Factual error: The entire time the volcano is shown erupting, the volcanic lava is instantly melting everything it touches despite the fact that in real life volcanic lava would not have any human being melting because lava is more dense than water. Also lava in real life cannot instantly melt things. And no object should be sinking if caught in a lava flow.21collaw
Factual error: The volcanic ash that can be seen throughout the film is too big to be real. The ash shown is something like 1-2 cm in diameter. Volcanic ash is not that big in real life; it is not more than 2 millimetres in diameter. (01:02:30)A Demon
Factual error: It is impossible for a volcano to form under Los Angeles. The San Andreas Fault traveling through the west coast is a "Transform" fault in which the Pacific plate and the North American plate creates a fault line that only slide past each other. In order for a volcano to form, two plates need to subduct. Because there is no ripping apart or subduction taking place along a transform fault, there isn't any magma formation to lead to volcanoes.
Factual error: Mount Wilshire is basically a shield volcano in the form of a tall stratovolcano because shield volcanoes only shoot ashes, gas, and lava (and pyroclastics for pyroclastic shield volcanoes). The lava itself behaves as if it was sentient. During the lava geyser scene, small particles of molten lava seem to be falling directly on people despite the fact that no-one was anywhere near it except for Mike, his daughter, and the little boy.21collaw
Factual error: During specific scenes, volcanic gas is shown to have vaporized several city workers entirely, burned half a workers' head to the point where it appeared coal black, steamed rats, and vaporized Amy's friend. Volcanic gas in real life could burn a subject, but not instantly vaporize them like it did in the movie.21collaw