The following conversation is between Susan Russell-Robinson from the US Geological  Survey and Barbara Reynolds from USA Today . Q: Why , after 600 years of no activity , did Mount Pinatubo in Philippines erupt in 1991 ? A: Volcanoes  each have their own eruption styles . This volcano probably has a rule which makes it erupt in the order of every 500 to 1000 years , but a volcano in Hawaii seems to erupt every year , and some of the Alaskan volcanoes might erupt every 10 or 20 years . Q: So nothing caused it ? A: There's nothing out of the ordinary . If you were to take an ordinary calendar year , 50 to 75 or 80 volcanoes erupt around the world every year . There are 20 to 30 volcanoes every month that show signs of unrest . That might be a full-blown eruption or a whole host of activities like that . Q: What is "the ring of fire" ? A: If you look at where active volcanoes are placed around the world , there are somewhere between 500 and 600 of them . There is what appears to be almost a necklace that goes around the Pacific Ocean . It makes a ring where 60% of the world's volcanoes lie . Q: Why such a concentration there ? A: That's based on a theory that the oceans and the continents are like separate pieces . When continents and when that happens it seems to produce magma at depth and then you have volcanoes in the same ring . Barbara Reynolds' main purpose here is    _    .
to show how dangerous volcanoes are to the world to learn what signs a volcano gives us before its eruption to warn the world of the existence of "the ring of fire" to introduce some general idea of volcanoes to the public