Impressive Pyrotechnics

""What an odd-sized explosion.""

- J.D., Scrubs, "My Unicorn"

Stuff Blowing Up on TV and movies are usually large, visually impressive fireballs, that appear to be fueled by gasoline, propane or another similar such fuel, even when the object in question has no right to explode at all, much less spectacularly. Sometimes they are (or include) a shower of sparks.

In reality, a fireball is often a minor part of an explosion (though not always, as is the case with incendiaries, like phosphorus, uranium, gelled fuels like napalm, etc). The real devastation from an explosion often comes from the force (overpressure) of the blast and the flying debris and shrapnel. Very rarely will explosions look like what they do in real life, an expanding cloud of dust and debris with very little light—and very little left behind. Those also can look visually impressive (since you see a huge cloud in just a second), but those can be difficult to film, while fireballs done in movies are a lot safer. Video games and animation have the excuse of all that debris being almost too much to draw or render.

The explosions also burn at a ridiculously slow rate, which conveniently enables one to Outrun the Fireball.

In addition, the more powerful the explosion, the more quickly it uses up or blows apart the reactants involved in any combustion, so there's an inverse relationship between how powerful the blast is and how much fire there is. Film pyrotechnicians favor big fireballs not only because they're more spectacular, but because they're low-powered and thus relatively safe to work with.

Volcanoes are often part of this. The lava is the scariest part shown in fiction (ironically with Convection, Schmonvection involved), but the lava usually stays in a (relatively) short range around the volcano and most flows are slow enough that they can easily be avoided by appropriately wary individuals. People in Hawaii live around lava spewers. The really devastating effects are the cloud effects such as pyroclastic flows and Plinian eruptions (both happened when Mt. Vesuvius erupted in 79 AD).

Oh, and fireballs in space. Perhaps some chemical mixtures can make fireballs in a near vacuum, but it's unlikely most space explosions are like that.

Occurs in 90% of all onscreen explosions.

Compare Koosh Bomb, Every Car Is a Pinto.

Anime & Manga

 * Tengen Toppa Gurren Lagann has explosions going even further over the top than the show takes most things. The very first thing you see after the opening credits? A galaxy blowing up. It sets the tone quite nicely.

Film

 * The DVD commentary for Dark Blue World (2001) mentions that the SFX man asked the director if he wanted a realistic look or huge fireballs for the scene where the airfield is bombed at night. He decided to go for the unrealistic visual effect.
 * At the end of I Am Legend, Robert Neville destroys the monster horde in the laboratory with a hand grenade which produces a large fireball.
 * In the 6th Kara no Kyoukai: film, The climax of Azaka's fight against Ouji consists of  detonating very colourfully. See here, at approximately 4:08.
 * Casualties of War.

Live-Action TV

 * Demonstrated in an episode of MythBusters, where they specifically compared blowing up a car with a Hollywood style explosion, and blowing it up properly. The Hollywood one caused an impressive fireball, and scorched the car while leaving it mostly intact. The proper one blew the car into pieces scattered over a wide radius.
 * Of course the proper one did have a very impressive fireball of its own. You just needed to watch in Slow Motion to really appreciate it.
 * They also tested whether tracer bullets + petrol tank = boom. The answer? Sort of, but not really an explosion, so much as just a fire. But a bad guy in a movie is unlikely to be using tracer bullets, especially since most movies take place in Los Angeles, and tracer bullets are illegal in California.
 * Because legality is always such a problem for those poor criminal types...
 * Well, to the producers it is...
 * Professional Wrestling must have an infinite budget supply for this...
 * The Scrubs episode "My Unicorn" parodies this when an explosion occurs after a toy airplane crash.
 * Any kind of monster in any tokusatsu.
 * Der Clown -- Payday:, so huge is the fireball.
 * Didn't from quarter to half of the series in any given season contain some or other fireworks?
 * Much beloved of Jeremy Clarkson on Top Gear, which earned him the ire of co-presenter James May when he set off fireballs while May was trying to film an ad for a tame diesel Volkswagen Scirocco.
 * Stargate SG-1 featured plenty of explosions, but happily mocked the need for bigger explosions with each season in one episode. No matter how large the special effects team made their fireballs, the director constantly demanded larger, requiring increasingly elaborate setups to get the desired effect.

Video Games

 * Star Fox 64 was great at this. Best examples include: Either ending of Katina (Bill comments on the fireworks if you destroy the Saucerer, but seeing it vaporize the Katina base is probably even cooler looking), Macbeth (where you get to blow up an enemy weapons facility by smashing a supply train into it), Bolse (the entire Bolse satellite blows up, and since it's described in the Guide as being planet-sized, this may also qualify for Earthshattering Kaboom), Area 6 (everything explodes in Area 6. Especially at the very end) and of course, Venom, where
 * Also, the underwater level, in which it is proven that giant clams are made of nitroglycerine.
 * Crysis is actually the ultimate king of Impressive Pyrotechnics. Any object that can explode will explode with incredible graphics effects. The pinnacle being
 * Surprisingly, the video game Borderlands subverts this trope, as the grenades and rockets the player acquires behave like normal, boring, Real-Life explosions.
 * Er, that is, until you pick up 'Grenade M.O.D.s', which add features like Incendiary, Shock, and Corrosive damage to your otherwise normal frag grenades. Which causes them to explode in Spectacular Fireballs, Static Discharges, and Chemical Plumes, respectively.

Multiple media

 * There are a lot of science fiction space battles that are just full of this.
 * Almost any fictional appearance in any media of dynamite will be portrayed with massive fireballs.