At this point in the series, we understand the structure of the Earth, which includes the crust,
and we know that there is crust below our feet, but also oceanic crust at the bottom of the ocean.
We mentioned a few of the features of oceanic crust when we introduced plate tectonics,
those being mid-ocean ridges and trenches. But to fully understand the structure of oceanic
crust, it will be useful to learn more about the layers of the ocean itself.
Water covers over 70% of the Earth’s surface, so if we want to know about
every area of Earth’s surface, this will indeed be a critical section to include.
Let’s take a deep dive into the Pacific Ocean. The western edge of the Pacific Ocean starts with
a ramp of continental crust that extends off the coast of mainland Japan to the deep Japan trench.
Scientists call this ramp of continental crust beneath the ocean the continental
shelf. Typically, continental shelves are broad, sloping plains covered in shallow ocean water less
than 250 meters deep, but in the Pacific Ocean, the gentle gradient of the continental shelf
drops off quickly into a steep incline called the continental slope. Off the coast of Japan,
the continental slope extends all the way down to the bottom of the Japan trench,
over 7 kilometers in depth. As the depth changes along
the continental shelf and slope, the amount of sunlight that penetrates through the ocean water
changes drastically. At the shallow depths along the continental shelf, between 0 and 200 meters,
there is enough light present for photosynthesis to take place. Physical oceanographers,
scientists who study the interactions of the ocean with light, wind, and rock, have called
this top layer the photic or epipelagic zone. Pelagic means open ocean and refers only to
areas that are far from the influence of the shore and the ocean floor. 90% of ocean life,
from whales and dolphins to green algae and phytoplankton, lives within the epipelagic zone.
Beneath the epipelagic zone is the mesopelagic zone, which extends from 200 to 1000 meters,
where sunlight can no longer reach. Small bristlemouth fish make their home here
and migrate up to the epipelagic zone under the cover of night to feed. Although most people have
never even seen one, the tan bristlemouth fish is the most abundant vertebrate on Earth, so they are
clearly thriving in their deep-sea environment. Next is the bathypelagic zone, from 1000 to 4000
meters. Sunlight certainly can’t reach the bathypelagic zone and the organisms
there live in complete darkness, unable to see anything. Some species, like the anglerfish,
use the lack of sunlight to their advantage and create specks of light from symbiotic,
bioluminescent bacteria living at the tip of their dorsal fin to attract prey.
Even deeper is the abyssopelagic zone, extending from 4000 to 6000 meters.
Temperatures in the abyssopelagic zone stay at a chilly 4 degrees Celsius, and
pressures reach 200 to 600 times the atmospheric pressure at sea level. Incredibly, there are some
organisms that can withstand these conditions. Sea spiders with spindly, 30-centimeter-long legs
and mud-eating sea pigs scuttle across the bottom of the ocean floor, making an unlikely home.
Finally, the last zone is the hadopelagic zone, and it extends from 6000 to as far
as 11,000 meters. The hadopelagic zone is only found in deep ocean trenches,
like the Mariana, and it is occupied only by a few species of tube worms and sea cucumbers.
Now it is certainly the case that we could talk much more extensively about all of the fascinating
living organisms that inhabit each of these zones. But that is the subject of the zoology series,
so if you want to learn more about aquatic animal life, be sure to visit that playlist
in the near future. For our purposes in this geology series, it is merely important to have
a basic understanding of the layers of the ocean and the depths they correspond with, as well as
the general features of each layer. After all, the ocean is what sits atop the oceanic crust, filling
in every nook and cranny, from the deepest trench all the way up to sea level. And with the ocean
itself now covered, let’s head all the way down to the ocean floor and discuss oceanic crust next.