An estimated 75% of volcanic eruptive volume is generated by submarine eruptions near mid ocean ridges alone, however problems detecting deep sea volcanics meant they remained virtually unknown until advances in the 1990s made it possible to observe them.Submarine eruptions may produce seamounts, which may break the surface and form volcanic islands. Submarine volcanism is driven by various processes. Volcanoes near plate boundaries and mid-ocean ridges are built by the decompression melting of mantle rock that rises on an upwelling portion of a convection cell to the crustal surface. Eruptions associated with subducting zones, meanwhile, are driven by subducting plates that add volatiles to the rising plate, lowering its melting point. Each process generates different rock; mid-ocean ridge volcanics are primarily basaltic, whereas subduction flows are mostly calc-alkaline, and more explosive and viscous.Spreading rates along mid-ocean ridges vary widely, from 2 cm (0.8 in) per year at the Mid-Atlantic Ridge, to up to 16 cm (6 in) along the East Pacific Rise. Higher spreading rates are a probable cause for higher levels of volcanism. The technology for studying seamount eruptions did not exist until advancements in hydrophone technology made it possible to "listen" to acoustic waves, known as T-waves, released by submarine earthquakes associated with submarine volcanic eruptions. The reason for this is that land-based seismometers cannot detect sea-based earthquakes below a magnitude of 4, but acoustic waves travel well in water and over long periods of time.
Causes of Metamorphopsia
Metamorphopsia can be a symptom of a number of eye disorders involving the retina or macula. Some of these conditions include the following:

Age-related macular degeneration
Epiretinal membrane and vitreomacular traction
Posterior vitreous detachment
Macular hole

Diagnosis
Tests used for diagnosis of Metamorphopsia mostly make use of subjective assessments of how a person views regular patterns. Many of these tests have a poor ability to accurately diagnose or identify a person with the disease (i.e.,poor sensitivity). The use of assessments such as a psychophysical test called preferential hyperacuity perimetry, which assesses a person’s ability to any misalignments of visual objects, may permit a more sensitive diagnosis of Metamorphopsia. Treatment and Prognosis
Metamorphopsia is a symptom of several common retinal and macular diseases, therefore treating the underlying disorder can improve symptoms. For people who have conditions such as Epiretinal membrane (ERM), Macular Holes and Retinal Detachment, decreased metamorphopsia is associated with an increase in visual acuity. Quantitative evaluation of metamorphopsia is an important step in understanding visual functions of individuals with macular disorders and is an essential tool for physicians in evaluating treatment results. Types
Dry (non-exudative, > 80%)—deposition of yellowish extracellular material in and between Bruchs membrane and retinal pigment epithelium (“drusen”) with gradual loss in vision.Wet (exudative, 10–15%)—rapid loss of vision due to bleeding secondary to choroidal neovascularization. Etymology
Gk, meta + morphe, form, opsis, sight

See also
Dysmorphopsia
Hallucination


== References ==