A large open pit blast releases an enormous amount of stored chemical energy, and it is over in less than a second. What looks like one event is actually hundreds of separate detonations, sequenced to the millisecond. Rock breaks in two stages. - Detonation front: the explosive column reacts at 3,000 to 6,000 m/s, generating borehole pressures in the gigapascal range. The shock wave crushes rock immediately around the hole, then radiates outward. Rock is roughly ten times weaker in tension than compression, so much of the fracturing happens when that wave reflects off a free face and returns as a tensile pulse. - Gas expansion: microseconds later, the detonation products expand into the new cracks, wedge them open, and heave the broken mass forward into a muckpile. Design controls everything. - Burden is the distance from the hole to the nearest free face. Too large and the charge is confined, producing oversize rock and heavy ground vibration. Too small and gas vents early, throwi...