It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. How do volcanologists measure how big an eruption is? There is not any single feature that determines the "bigness", but the following eruption magnitude scale - called the Volcanic Explosivity Index or VEI - is based on a number of things that can be observed during an eruption. Merapi, one of Indonesias most active volcanoes, lies in one of the worlds most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. Every year about 60 volcanoes erupt, but most of the activity is pretty weak. is a Philippine national institution dedicated to provide information on the activities of volcanoes, earthquakes, and tsunamis, as well as other specialized information and services primarily for the protection of life and property and in support of economic, productivity, and sustainable development.