An explosion occurs when a large amount of energy is released into a small volume of area in a very short time. The energy released comes in many forms, including chemical (artificial explosives), nuclear and hydrothermal (volcano eruptions).
Burning very rapidly, explosive material releases concentrated gas that expands quickly to fill the surrounding air space and apply pressure to everything in it. That is, it explodes in a blast powerful enough to blow away nearby trees, cars, buildings and anything else in its path [sources: Muller, Harris].
On the set of an action flick, blasts are often cultivated using nifty camera angles, CGI and miniature explosive devices, with notable exceptions like 2009 Oscar winner "The Hurt Locker" which used actual full-sized explosives. In real life, movie-type blasts require an explosive like C-4, TNT or gasoline, which cause explosions when they burn and mix with oxygen. C-4, for example, combines combustible material with plastic, often in a block form. The explosive is ignited using a detonator, which burns and causes the block to release a number of gases, including nitrogen and carbon oxides at a very fast rate and with a whole lot of force (the explosion) [sources: Harris, Failes].
And those fiery automobile explosions that are the centerpiece of any good car chase scene? More likely caused by strategically placed C-4 than an explosion in the gas tank. These days, cars on the street are equipped with highly durable gas tanks precisely to prevent them from detonating in the event of a crash. It is also important to remember that gas burns in vapor, but not liquid form. That means that, even if a car's tank were ruptured, the liquid gas in it would have to convert to vapor, mix with the air in the proper proportion, and be ignited in order to the car to blow up.
When an explosion of big screen proportions does happen, however, it engulfs the surrounding area in a heartbeat. Think you can outrun it? Read on, and then think again.