The lava was primarily made of methylcellulose, the thickening agent used in fast-food milkshakes.
The ash was made mostly of ground newspaper.
(at around 23 mins) Rachel mentions an incident in Paricutin. She's referring to the 1,500 foot (457.2 meters) volcano that emerged over a few months in 1942 in Paricutin, Michoacan, Mexico.
This was the second movie of 1997 based on volcanic eruption; the first one, Le Pic de Dante (1997), is considered to be a more accurate depiction of volcanic eruptions to happen in nature. Dante's Peak was made on an accelerated shooting schedule in order to reach cinemas first, and had a better box office performance as a result. This almost became a trend in following years: big studios often had competing summer blockbusters about similar subjects, e.g. in 1998, both Deep Impact (1998) and Armageddon (1998) dealt with asteroids threatening to hit Earth.
An elaborate miniaturized portion of the doomed city, which was destroyed as the lava flowed, was constructed inside hangar number one at the San Bernardino International Airport.