Forbidden Zone

The film begins on "Friday, April 17" at 4 pm in Venice, California. Huckleberry P. Jones, local pimp, narcotics peddler and slumlord, enters a vacant house that he owns. While stashing heroin in the basement, he stumbles upon a mysterious door and enters it, falling into the Sixth Dimension, from which he promptly escapes. After retrieving the heroin, he sells the house to the Hercules family. On their way to school, Frenchy Hercules and her brother Flash have a conversation with Squeezit Henderson, who tells them that, while being violently beaten by his mother, he had a vision of his transgender sister RenÃ©, who had fallen into the Sixth Dimension through the door in the Hercules' basement.
Frenchy returns home to confide in her mother, and decides to take just a "little peek" behind the forbidden door in the basement. After arriving in the Sixth Dimension, she is captured by the perpetually topless Princess, who brings Frenchy to the rulers of the Sixth Dimension, the midget King Fausto and his queen, Doris. When the king falls for Frenchy, Doris orders their frog servant, Bust Rod, to lock her up. In order to make sure that Frenchy is not harmed, Fausto tells Bust Rod to take Frenchy to Cell 63, where the king keeps his favorite concubines (as well as RenÃ©).
The next day at school, Flash tries to convince Squeezit to help him rescue RenÃ© and Frenchy. When Squeezit refuses, Flash enlists the help of Gramps instead. In the Sixth Dimension, they speak to an old Jewish man who tells them how to help Frenchy escape, but they soon are captured by Bust Rod. Doris interrogates Flash and Gramps and then lowers them into a large septic tank. She then plots her revenge against Frenchy, relocating all the denizens of Cell 63 to a torture chamber. She leaves the Princess to oversee Frenchy's torture and execution, but when a fuse is blown, the torture is put on hold and the prisoners from Cell 63 are relocated to keep the King from finding them.
After escaping the septic tank, Flash and Gramps come across a woman who tells them that she was once happily married to the king, until Doris stole the throne by seducing her, "even though she's not my type". The ex-queen has been sitting in her cell for 1,000 years, and has been writing a screenplay in order to keep her sanity. Meanwhile, Pa Hercules is blasted through the stratosphere by an explosion caused by improperly extinguishing his cigarette in a vat of highly flammable tar during his work break at the La Brea Tar Pit Factory. After re-entry, Pa falls through the Hercules family basement and into the Sixth Dimension, where he is imprisoned.
Finding a phone, Flash calls Squeezit and again asks for his help. Finally, Squeezit agrees to go into the Sixth Dimension to help rescue Frenchy and RenÃ©. There, he is captured by Satan, with whom he makes a deal to bring him the Princess in exchange for Satan's help freeing RenÃ© and Frenchy. Squeezit accomplishes this task, but has failed to include himself in the deal to rescue his friends, and the devil has him decapitated. Queen Doris sends Bust Rod to keep an eye on the king, and to ensure he doesn't find out where she's hidden Frenchy.
Fausto catches Bust Rod and forces him to lead him to Frenchy and RenÃ©, whom he orders to leave the Sixth Dimension to avoid the Queen's wrath. However, en route to safety, RenÃ© is stricken with pseudo-menstrual cramps, and they are again captured by the frog. Squeezit's head, which has now sprouted chicken wings, finds the king and informs him of what has happened.
While preparing to kill Frenchy, Doris is confronted by the ex-queen, and the two engage in a cat-fight, with Doris eventually coming out as the victor. Just as she is about to kill Frenchy, Fausto stops her, explaining that Satan's Mystic Knights of the Oingo Boingo are holding the Princess hostage, and will kill her should anything befall Frenchy. Flash and Gramps arrive, and Flash is knocked down by Gramps. Ma Hercules enters and, seeing a seemingly dead Flash, shoots Doris. Fausto mourns Doris, then marries Frenchy.
The surviving characters look toward a great future as they plan to take over everyone and everything in the Galaxy.

Question: What caused the explosion at the La Brea Tar Pit Factory?
Answer: Extinguishing a cigarette in a vat of tar.