Mid-ocean ridge

The spreading center or axis commonly connects to a transform fault oriented at right angles to the axis. The flanks of mid-ocean ridges are in many places marked by the inactive scars of transform faults called fracture zones. At faster spreading rates the axes often display overlapping spreading centers that lack connecting transform faults.^[2][14] The depth of the axis changes in a systematic way with shallower depths between offsets such as transform faults and overlapping spreading centers dividing the axis into segments. One hypothesis for different along-axis depths is variations in magma supply to the spreading center.^[2] Ultra-slow spreading ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults.^[13]

Volcanism

Mid-ocean ridges exhibit active

The crystallized magma forms a new crust of

Driving mechanisms

The oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at ocean

Increased rates of seafloor spreading (i.e. the rate of expansion of the mid-ocean ridge) have caused the global (eustatic) sea level to rise over very long timescales (millions of years).^[32][33] Increased seafloor spreading means that the mid-ocean ridge will then expand and form a broader ridge with decreased average depth, taking up more space in the ocean basin. This displaces the overlying ocean and causes sea levels to rise.^[34]

Sealevel change can be attributed to other factors (thermal expansion, ice melting, and mantle convection creating dynamic topography^[35]). Over very long timescales, however, it is the result of changes in the volume of the ocean basins which are, in turn, affected by rates of seafloor spreading along the mid-ocean ridges.^[36]

The 100 to 170 meters higher sea level of the Cretaceous Period (144–65 Ma) is partly attributed to plate tectonics because thermal expansion and the absence of ice sheets only account for some of the extra sea level.^[34]

Impact on seawater chemistry and carbonate deposition

Seafloor spreading on mid-ocean ridges is a global scale ion-exchange system.^[37] Hydrothermal vents at spreading centers introduce various amounts of iron, sulfur, manganese, silicon, and other elements into the ocean, some of which are recycled into the ocean crust. Helium-3, an isotope that accompanies volcanism from the mantle, is emitted by hydrothermal vents and can be detected in plumes within the ocean.^[38]

Fast spreading rates will expand the mid-ocean ridge causing basalt reactions with seawater to happen more rapidly. The magnesium/calcium ratio will be lower because more magnesium ions are being removed from seawater and consumed by the rock, and more calcium ions are being removed from the rock and released into seawater. Hydrothermal activity at the ridge crest is efficient in removing magnesium.

Slow spreading at mid-ocean ridges has the opposite effect and will result in a higher Mg/Ca ratio favoring the precipitation of aragonite and high-Mg calcite polymorphs of calcium carbonate (aragonite seas).^[41]

Experiments show that most modern high-Mg calcite organisms would have been low-Mg calcite in past calcite seas,^[42] meaning that the Mg/Ca ratio in an organism's skeleton varies with the Mg/Ca ratio of the seawater in which it was grown.

The mineralogy of reef-building and sediment-producing organisms is thus regulated by chemical reactions occurring along the mid-ocean ridge, the rate of which is controlled by the rate of sea-floor spreading.^[39][42]

History

Discovery

The first indications that a ridge bisects the Atlantic Ocean basin came from the results of the British Challenger expedition in the nineteenth century.^[43] Soundings from lines dropped to the seafloor were analyzed by oceanographers Matthew Fontaine Maury and Charles Wyville Thomson and revealed a prominent rise in the seafloor that ran down the Atlantic basin from north to south. Sonar echo sounders confirmed this in the early twentieth century.^[44]

It was not until after

Alfred Wegener proposed the theory of continental drift in 1912. He stated: "the Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima [rising] from depth".^[48] However, Wegener did not pursue this observation in his later works and his theory was dismissed by geologists because there was no mechanism to explain how continents could plow through ocean crust, and the theory became largely forgotten.

Following the discovery of the worldwide extent of the mid-ocean ridge in the 1950s, geologists faced a new task: explaining how such an enormous geological structure could have formed. In the 1960s, geologists discovered and began to propose mechanisms for seafloor spreading. The discovery of mid-ocean ridges and the process of seafloor spreading allowed for Wegener's theory to be expanded so that it included the movement of oceanic crust as well as the continents.^[49] Plate tectonics was a suitable explanation for seafloor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a major paradigm shift in geological thinking.

