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(with sticky lava and explosive violence) forming where two plates collide.
The nature of volcanic
activity
Trying to understand volcanoes is difficult
because of the complex way in which they
behave. A volcanic eruption may involve
rivers of lava or violent explosions. The
same volcano may, on one occasion, erupt
lava and, the next time, send up towering clouds of ash and gas and no lava, or be explosive at the start and change to producing lava at the end.
Volcanoes are very unpredictable not just in the materials they spit out, but in how often they erupt. They may erupt every few days, every few years, or every few centuries. This not only makes it difficult to know how the volcano will behave in the future, but also whether it has become inactive. Volcanoes that have erupted within the last ten or fifteen years are generally called active volcanoes; those that have not erupted for decades, but whose lava and ash is still relatively fresh, are thought of
At spreading boundaries the splitting of the crust provides an opportunity for magma to well up to the surface and spread out from fissures rather than from single vents.
(Below) The convection cells in
the upper part of the earth could be the cause of crustal movement. Volcanoes appear near both spreading and colliding boundaries.
The volcanoes that erupt at this boundary are usually explosive.
Tectonic plate moving away from spreading boundary and between the rising and sinking parts of the convection cell
Convection currents
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Sinking current