Although Pompeii is neither in the earthquake zone nor in the world’s "Ring of Fire," earthquakes and volcanic eruptions can, and do, occur outside those zones. Vesuvius had erupted before 79 A.D. (notably in 5960 and 3580 B.C.) It has erupted several times since (in 1631, 1779, 1906 and 1944).
Using Pliny’s letters with evidence from subsequent Vesuvius eruptions and similar volcanic explosions like Mt. Pinatubo and Mt. St. Helens, we can piece together how Pompeii was destroyed. And, using the ruins themselves as an incredible living museum, we can see what was left when Vesuvius froze Pompeii in time.
Pliny’s mother first noticed an unusual cloud between 2 and 3 in the afternoon. From their distance of 18 miles, or so, the cloud looked like "a pine tree" rising from a mountain. (They could not tell it was from Vesuvius.)
Some of the cloud was white, in other parts there were dark patches of earth and ash.
Fire was not shooting out of the mountain. That came later.
Monstrous smoke clouds dumped debris on Pompeii during the afternoon and early evening of August 24th. Dust, ashes, cinders, tephra and rocks fell on the town for about 8 hours. Heaps of tephrite (rocks) landing on houses caused roofs to collapse. Buildings began to crack diagonally from in-plane loading and shear deformation.
Pliny was the first person to describe the phenomenon scientists now call a "Plinian" event. It is a sustained, explosive eruption which generates a high-altitude column of volcanic material and blankets large areas with ash. The "pine tree" effect, described in Pliny’s letter, is called a Plinian column.
The driving force behind all of this activity is volcanic gas which pushes hot, molten material like magma (located below the earth’s surface) out of the mountain. What we first see are monstrous clouds of smoke and ash. (Follow the link to see an amazing USGS picture of one particle of volcanic ash as viewed through the lens of a scanning electron microscope.)