|MadSci Network: Astronomy|
It just so happens I was pondering a question very similar to this just yesterday. I am very glad to answer your question!
Meteorites do not generally explode. They can break apart, ablate into tiny dust particles, and land in one piece. Every once in a while, though, they do exhibit behavior that no one but an astronomer would fail to say was an explosion!
Recently a meteorite caused quite a stir in the Northeast of the USA. It created sonic booms, left a smoky trail in the sky, and some even said it levelled a cornfield! It turns out no cornfields were harmed in the making of the event, but it's easy to see how folks would think the thing that they saw in the sky could create a fire.
Most meteorites break apart or ablate away when they enter the earth's atmosphere. Ablation just means it heats up and flakes off. Some meteorites make it all the way to the ground, where they can leave a crater. The meteorites themselves tend not to be very hot...though the material projected from the impacting crater can be very hot! (There is a mineral called stishovite that is quartz that has been subjected to the high pressures and temperatures of a meteor impact!)
Here is an article from the excellent folks at NASA that talks about your
question and others:
Thanks for your question!
Steve E. Williams Science Demonstrator and Developer Pacific Science Center Seattle
I think what you are asking is, "How do meteors explode?" An object isn't a "meteorite" until it hits the ground.
When celestial objects enter the Earth's atmosphere they are strongly heated by friction. This not only causes them to heat up and glow, but often vaporizes parts of the object, forming a glowing ball of gas around the meteor (remember the re-entry scene in Apollo 13?). Such an object is visible crossing the sky. A meteor brighter than magnitude -4 (magnitude is a negative-logarithmic scale, like pH, so a negative magnitude is pretty bright) is called a fireball; here's a recent report from space.com.
I wasn't able to find anything specific to HOW meteors explode. But here's my educated opinion:
If the meteor is heated strongly enough, it will melt or break up, and that's what we see as an explosion. I don't think the meteor detonates, but it has been softened by heat to the point that it can't hold together any more, and it disintegrates into glowing bits that scatter across the sky. Comets are particularly prone to explosions. That's because they are loose agglomerations of rocky material held together by ice, and the violent heating caused by entering the earth's atmosphere is enough to make them explode rather violently. The explosion is a physical process caused by drastic and rapid heating, not a chemical explosion like a stick of dynamite.
For an account of a really spectacular explosion, probably of a comet, check out this website on the Tunguska explosion of 1908.
Try the links in the MadSci Library for more information on Astronomy.