It is estimated that along Earth's mid-ocean ridges every year 2.7 km² (1.0 sq mi) of new seafloor is formed by this process.^[50] With a crustal thickness of 7 km (4.3 mi), this amounts to about 19 km³ (4.6 cu mi) of new ocean crust formed every year.^[50]

• 
  Oceanic ridge and deep sea vent chemistry
• 
  Plates in the crust of the earth, according to the plate tectonics theory
• 
  Seafloor

  magnetic striping
• 
  A demonstration of magnetic striping

List of mid-ocean ridges

• Aden Ridge – Part of an active oblique rift system in the Gulf of Aden, between Somalia and the Arabian Peninsula
• Cocos Ridge
   – Pacific volcanic hotspotPages displaying short descriptions of redirect targets
• Explorer Ridge – Mid-ocean ridge west of British Columbia, Canada
• Galapagos Spreading Center - an east-west trending mid-ocean ridge east of the eponymous islands between the Nazca and Cocos plates
• Gorda Ridge – Tectonic spreading center off the northern coast of California and southern Oregon
• Juan de Fuca Ridge – Divergent plate boundary off the coast of the Pacific Northwest region of North America
• South American–Antarctic Ridge – Mid-ocean ridge in the South Atlantic between the South American Plate and the Antarctic Plate
• Chile Rise
   – Submarine oceanic ridge in the Pacific OceanPages displaying short descriptions of redirect targets
• East Pacific Rise – Mid-oceanic ridge at a divergent tectonic plate boundary on the floor of the Pacific Ocean
• Gakkel Ridge – Mid-oceanic ridge under the Arctic Ocean between the North American Plate and the Eurasian Plate(Mid-Arctic Ridge)
• Pacific-Antarctic Ridge – Tectonic plate boundary in the South Pacific Ocean
• Central Indian Ridge – A north-south-trending mid-ocean ridge in the western Indian Ocean
  • Carlsberg Ridge – Tectonic plate ridge
• Southeast Indian Ridge – Mid-ocean ridge in the southern Indian Ocean
• Southwest Indian Ridge – A mid-ocean ridge on the bed of the south-west Indian Ocean and south-east Atlantic Ocean
• Mid-Atlantic Ridge – Atlantic Ocean tectonic plate boundary
  • Kolbeinsey Ridge – Segment of the Mid-Atlantic Ridge north of Iceland in the Arctic Ocean
  • Mohns Ridge – Geographical region in the Atlantic basin
  • Knipovich
     – Russian zoologistPages displaying short descriptions of redirect targets Ridge (between Greenland and Spitsbergen)
  • Reykjanes Ridge
     – Atlantic Ocean tectonic plate boundaryPages displaying short descriptions of redirect targets (south of Iceland)

List of ancient oceanic ridges

• Aegir Ridge – Extinct mid-ocean ridge in the far-northern Atlantic Ocean
• Alpha Ridge – Major volcanic ridge under the Arctic Ocean
• Kula-Farallon Ridge – Ancient mid-ocean ridge
• Mid-Labrador Ridge – Mid-ocean ridge in the Labrador Sea
• Pacific-Farallon Ridge – Spreading ridge during the Late Cretaceous
• Pacific-Kula Ridge – Mid-ocean ridge between the Pacific and Kula plates in the Pacific Ocean during the Paleogene period
• Phoenix Ridge
   – Ancient mid-ocean ridge between the Phoenix and Pacific platesPages displaying short descriptions of redirect targets

See also

• Afar Triangle – Geological depression caused by the Afar Triple Junction
• Geography of Iceland – Geographical features of Iceland
• List of oceanic landforms
   – Links to Wikipedia articles on landformsPages displaying short descriptions of redirect targets
• Ocean chemistry
• Oceanic crust
• Petrological Database of the Ocean Floor
• Project FAMOUS – first crewed submersible study of the rift valley of the Mid-Atlantic Ridge
• RISE project
  – discovery of black smokers hydrothermal systems on the East Pacific Rise
• Slab window – Type of gap in a subducted oceanic plate
• Submarine volcano – Underwater vents or fissures in the Earth's surface from which magma can erupt
• Vine-Matthews-Morely hypothesis; explains relation of marine magnetic anomalies to seafloor spreading.

References

External links

Wikimedia Commons has media related to Mid-ocean ridges.

• An explanation of relevant tectonic forces
• Mid-Oceanic ridge, like baseball seam (The Dynamic Earth, USGS)
• Ridge2000, Studying Mid-Ocean Ridges from Mantle to Microbe