Physical Oceanography: An Overview
2.11 The Sea Floor and Its Dynamics
Fig. 2.25 Spreading of high-salinity Mediterranean water across the Atlantic at in- termediate depths. [Adapted from Worthington, L. V., and W. R. Wright, North Atlan- tic Ocean Atlas (1970).]
great enough t o balance the downward diffusion of heat and thereby main- tain the vertical temperature gradients.
This sinking of cold water also aids in the removal of heat from the tropics by providing a conduit for surface waters, their thermal free energy now ex- hausted to their surroundings, t o return t o the subsurface system and thus complete the cycle started in the equatorial regions under the forcing of winds that the currents themselves have played a major role in establishing.
Fig. 2.26 The gross depth structure of the world’s oceans, showing three depth ranges and the axes of deep sea trenches. [From Dietrich, G., General Oceanography (1980); originally due to Wust (1940).]
traverse and on which the fluid boundary conditions must be satisfied, to the solid earth geophysicist or marine geologist, the sea floor is an immense- ly rich and dynamic region whose study has yielded one of the major intellec- tual advances in geophysics of the century. These advances are summarized in the topics of sea floor spreading, plate tectonics, and continental drift.
Before launching into their study, however, a short diversion into a view of the morphology of the ocean floor is in order.
Figure 2.26 is a bathymetric map of the world’s oceans showing broad depth categories. A study of the oceanic relief shows several distinct regimes of the sea floor. The continental shelves are gently sloping seaward projec- tions of the continents themselves, and extend into the oceans for distances that range from tens to a few hundreds of kilometers; the shelf is conven- tionally considered to end at a depth of approximately 200 m, which con- tour forms the shelfbreak. Still further at sea, the shelfslope or continental margin exists, often cut through by submarine canyons and drowned river beds; the slope is the site of turbidity currents, mixtures of sediment and water that occasionally tumble down the slope, sometimes with enough force to sever submarine communication cables. The margin generally terminates at the abyssal deep, the flat, sediment-filled plain that occupies much of the sea floor. The accumulation of the organic detritus that constantly rains down onto the sea floor from above at rates of millimeters per year has filled the abyss with sediments whose depths may approach several kilometers. Lo- cated out in the breadth of the sea floor but sometimes intersecting the con- tinents and islands are the more shallow mid-ocean ridges, which are regions of mountains, volcanoes, seismic activity, and bottom heat flow, and which are also the sites of new sea floor production. The ridges generally lie at depths near 1 to 2 km, but actually broach the surface in a few places such as Ice- land. Scattered at various locations throughout the breadth of the sea are volcanic islands, atolls, and seamounts produced by vulcanism, wherein heat- ed material from the interior of the earth has been transported to the sur- face by internal forces. Closer to the continents are islands and undersea structures that are not volcanic in their origin but rather are continental frag- ments that have broken off from the main land masses. Also near the conti- nents are the deep ocean or submarine trenches, regions where the ocean bottom reaches its greatest depths-the deepest being more than 11 km- under the influence of subduction forces occurring between the sea floor and the adjacent continents. The trenches are the sites of numerous deep earth- quakes that extend to perhaps 700 km beneath the surface.
In order to understand the theoretical origin of the motions of the solid upper layers of the earth implied by the submarine features of Fig. 2.26, it is necessary to know something of the hypothesized interior distribution and state of matter of the planet. Figure 2.27 represents a vertical wedge of the
50 Principles of Ocean Physics
interior showing divisions into solid and liquid cores, a solid but somewhat plastic mantle, and a crustal region consisting of several distinct layers whose thicknesses in the figure have been somewhat exaggerated. Pressures within the interior of the earth are largely hydrostatic and the approach of the crust
Fig. 2.27 A wedgeshaped sector of the earth’s interior, illustrating solid and Ii- quid cores and near-surface crustal differentiat.ions. [Adapted from Geopotenfial Research Mission, National Aeronautics and Space Administration (1984).]
to hydrostatic equilibrium is termed isostatic adjustment; it is not perfect, but results in gravity anomalies and dynamical motions. The shallow layer termed the lithosphere can be considered as supporting both continental rock masses and the ocean floor, all viewed as floating on the upper mantle. Al- though the lithosphere cannot support vertical stresses well, it nevertheless transmits horizontal stresses readily, somewhat as a sheet of thin ice might.
