narrow Shelf
Volcanic Arc
sea surface
= earthquake
Plate Boundary
Subduction-type continental margin (active).
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Continental Borderland
(block-faulted shelf)
Julian
Salton Trough
San Diego
Sea surface
-
+ - +
-
-
+
+
-
+
San Andreas Fault
Transform Plate Boundary
(wide & diffuse)
Transform-type continental margin (active).
In the above cross section of a transform margin, most of the motion between the Pacific and
North American plates occurs along the San Andreas Fault in eastern San Diego County.
However, the transform boundary is actually wide and diffuse, with a significant amount of
sideways motion occurring along many other faults, including several active faults offshore of
San Diego, in the Continental Borderland.
These faults display both strike-slip, horizontal motion (represented by the “+” and “-“ signs
indicating out of [+] and into [-] the plane of the page) and dip-slip, vertical motion (represented
by the block arrows). In effect, the continental borderland is broken up into a series of uplifted
and down-dropped tectonic blocks.
The uplifted blocks create offshore mountains, some of which crest the sea surface, such as the
Coronado Islands off the coast of Baja, and the Channel Islands, off the coast of Santa Barbara.
The down-dropped blocks marine create basins, like the one just west of Point Loma, where
San Diego’s sewage is pumped.
Make sure you can summarize some of the main differences between passive and active
continental margins.
Ocean Basins
As one moves out to sea, away from the continental margins, a whole other mix of sea floor
features will be encountered, including mid-ocean ridges, rift valleys, black smokers, fracture
zones, transform faults, abyssal plains, deep-marine trenches, seamounts, guyots, and reefs.
Your book does a nice job of describing these features.
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Mid-ocean Ridges
As you know by now, mid-ocean ridges represent the sea floor expression of a divergent plate
boundary, where new lithosphere is being created. Here’s a map of the world mid-ocean ridge
system (red stitches):
Mid-ocean ridge system.
(image courtesy of Wikipedia; http://en.wikipedia.org/wiki/Mid-ocean_ridge).
Here’s a computer generated image of the mid-atlantic ridge. Whereas trenches are
characterized by regions of deep water, ridges rise up to several kilometers above the deep sea
floor in positive relief. In the image below, the mid-Atlantic Ridge is clearly visible along the
divergent/transform boundary between the South American and African plates. Fracture zones
are major lines of weakness and breakage that cross the mid-ocean ridges at approximately 90º
angles. Note that transform plate boundaries occur along a fracture zone between two ridge
segments, as shown below:
N
Mid-Atlantic Ridge segment
African Plate
Fracture Zone
Transform plate
boundary
Fracture Zone
South American Plate
350 mi approx
Southern Atlantic Ocean sea floor.
(image courtesy of Google Earth).
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As shown in Figure 3.19A, slow-spreading ridges typically possess a deep rift valley in the
center of the ridge, where seafloor spreading actually occurs. As you might expect, extensive
normal faulting has been observed along the margins of the rift valley, where tensile stress is
concentrated, as shown here:
Rift Valley
Hydrothermal
circulation
Cross sectional sketch of a typical rift valley.
Here’s a link to a spectacular video clip of seafloor pillow lava erupting onto the sea floor off
the coast of Hawaii (requires Quicktime). This isn’t an actual clip of pillow lava forming within a
rift valley, but the process shown in this clip is thought to be similar to how pillow lava erupts
within the rift valley.
As shown in the sketch above, circulating sea water seeps into the fractured ridge, becomes
superheated, and exits the ridge, onto the seafloor within the rift valley, creating tall chimneys
(hydrothermal vents) called black smokers (Figure 3.19B). Check out this video clip of a black
smoker (requires Quicktime). Black smokers were discovered in the late 1970s when
researchers first visited the mid-ocean ridges in piloted submersibles. The black “smoke”
belching out of the smokers is a mixture of superheated water and hydrogen sulfide compounds,
which cloud the water, turning it black.
Exotic ecosystems are found around black smokers, where sulfur-digesting bacteria extract
energy directly from the hydrogen sulfide in the hot water exiting the black smokers.
One of the more bizarre life forms that lives around black smokers is a tube worm. Check out
the anatomy of a tube worm. These strange creatures have no mouth, no stomach, and no
intestine. They’re basically hosts for specialized bacteria that extract chemical energy from the
hydrogen sulfide gushing out of the black smokers.
Trenches
As you already know, deep marine trenches mark the location on the sea floor where
subduction occurs. Trenches are typically hundreds to thousands of kilometers long but very
narrow (only a few tens of kilometers wide). The Challenger Deep portion of the Mariana trench
south of Japan reaches a depth of almost 11,000 meters. That’s over 35,000 feet deep, deeper
below sea level than Mt. Everest rises above sea level!
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In the image below, the line of dark blue (very deep water) marks the location of the Aleutian
trench.
Aleutian Islands (volcanic arc)
N. American Plate
Aleutian Trench
(subduction zone)
N
Pacific Plate
(moving NW)
300 mi approx
Northern Pacific Ocean sea floor.
