THE OCEAN BASINS
ocean ridge system
is by far the world’s longest mountain chain, starting near Iceland and
extending south through the Atlantic, east through the Indian Ocean and into the Pacific Ocean, with
various additional offshoots.
On the ocean floor, the rift system has a very distinctive set of landforms.
The ridges themselves are broad topographic highs, rising as much as 3-4 km above the abyssal plain,
but across a width of 100’s of km perpendicular to the ridge axis.
Often the central axis of the ridge
system is a deep valley, and off to the sides are additional parallel ridges and valleys.
are rather straight, and different sections are offset from each other along
scars occur at near-right angles to the ridges, both connecting pairs of ridge segments and extending
The ridge and transform system is seismically active, although the earthquakes are not as strong as in
some other areas.
Seismic motion solutions (derived from analyzing 3-component seismographs) reveal
most faults near ridges are extensional, but those on transforms are mostly
However, the strike-slip motions are in the opposite direction one would expect if the ridge
sections were once directly connected and are now moving away from each other.
Iceland sits on top of the mid-Atlantic ridge, and the geological processes we see there, including
volcanism, hydrothermal activity, and rifting, occur all along the ridge system, both above and below
Deep-sea hydrothermal systems often feature
, geysers of hot, mineralized water
that can have associated colonies of incredibly specialized organisms that are adapted to live under great
pressure and without any energy from the sun.
Locations where three ridge systems come together are a form of
The ridge system
extending north from a triple junction in the mid-Indian Ocean leads into the Gulf of Aden, where there
is another apparent triple junction, with one arm constituting the Red Sea and the other leading into a
region known as the East African Rift, which features extensional faulting, volcanism, and hydrothermal
activity similar in many ways to what we see on Iceland.
The Red Sea features a central valley similar
to well-developed ocean ridges.
are long, deep depressions in the ocean floor. The trench off of the Marianas Islands, at
11035 meters below sea level, is deeper than Mount Everest is high (8848 meters above sea level).
Trenches are associated with particularly intense seismic and volcanic activity.
Most of the world’s
trenches rim the Pacific Ocean, creating the so-called “
Ring of Fire
The largest earthquakes recorded
are near trenches.
When “looked at” from the side, parallel to the trench axis, the set of earthquake foci
recorded over several years appear to form a linear band that can extend down as deep as ~700 km, the
deepest earthquakes known.
Active volcanoes associated with trenches include those in the Cascades