Mid-Ocean Ridges
Mid-Ocean ridges
They are mountain range rising 2-3 km above the abyssal plains. Mid-Ocean Ridges are places where new oceanic listosphere forms and seafloor spreading takes place. They cover 1/3 of the acean area. They are more than 60.000 km long. Associated with volcanism and shallow earthquakes (<60 km deep) They have central valleys episodically filled with lava and intruded by dikes Magma chambers 2-3 km below surface; magma is supplied from 30-60 km depth within the mantle Type of magma: Low-K tholeiite, low in incompatible elements They are cut by fracture zones that segments and offset the ridge crest
In Iceland, it crops out above sea level because of the presence of a mantle hot spot there. Mid-Ocean Ridges: topography
Nasca Plate Two Types of mid-ocean ridge according to rate of spreading that varies from 1 to 12 cm/yr:
Slow spreading ridges (1-5 cm/yr) High topographic contrast that tapers out away from the ridge crest Association with a prominent rift valley 1 km deep and 30-50 km wide) Example: Mid-Atlantic Ridge margins are rising
Fast spreading ridges No large rift valley, instead there is an axial summit with or without a narrow graben Example: East Pacific Rise
As the seafloor spreads away from the crest then lithosphere cools and sinks. The sinking rate is 1km per 10 Ma and the next 1km sinking in 26 Ma Depth below crest = k. Sqrt (age) where k= 1000m/sqrt(10) k= 2000/sqrt(10+26)
For an average depth of 5000 m (2400 m below the ridge crest) k= 60 Ma.
Magma formation below oceanic ridges Magnetic stripes parallel to the trend of the ocean ridge: evidence of sefloor spreading.
First explained by Vine and Mathews (1963)
Magnetostatigraphy
Age of the seafloor: It gets older away from the central Ridge valley
Seismo-tectonic setting
Seamounts, Guyots, Island chains
Seamounts Most are volcanic in origin Some occur as linear chain (e.g., Hawaian volcanoes) Seamount with a flat top (table mount) is a guyot. Flat tops are due to wave erosion
Hot spot activity and formation of mountain Chains
Example: Hawaii volcanic chain
Age of islands get older towards NW