Page 185 - (DK) Ocean - The Definitive Visual Guide

ABYSSAL PLAINS, TRENCHES, AND MID-OCEAN RIDGES 183

Ocean Trenches Japan Mariana Trench seamounts
China
Ocean trenches are created by a process called subduction. Where oceanic and Pacific
Ocean
continental tectonic plates collide, the denser but thinner oceanic plate is forced
down beneath the thicker but less dense continental plate, and plunges to its
destruction in the mantle deep below. Where two oceanic plates collide, the older
plate is subducted beneath the younger. The buckling where the plates collide causes
a deep depression at the point of impact—an ocean trench. These are the deepest
places on the ocean floor. Trenches are typically V-shaped, with steeper slopes on the
continental side. The Pacific is the region of most active subduction, with 17 of the
20 major ocean trench systems. The Atlantic has
two major trenches, the Puerto Rico and South MARIANA TRENCH
Sandwich trenches, and the Java Trench is the only The Mariana Trench is roughly
major trench in the Indian Ocean. The deepest 1,600 miles (2,500 km) long and 40 miles
(70 km) wide. It lies in the western
trench on Earth is the Mariana Trench, located in Pacific, around 1,000 miles (1,600 km)
the Pacific Ocean, near the Mariana Islands. to the south and east of Japan.

Life in the Ocean Trenches
THE DEATH OF A WHALE
Animals have been found at great depths in the ocean trenches. The
Occasionally, a dead whale sinks years to strip the flesh from a depth record for a fish belongs to a cuskeel, Abyssobrotula galathea. This
to the abyssal plain and provides blue whale. Later, bacteria invade and was dredged in 1970 from a depth of 27,453 ft (8,370 m) in the Puerto
a feast. Scientists have counted decompose the remaining bones. Rico Trench. In 1998, the unmanned Japanese submersible Kaiko
12,000 animals of 43 species This process leaches out sulfides collected some large amphipods (shrimplike crustaceans) called
feeding on the bones of a single that sustain a complex community Hirondellea gigas from the bottom of the Mariana Trench. These
whale. It may take them 11 of seafloor life.
were later found to harbor wood-dissolving enzymes in their gut,
suggesting they can digest woodfall (tree debris swept into the ocean
that eventually sinks to the bottom). Kaiko also collected sediment
samples that contained 432 different species of foraminiferans and
a range of bacteria.
Since 2010, some giant
unicellular organisms more than
4 in (10 cm) across, belonging to
a class called xenophyophores
(a form of foraminiferan), have
been observed in the Mariana
Trench and elsewhere. Cameras
on board the Deepsea Challenger
that descended to the bottom GELATINOUS BLINDFISH
of the Mariana Trench in A small number of these curious fish have been
2012 detected sea cucumbers collected from the seabed in the Atlantic,
the abyssal plain Pacific, and Indian Oceans, at depths of at least
is a flat expanse volcanic islands each volcanic island and a jellyfish as well as 10,000ft (3,000m). Like many deep-water fish,
of mud that form an arc is the above-water xenophyophores and amphipods. they are almost transparent, with tiny eyes.
covers a vast parallel to the part of a huge
area of seafloor ocean trench undersea volcano
THE OCEAN FLOOR
DISCOVERY
The seafloor lies about 2.3 miles (3.7 km)
below the sea surface. It is made of a THE TRIESTE
layer of dark-colored rock, called oceanic
crust, which is covered in muddy sediment. EXPEDITION
Tectonic plates are generally made of this
oceanic crust and continental crust, In 1960, two oceanographers,
along with part of Earth’s Don Walsh and Jacques Picard,
deeper mantle layer. dived to 35,797 ft (10,911 m)
Features such as in the Challenger Deep section
volcanic islands and of the Mariana Trench in the
seamounts are caused bathyscaphe Trieste—still the
by erupting magma.
greatest depth reached by humans.
It took five hours to descend to
that depth, and after just 20
minutes hanging there, the crew
began their return to the surface.
oceanic crust
is thinner than
continental crust,
and made of
dark-colored rock OCEAN ENVIRONMENTS

the ocean trench forms magma pools in a a volcano forms from
where one tectonic plate chamber beneath a buildup of lava
moves under another a volcano when magma erupts
at the surface