Ocean Basin Physiography Courtesy: UCLA, ESS ISOSTASY The surface of the earth can be subdivided into two major areas: 1) the ocean basins and 2) the continents. Although the oceans cover about 71% of the earth's surface, a large fraction of this area is underlain by relatively shallow continental margins. The oceans have an average depth of 3800 meters below sea level and the continents have an average elevation of 840 meters above sea level. These relationships are summarized in Figure 1, which plots the relative proportions of the surface area of land and ocean floor lying at different elevations above and below sea level. Interestingly, the plot reveals that elevations have a bimodal distribution with each mode being close to one of the mean values quoted above. Figure 1. The distribution of levels on the earth's surface. The hypsographic curve, based on the histogram showing frequency distribution, indicates the percentages of the earth's surface that lie above, below, or between any levels. (Data from Sverdrup, Johnson, and fleming, 1966; after W.A. Anikouchine and R.W. Sternberg, 1973) An important concept for understanding the shape of the bimodal distribution in Figure 1 is the theory of isostasy. This theory suggests that the earth consists of blocks of rigid lithosphere, about 100 km thick, which are "floating" in isostatic equilibrium on a plastic region of earth's mantle called the asthenosphere (Figure 2). Archimedes, a Greek mathematician (circa 212 187 BC), was the first to note that a rigid body floating on a fluid will sink into
Figure 2. Schematic of the respective positions of the asthenosphere and lithosphere (Tarbuck and Lutgens, 1990). the fluid until the mass of the displaced fluid exactly equals the total mass of the rigid body. This concept of buoyancy is essential to an understanding of isostasy. The flotation of ice on water provides a useful illustration. Density is defined as the mass of a substance divided by its volume, and is measured in grams per cubic centimeter (g/cm 3 ). The density of ice is about 0.92 g/cm 3, while water is about 1.0 g/cm 3 (water is exactly 1.0 g/cm 3 at 4 C). Various-sized blocks of ice of the same density float at an equilibrium elevation in water that depends upon their shape and volume with 92 % of the block's volume submerged (Figure 3). Larger blocks float higher than smaller ones because they displace a larger volume of water, and the buoyant force is therefore greater. Recall that in this particular example, density is held constant, and volume is allowed to vary. Let us now consider the effect of allowing density to vary. Wooden blocks of the same shape and volume float at different depths in water, depending on their density; hardwoods (e.g., oak) are denser woods, and float at a deeper level than softwoods (e.g., pine or balsa). If Archimedes were in lab today and we asked him to explain the bimodal distribution in Figure 1, he would probably predict that 1) the earth's interior is deformable (not rigid) and 2) the materials of the ocean basins are denser and/or thinner than the materials composing the continents. He would be right on all counts! Yee haw! On geologic timescales, the Earth's interior behaves like a deformable, high-viscosity fluid. Continental crust averages 35 km in thickness and has a composition close to granite (density 2.7 2.8 g/cm 3 ). Oceanic crust, on the other hand, is much thinner (avg. 5km) and is composed of basalt (density 3.0 g/cm 3 ). Thus, from the viewpoint of isostasy, the continents stand at a higher elevation because they are composed of thick masses of lower density materials. Large mountain ranges on land are believed to stand high because they are supported by thick roots of granitic rock (up to 70 km
Figure 3. Isostasy can be understood by considering the situation in the crust of the earth and comparing it with ice that floats in water. The thick pieces of ice are higher above the water level than the thin ones and they also extend further below sea level than the thin pieces. (William A. Anikouchine/Richard W. Sternberg, The World Ocean: An Introduction to Oceanography, 1981, pp. 21, 23. Reprinted by permission of Prentice-Hall, Englewood Cliffs, New Jersey.) thick). Large mountain chains on the ocean floor (i.e., mid-ocean ridges) stand high because the lithosphere is hot and therefore has a lower density than lithosphere of the deeper ocean basins. As we will discuss in lab, the lithosphere comprises more than simply the oceanic and continental crust. The remainder is upper mantle material with a density of 3.3 g/cm 3. GEOMORPHIC FEATURES OF THE OCEAN FLOOR Now that we have discussed the first order subdivision of the earth's surface into continents and ocean basins, and the theory of isostasy explaining their existence, we will turn our attention to a more detailed consideration of ocean floor features. The ocean floor can be
subdivided into several smaller scale physiographic features (Table 1) that bear important relationships to plate tectonic processes that have shaped the ocean floor. We will review Plate Tectonics later in the quarter. As a prelude to plate tectonics we will now consider some of the important second order features: Continental Margins: Two types of margins are recognized: 1. The Atlantic type is characterized by a wide, gently sloping (0.1 ) continental shelf, a steeper (4 ) continental slope descending to the deep sea, and a flatter (0.2 ) continental rise at the base of the slope formed by accumulation of sedimentary materials (Figure 4). This type of margin is most common in the Atlantic Ocean, but it also occurs in other oceans. 