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The
global mid-ocean ridge system is the largest single volcanic feature on
the Earth, encircling it like the seams of a baseball. Here the Earth’s
crust is spreading, creating new ocean floor and literally renewing the
surface of our planet. Older crust is recycled back into the mantle elsewhere
on the globe, typically where plates collide. The mid-ocean ridge consists
of thousands of individual volcanoes or volcanic ridge segments which
periodically erupt.
Beneath
a typical mid-ocean ridge, mantle material partially melts as it rises
in response to reduced pressure. This melted rock, or "magma",
may collect in a reservoir a few kilometers below the seafloor, awaiting
eruption. Much of the magma eventually freezes in place there within the
crust, forming the bulk of the new oceanic crust without erupting at all.
Average oceanic crust is about 10km thick, but only the upper 1 to 3 km
are formed by eruption processes. When magma pressure builds up enough
to force its way out to the seafloor, eruption occurs. "Dikes"
are magma-filled cracks and are the conduits that magmas flow through
to reach the surface. A typical ridge eruption leaves behind a dike that
is 10s of centimeters up to 2 meters in width, extending between the crustal
magma chamber and the eruptive fissure at the surface. Lavas pour from
the fissure across the surface of the volcanic seafloor, adding a thin
coat of new lava (typically <10 m thick) with each eruption. This process
of magma ascending and lavas erupting is on-going and perpetual. At the
Juan de Fuca Ridge, the spreading process creates an average width of
~6 m of new crust in 100 years.
Because
the ridge is the site of focused volcanic activity, seawater circulates
actively to cool the new crust. This heated water reacts with the volcanic
rock, dissolving out metals and depositing them around seafloor hot springs.
Within the volcanic upper crust, subterranean chambers and fractures filled
with heated water act as incubators for microbes that live in some of
the harshest conditions ever discovered to support life. These microbes
are the foundation for a rich ecosystem that thrives only at these hydrothermal
vents.
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