Write a statement describing how spreading rates affect ridge topography.
The greatest volume of magma (more than 60 percent of Earth’s total yearly output) is produced along the oceanic ridge system in association with seafloor spreading. As plates diverge, fractures created in the oceanic crust fill with molten rock that gradually wells up from the hot mantle below.
This molten material slowly cools and crystallizes, producing new slivers of seafloor. This process repeats in episodic bursts, generating new lithosphere that moves away from the ridge crest in a conveyor belt fashion.
Harry Hess of Princeton University formulated the concept of seafloor spreading in the early 1960s. Later, geologists were able to verify Hess’s view that seafloor spreading occurs along the crests of oceanic ridges, where hot mantle rock rises to replace the material that has shifted horizontally. Partial melting of ultramafic mantle rock produces basaltic magma that has a surprisingly consistent chemical composition. The newly formed melt separates from the mantle rock and rises toward the surface. Along some ridge segments, the melt collects in small, elongated reservoirs located just beneath the ridge crest. Eventually, about 10 to 20 percent of the melt migrates upward along fissures and erupts as lava flows on the ocean floor, while the remainder crystallizes at depth to form the lower crust. This activity continuously adds new basaltic rock to diverging plate margins, temporarily welding them together; they are then broken as spreading continues.
Along some ridges, outpourings of pillow lavas build submerged shield volcanoes (seamounts) as well as elongated lava ridges. At other locations, more voluminous lava flows pave the surface to create a relatively subdued topography.
Why Are Oceanic Ridges Elevated?
The primary reason for the elevated position of the ridge system is that newly created oceanic lithosphere is hot and therefore less dense than cooler rocks of the deepocean basin. As the newly formed basaltic crust travels away from the ridge crest due to seafloor spreading, it is cooled from above as seawater circulates through the pore spaces and fractures in the rock. Further cooling occurs as it gets farther and farther from the zone of hot mantle upwelling. As a result, the lithosphere gradually cools, contracts, and becomes denser. This thermal contraction accounts for the greater ocean depths that occur away from the ridge. After about 80 million years of cooling and contraction, rock that was once part of an elevated ocean-ridge system becomes part of the deepocean basin.
As lithosphere is displaced away from the ridge crest, cooling also causes a gradual increase in lithospheric thickness. This happens because the boundary between the lithosphere and asthenosphere is a thermal (temperature) boundary. As material in the uppermost asthenosphere ages (cools), it becomes stiff and rigid. Thus, the upper portion of the asthenosphere is gradually converted to lithosphere simply by cooling. Oceanic lithosphere continues to thicken until it is about 80 to 100 kilometers (50 to 60 miles) thick. Thereafter, its thickness remains relatively unchanged until it is subducted.
Spreading Rates and Ridge Topography
When researchers studied various segments of the oceanic ridge system, it became clear that there were topographic differences. These differences appear to result from differences in spreading rates—which largely determine the amount of melt generated at a rift zone. More magma wells up from the mantle at fast spreading centers than at slow spreading centers. This difference in output causes differences in the structure and topography of various ridge segments.
Oceanic ridges that exhibit slow spreading rates from 1 to 5 centimeters per year have prominent rift valleys and rugged topography (Figure 1A).
The Mid-Atlantic and Mid-Indian Ridges are examples. The vertical displacement of large slabs of oceanic crust along normal faults is responsible for the steep walls of the rift valleys.
Furthermore, volcanism produces numerous cones in the rift valley, which enhance the rugged topography of the ridge crest.
By contrast, along the Galapagos Ridge, an intermediate spreading rate of 5 to 9 centimeters per year is the norm. As a result, the rift valleys that develop are relatively shallow—often less than 200 meters (660 feet) deep. In addition, their topography is more subdued compared to ridges that have slower spreading rates.
At fast spreading centers (greater than 9 centimeters per year), such as along much of the East Pacific
Rise, rift valleys are generally absent (Figure 1B). Instead, the ridge axis is elevated. These elevated structures, called swells, are built from lava flows up to 10 meters (30 feet) thick that have incrementally paved the ridge crest with volcanic rocks (see Figure 1B).
In addition, because the depth of the ocean depends largely on the age of the seafloor, ridge segments that exhibit faster spreading rates tend to have more gradual profiles than ridges that have slower spreading rates (Figure 2). Because of these differences in topography, the gently sloping, less rugged portions of fastspreading ridges are called rises.
How Does Oceanic Crust Form?
The molten rock that forms new oceanic crust originates from partial melting of the ultramafic mantle rock. This process generates basaltic melt that is less dense than the surrounding solid rock. The newly formed melt rises through the upper mantle along thousands of tiny conduits that feed into a few dozen larger, elongated channels, perhaps 100 meters (300 feet) or more wide. These structures, in turn, feed lens-shaped magma chambers located directly beneath the ridge crest. The addition of melt from below steadily increases the pressure inside the magma chambers. As a result, the rocks above these reservoirs periodically fracture, allowing the melt to ascend along numerous vertical fractures that develop in the ocean crust. Some of the melt cools and solidifies to form dikes. New dikes intrude older dikes, which are still warm and weak, to form a sheeted dike complex. This portion of the oceanic crust is usually 1 to 2 kilometers thick. Roughly 10 to 20 percent of the melt eventually erupts on the ocean floor. Because the surface of these submarine lava flows is chilled quickly by seawater, these flows generally travel no more than a few kilometers before completely solidifying. The forward motion occurs as lava accumulates behind the congealed margin and then breaks through. This process occurs repeatedly, as molte basalt is extruded—like toothpaste from a tightly squeezed tube. The result is protuberances resembling large bed pillows stacked one atop the other, hence the name pillow lavas (Figure 3).
In some settings, pillow lavas may build volcano-size mounds tha resemble shield volcanoes, whereas in other situations they form elongated ridges tens of kilometers long. These structures are eventually separated from their supply of magma as they are carried away from the ridge crest by seafloor spreading. The lowest unit of the ocean crust develops from crystallization within the central magma chamber itself. The first minerals to crystallize are olivine, pyroxene, and occasionally chromite (chromium oxide), which settle through the magma to form a layered zone near the floor of the reservoir. The remaining melt tends to cool along the walls of magma chambers to form massive amounts of coarse-grained gabbro. This portion accounts for up to 5 of the 7 kilometers (3 to 4.5 miles) of ocean crust thickness. Although molten rock rises continuously from the mantle toward the surface, seafloor spreading occurs in pulse-like bursts. As the melt begins to accumulate in the lens-shaped reservoirs, it is blocked from continuing upward by the stiff overlying rocks. As the amount of melt ntering the magma reservoirs ncreases, pressure rises. Periodically, the pressure exceeds the strength of the overlying rocks, which fracture and initiate a short episode of seafloor spreading.