(image courtesy of Google Earth)
A True Story…
On January 23, 1960, U.S. Navy Lieutenant Don Walsh and Swiss oceanographer Jacques Piccard
descended to the bottom of the Challenger Deep, the deepest part of the Mariana Trench, in a metallic
submersible dubbed the Trieste. The descent took approximately 4 hours. At a depth of about 30,000
feet, a sharp crack rang through the ship and shook it violently as one of the outer Plexiglas window
panes cracked under the immense pressure. The inner hull remained intact. As the Trieste came to rest
on the sea floor at a depth of approximately 10,900 meters (35,761 feet), it kicked up a cloud of silt.
Looking out the Plexiglas viewing port, Piccard and Walsh identified sole and flounder, thus proving that
marine vertebrates could withstand the intense pressures at even the greatest depths. After about 20
minutes, the Trieste began its ascent. Since this dive, no inhabited craft has ever returned to the
Challenger Deep (http://en.wikipedia.org/wiki/Bathyscaphe_Trieste). Here’s a photo of the Trieste:
The Trieste.
(courtesy of Wikipedia; http://en.wikipedia.org/wiki/Bathyscaphe_Trieste)
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Abyssal Plain
Abyssal plains are very flat regions of the sea floor extending out from the base of the
continental slope out into the deep ocean basins.
As your book points out, seismic profiling has shown that abyssal plains are formed by marine
sedimentation, which blankets the irregular, volcanic topography like a snowfall, covering it up
(Figure 3.13).
Abyssal plains cover about 40% of the sea floor and reach depths of about 5 kilometers.
Before plate tectonic theory was developed, marine geologists were puzzled by the fact that
abyssal plain sediments deposited on volcanic bedrock were fairly thin (typically only a few
hundred meters thick) and not very old (less than 200 million years).
With the development of plate tectonic theory, these observations began to make sense: due to
the subduction process, sea floor is consumed before it has a chance to get too old and before
the overlying abyssal plain sediments can become very thick.
In the image below, the abyssal plain is clearly visible off the west coast of Africa. It’s easy to
recognize, because sea floor sediments have completely buried the irregular topography
associated with the mid-Atlantic ridge.
N
Mid-Atlantic
Ridge
Abyssal Plain
750 mi approx
Abyssal Plain (image courtesy of Google Earth).
Seamounts
Conical mountains that rise 1 kilometer or more above the sea floor are called seamounts.
Here’s a 3-D rendering (below) of a seamount, basically an undersea mountain rising from the
sea floor that doesn’t reach the sea surface. This volcanic seamount is called Lo’ihi and occurs
southeast of the Big Island of Hawaii.
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In the image below, the horizontal lines are bathymetric contours, or lines of equal water depth.
Lo’ihi Seamount.
(image courtesy of Wikipedia; http://en.wikipedia.org/wiki/Loihi_Seamount)
Guyots are flat-topped seamounts that once crested the sea surface and were eventually
eroded flat by wave energy.
The 3-D image below is the Bear seamount (actually a guyot), located just off the coast of
Massachusetts.
Bear Seamout, a Guyot.
(image courtesy of Wikipedia; http://en.wikipedia.org/wiki/Guyot)
As you may recall, Dr. Harry Hess was the first person to explain the origin of guyots.
As the sea floor spreads laterally away from a mid-ocean ridge, volcanic islands that once stood
above sea level are severed from their magma source, and wave erosion reduces the island to
a flat platform slightly below sea level. For Hess, the existence of guyots was evidence that the
sea floor had moved.
Reefs
Coral reefs are massive structures produced by living organisms, mainly coral--small, colonial
organisms that secrete an exoskeleton of calcium carbonate.
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The accumulation of skeletal material, broken up by wave action and the erosive action of
various organisms, produces a massive calcium carbonate structure that rises above the sea
floor to nearly the sea surface and supports a great variety of plant and animal life.
Shallow-water reefs are found throughout the tropical oceans, between approximately 30º N and
30º S latitude, and also occur as fringing reefs surrounding islands and continents.
As shown in the image below, biodiversity is high around coral reefs:
Coral Reef.
(photo courtesy of Wikipedia; http://en.wikipedia.org/wiki/Coral_reef)
Most coral reefs occur in shallow, tropical waters, although deep-water reefs do occur on
smaller scales. Shallow, tropical coral reefs require fairly warm water of approximately 20º C or
greater in which to thrive. Here’s a map (below) showing the worldwide distribution of shallowwater reefs (blue) bounded by the 20º C isotherm. Most of the worlds’ tropical reefs are found
within this band:
Occurrence of tropical coral reefs (blue shading).
(Courtesy of Wikipedia; http://en.wikipedia.org/wiki/Coral_reef)
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Don’t worry about the section in your textbook on ophiolites, ancient fragments of sea floor that
have been thrust onto the continents.
That’s it for Chapter 3!
References Cited:
Mendard, Henry. 1969. Anatomy of an Expedition. Henry H. Mendard Institute of Marine Resources and
Scripps Institution of Oceanography. McGraw Hill Book Company (download paper here).
Bathyscape Trieste. Wikipedia: the free Encyclopedia; http://en.wikipedia.org/wiki/Bathyscaphe_Trieste.
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