A
mountain rising from the
ocean floor whose top does not break the
surface of the
ocean to form an
island.
Seamounts are created in bunches; wherever there is one seamount, you are likely to find a hundred.
Of course, the distinction between an island and a seamount is meaningful only to the
life forms that want to live on islands. The distinction is irrelevant to the
geological processes involved. These processes create mountains; some become islands and others do not.
Seamounts are generally volcanic in origin; although some (such as
Loihi) are active
volcanoes that are building themselves up from the ocean floor, most are
extinct volcanoes that either never lasted long enough to break the surface, or had once been islands but have been completely eroded away, and/or drowned by sea level rising
1.
Near the margins of an undersea
subduction zone, a line of seamounts will form in much the same fashion that a line of volcanoes will form near a continental subduction zone
2. Examples of this are the
Aleutian Islands, the
Solomon Islands, and the
Aeolian Arc in the
Tyrrhenian Sea.
Alternatively, as a
tectonic plate drags itself over a
hot spot, the hot spot will periodically eat holes through the plate and form a line of seamounts/islands. For example, the same hot spot that created the Hawaiian Islands also created the
Emperor Seamounts. These stretch west from
Midway Island, and then north, forming a chain of undersea mountains stretching all the way to the point where the Aleutian Islands meet the coast of
Kamchatka.
1If an island or a continent has a
coral reef, sometimes the reef can grow fast enough to keep up with sea level change, and an
atoll results. The
Great Barrier Reef is also an example of this. However,
drowned coral reefs are numerous. The Emperor Seamounts contain many.
2For example, the
Cascade Mountains on the west coast of
North America.