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Now that we have the understanding
of Plate Tectonics that we developed in last week’s lecture, let’s go down to the bottom of the
seafloor and explore its depths a little further. First, it’s important to note that less than 1% of
the surface of the seafloor has ever been directly sampled. What we know about the seafloor is,
for the most part, determined through indirect sampling techniques such as determining from
the sea surface the magnetism of seafloor rocks below, which we used to determine the rocks’ age,
as discussed in our last lecture. Bathymetry is the term we use to describe the vertical
relief present on the floor that underlies a body of water. Topography is the term we use
to describe vertical relief on land. Satellites cannot determine seafloor bathymetry remotely,
because of the overlying seawater. However, indirectly we can approximate bathymetry of
the seafloor by using satellites to measure the strength of gravity, which can then be correlated
directly to seafloor high and low elevations or depths. The most accurate and direct way to
learn about the ocean floor bathymetry is to use sonar or sound waves, which are sent down from a
ship atop the water. These waves then reflect off the seafloor and return to the surface, providing
us depth based on the time the waves took to travel. Seismic waves can be similarly used to
tell us more about crustal rock layer thickness. Both of these techniques – sonar and seismic
waves – require that a ship move over the seafloor that is being imaged – both sending out the waves
and receiving the reflections back. This image here was taken using sidescan sonar technology
by the U.S. Geological Survey. It was developed as part of a larger many-year-long project to map
the continental shelf offshore of U.S. states and territories. To get these images, scientists had
to drag a sonar sled behind their boat and then travel back and forth across the surface of the
ocean, like mowing a lawn, piecing together all the strips of information into a larger map – a
long and tedious process! However, the effort provides us with detailed maps, like this one
that clearly shows the bathymetry under the Golden Gate Bridge, with the deepest areas in blue,
and the shallowest in yellow. Most of the seafloor images that you will see in this class were
developed using some kind of sonar technique.
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So where to start our seafloor explorations? Let’s
begin at the coast where humans interact most commonly and directly with the oceans. Moving
across the flat and shallow continental shelf, we reach the edge or break before falling down
the slope to the base of the deep sea floor below or abyssal plain. The continental shelf
is made from sediment that collects along the coast by rivers and gravity carrying
it to the oceans. Sediment is just another way of saying debris – bits and pieces of rock, mineral,
and organic material that collects atop low-lying areas. Since the ocean is the lowest-lying area in
the world, it should be completely covered with the constant raining down and piling up of this
debris or sediment. And it is! The more sediment that is carried to the coast, the bigger and
wider the continental shelf becomes. Remember, the continental shelf is an extremely flat, shallow,
currently flooded extension of the continent. It’s basically just a big pile of debris that sloughs
off the edges of our continent and accumulates. Where plate boundaries coincide with these edges
or margins of the continents, the shelves are narrow. Trenches are found just offshore,
and the sediment that piles up on the shelf and periodically falls in slides,
will partially or completely fill the trenches depending on the size. We call this type of
margin (where the edge of the continent meets a subducting ocean plate) active.
Where continental margins have no active subduction, because the nearby ocean crust
is fused to the continent – all part of the same plate moving together – we call them passive. At
passive margins, the sediment that collects along the shelf can extend quite a distance
offshore. Sometimes these shelves can extend up to 700 km! The deep flat seafloor that sits
offshore of the shelves is called the abyssal plain. The continental slope extends from the
shelf break down to the abyssal plains. The base of the shelf is covered in an apron of sediment
that has fallen off the shelf above. That apron of sediment is called the continental rise. Why? If
we journey from the deep abyssal plains (which are, on average, 5,000 m or 5 km deep) towards
the margins, returning to the shore, the first thing we see is a gentle “rise” of sediment,
then a much steeper slope that rises up to the flat continental shelf above. Because
passive continental margins have no trenches or subduction, the sediment can collect in large
amounts. Like any large pile of material that sits high above its surroundings (think snow
piled atop a mountain in winter), there will be a natural instability at the edge or break,
and regular slides will happen down the slope to settle debris in the continental rise apron of
sediment. When large avalanches of sediment occur, which we call turbidity currents,
they carve out deep channels or valleys called submarine canyons. These canyons can get quite
deep and extensive, as successive turbidity currents continue to occur in the same location
and carve the valleys out deeper and deeper. So where do we find passive versus active margins?