Radioactive decay within the earth is thought to provide the heat neces- sary to keep much of the interior plastic or liquid, as well as the potential and kinetic energy needed to drive the very slow but extensive motions with- in the liquid core and mantle. Enormous hydrostatic pressure is thought to have solidified the inner core, in spite of its high temperature. These mo- tions, which must also obey the laws of fluid dynamics, are exceedingly vis- cous (in fluid terms), but are also conditioned by rotational and convective forces, as are the oceans and atmosphere-although the time scales for in- terior motions are millions of times longer than for the surface fluids. The existence of cellular convection currents within the mantle has been hypothe- sized, of somewhat the same genre as the combined Walker-Hadley cells in the atmosphere, and it is thought that these convection cells bring heat and molten material from the deep interior upward to the crustal complex, and exert horizontal and vertical forces on the athenosphere and lithosphere, as suggested in Fig. 2.28. The molten interior material broaches the crust along the rift valleys centered on the mid-ocean ridges, and in convective plumes, or “hot spots,” where undersea mountain building, volcanoes, and shallow
-
Sea floor spreading- OceanFig. 2.28 Schematic vertical cross section of the crust under the ocean. Convec- tion cells advect the lithosphere away from mid-ocean ridges as on a conveyor belt.
The crust is consumed under the continental edges beneath deep ocean trenches.
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earthquakes (depths of 1 to 3 km) occur. New sea floor is created at the mid- ocean ridges and is slowly carried away from the regions of formation, called spreading centers, by the motion of the lithosphere, somewhat as on a con- veyor belt. Speeds range from a very few to perhaps 10 cm yr-' (10 m per century). This is the process of sea floor spreading, and takes place along ridges such as the Mid-Atlantic Ridge and the East Pacific Rise, for example (see Fig. 2.26). These ridges comprise the oceanic legs of the major crustal fault zones of the earth; together with their landward extensions (such as the San Andreas Fault and the East African Rift Valley), they physically split the surface of the globe into perhaps 14 major continental plates (Fig. 2.29).
On a spherical earth, the motion of the rigid lithospheric plates away from the spreading centers must be considered, by Euler's theorem on the general displacement of a rigid body with one point fixed, to be a rotation about a fixed axis called the Euler pole, as suggested in Fig. 2.30. The Euler pole does not generally coincide with the spin axis. Plate elements close to the pole .move with lower azimuthal velocities than do those closer to the Euler equator, varying as the sine of the colatitude, 8 . Under these differential ve- locities, the spreading centers are thought to split zonally into a large num- ber of ridge-ridge transform faults, which then become secondary sites of mountain building, volcanoes, and earthquakes.
During the process of sea floor spreading, undersea volcanoes and vol- canic islands are also carried away from the spreading centers or hot spots by the motion of the lithosphere, cooling in the meantime. Over a period measured in a few tens of millions of years, the newly formed sea floor and volcanoes are conveyed away from their sources over convective plumes, usually to become inactive, then erode, then accumulate sediments from the rain of ocean biogenic and terrigenous (land-originated) material, and sink into the underlying mantle. This progressive sinking of the older bottom is due to the increased weight from accumulated benthic material, the cooling of the extruded matter, and the isostatic adjustment process. Often the vol- canoes in the tropics that slowly sink beneath the sea surface along with the sea floor become atolls as corals build around their lips, or become flat-topped seamounts called guyots.
If the sub-crustal heat source happens to be a localized hot spot fixed in the mantle, the plate motion past the spot may result in a long series of vol- canic islands and sea mounts; such is the case of the combined Hawaiian- Midway-Emperor Seamount chain extending northwest from the big island of Hawaii. This chain has formed over a period of approximately 70 million years as the Pacific Plate drifted away from the East Pacific Rise at a mean velocity of some 6 cm y r - ' . It is clear that the island building has been epi- sodic and quasi-random, but has taken place over this entire time, continu- ing on to the present. This same plate motion is expected to carry Baja
subduction zones. [Adapted from Morgan, W. J., Studies in Earfh and Space Sciences (1972).]
54 Principles of Ocean Physics Spin
axis
Euler pole
Fig. 2.30 Motion of a rigid lithospheric plate about the Euler pole. The variation in azimuthal velocity is thought to create ridge-ridge transform faults.
