Operation Fiery Vigil

Operation Fiery Vigil was the emergency evacuation of all non-essential military and United States Department of Defense civilian personnel and their dependents from Clark Air Base and U.S. Naval Base Subic Bay during the June 1991 volcanic eruption of Mount Pinatubo in the Philippines. This noncombatant evacuation operation (NEO) resulted in the transfer of roughly 20,000 people from Clark Air Base and U.S. Naval Base Subic Bay back to CONUS, by way of Cebu, Philippines. The Commanding General, 13th USAF, was in command of the Joint Task Force.

Timeline of events

 * 16 July 1990: a magnitude 7.8 earthquake struck the island of Luzon, Philippines. The epicenter was near the town of Rizal, Nueva Ecija, roughly 60 kilometers from Mount Pinatubo. This earthquake caused a landslide, some local tremors, and a brief increase in steam emissions from a preexisting geothermal area at Mount Pinatubo.
 * March–early June 1991: magma rising toward the surface from more than 32 kilometers beneath Mount Pinatubo triggered small earthquakes and caused powerful steam explosions that blasted three craters on the north flank of the volcano. Thousands of small earthquakes occurred beneath Pinatubo, and many thousands of tons of sulfur dioxide gas were emitted by the volcano.
 * 7 June 1991: the first magmatic eruptions took place, resulting in the formation of a 660 ft high lava dome at the summit of the volcano.
 * 10 June 1991: after receiving final authorization from the Secretary of Defense, all non-essential military and Department of Defense civilian personnel and their dependents initiated land evacuation from Clark Air Base at 0600 local time. This land evacuation brought an estimated 15,000 personnel and several thousand vehicles onto U.S. Naval Base Subic Bay.
 * 12–14 June 1991: several waves of eruptions generated eruption columns up to 80000 ft in altitude and pyroclastic flows (high speed avalanches of superheated gas and tephra) extending out to 4 kilometers from the summit. These eruptions were accompanied by nearly continuous seismic activity and expulsion of huge quantities of ash, tephra, and volcanic bombs.
 * 15 June 1991: the major eruption of Mount Pinatubo occurred, sending ash and tephra over 100000 ft into the air. Clark Air Base and Subic Bay Naval Station, the two largest U.S. military bases in the Philippines, were heavily damaged by ash from this volcanic eruption. Nearly one foot of ash accumulated on both Clark Air Base and U.S. Naval Base Subic Bay. Many buildings collapsed under the weight of the accumulated ash, and all flight operations were suspended at both bases for many days or even weeks afterwards.

Aftermath
The 1991 Plinian/Ultra-Plinian eruption of Mount Pinatubo was the second largest terrestrial eruption of the 20th century (surpassed only by the 1912 eruption of Novarupta), and the largest eruption in living memory. The eruption produced high-speed pyroclastic flows, giant lahars, and a cloud of volcanic ash hundreds of miles across. 20 million tons of sulfur dioxide and roughly 11 km3 of tephra are estimated to have been ejected in total, which corresponds to a Volcanic Explosivity Index (VEI) of 6. By contrast, roughly 4 km3 of material was ejected in the 1980 eruption of Mount St. Helens; this corresponds to a VEI of 5.

Very few of the estimated 20,000 who left the base ever returned. The vast majority were evacuated to Andersen Air Force Base, Guam and processed for return to the continental United States. This figure includes approximately 5,000 who were evacuated to Cebu City on the USS Midway (CV-41), USS Abraham Lincoln (CVN-72), USS Peleliu (LHA-5), and twenty other U.S. Navy ships of the task force.
 * 22 June 1991: a team of 11 engineers and utility systems specialists from Headquarters Pacific Air Forces (PACAF) and the 554th Rapid Engineer Deployable Heavy Operational Repair Squadron Engineers (RED HORSE) arrived at Clark Air Base to assess the damage caused by Mount Pinatubo to determine the fate of the base.
 * 12 July 1991, the United States Secretary of the Air Force announced that the U.S. Air Force would leave the Philippines no later than 16 September 1992.
 * 4 September 1991: a lahar, 20 ft to 40 ft high and almost 200 ft wide, smashed along the southern boundary of Clark Air Base, sweeping away a security policeman who was subsequently rescued.
 * 5 November 1991: the Secretary of the Air Force visited Clark Air Base and paid tribute to the "Ash Warriors", the personnel who had remained throughout the volcanic activity and cleanup.
 * 26 November 1991: the American flag was lowered for the last time by the Ash Warriors, and Clark Air Base was turned over to the government of the Philippines, ending over 90 years of US presence at the base.