Hopefully you remember from last week’s lecture where to go to find subduction zones in the world.
Let’s review.
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Starting with the Atlantic Ocean – where are the subduction zones? Aside from the
ones in the Caribbean Sea, there are none, so what does that mean? The entire Atlantic Ocean
is surrounded by passive margins. Ocean plate is fused to continental plate – typical for a
relatively young ocean that is still growing and spreading. What does that mean? No major
earthquakes. No volcanism. No coastal terrane accretion and mountain building. And really
wide continental shelves! In fact, what else do you remember will happen on these passive
margins as sediment collects each day? Isostatic subsidence or sinking under the weight. Minor
earthquakes associated with this isostatic sinking might occur. And not much else.
Let’s stay in the Atlantic and move outward across the abyssal plains. What will we encounter in
the middle? The seafloor should become more rugged as it approaches the mid-ocean ridge
that marks the divergent plate boundary and seafloor spreading center. What will that look
like? Rough topography and rising mountains that have a down-dropped square-shaped rift valley at
the very top – sort of like rising bread that splits at the top. Instead of butter drizzling
down along this split, we instead get eruptions of basalt (pillow basalts), hydrothermal vents,
and shallow earthquakes. Each half of the Atlantic Ocean is a mirror image of the other,
due to the nature of seafloor spreading and lack of subduction zones. Thus when travelling
from New York to London, we’ll experience wide continental shelves on both margins – deep abyssal
plains – and a mid-ocean ridge in the center. Let’s look more closely at the abyssal plains.
Why are these the flattest plains on the planet? As new seafloor forms at the ridges and is pushed
away on either side, the crust on both sides gets slowly colder and older. As the lithosphere leaves
the upwarped push of the ridge, and attaches more mantle to its base, it becomes denser
and sinks down lower into the asthenosphere. Sediment collects atop – the smallest amount
at the ridge where new seafloor is exposed. But as this sediment collects at rates of about 1 cm
every 1,000 years, eventually, over millions of years, the sediment will complete cover any rough
topography and leave a flat surface that hides underlying irregularities. Imagine sifting flour
over a countertop covered in pots and pans. If you keep at it long enough, the flour will cover
everything, and the surface will be smooth. As this Google Oceans flyby confirms for us,
the Atlantic Ocean is surrounded by continental shelves, slopes,
and rises, riddled with numerous submarine canyons. The ocean also has extensive flat,
deep abyssal plains with an ocean ridge along the center with a well-developed rift valley.
Now let’s review the Indian Ocean – what kind of margins dominate this ocean? As you can see,
there is only one subduction zone, over here by Indonesia. Let’s look more closely
at this trench with a Google Ocean flyby. Not only will this subduction cause large
earthquakes like the one that caused the Dec. 26, 2004 tsunami, but the islands above are
all volcanic and thus volcanic eruptions are a continual hazard for Indonesia. The rest of the
Indian Ocean margins are all passive and carved on their edges by submarine canyons. And again,
hopefully you remember the triple junction discussed in last week’s lecture, where three
mid-ocean ridges come together. Let’s look more closely at that and the rift valleys
that sit atop, caused by divergence. Now let’s head to the Pacific Ocean,
where you’ll remember the Ring of Fire means subduction zones, large deep earthquakes,
and volcanic arcs all around its margins. Let’s do a Google Ocean flyby up the Peru-Chile trench,
and then out to the spreading centers of the East Pacific Rise,
before continuing north up through the Gulf of California or Sea of Cortez spreading centers
where we hit the large transform known as the San Andreas Fault system. Continuing northward,
we are now on what kind of margin? No subduction. Therefore, by our definition, passive! Sediment
collects. No deep earthquakes. No volcanoes. Wide continental shelf. And deep submarine canyons,
like this one here in Monterey Bay – one of the largest and most well-studied submarine canyons
in the world.
Pause now. [music]
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