They make a big kaboom. A rock a mile across (depending of course on the velocity) can deliver more pop that all the nuclear arsenals put into a pile and set off simultaneously. The Meteor Crater / Barringer Crater in Arizona was formed by a rock about 160 feet across (about 1/33rd of a mile, hence, other things being equal, about 1/35937 of a mile diameter rock, that's 33 in three dimensions). Boom.
The crater stretches 1200 meters across and 170 meters deep, with the rim of the crater rising 45 meters above the surrounding landscape. The area around the impact site is tinted red with oxidized iron from the nickel-iron meteor which impacted there almost 50,000 years ago. | University of Iowa
