Iceland covers only about 103,000 kilometers and has about twenty-seven active volcanoes. Directly beneath, the American and Eurasian tectonic plates are measurably spreading apart.
The violent eruption that obliterated Krakatoa in 1883 was caused by different plate movements. Types of volcanic eruptions are determined by plate tectonics, a science established in the 1960s.
In his fascinating book Krakatoa (New York, Harper Perennial, 2003), Simon Winchester explains what led to that cataclysmic explosion. Krakatoa, a stratovolcano, was of a tall conical type prone by its nature to erupt suddenly and violently.
In the same dangerously volcanic region, Mount Toba in northern Sumatra blew about 74,000 years ago. This would have measured 8 on the Volcanic Explosivity Index now used to classify eruptions. (307)
Nearby Tambora, in Indonesia, exploded in 1815 with an explosivity index of 7 and Mount Taupo, in New Zealand, which has not erupted since 180 CE, is considered the third most violent. (308)
Clearly, not all volcanoes are created equal. Their ash can be different too. Lighter ash floats much further. Carried by the prevailing winds, it creates a miasma that lasts for days and weeks.
This happened when Eyjafjallajokull erupted in Iceland last May, causing a wave of airport closures across Europe. The recent crop of ash from Iceland's Grimsvotn was heavier and sank more quickly, so air traffic was not disrupted.