Mid Ocean Ridges
My Blogs (olelog) are mainly based on my daily reading of earth science news. Here on whatonearth.olehnielsen.dk I try to weave some of the pieces together to a greater whole with added background info.

 Plate Tectonics    Triple Junctions    Terranes 

Mid Ocean Ridges:

Mid-Ocean Ridges are places where the Earth's tectonic plates are gradually moving apart, and as they do, upwelling magma rises up to fill the gap, so that new oceanic crust is formed. This magma provides a heat source that creates many hyrothermal vents ("black smokers") along the ridges which transport heat and chemicals into the ocean. (Image by Robert Simmon, NASA GSFC)

The Mid-Atlantic Ridge, which splits nearly the entire Atlantic Ocean north to south, is probably the best-known and most-studied example of a divergent-plate boundary. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today.

The Atlantic Ocean started to form in the late Triassic Period. By Early Jurassic time (about 190 million years ago) rifting separated West Africa and the Eastern U.S. and began opening the Atlantic Ocean. By Early Cretaceous (about 120 million years ago) Africa separated from South America; and by Late Cretaceous (about 80 million years ago) North America separated from Greenland.

Southwest Indian Ridge

The Southwest Indian Ridge separates the Antarctic and African plates and extends from the Bouvet Triple Junction in the South Atlantic to the Rodriguez Triple Junction in the Central Indian Ocean. One of the most important characteristics of the Southwest Indian Ridge, from a global perspective, is that it is associated with extremely slow spreading rates (13 - 18 mm/year). The two triple junctions bounding the Southwest Indian Ridge differ in their characteristics: the fairly stationary Bouvet Triple Junction is associated with a hotspot, while the Rodriguez Triple Junction is actively migrating. The Southwest Indian Ridge axis is marked by a rift valley, the dimensions of which vary along-strike in a poorly understood manner. It also displays variations in segmentation geometry along-axis with long sections that are very linear and orthogonal to the spreading direction and sections that are segmented by transforms at intervals of 50 - 100 km and oblique to the spreading direction. The Southwest Indian Ridge is influenced by its proximity to the Bouvet and Marion hotspots.

Gakkel Ridge

The Gakkel Ridge (also known as Nansen Ridge) is a mid-oceanic ridge located in the Arctic Ocean between Greenland and Siberia with a length of about 1800 kilometres. It is part of the North American - Eurasian plate boundary. The Gakkel Ridge is the slowest spreading ridge on earth with a rate of less than one centimetre per year. It is also the deepest ocean ridge, ranging from 3-5 km deep. Rifting started 55 million years ago, when the Eurasian Basin opened, and now The Gakkel Ridge separates the approximately 4000 m deep Eurasian Basin into the Amundsen Basin and the Nansen Basin. The southern end of the ridge is spreading faster than the northern end, meaning that at the eastern termination the spreading rate is only ~6 mm/year and that the rate increases towards the western termination to ~13 mm/year. The width of the central rift valley varies between 20 and 40 km.

Gakkel Ridge is named after the Soviet Arctic explorer Yakov Yakovlevich Gakkel.

Fridtjof Nansen (1861 - 1930) was a Norwegian explorer of the Arctic, an oceanographer, and a Nobel Peace Prize-winning humanitarian. His Arctic accomplishments include: the first crossing of Greenland (1888), proof of the existence a polar oceanic current, and closest explorer to the North Pole at that time.

The East Pacific Rise

The East Pacific Rise is a long north-south seafloor spreading ridge under the eastern Pacific Ocean from near Antarctica in the south northward to its termination at the northern end of the Gulf of California in the Salton Sea basin in southern California. The rise is a constructive tectonic plate margin or divergent boundary lying along the eastern margin of the Pacific Ocean basin. The spreading zone separates the Pacific Plate to the west from (south to north) the Antarctic Plate, the Nazca Plate, Cocos Plate, and the North American Plate.

The main portion of the rise lies generally about 3,200 km off the coast, and it lies about 1,800 – 2,700 m above the surrounding seafloor. The East Pacific Rise has a generally smooth and flattish surface, and it drops sharply away at the sides.

Link to the full Britannica Online Article on the East Pacific Rise

The East Scotia Ridge is an active back-arc spreading centre located to the west of the South Sandwich island arc in the South Atlantic Ocean

The Macquarie Ridge runs from New Zealand to its junction with the Antarctic Plate. Some 12 million years ago. basalt magma started to flow from a fissure on the seabed at the abutting edges of the Australian Plate and the Pacific Plate. A third tectonic plate, Antarctic, abuts the other two plates some 650 kilometres south of the Macquarie Island.

The Southeast Indian Ridge

Lomonosov ridge

An undersea ridge which subdivides the Arctic Basin, extending from Ellesmere Land to the New Siberian Islands.

Black smokers

Black smokers are chimneylike structures made up of sulfur-bearing minerals or sulfides that come from beneath Earth's crust. They form when hot (roughly 350°C, but can be as hot as 400°C), mineral-rich water flows out onto the ocean floor through the volcanic lava on a mid-ocean ridge volcano. Such hydrothermal vents were first discovered in 1977, and look a bit like geysers on the sea floor. Sea water deep inside the seabed mixes with high-temperature magmatic gas, picking up a high mineral concentration from the surrounding rocks, and jets out from a crack in the sea floor. The hot fluid can feed a variety of weird marine life. The colour of the fluid is determined by a number of things, including the temperature and chemistry of the magmatic chamber. At high temperatures, heavy-metal ions can create black sulphides that look like smoke from a chimney.

There are also white smokers. White smoker fluid is usually cooler (250-300°C) and flows more slowly than the black smoker fluid. The chimneys generally are smaller as well. The white colour comes from minerals that form when the fluid exits the chimney and mixes with seawater. Unlike the black minerals in black smokers, these minerals don't contain metals, but are often compounds of barium, calcium, and silicon, which are white.

Hydrothermal vents can also spit out clear bubbles or grey smoke.

But a 'blue smoker' had never been reported until august 2006 - in a lava dome in the Okinawa Trough, 1,470 metres deep in the southern waters of Japan. The colour change to blue is undoubtedly due to a change in chemistry in the water, probably caused by a change in magma activity below. But why it is blue is unclear so far The researchers plan to investigate the vent more thoroughly from March.

The diagram shows the principle of a hydrothermal vent. Magmatic processes provide the driving mechanism for hydrothermal circulation through oceanic rocks. Seawater with a starting temperature of around 2°C seeps through the lavas above a magmachamber and are heated to around 400°C. The hot water dissolves minerals in from the surrounding rocks. We now have what is called a hydrothermal fluid. When the hydrothermal fluids exit the chimney and mix with the cold seawater sulfide and sulfate precipitates.

Most black smokers are found at an average depth of about 2,100 meters in areas of seafloor spreading along the Mid-Ocean Ridge System. The vents are formed in fields hundreds of meters wide.

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