We often think of Earth’s surface as a well-explored place that is thoroughly colonized
by humans. To some, it may seem as though we know all there is to know about this land we inhabit.
However, the truth is that most of the surface of our planet is unexplored,
due to it being covered with kilometers of water. Of course, we are talking about the ocean floor.
It sounds crazy, but we know more about the lunar surface than we do about the ocean floor.
So, why is the ocean floor so difficult to explore? The primary difficulty is withstanding
the high pressures that result from the weight of up to 11 kilometers of water,
yielding pressures of more than 1000 atmospheres. Though the deepest areas of the ocean, trenches,
only represents a small area of the ocean, its average depth is still a substantial 3.5
kilometers. But, as we know from studying Earth’s layers, we do not have to visit an area of the
Earth to know something about it. Oceanographers use methods like sonar, which reflects
sonic waves off the ocean floor, to determine its topography and create detailed bathymetric maps.
Instruments called echo sounders emit a sound pulse, and then measure the amount of time it
takes the pulse to reach the ocean floor, bounce off of it, and return to the sounder.
Since the speed of the pulse can vary with ocean water’s salinity, pressure, and temperature,
engineers developed multibeam echosounders that send out hundreds of sound pulses and can measure
the angle of return from each pulse. With the high angular resolution of the multibeam echosounder,
scientists can measure ocean floor features from multiple sides and create a more realistic
picture of what these features look like. So, what type of landforms are on the ocean floor?
For the most part, the very bottom of the ocean floor, called the abyssal plain,
is flat and featureless. At the center of an ocean basin lies a mid-ocean ridge, where new oceanic
crust is being created. On either side of an ocean lies a continental margin, which is
simply the transition zone from land to sea. The continental margin is composed of three areas,
the continental shelf, which is a gently sloping surface that makes up all shorelines, the
continental slope, a steep drop-off that marks the edge of the continental crust, and the continental
rise, which is an area where sediment from the continental slope piles up during underwater
avalanches called turbidity currents. The abyssal plane, which is probably what you imagine when you
think of the ocean floor, is located between the mid-ocean ridge and continental margin.
The abyssal plane ranges in depth from 3 to 6 kilometers and covers 70% of the ocean floor.
As we mentioned, it is predominately flat and featureless, with the occasional trench
near areas experiencing subduction, or volcanic seamount above hotspots, breaking up the monotony.
Lying atop the oceanic crust is a thick layer of sediment, blanketing the ocean floor and providing
a habitat for many types of burrowing organisms. Sediment thickness varies based mainly on the
age of the crust and the distance from land, with the thickest sediments occurring closest to land,
where both the sediment supply is greatest and oceanic crust is oldest. The largest sediment
deposits in today’s oceans are in the Bay of Bengal and Gulf of Mexico, where they approach
20 kilometers thick in some spots. For comparison, atop the youngest crust near mid-ocean ridges,
sediment may only be millimeters thick. As sediment is progressively buried in basins,
the weight of the overburden causes the sediment to compact and minerals such as
quartz and calcite begin to precipitate from the mineral-laden pore waters, cementing sediment
grains together to form a sedimentary rock. This combined process of compaction
and cementation is called lithification. To fully understand sedimentary rocks, we need
to understand sediments, where they come from, how they are transported, and what happens to them
during transport. There are two types of sediment found in the ocean: terrigenous and biogenic.
Terrigenous sediment is made up of minerals and small rocks that eroded from continental crust,
or more simply, the land. Wind and water, though mainly water, carry sediment from the mountains
to the oceans. The reason that the Bay of Bengal has such a thick sediment layer is that several
large river systems are dumping millions of tons of eroded sediment from the rising Himalayas.
In a similar manner, the Mississippi river carries loads of sediment from the eroding Rockies and
Appalachians into the Gulf of Mexico. This is terrigenous sediment. Most terrigenous sediment
is transported to the oceans during large storms and floods when river flow is fastest. This is
the same reason why you probably shake your orange juice before pouring a glass. Shaking it increases
its velocity, allowing the juice to lift the pulp into suspension. Once the sediment reaches the
open ocean, flow velocity drops, and it begins to settle out by mass. Sand settles out first,
followed by silt, then mud, with clay being the last to settle out, sometimes remaining suspended
in the ocean for hundreds to thousands of years. Since the largest grains settle out first, sand
and silt are only found near the shoreline, with mud accumulating further offshore, and
only the smallest clay particles reaching the open ocean, slowly accumulating on the abyssal plane.
Biogenic sediments, or “oozes”, are made of materials created by living organisms.
There are two categories of biogenic sediment that accumulate on the ocean floor:
calcareous and siliceous. Calcareous biogenic sediments are comprised of the calcium carbonate
skeletons from single-celled marine organisms like foraminifera. However, since calcium carbonate
dissolves readily in colder, deeper water, calcareous oozes are primarily found in warm,
shallow seas. The depth at which calcium carbonate starts to dissolve in ocean water
is called the carbonate compensation depth, abbreviated as CCD. Since the CCD is a function
of temperature and seawater chemistry, its exact depth varies, being deeper in equatorial regions
and shallower near the poles. By mapping this depth across the ocean, scientists created the
lysocline, which shows how the CCD changes with location. Siliceous biogenic sediments are made
of the silica skeletons from single-celled marine organisms like diatoms and radiolarians. Siliceous
oozes are essentially the opposite of calcareous oozes in that the silica tends to dissolve
rapidly in the shallow ocean, whereas carbonate rapidly dissolves in deep water, as we mentioned.
Given the high solubility in seawater, siliceous oozes only form in areas where
diatoms and radiolarians are thriving, and sedimentation can exceed dissolution.
Unlike the calcareous, siliceous oozes are likely to be found in the cold, deep water at the poles
or in areas where upwelling occurs. Upwelling is a process where strong winds and the Earth’s
rotation cause shallow waters to diverge, allowing deep, cold water to rise to the surface, filling
the space created by the diverging surface waters. Upwelling tends to occur along western shorelines
where winds blow from the North, which when modified by the Coriolis Force,
cause an ocean current that moves away from land, or 90 degrees to the right of the wind.
This movement of water at right angles to the prevailing winds is called Eckman Transport.
The Coriolis Force deflects moving objects to the right in the Northern Hemisphere and
to the left in the Southern Hemisphere. This applies to all moving fluids on Earth’s surface,
including the atmosphere. With these different types
of sediments understood, let’s move forward and get a closer look at the oceanic crust itself.