Constructive: Plates move away from each other (N.American plate & Eurasian plate)
Features: Mid ocean ridges, Sea floor spreading & Rift valleys
Lava extruded is alkaline and free flowing (basaltic lava), volvanoes have gentle sides forming shiled volcanoes, vent does not block so eruptions are frequent and non-violent.
Destructive: 3 types (Oceanic subducted under continental - two continents meet creates fold mountains and marks the end of an ocean - two oceanic plates meet the older denser plate subducts.
Features: Deep ocean trenches, Deep focus earthquakes, Island arcs & Volcanoes
Lava reaches the surface through fissures in continental crust, it absorbs silica and becomes acidic and viscous, it is slow moving so forms steep sided cones (composite volcanoes) lava can solidify and block the vent forming a plug, pressure must build up before eruption e.g. Mount St. Helens.
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