Morphology of the Trenches
25
Equation (1.1) occupies a special place in marine geophysics, geology, and paleoceanography. It is one of the very few statements about Earth history that is both
quantitative and simple, and at the sarne time also valid and useful.
During sinking of the sea floor, the rough topography produced by volcanism and
faulting moves down the ridge flanks and is then gradually smoothed by the sediment
cover (see Fig. 1.7). However, abyssal hills with a relief in the 50 to 1000 m range
and with slopes of 1 ° to 15° remain as expressions of the underlying basement
morphology over large regions. Abyssal-hill morphology is the most common type of
landscape on the face of Earth: in the Pacific Ocean about 80 % of the sea floor
belongs to this category.
1.5 Morphology of the Trenches
In general, trenches are found near the margins of ocean basins, notably the Pacific
Basin. It is not obvious why there are not more mid-ocean trenches. To answer this
question, we need to learn more about processes within the mantle. First, some
observations: trenches are roughly 100 km wide (in their shallower part) and from
hundreds to thousands of kilometers long. For example, the Aleutian Trench is 2900
km long. The cross-section is usually V-shaped (Fig. 1.10.a), and the deepest part
may be flat due to ponded sediment. Such sediments generally show undisturbed
horizontal layering - an observation which was sometimes used as an argument
against subduction when the concept of sea-floor spreading was new. The trench
walls usually have slopes between 8° and 15°. However, steep sides (up to 45°) as
well as steps have also been mapped. In cases, outcrops of basalt have been observed
by dredging and deep-sea photography.
The greatest depths are in the western Pacific, in sediment-starved trenches off
island arcs: Mariana Trench maximally 10 915 m; Tonga Trench, 10 800 m; Philippine Trench, 10055 m; Japan Trench, 9700 m; Kermadec Trench, 10 050 m. The
values are not exact; they were determined from echo soundings which include
corrections for effects of regional temperature and salinity distribution on the velocity
of sound in the water. Any errors in the determinations, however, cannot mask the
similarity in these depths. As in the similarity of ridge-crest elevations, this coherence
points to the action of similar processes in each of the trenches in the western Pacific.
Elsewhere, trenches are shallower: Puerto Rico Trench about 8600 m; South Sandwich, 8260 m, Sunda 7135 m. The eastern Pacific is characterized by trenches directly adjacent to continents, without intervening island arcs. These trenches are filled
with continental debris and this, presumably, is the reason why they are distinctly
shallower than their western counterparts.
The ring of trenches girdling the Pacific is the site of most of the earthquakes on
Earth: more than 80 % of the shallow earthquakes « 60 km deep), 90 % of the
intermediate ones (60 to 300 km deep), and almost all of the deep quake centers (300
to 700 km) are concentrated here (Fig. 1.11).
The rest of the deep and intermediate quakes also occur largely in trenches; some
occur in the Mediterranean, in Iran, and in central Asia at the northern boundary of
25
Equation (1.1) occupies a special place in marine geophysics, geology, and paleoceanography. It is one of the very few statements about Earth history that is both
quantitative and simple, and at the sarne time also valid and useful.
During sinking of the sea floor, the rough topography produced by volcanism and
faulting moves down the ridge flanks and is then gradually smoothed by the sediment
cover (see Fig. 1.7). However, abyssal hills with a relief in the 50 to 1000 m range
and with slopes of 1 ° to 15° remain as expressions of the underlying basement
morphology over large regions. Abyssal-hill morphology is the most common type of
landscape on the face of Earth: in the Pacific Ocean about 80 % of the sea floor
belongs to this category.
1.5 Morphology of the Trenches
In general, trenches are found near the margins of ocean basins, notably the Pacific
Basin. It is not obvious why there are not more mid-ocean trenches. To answer this
question, we need to learn more about processes within the mantle. First, some
observations: trenches are roughly 100 km wide (in their shallower part) and from
hundreds to thousands of kilometers long. For example, the Aleutian Trench is 2900
km long. The cross-section is usually V-shaped (Fig. 1.10.a), and the deepest part
may be flat due to ponded sediment. Such sediments generally show undisturbed
horizontal layering - an observation which was sometimes used as an argument
against subduction when the concept of sea-floor spreading was new. The trench
walls usually have slopes between 8° and 15°. However, steep sides (up to 45°) as
well as steps have also been mapped. In cases, outcrops of basalt have been observed
by dredging and deep-sea photography.
The greatest depths are in the western Pacific, in sediment-starved trenches off
island arcs: Mariana Trench maximally 10 915 m; Tonga Trench, 10 800 m; Philippine Trench, 10055 m; Japan Trench, 9700 m; Kermadec Trench, 10 050 m. The
values are not exact; they were determined from echo soundings which include
corrections for effects of regional temperature and salinity distribution on the velocity
of sound in the water. Any errors in the determinations, however, cannot mask the
similarity in these depths. As in the similarity of ridge-crest elevations, this coherence
points to the action of similar processes in each of the trenches in the western Pacific.
Elsewhere, trenches are shallower: Puerto Rico Trench about 8600 m; South Sandwich, 8260 m, Sunda 7135 m. The eastern Pacific is characterized by trenches directly adjacent to continents, without intervening island arcs. These trenches are filled
with continental debris and this, presumably, is the reason why they are distinctly
shallower than their western counterparts.
The ring of trenches girdling the Pacific is the site of most of the earthquakes on
Earth: more than 80 % of the shallow earthquakes « 60 km deep), 90 % of the
intermediate ones (60 to 300 km deep), and almost all of the deep quake centers (300
to 700 km) are concentrated here (Fig. 1.11).
The rest of the deep and intermediate quakes also occur largely in trenches; some
occur in the Mediterranean, in Iran, and in central Asia at the northern boundary of