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 Plate Tectonic Stories

Mount Pinatubo, Philippines

Mount Pinatubo

The Mount Pinatubo Eruption Column from Clark Air Base: ©  USGS

Mount Pinatubo in the Philippines is a modern day example of the highly explosive eruptions and deposits in a similar (but not exact) plate setting to that associated with Cwm Idwal. Mount Pinatubo is an active volcano found in the Zambales Mountains on the northern island of Luzon and is part of the Luzon island arc chain at a complex intersection of micro-plates between the Eurasian and Philippine plate. It is a subduction-related volcano, formed by the Eurasian Plate sliding under the Philippine Mobile Belt along the Manila Trench to the west. Molten material related to the complex tectonics associated with the subducting slab, rises through the lithosphere and generates the volcanism typical of subduction.

  Philippine Plate Tectonics
  The complex plate tectonics in the Philippines region: © Eric Gaba
The earliest volcanic activity at Mount Pinatubo is thought to have begun around 1.1 million years ago but then ended several tens of thousands of years before the start of this second, modern phase of activity which started around 33,000 BC, after a long period of dormancy. Eruptions at Pinatubo have a long history of being explosive and cataclysmic. Each eruption ejected large amounts of material and, like the eruption at Cwm Idwal, covered enormous areas with pyroclastic flow deposits. Pinatubo’s most famous eruption came in 1991, though this was one of the smaller ones in its modern eruptive history.  

The 1991 eruption of Pinatubo is an incredibly well documented one and is particularly famous for its successful evacuation of 58,000 people, due to the combined efforts of Philippine Geologists and the USGS. It was the second-largest volcanic eruption of the 20th century with a big impact on a very densely populated area. As with Cwm Idwal, the eruptive style at Pinatubo was explosive with a large volume of gas, bombs, ash, tephra (lumps of volcanic rock) and pyroclastic density currents (hot mixtures of gas and rock). In total, the eruption ejected more than 5 km³ of material. This quantity of volcanic rock dramatically changed the landscape of Luzon, filling some valleys while emptying others. Some sections of the volcano expanded, while the top collapsed into a 2.5 km diameter caldera. The deposits of the pyroclastic density currents (ignimbrites) were 200 m thick and similar to those seen in the Pitts Head Tuff of Cwm Idwal, though much thicker and more chemically weathered due to the tropical climate.

 The crater lake at Mount Pinatubo: © Neureiter

The gas output was equally as impressive. 18 million tons of sulfur dioxide was injected into the atmosphere, the cloud circling the globe several times. Sulfur dioxide is a short term global coolant because of the effect it has on our ozone layer. The quantity released at Pinatubo reduced global temperature by 0.5 °C for two years.  

Information on the 1991 eruption is easily accessible online and has far more on the human preparation and response to the event. Below are just a couple of many potential links:

USGS - The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines

USGS - Remembering Mount Pinatubo 25 Years On