California and Southern California northwest at a rate such that in approxi- mately 50 million years, that region will be located off the Pacific North- west (Fig. 2.31).
The linear regions where oceanic plates thrust up against or otherwise in- tersect continental plates are areas of active destruction of the lithosphere, deep earthquakes, and mountain building (Fig. 2.28). Such a region extends around a major portion of the entire Pacific from the tip of South America, through North America, Northeastern Asia, Japan, on to the Marianas and Tonga Trenches, and past New Zealand; it is sometimes referred to as the Pacific “Rim of Fire.” Seismic sea waves or tsunami are produced by earth- quakes near these trenches and often propagate across the entire Pacific.
Present
I
50 MyFig. 2.31 Pacific Plate motion past the North American Plate will transport Baja California and Southern California to a location abreast of the Pacific Northwest if present rate continues for 50 million years. [Adapted from Mini-Micro Systems (1981).]
56 Principles of Ocean Physics
The collision of the sea floor with the lighter continental land mass gener- ally results in underthrusting and consumption of the lithosphere, a process termed subduction, and is accompanied by the return of oceanic sediments to the continental roots and the up-thrusting of continental material, often producing coastal mountain ranges thereby. The Peru-Chile Trench and the Andes Mountains are examples of this type of tectonic activity. Plate colli- sion and orogeny (mountain building) over geological time also explain why erosion and weathering have not long since worn down the earth’s surface to a smooth, water-covered sphere, and also explain the appearance of oceanic fossils high atop mountains.
In other regions where the oceanic plate moves more or less parallel to the continental plate, a strike-slip fault is said to occur, with the San An- dreas Fault being the best known example. The relative motion here is usually episodic, with long periods during which no slippage occurs but during which considerable strain is accumulated along the fault. The sudden release of this strain via differential motion between segments of the fault results in earth- quakes and sharp elevation changes. In some regions, the friction produced may result in remelting of subsurface material, with the associated faulting allowing magma to flow to the surface. The chain of volcanoes in the Cas- cade Mountains have their origin in this type of motion.
Figure 2.29 is a map that summarizes the principal plate tectonic features and their manifestations on a global scale. Over geological time, the plate tectonic process appears capable of transporting entire continental and oceanic masses about the earth, a mechanism that now gives credence to the earlier continental drift theory advanced by Wegener in 1915, based on the geomet- ric fit of the Atlantic coastlines of South America and Africa; however, Wegener’s hypothesis was rejected by his contemporaries for lack of any known process by which the continents could move. Since then, by linking his idea to the concepts of sea floor spreading and plate tectonics, geologists and geophysicists have been able to legitimize the hypothesis and even to reconstruct the probable distribution of the crustal plates in the past, back to approximately 200 million years before present (Mybp). Figure 2.32 shows such a reconstruction, with South America and Africa fitting together in the Southern Hemisphere along the lines of the continental shelf break, and North America, Africa, and Eurasia nestling together in the Northern Hemisphere.
Near the south pole, one sees that Antarctica, India, and Australia may also have been conjoined with South America and Africa to form a superconti- nent named Pangaea (all earth), with a superocean called Panthalassa (all ocean) that would have occupied some 60% of the earth’s surface. The cli- mate interior to the continents was probably exceedingly dry and the ocean- ic circulation quite different from that of today. As time went on, under the influence of the conjectured mantle convective cells, rifts developed that
ocean fanthalassa. [From Dietz, R., and J. Holden, J. Geophys. Res. (1970).]
sa Principles of Ocean Physics
split the continents along lines that are still traceable today, and the subse- quent spreading of the sea floor carried them apart at speeds up to the order of 0.1 km per millennium. By 65 Mybp, the oceans as we know them had more or less opened up, while Panthalassa had shrunk to become the proto- Pacific. To some extent, the changing oceanic circulation patterns of the past can be discerned in the deposition and type of sediments laid down on the sea floor during that time.
The process of plate motion continues, and in some areas its drift rates have been directly measured over a decade or more by geodetic and satellite means. Projections of the future distributions of land masses have been made based on extrapolations of the present motions. While these projections are no more certain than the reconstructions of the past, what is certain is that in another 100 million years, the surface of the earth and the configuration of the oceans will be quite different from what they are today.
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60 Principles of Ocean Physics
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