Another possibility (and not at all mutually exclusive) is that we are not being told about most of the volcanic eruptions that are occurring. Think about it. There are 3 million volcanoes on Earth, but land only covers 30% of the surface area. In other words, most volcanoes are underwater, yet we hear about VERY FEW of those - generally only when they breach surface with eruptions or when they create a new island. So if there are 50 volcanoes erupting that we know about then there may very well be another HUNDRED that are erupting underwater.