Mid Oceanic Ridges PDF

5. Mid Oceanic ridges Introduction The rigid surface layer (generally about 70–100 km thick) of the Earth comprising the crust and uppermost mantle, termed as lithosphere, is divided into a number of tectonic plates. Presently, there are about eight primary plat...

5. Mid Oceanic ridges Introduction The rigid surface layer (generally about 70–100 km thick) of the Earth comprising the crust and uppermost mantle, termed as lithosphere, is divided into a number of tectonic plates. Presently, there are about eight primary plates, namely, North American, South American, African, Indian, Australian, Antarctica, Eurasian, and Pacific; seven secondary plates; and several more tertiary plates. These lithospheric plates are bounded by one of the three main types of geological features: (1) mid oceanic ridges, (2) subduction zones, and (3) transform faults. The boundaries are narrow deforming zones and are associated with intense seismicity, whereas the plate’s interiors are relatively stable. The plates upon which continents and ocean floor lie are in continuous motion at a speed of few centimeters per year. Each plate is in relative motion with respect to the other on the surface of the Earth. This relative motion produces new crust at mid oceanic ridges and consumes old lithosphere at subduction zones. Apart from these tectonic processes, plates do undergo breakups and unifications through geologic time. Morphology of mid oceanic ridge A mid oceanic ridge is an underwater linear physiographic feature and serves as a divergent tectonic boundary between two lithospheric plates. The mid oceanic ridges, encircle the entire globe, are physically connected and extend for a total length of about 65,000 km as a single global entity in the world oceans (Fig. 1). PHY (NIO): 3- 4202 Continental Margins and Ocean Basins by Dr. Maria Ana Desa. Asst. Prof. AcSIR Fig. 1: The location of mid oceanic ridges in the world oceans The mid ocean ridge consists of thousands of individual volcanoes or volcanic ridge segments where magma is continuously upwelling and being added to the previously formed oceanic crust on either side causing seafloor spreading. The magma upwelling and seafloor spreading rates along the global mid oceanic ridges are not fixed; they greatly vary with a range from 6 to 200 mm/year. Based on the seafloor spreading rates, the mid oceanic ridges are classified into 3 categories, i.e. slow, intermediate, and fast spreading ridges. The Gakkel mid oceanic ridge located in the Arctic Ocean between Greenland and Siberia is the slowest spreading ridge with an spreading rate of about 6 mm/year. On the other hand, the East Pacific Rise particularly in equatorial zone generates magma at the fastest rates of up to 200 mm/year. The seafloor morphology on the flanks of the mid oceanic ridge is largely controlled by the rate of seafloor spreading. Fast spreading rates contribute to generally smooth and flat seafloor surfaces in the vicinity of the ridge, while the slow spreading rates cause exceptionally rough surfaces. The slow spreading ridges are generally marked in their crest by an axial valley, called rift, which is clearly distinguishable along the Carlsberg Ridge in northern Indian Ocean. Here, the ridge crest is characterized by rugged topography, steep valley walls, and wide rift valley (Fig. 2). In contrast at the fast PHY (NIO): 3- 4202 Continental Margins and Ocean Basins by Dr. Maria Ana Desa. Asst. Prof. AcSIR spreading ridges, the rift valley is absent and a smooth volcanic summit with a crack along its crest is observed, eg. East Pacific Rise. Fig. 2: Fast and slow spreading at mid oceanic ridges Internal structure of the mid oceanic ridge Beneath a typical mid oceanic ridge, mantle material partially melts as it rises in response to reduced pressure. This melted rock, or "magma", may collect in a reservoir a few kilometers below the seafloor, awaiting eruption. Much of the magma eventually freezes in a place within the crust, forming the bulk of the new oceanic crust without erupting at all. Average oceanic crust is about 7 km thick, but only the upper 1 to 3 km is formed by eruption processes. When magma pressure builds up enough to force its way out to the seafloor, eruption occurs. "Dikes" are magma-filled cracks and are the conduits through which magma flows to reach the surface. A typical ridge eruption leaves behind a dike that is tens of centimeters up to 2 meters in width, extending between the crustal magma chamber and the eruptive fissure at the surface. Lavas pour from the fissure across the surface of the volcanic seafloor, adding a thin coat of new lava (typically 60 mm yr-1 full rate) and “slow” (< 60 mm yr-1 full rate). An “intermediate” type is often placed between them. Both types of ridges share certain characteristics: (1) they have PHY (NIO): 3- 4202 Continental Margins and Ocean Basins by Dr. Maria Ana Desa. Asst. Prof. AcSIR roughly the same crustal thickness (6–7 km), (2) in plan view they have a characteristic stair-step geometry of volcanic rifts separated by perpendicular transform offsets, and (3) they generate a characteristic outcrop pattern of elongate, fault-bounded abyssal hills trending normal to the spreading direction. Their differences lie primarily in their across- axis morphology: fast-spreading ridges have an axial rise with a very narrow summit graben that is the locus for most volcanic and tectonic activity, whereas slow-spreading ridges have rugged rift mountains enclosing a broad axial valley. Fast-spreading ridges tend to be dominated by volcanism, while the morphology of slow-spreading ridges is dominated more by tectonics Two of the most carefully studied mid-ocean ridges are the Mid-Atlantic Ridge and the East Pacific Rise. The Mid-Atlantic Ridge runs down the center of the Atlantic Ocean, slowly spreading at a rate of two to five centimeters (0.8 to 2 inches) per year and forming a rift valley that is about the depth and width of the Grand Canyon. In contrast, the East Pacific Rise spreads fast at rates of 6 to 16 centimeters (three to six inches) per year. Due to the fast spreading rates, there is no rift valley in the Pacific, just a smooth volcanic summit with a crack along the crest that is much smaller than the Atlantic rift valley. Ultraslow-spreading ridges (< 20 mm yr-1 full rate) represent a major departure from the style of crustal accretion seen in the rest of the ocean basins, e.g. the SWIR and Gakkel Ridge. At ultraslow-spreading ridges, the seismic crust is thinner (~ 1–4 km) than at other ridges. The changeover from slow- to ultraslow-spreading characteristics is not strictly a function of spreading rate. Ultraslow-spreading ridges uniquely possess amagmatic rifts that expose mantle peridotite directly on the seafloor, with only scattered basalt and gabbro. Unlike magmatic segments, they have mantle peridotite walls formed by long sloping scarps or irregular uplifts rather than the basaltic-block, normal faulting of Penrose crust PHY (NIO): 3- 4202 Continental Margins and Ocean Basins by Dr. Maria Ana Desa. Asst. Prof. AcSIR