Herculaneum was buried under a huge mass of mud, approx 20m/60'. Pyroclastic surges hit
Herculaneum at temperatures around 400°C to 500°C. Because the hot ash and mud
enveloped the city instantaneously, oxygen was cut off, turning wood into stable charcoal
rather than burning it away. That fact makes Herculaneum completely different from Pompeii.