A convergent plate margin is where “plates moving in opposite directions meet, and the result of the collision normally is a vast crumpling of the edges as one plate subducts under the other.” (McKnight, 1999, p.391). When a plate is subducted it is forced under the plate it is colliding with and into the asthenosphere below. At converging margins between continental and oceanic lithosphere it is usually the oceanic crust which is subducted. This is because “oceanic plate is comparatively thin and dense, in contrast to the thick, buoyant continental plate”. (Strahler and Strahler 2003, p408). Once the edge of the plate has started to be subducted gravity starts to have an influence “the descending plate is cooler and therefore denser than the surrounding hot, soft asthenosphere. As a result, the slab “sinks under its own weight “, once subduction has begun.” (Strahler and Strahler 2003, p408).
Specifically it is the layer of sediments lying on top of oceanic crust, which is superheated and melted by the surrounding asthenosphere, that becomes magma. Because this magma is less dense than the surrounding material it tends to rise, which results in the formation of a magma chamber and ultimately a volcano. (Strahler and Strahler 2003)
The magma that is formed from the melting of these sediments and the lithosphere results in a volcano arc being formed on the continental plate. Monroe and Wicnder (1992, p364) illustrate this. “This magma rises beneath the overriding continental plate and can extrude at the surface, producing a chain of andesitic volcanoes (also called a volcanic arc).”
The Andes Mountains are an excellent example of a volcanic mountain arc. They are the resulting volcanic mountain chain on the overriding plate, in this case the South American plate under which the Nazca plate is being subducted. (Monroe and Wicnder, 1992).
“Where oceanic crust collides with oceanic crust an island arc may be formed above a subduction zone”. (Selby, 1985, p54). Again, Monroe and Wicnder (1992) effectively explain the processes leading to the formation of a volcanic island arc;
As the subducting plate descends into the asthenosphere, it is heated and partially melted, generating a magma, commonly of andesitic composition. This magma is less dense than the surrounding mantle rocks and rises to the surface through the nonsubducting or overriding plate where it forms a curved chain of volcanoes called a volcanic island arc.
Sea-floor spreading is also related to volcanic activity but in a very different way to subduction zones. This theory originated to explain the presence of oceanic ridges and was most notably propounded by the American oceanographer Harry Hess. (Mcknight 1999, p388). Sea-floor spreading occurs at divergent plate boundaries. It is generally accepted that the main driving force behind sea-floor spreading is convection currents in the mantle “stirred by local thermogenesis and heat conduction from the core.” (Smithson et. al. 2002). This is agreed by Davies (1998, cited in Cañón-Tapia and Walker, 2004) “In divergent-plate boundaries vertical movements of mantle are likely to be related to large-scale convective motions”. Smithson et. al. (2002, p190) propose that this results in several processes that ultimately cause sea-floor spreading. Firstly the viscous drag or friction of the convecting mantle pulls the oceanic lithosphere apart. The mantle plume leads to the development of a thermal bulge in the lithosphere. This means that gravity then acts on the oceanic lithosphere either side of the fissure and drags them apart, a process called slab pull. This is acting in conjunction with ridge push, where the formation of new lithosphere at the ridge is pushing the crust apart.
Mid-ocean ridges are associated with incredibly intense volcanic activity. This is highlighted by Thouret (1999) “Recent investigations confirm that the ocean floors, in particular the mid-ocean ridges, are home to over 60% of the Earth's volcanoes”.
Mid-ocean ridges are also themselves the result of the volcanic activity associated with sea-floor spreading. Lutgens and Tarbuck (2002, p127)
Here as the plates move away from the ridge axis, the fractures created are immediately filled with molten rock that oozes up from the hot asthenosphere. This hot material cools to hard rock, producing new slivers of seafloor. In a continuous manner, successive plate spreading and upwelling of magma add new oceanic crust (lithosphere) between the diverging plates.
This mechanism is called sea-floor spreading.
The type of volcanic activity that occurs at mid-ocean ridges is very different to that associated with convergent plate margins. Basaltic lava is extruded much in the same way as volcanic eruptions occurring at hot pots such as Hawaii. Formations include pillow lavas, lava tubes and sheet lava flows. (Monroe and Wicnder, 1992, p327)
There is rapidly increasing understanding in some areas as I found when cross-examining sources. When looking at Selby from 1985 he only mentioned one driving force for sea-floor spreading. Presumably this is because no other explanations had been discovered or hypothesised. This was however probably the accepted view at the time. Compare this to Smithson et. al. from 2002 which explains how one force alone could not possibly be responsible for sea-floor spreading as was once believed by Selby and others. He goes on to give a more detailed and thorough explanation of the processes behind sea-floor spreading.
