Geology Reference
In-Depth Information
What Would You Do
?
geologic features on the continents are best explained by the
extension of these fractures into continents.
Shallow-focus earthquakes take place along these frac-
tures, but only between the displaced ridge segments. Further-
more, because ridges are higher than the adjacent seafl oor, the
offset segments yield nearly vertical escarpments 2 or 3 km
high (Figure 9.13). The reason oceanic ridges have so many
fractures is that plate divergence takes place irregularly on a
sphere, resulting in stresses that cause fracturing. We discussed
these fractures between offset ridge segments more fully in
Chapter 2, where they are termed
transform faults.
Hydrothermal vents on the seafl oor are known sites of sev-
eral metals of great importance to industrialized societies.
Furthermore, it appears that these metals are being depos-
ited even now, so if we mine one area, more of the same
resources form elsewhere. Given these conditions, it would
seem that our problems of diminishing resources are solved.
So why not simply mine the seafl oor? Also, many chemical
elements are present in seawater. The technology exists to
extract elements such as gold, uranium, and others, so why
not do so?
As noted, the seafl oor is not a fl at, featureless plain except
for the abyssal plains, and even these are underlain by rug-
ged topography (Figure 9.11). In fact, a large number of vol-
canic hills, seamounts, and guyots rise above the seafl oor in
all ocean basins, but they are particularly abundant in the
Pacifi c. All are of volcanic origin and differ mostly in size.
Seamounts
rise more than 1 km above the seafl oor, and if
fl at topped, they are called
guyots
(
during the eruption. And in a nearby area, a new fissure
opened in the seafl oor; by December 1993, a new hydrother-
mal vent community had become well established.
In 2001, scientists announced another kind of seafl oor
vent in the North Atlantic responsible for massive pillars
and spires as tall as 60 m. Unlike the black smokers, though,
these vents are 14-15 km from spreading ridges, and they
consist of light-colored minerals that were derived by chem-
ical reaction between seawater and minerals in the oceanic
crust.
Figure 9.14). Guyots
are volcanoes that originally extended above sea level. How-
ever, as the plate upon which they were located continued to
move, they were carried away from a spreading ridge, and
as the oceanic crust cooled, it descended to greater depths.
Thus, what was once an island slowly sank beneath the sea,
and as it did, wave erosion produced the typical fl at-topped
appearance (Figure 9.14). Many other volcanic features
smaller than seamounts exist on the seafl oor, but they prob-
ably originated in the same way. These so-called
abyssal hills
average only about 250 m high.
Other common features in the ocean basins are long,
narrow ridges and broad plateau-like features rising as much
as 2 to 3 km above the sur-
rounding seafloor. These
aseismic ridges
are so called
because they lack seismic
activity. A few of these ridges
are probably small fragments
separated from continents
during rifting and are referred
to as
microcontinents.
The Jan
Mayen Ridge in the North
Atlantic is probably a micro-
continent (Figure 9.10).
Most aseismic ridges
form as a linear succession
of hot-spot volcanoes. These
may develop at or near an oce-
anic ridge, but each volcano
so formed is carried laterally
with the plate upon which it
originated. The net result is
a line of seamounts/guyots
extending from an oceanic
ridge (Figure 9.14); the Walvis
◗
Oceanic ridges are not continuous features winding with-
out interruption around the globe. They abruptly terminate
where they are offset along fractures oriented more or less
at right angles to ridge axes (
Figure 9.13). These large-
scale fractures are hundreds of kilometers long, although
they are diffi cult to trace where they are buried beneath sea-
fl oor sediments. Many geologists are convinced that some
◗
Plate boundary
Oceanic
ridge
Adjacent sections
here move in same
directions
Sections here
move in opposite
directions
Adjacent sections
here move in same
directions
Fracture zone
(inactive)
Transform fault
(active part of
fracture zone)
Fracture zone
(inactive)
Lithosphere
Direction of
plate movement
Asthenosphere
◗
Figure 9.13
Seafl oor Fractures Diagrammatic view of an oceanic ridge offset along a fracture.
That part of the fracture between displaced segments of the ridge is a transform fault. Recall
from Chapter 2 that transform faults are one type of plate boundary.
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