The Seafloor - The Changing Earth: Exploring Geology and Evolution - page 232

◗

Rift

Figure 9.12 Central Rift and
Submarine Hydrothermal Vents

Faults

Pillow lava mound

0

Lava flows

Actively
forming
dike
Sheeted
dikes

a Cross section of the Mid-Atlantic
Ridge showing its central rift where recent
moundlike accumulations of volcanic
rocks, mostly pillow lava, are found.

1

Magma

0 1 2

Distance (km)

Bottom current

Chimney

Precipitation
FeO(OH) MnO 2

Precipitation
CaSO 4 , FeS

Sedimentation

Seawater seepage

H 2 S in water

Basalt

Basalt

Precipitation
FeS, FeS 2, CuFeS 2,

350°C 350°C

Magma

b Cross section showing the origin of a submarine hydrothermal
vent called a black smoker.

organisms depend on bacteria that oxidize sulfur com-
pounds for their ultimate source of nutrients. The vents are
also interesting because of their economic potential. The
heated seawater reacts with oceanic crust, transforming it
into a metal-rich solution that discharges into seawater and
cools, precipitating iron, copper, and zinc sulfi des and other
minerals. A chimney-like vent forms that eventually col-
lapses and forms a mound of sediments rich in the elements
mentioned above.
Apparently, the chimneys through which black smokers
discharge grow rapidly. A 10-m-high chimney accidentally
knocked over in 1991 by the submersible Alvin grew to 6 m in
just three months. Also in 1991, scientists aboard Alvin saw
the results of a submarine eruption on the East Pacifi c Rise,
which they missed by less than two weeks. Fresh lava and ash
covered the area, as well as the remains of tube worms killed

c This black smoker on the East Pacifi c Rise is at a depth of
2800 m. The plume of “black smoke” is heated seawater
with dissolved minerals.