Volcanologists rely on first-hand observations of volcanic eruptions to gain insights into how volcanoes work. Often these observations are made by individuals who are not trained scientists but who just happen to be in the right place at the right time. A good example of this is a series of letters that Pliny the Younger wrote describing the 79 AD eruption of Vesuvus and the death of his uncle, Pliny the Elder. Pliny the Younger was a young student who happened to be within sight of the volcano when it erupted. His letters are, in fact, the first written accounts of a explosive eruption.

Below are two excerpts from his letters. Read them, or watch the videos, and note the amount of detail and the kinds of features that he described. In subsequent exercises you will be making observations of explosive eruptions in much the same way as Pliny did in 79 A.D.

"Its general appearance can best be expressed as being like an umbrella pine, for it rose to a great height on a sort of trunk and then split off into two branches, I imagine because it was thrust upward by the first blast and then left unsupported as the pressure subsided, or else it was borne down by its own weight so that it spread out and gradually dispersed. Sometimes it looked white, sometimes blotched and dirty, according to the amount of soil and ashes it carried with it."

"The carts that we had ordered brought were moving in opposite directions, though the ground was perfectly flat, and they wouldn't stay in place even with their wheels blocked by stones. In addition, it seemed as though the sea was being sucked backwards, as if it were being pushed back by the shaking of the land. Certainly the shoreline moved outwards, and many sea creatures were left on dry sand. Behind us were frightening dark clouds, rent by lightning twisted and hurled, opening to reveal huge figures of flame. These were like lightning, but bigger....... It wasn't long thereafter that the cloud stretched down to the ground and covered the sea. It girdled Capri and made it vanish, it hid Misenum's promontory. Then my mother began to beg and urge and order me to flee however I might, saying that a young man could make it, that she, weighed down in years and body, would die happy if she escaped being the cause of my death. I replied that I wouldn't save myself without her, and then I took her hand and made her walk a little faster. She obeyed with difficulty, and blamed herself for delaying me.
Now came the dust, though still thinly. I look back: a dense cloud looms behind us, following us like a flood poured across the land. "Let us turn aside while we can still see, lest we be knocked over in the street and crushed by the crowd of our companions." We had scarcely sat down when a darkness came that was not like a moonless or cloudy night, but more like the black of closed and unlighted rooms. You could hear women lamenting, children crying, men shouting................ It grew lighter, though that seemed not a return of day, but a sign that the fire was approaching. The fire itself actually stopped some distance away, but darkness and ashes came again, a great weight of them. We stood up and shook the ash off again and again, otherwise we would have been covered with it and crushed by the weight. I might boast that no groan escaped me in such perils, no cowardly word, but that I believed that I was perishing with the world, and the world with me, which was a great consolation for death. At last the cloud thinned out and dwindled to no more than smoke or fog. Soon there was real daylight. The sun was even shining, though with the lurid glow it has after an eclipse. The sight that met our still terrified eyes was a changed world, buried in ash like snow."