Deep-sea trenches generally lie
seaward and parallel to
arcs or mountain ranges of
the continental margins.
They are closely associated with
and found in subduction zones—
that is, locations where a
lithospheric plate bearing oceanic
crust slides down into the upper
mantle under the force of gravity.
Both types of subduction zones are
large earthquakes that originate at
a depth of as much as 700 km (435
The deep earthquakes below
subduction zones occur in a plane
that dips 30° or more under the
Typical trench depths are 8 to 10
km (5 to 6 miles). The longest
trench is the Peru-Chile Trench,
which extends some 5,900 km
(about 3,700 miles) along the west
coast of South America.
Ocean trenches are the deepest
parts of the ocean. They are also
called submarine valleys.
An ocean trench is a long,
deep depression in the ocean floor,
similar to deep chasms on the
Earth’s dry land.
Some trenches are near
continental shelves. Others are
found near chains of volcanic
islands, often called volcanic arcs.
Some volcanic arcs include the
Aleutians, and the island nations of
Japan and the Philippines
Trenches are formed as a result
of plate tectonics, or the
movement of the Earth’s crust.
Tectonic plates slip underneath
each other in a process known
When the leading edge of a heavy
plate meets the edge of a lighter
plate, the heavier plate bends
This place where the heavier plate
melts (subducts) beneath the
lighter one is called the subduction
Ocean trenches can be formed by
subduction between continental
crust and oceanic crust.
Continental crust is always lighter.
The long series of Peru-Chile
Trenches off the west coast of
South America is formed by the
oceanic crust of the Nazca plate
subducting beneath the
continental crust of the South
Ocean trenches can also be formed
when two plates carrying oceanic
crust meet. These are more rare.
The Mariana Trench, in the South
Pacific Ocean, is formed as the
massive Pacific plate subducts
beneath the Philippine plate.
The deepest place on Earth is
called the Challenger Deep. It is
found in the Mariana Trench in the
Pacific Ocean, near the island of
The Challenger Deep is 10,994
meters (36,070 feet) below the
For comparison, Mount Everest,
the world’s tallest mountain, is
8,850 meters (29,035 feet) above
sea level. Mount Everest could fit
inside the Mariana Trench with
more than 2 kilometers (1 mile) to
In June 2009, scientists sent
an unmanned deep-sea robot,
the Nereus, to explore the Mariana
Trench and Challenger Deep.
The vehicle had to be designed to
withstand extreme pressure of
15,000 pounds per square inch—
more than 1,000 times the
pressure felt at sea level.
Scientists want to learn more
about the subduction process that
created the trench.
The deepest part of the Atlantic
Ocean is the Puerto Rico Trench.
It is just over 8,600 meters (28,232
feet) deep and is about 280
kilometers (175 miles) long.
The Puerto Rico Trench was
formed by the North Atlantic plate
sliding beneath the Caribbean
Animals that live in ocean trenches
have to survive a habitat of
Most organisms collected from the
Challenger Deep have been
microscopic. The organisms, or
foraminifera, are similar
to algae or slime-molds.
Scientists believe the foraminifera
they found at the bottom of the
Challenger Deep are similar to
Earth’s earliest life forms.