Run away, run away…..
This week marks the 23rd anniversary of the second most powerful volcanic eruption of the 20th century, that of Mount Pinatubo in the Philippines. Hot flows of burning ash flowed down the mountain after a series of huge explosions. The results were hundreds of fatalities, a hundred thousand homeless, heavy destruction suffered by the US bases at Clark (later abandoned) and Subic bay, some terrifying incidents with airliners losing all engines and plummeting thousands of metres before successfully restarting and the huge surprise that an unknown mountain that wasn’t even known to be an active volcano could wreak such destruction. The eruption also left a legacy of lahars (heavy mudflows that flow down river valleys destroying everything in their path) from the combination of tropical and typhoon rainfall and very fine ash.
It also marked a triumph in eruption prediction, and timely evacuation saved many thousands as the activity grew between the 7th and 15th of June, culminating in this massive explosion. Just for extra luck, the eruption coincided with a typhoon, and the combinations of ash and rain collapsed many roofs for tens of kilometres around, causing most of the fatalities. Before the warning signs started, the peak was heavily eroded, obscured by forest and most people didn’t even realise it was a volcano, let alone an active one. Those that did thought it extinct, as it had not erupted for half a millennium.
It all started with a magnitude 7.8 earthquake eleven months before, followed by a growing crescendo of smaller quakes from April 91, as magma rose towards the edifice from its chamber some 30km down. This provoked some explosions and was accompanied by extensive degassing of smelly sulphur dioxide. Between June 7 and 12, the first magma breached the surface, and a lava dome started to grow, but the pressure was building ever higher below, until the first large explosive eruption on the 12th. During the final paroxysm on the 15th, an estimated 5 cubic km (90% of the eruption products) of pulverised rock and lava exploded out, sending an ash cloud 35km up into the stratosphere, which was blown all over the island by the typhoon’s powerful winds. The ash cloud then circled the globe several times, being tracked by satellites on its circumambulations. Meanwhile, the crater collapsed like a piston into the evacuated magma chamber, creating a caldera 2.5km across.
Pyroclastic flows such as this one filled river valleys with 200 metres of fine ash, some of which was still at a temperature of 500 Celsius five years later, which will now slowly cool for decades. The eruption was rich in sulphur dioxide, which cooled the global climate very effectively for a few years by around half a degree (C), masking the effects of an El Nino event. Many people remain displaced to this day by this nine hour culmination, and many areas that were once fields remain buried by lahar or ash deposits.
Luzon island includes a volcanic arc, a chain of volcanoes called the Zambales mountains, in which the magma results from the subduction of the Eurasian plate at the Manila trench. Pinatubo 1 grew around a million years ago, and its andesitic to dacitic lava covers the entire region. Several volcanoes in the area, including its modern incarnation are remnants of this long gone peak and its many satellite vents. The modern mountain was born in its largest recorded eruption some 35,000 years ago, which was estimated to have been five times larger than the 1991 event (one of the smallest recorded). Several large explosions at varying intervals have happened since then, and more will certainly do so in the future.
Image credit: Alberto Garcia