David Oebker, age 11, of Visalia, California, for his question:
Why are most volcanoes near coastlines?
Geographers made this clever observation long ago when they began viewing the world map as a whole. Later, earth scientists were able to relate this placement to the basic processes of global mountain building.
Way back around 200 B.C. Eratosthenes guessed that our planet was round and figured a way to measure its circumference. He did so without traveling far from home, and his figure was accurate within a mere 50 miles. Metals and minerals were classified in the Middle Ages, and 18th Century geologists learned how to interpret the rocky record of the earth's restless past. This groundwork set the stage for the planetary viewpoint of modern earth scientists. Modern scientific method tackles a major problem not only on one front, but on many. The study of volcanoes is approached from geology and geodesy, mineralogy, and even astronomy. And geography supplies the necessary global picture from every angle.
Most active volcanoes are located within a global pattern along the mountainous coasts of major continents. This association of volcanoes with mountain chains provides clues to the causes of volcanoes and adds as well to our global understanding of the earth's crust. Modern earth scientists see the crust as a thin, lightweight skin. With the mantle layer below, it seals the denser, hotter interior somewhat like the cap on a pop bottle. The global cap, however, seems unable to do a perfect job of sealing the mighty forces of heat and pressure that exist in the interior.
Most experts suspect that the internal forces rise and fall somewhat like the up and down convection currents in a boiling pot. In certain zones they well through the mantle and disrupt the crust, warping and cracking it in weak faults thousands of miles long. Here mountains are made with shuddering earthquakes and seething outpourings of volcanic lavas. The major zone of crustal weakness follows the islands and coastlines around the Pacific. This is the Ring of Fire where most volcanoes are active. Another weak zone underlying the Mediterranean reaches eastward through the lofty young mountains of Asia and joins the Pacific zone.
Volcanoes and earthquakes go with mountain making. And young mountains are still restless along the weak crustal slabs around the Pacific coastline. A smaller percentage of volcanoes are active in submarine mountains. In the massive Mid Atlantic Ridge they erupt in Iceland, around the Azores and other ocean islands. The Hawaiian Islands were built by volcanoes from faulted crustal blocks in the seabed 30,000 feet below the waves.
The basic causes of volcanoes seem to be the heat and pressure of the earth's interior and a crustal zone too weak to seal it within. Most of that internal heat is thought to come from buried radioactive minerals. These materials decay at fixed rates and finally cool down. Some of those originally trapped below are already cold. But internal supplies of uranium, thorium, and potassium are expected to yield heat through another five billion years.