The Melt Redemption

Fig.1 Mid-Ocean ridges are large mountain ranges
underwater, with intense heat activities

Where is the longest and largest mountain range in the world? Himalayas? Andes? No. It is under the sea. Known as Mid-Ocean Ridge, the huge underwater mountain system extends as long as 65000 km and connects the undersea mountains from Pacific to Atlantic, forming spectacular landscape in the midst of the vast seafloor. Mountaineers in the legendary kingdom of Atlantis, if this kingdom exists, might be excited to climb these mountains, but they’d better take good care of their feet, because mid-ocean ridges could be very hot. Black smokers with temperature ranging from 60 °C to as high as 464 °C have been discovered along mid-ocean ridges by deep-sea explorations. These heat and energy released by the black smokers are believed to have given birth to the life on Earth, and are closely related to the magma movement beneath mid-ocean ridges.

Like cut pie, the Earth’s surface is broken up into several pieces called plates, which move in relation with one another. Mid-ocean ridges, or tectonically known as divergent plate boundaries, are places where two plates are moving apart. The change of temperature and pressure in this process causes the rocks beneath mid-ocean ridges to melt and erupt and then solidify, forming new crust along mid-ocean ridges, and that is the mountain range we see under the sea. The axis of a mid-ocean ridge is always believed to be the exit for the upwelling melts to erupt. However, recent observations suggest that those naughty melts (or magma) have probably already found a secret passage to escape. Throughout the global mid-ocean ridge system, mid-ocean ridges are cut by huge fractures called transform faults. Previous theories believe that these fractures, with efficient cooling, may direct melts away, but recent research showed that there is thickened crust along transform faults, which indicates active magma activity in these regions.  

Hebert and Montesi (2011) apply a 3D model of melt movement to investigate the unusual thickened crust along an oceanic transform, and suggest that the faults and cracks in transform region might create connected channels and should be responsible for the melt redemption. In 2D, melt migration at mid-ocean ridge system can be modeled by a process with two steps: (1) vertical upwelling under buoyancy; (2) lateral migration along a low-permeability lid inclined towards the ridge axis. However, in 3D, the existence of the transform will affect the melt pathway to the ridge axis and result in thickened crust. Hebert and Montesi solve a 3D model based on a thermal structure that incorporates important parameters, and emphasize the potential structural controls on melt concentration in transform faults. Their results show that the structural damage such as faults and dikes, intersects the lid on the melt pathway, redistributes ridge crust to the transform domain, and accounts well for the unusual crustal thickness at transform faults observed. Their research gives a new perspective in the study of the origin of the huge mountain range under the sea, and can be used to explain the heat distribution along mid-ocean ridges, contributing to the efforts in unveiling the mysteries of this region.

Fig.2 Sketches for melt pathway beneath a ridge and a transform respectively. The melts are guided by a lid called permeability barrier and may be extracted by some structural damages to the surface, forming thickened crust

Reference

Hebert, L. B., and L. G. J. Montési (2011), Melt extraction pathways at seg- mented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform, Geophys. Res. Lett., 38, L11306, doi:10.1029/2011GL047206.