Why Do We Have Volcanoes?

So the earth can release vented up stress

The farther you go down from the surface of the earth the hotter it gets,  When you go down far enough, the earths materials (rock, sand, dirt, etc.) are so hot they are molten or liquid.  This heated molten rock can build up very high pressure like water boiling in a pot with a lid on it.  When the pressure gets great enough, some of this molten rock (magma or lava) pushes up at a weak point and relieves the pressurie by blowing a hole up to the surface and venting into the atmosphere.  Sometimes this occurs along the ocean floor and sometimes on land.  When it occurs in the same weak spot time after time, the cooling lava that remains on the survace and builds up creates what we call a volcano.  This process is what created the Hawaiian Islands, Burmuda, etc.  There is some corelation between volcanic eruptions and earthquakes which are also caused by underground pressure buildup.  Earthquakes that have their epicenter (point of the most force) under the oceans are what causes tidal waves or Tsunamis.  Hope this explains it for you.
Winston1