2. The Pacific type is characterized by a narrow shelf and slope descending into a deep marginal trough, or trench, generally parallel to the continental margin. An example is the area off Chile and Peru in South America. Pacific type margins are also characteristic of some large island arcs. These are typical of the western Pacific. Ocean trenches are the deepest places on the seafloor, typically about twice as deep as the ocean basins. The deepest is the Marianas Trench (11.22 km below sea level). Figure 4. Profile across a hypothetical ocean basin showing marine physiographic provinces. To the left of the oceanic ridge is a passive continental margin typical of the Atlantic Ocean with a wide continental shelf, slope and rise created by a thick wedge of continental sediments. Features to the right are more typical of active margins like those of the Pacific where trenches and associated volcanic island arcs are common. In the deep-ocean basin, abyssal plain development is shown to the left of the ridge, and abyssal hills interrupt the plain to the right. Most of the materials comprising continental margins are sediments. Those on the continental shelf are derived from rivers that deposit their materials in the nearshore environment or by coastal erosion. These sediments are redistributed in the nearshore environment currents or by gravity processes. Sediments which accumulate on the continental shelves and slopes may eventually become oversteepened and slump or slide downlsope. Such gravity-driven processes are very important in transporting sediments down the continental slope and onto the continental rise or abyssal plains (discussed below). Important agents in this process are turbidity currents. These are short lived, gravity-induced currents consisting of mixtures of sediment and water which flow downslope as a density current. Turbidity currents are the primary means by which terrigenous (land-derived) sediment is transported from shallow water onto the deep ocean
floor. Sediments deposited by turbidity currents are often triggered by earthquakes. A classic example of this kind of activity was provided during the Grand Banks Earthquake off Newfoundland on November 19, 1929 (Figures 5 and 6). Slumping and turbidity currents triggered by the earthquake resulted in breakage of 13 trans-atlantic telephone and telegraph cables over a period of about 12 hours. Accurate records of the times of breakage provided a basis for determining the velocity of the turbidity currents which traveled at estimated speeds up to 95 kilometers per hour down the continental slope. Submarine canyons are steep-walled, V-shaped valleys that incise into continental shelves and slopes and open out at depth onto the continental rise. Most submarine canyons are associated with the mouths of large rivers. Although their origin is still debated, submarine canyons are important here because they serve as major conduits which funnel turbidity currents and sediments from the continental shelf onto the deep ocean floor. The mouths of submarine canyons are often associated with large lobes of sediment called submarine fans (e.g., the Ganges and Indus Cones). Smaller submarine canyons are common features on the shelf of the Pacific coast of the United States. Monterey Canyon, at the head of the Monterey Bay, California, is one of the largest submarine canyons on the U.S. Pacific coast. Deep-Ocean floor: 1. Mid-ocean Ridge and Rise System: This chain of mountains is 60,000 km long. It often has a central rift valley as much as 1 km deep and a rugged topography on its flanks (Figure 7). The Ridge and Rise System stands 1 to 3 km above the deeper ocean basin and is frequently cut and offset by numerous fracture zones. These are linear zones of irregular topography on the sea floor, 10 to 100 km wide and up to 3500 km long. Fracture zones are characterized by escarpments that can be from 100 to 4000 m high and separate regions of the seafloor of different depths. 2. Abyssal hills typify the topographic province of most of the deeper ocean basins where water depths are typically ~5 km. This province is covered by much more sediment than the rugged flanks of the midocean ridge system and thus gently rolling hills are often found here. Most of the Pacific has abyssal hills morphology. 3. Abyssal plains are found near some "Atlantic" type continental margins where sediments from turbidity currents have flowed off the continental rise and spread out over the ocean floor producing extremely flat stretches of the ocean floor. Other features include seamounts (hills >1 km above the ocean floor) and islands which are large, often extinct volcanoes. These often form long chains with an active volcano at only one end (e.g., Hawaii). There are also some larger features called plateaus, ridges and rises (but not from the midocean Ridge and Rise System) which are either fragments of continental crust or volcanic edifices.
Figure 5. Profile (cross-section) of the sea floor showing the events of the November 18, 1929 earthquake off the coast of Newfoundland. The arrows point to cable breaks; the numbers to times of breaks in hours and minutes after the earthquake. Vertical scale is greatly exaggerated. (After B.C.Heezen and M. Ewing, "Turbidity currents and submarine slump and the 1929 Grand Banks earthquake," American Journal of Science 250: 867). From Tarbuck and Lutgens, 1990. Figure 6. Map of Grand Banks area where cable breaks signaled the beginning of a slump and turbidity current. (From B. Heezen and C.L. Drake, 1964, American Association of Petroleum Geologists.) Figure 6b. In addition to triggering undersea turbidity currents, the large 1929 earthquake and offshore slumping caused devastating tsunami waves in eastern Canada. This two story house (next to the ship) was found floating several kilometers out at sea. We ll learn more about tsunami later in the course. More information about this photo is available at http://earthquakescanada.nrcan.gc.ca/historic_eq/20th/1929/1929_e.php.
Figure 7. The upper figure is a close-up cross-section of the Mid-Atlantic Ridge. The width is 800 km. The three lower profiles have a much longer horizontal scale; about 4000 km. Note the range in relief from the high relief, rugged profile of the North Atlantic Ocean to the low relief of the East Pacific Rise. (After Heezen, B.C. and M. Ewing, 1963.)
Table 1. Major relief features of the ocean floor. I. First-order features of the earth's crust A. Continents B. Ocean basins II. Second- and third-order features of the ocean floor A. Continental margins 1. "Atlantic" type or passive margins -continental shelf cut by submarine canyons -continental slope cut by submarine canyons -continental rise 2. "Pacific" type or active margins -continental shelf -continental slope -trench (no rise) -subtypes continental trench at continent (e.g., Peru Chile) "island arc" (e.g., Aleutians, Japan, Marianas) B. Deep-ocean Provinces 1. Mid-ocean Ridge and Rise System -central rift valley -rugged flanks -cut by fracture zones 2. Abyssal Hills -sediment covered -topography more gentle than ocean ridge flanks 3. Abyssal Plains -near some "Atlantic" type margins -extremely flat 4. Other features -seamounts (>1 km above the ocean floor) -islands -volcanic island (and seamount) chains (e.g., Hawaiian-Emperor) -submarine plateaus, ridges and rises (not part of Ridge and Rise System)