16 Origin and Morphology of Ocean Basins
On H. M. S. Challenger, depth soundings were done by laboriously sending a
weight to the ocean floor and measuring the length of the wire paid out. When the
scattered soundings were connected in drawing depth contours, the ocean floor
looked smooth. Only when echo sounding was used routinely did it become obvious
that large parts of the ocean floor consist of immense mountain ranges whose cragginess rivals that of the Alps and the Sierra Nevada. Perhaps the most impressive of
these ranges is the Mid-Atlantic Ridge, first discovered by the famous Meteor Expedition (1925-1927) (Fig. 1.2 b).
More recently, it has been shown, largely through the work of M. Ewing and
co-workers, at Lamont Geological Observtory, that the seemingly endless Mid-Atlantic Ridge (Fig. 1.3) is itself only a portion of a world-encircling Mid-Ocean Ridge.
This was, of course, a discovery of immense importance. It identified the one
unifying morphological feature of the planet, the central template to which the
various scattered puzzle pieces of knowledge about the sea floor had to be fitted. The
only other feature of the ocean floor of comparable magnitude is the line of trenches
ringing the Pacific (Figs. 1.4 and 1.13). The complementary significance of these two
features - the Ridge System and the Trench System - became obvious in the 1960s,
from the study of magnetic properties of the sea floor, from earthquakes, and from
heat flow distribution. In the late 1960s, the hypothesis that new sea floor forms at the
center of the Mid-Ocean Ridge and that it travels toward the trenches where it sinks,
gained general acceptance, as outlined in the introduction.
The hypothesis, called sea-floor spreading, explains in an elegant fashion the
major features of the depth distribution of the sea floor.
Before we discuss this striking concept of sea-floor spreading in some detail,
however, let us go back to consider the basic processes which shape Earth's surface,
including the floor of the ocean.
1.2 Endogenic Processes
As is true of all of the face of the Earth, the sea floor is shaped by two kinds of
processes, those deriving their energy from inside the Earth, called endogenic, and
those driven by the Sun, called exogenic.
The forces inside the Earth produce volcanism and earthquakes; we meet them in
the eruptions on Hawaii, in the geysers of Yellowstone Park, in the quakes in California. Working over long periods of time, the endogenic forces, fueled by heat sources
within the Earth, build mountain ranges such as the Sierra Nevada and the Himalayas,
or create gigantic rifts, such as Death Valley and the Rhine Graben. It is reasonable to
suppose that the undersea mountains represent uplift, and that the great trenches
result from down warping of the sea floor by endogenic forces. Such motion, of
course, requires flow of material within the Earth. Thus, matter has to rise to make
the undersea mountain ranges, and must sink to make the trenches. The mental jump
in formulating the hypothesis of sea-floor spreading was to see these necessary motions as part of a convection system (Fig. 1.5).
On H. M. S. Challenger, depth soundings were done by laboriously sending a
weight to the ocean floor and measuring the length of the wire paid out. When the
scattered soundings were connected in drawing depth contours, the ocean floor
looked smooth. Only when echo sounding was used routinely did it become obvious
that large parts of the ocean floor consist of immense mountain ranges whose cragginess rivals that of the Alps and the Sierra Nevada. Perhaps the most impressive of
these ranges is the Mid-Atlantic Ridge, first discovered by the famous Meteor Expedition (1925-1927) (Fig. 1.2 b).
More recently, it has been shown, largely through the work of M. Ewing and
co-workers, at Lamont Geological Observtory, that the seemingly endless Mid-Atlantic Ridge (Fig. 1.3) is itself only a portion of a world-encircling Mid-Ocean Ridge.
This was, of course, a discovery of immense importance. It identified the one
unifying morphological feature of the planet, the central template to which the
various scattered puzzle pieces of knowledge about the sea floor had to be fitted. The
only other feature of the ocean floor of comparable magnitude is the line of trenches
ringing the Pacific (Figs. 1.4 and 1.13). The complementary significance of these two
features - the Ridge System and the Trench System - became obvious in the 1960s,
from the study of magnetic properties of the sea floor, from earthquakes, and from
heat flow distribution. In the late 1960s, the hypothesis that new sea floor forms at the
center of the Mid-Ocean Ridge and that it travels toward the trenches where it sinks,
gained general acceptance, as outlined in the introduction.
The hypothesis, called sea-floor spreading, explains in an elegant fashion the
major features of the depth distribution of the sea floor.
Before we discuss this striking concept of sea-floor spreading in some detail,
however, let us go back to consider the basic processes which shape Earth's surface,
including the floor of the ocean.
1.2 Endogenic Processes
As is true of all of the face of the Earth, the sea floor is shaped by two kinds of
processes, those deriving their energy from inside the Earth, called endogenic, and
those driven by the Sun, called exogenic.
The forces inside the Earth produce volcanism and earthquakes; we meet them in
the eruptions on Hawaii, in the geysers of Yellowstone Park, in the quakes in California. Working over long periods of time, the endogenic forces, fueled by heat sources
within the Earth, build mountain ranges such as the Sierra Nevada and the Himalayas,
or create gigantic rifts, such as Death Valley and the Rhine Graben. It is reasonable to
suppose that the undersea mountains represent uplift, and that the great trenches
result from down warping of the sea floor by endogenic forces. Such motion, of
course, requires flow of material within the Earth. Thus, matter has to rise to make
the undersea mountain ranges, and must sink to make the trenches. The mental jump
in formulating the hypothesis of sea-floor spreading was to see these necessary motions as part of a convection system (Fig. 1.5).