At a tensional or constructive boundary the plates [Plates: Large areas of the Earth's crust that move slowly on the upper part of the mantle, often colliding and moving away from each other. ] are moving apart. The plates move apart due to convection currents [convection currents: Radioactive decay in the core of the Earth causes hot circular convection currents to rise and cold currents to fall. These currents cause the motion of tectonic plates. ] inside the Earth.
As the plates move apart (very slowly), magma [magma: Magma is molten rock. ] rises from the mantle. The magma erupts to the surface of the Earth. This is also accompanied by earthquakes.
When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock. This process is repeated many times, over a long period of time.
Eventually the new rock builds up to form a volcano. Constructive boundaries tend to be found under the sea, eg the Mid Atlantic Ridge. Here, chains of underwater volcanoes have formed along the plate boundary [plate boundary: The region where two or more tectonic plates meet. It is a zone of intense seismic activity. ]. One of these volcanoes may become so large that it erupts out of the sea to form a volcanic island, eg Surtsey and the Westman Islands near Iceland.
The diagram below to see how magma pushes up between the two plates, causing a chain of volcanoes along the constructive plate boundary.
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