The earth is made up of tectonic plates, these push and pull away from each other. When these plates meet, one of them will slide under the other and melt. The melted rock then travels to the surface of the earth and the hot magma (lava) erupts. The land rises up when this happens and a volcano is formed. If you live above the place where the two plates meet, like Japan, there are likely to be volcanoes or earthquakes quite often.
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
According to the now generally accepted "plate-tectonics" theory, scientists believe that the Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called "plate-boundary" volcanoes. However, some active volcanoes are not associated with plate boundaries, and many of these so-called "intra-plate" volcanoes form roughly linear chains in the interior of some oceanic plates. The Hawaiian Islands provide perhaps the best example of an "intra-plate" volcanic chain, developed by the northwest-moving Pacific plate passing over an inferred "hot spot" that initiates the magma-generation and volcano formation process. The peripheral areas of the Pacific Ocean Basin, containing the boundaries of several plates, are dotted by Nearly 80% of the earth’s volcanoes that form the so-called "Ring of Fire." The "Ring" provides excellent examples of "plate boundary" volcanoes, including Mount St. Helens. The oceanic crust is subducting, or plunging beneath, the continental crust in this region.
