Geology Reference
In-Depth Information
Inactive sinking
volcano being "shaved"
at ocean surface
Active
volcanoes
Older, extinct
volcanoes
Older, extinct
volcanoes
Spreading
center
Sea level
G
S
G
G
G
S
G
G
Magma
chambers
Lithosphere
Asthenosphere
G = guyot
S = seamount
50
40
30
20
10
0
10
20
30
40
50
a
Age of ocean floor (millions of years)
◗
Figure 9.14
The Origin of Seamounts and Guyots As the plate on which a volcano rests moves into greater
water depths, the submerged volcanic island is called a seamount. Those that are fl at-topped are called guyots.
Ridge in the South Atlantic is a good example (Figure 9.10).
Aseismic ridges also form over hot spots unrelated to ridges—
the Hawaiian-Emperor chain in the Pacific, for example
(Figure 9.10).
ON THE DEEP SEAFLOOR
Sediments on the deep seafl oor are mostly fi ne grained, con-
sisting of silt- and clay-sized particles, because few processes
transport sand and gravel very far from land. Certainly, ice-
bergs carry sand and gravel, and, in fact, a broad band of
glacial-marine sediment is adjacent to Antarctica and Green-
land. Floating vegetation might also carry large particles far
out to sea, but it contributes very little sediment to the deep
seafl oor.
Most of the fine-grained sediment on the deep sea-
fl oor is derived from (1) windblown dust and volcanic ash
from the continents and volcanic islands, and (2) the shells
of microscopic plants and animals that live in the near-sur-
face waters. Minor sources are chemical reactions in seawater
that yield manganese nodules found in all ocean basins
(
◗
Figure 9.15
Manganese Nodules Chemical reactions in seawater
yield manganese nodules on the seafl oor.
oceanic islands.
Ooze
, in contrast, is made up mostly of tiny
shells of marine organisms.
Calcareous ooze
consists primar-
ily of calcium carbonate (CaCO
3
) skeletons of marine organ-
isms such as foraminifera, and
siliceous ooze
is composed of
the silica (SiO
2
) skeletons of single-celled organisms such as
radiolarians (animals) and diatoms (plants) (Figure 9.16).
Figure 9.15) and cosmic dust. Researchers think that as
many as 40,000 metric tons of cosmic dust fall to Earth each
year, but this is a trivial quantity compared to the volume of
sediment derived from the two primary sources.
Most sediment on the deep seafl oor is
pelagic
, meaning
that it settled from suspension far from land (
◗
The term
reef
has a variety of meanings, such as shallowly
submerged rocks that pose a hazard to navigation; however,
here we restrict it to mean a moundlike, wave-resistant struc-
ture composed of the skeletons of marine organisms (see
Geo-inSight on pages 236 and 237). Although commonly
Figure 9.16).
Pelagic sediment is further characterized as pelagic clay and
ooze.
Pelagic clay
is brown or red and, as its name implies,
is composed of clay-sized particles from the continents or
◗
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