According to the plate tectonic theory, there are three principal zones of volcanic activities: (i) spreading centers, (ii) subduction zones, (iii) regions within the plates themselves. Let us first study the origin of magma and its relations to plate tectonics.
Origin of Magma:
- Basaltic Magma: Most basaltic magma is believed to originate in partially molten asthenosphere at depths exceeding 200 kilometers. Since magma is lighter is lighter than the surrounding rocks, it has a tendency to rise.
- Andesitic magma: Andesitic magma, which is of intermediate composition is believed to originate mainly by the process of differentiation of basaltic magma. However, the close association of andesitic magma with the subduction zone indicates that it may have formed due to melting of the lithosphere. Lithosphere contains layers of sediments over basalt. It can yield a magma of andesitic composition on partial melting.The volcanic Andes Mountains of South America are thought to ha ve formed in this way.
- Rhyolitic Magma: Rhyolitic lavas erupt only from volcanoes located on continental crust. This magma is believed to have formed due to remelting of the continental crust.
Zones of Volcanic Acitivites:
- Volcanic Activity at Spreading Centers: Greatest volcanic extrusion occur at mid-oceanic ridges where the seafloor spreading is active. As the lithosphere moves apart, basaltic magma rises from the asthenosphere and fills the gap. Many successive eruptions, sometimes produce a volcanic cone which may rise above sea level.
- Intraplate volcanic activity: Volcanism that occurs on continents away from plate boundaries is the most difficult to explain. Some of the volcanic activities that have occurred within a continent may be associated with a spreading center, which is in the most initial stage of growing. The rift zone in Africa is considered to be the beginning stage of such a breakup.
- Volcanic activity at Subduction Zone: In convergent zones, oceanic crust is being subducted and melted. The andesitic magma thus generated moves upward and builds a volcanic arc adjacent to the trenches.This chain of volcanoes subsequently develops into and island arc.
The present day island arc systems contain igneous rocks of mostly andesitic to granitic composition. Magma of this composition often produces explosixe eruptons. Hence, some of the most violent volcanic activity has been associated with island arc systems.
The composition of the intermediate igneous rocks that form an island arc, resembles closely with the average composition of continental crust.Thus a developing island arc is a site where new continental crust is generated from the material of asthenosphere. This idea can be explained as follow.
- Partial melting of asthenosphere produces basaltic magma. On solidification it forms new oceanic crust at the mid-oceanic ridges.
- The newly formed crust is continuously pushed away by the process of seafloor spreading. Ultimately, it reaches a trench where it is subducted and partially melted thereby producing the felsic magma.
- The felsic magma rises up and forms island arc systems.
Since the continental rocks are less dense than the underlying material they remain afloat for ever. This mean that continental crust appears to be growing larger at the expense of oceanic crust.