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A volcano is a rupture on the crust of a planetary mass object, such as the Earth, which allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earth's volcanoes occur because the planet's crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in the Earth's mantle.[1] Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together. Volcanoes can also form where there is stretching and thinning of the crust's interior plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism.[2] Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere (or troposphere); however, they also absorb heat radiated up from the Earth, thereby warming the upper atmosphere (or stratosphere). Historically, so-called volcanic winters have caused catastrophic famines. Active volcanoes in the Philippines, as categorized by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), include volcanoes in the country having erupted within historical times (within the last 600 years), with accounts of these eruptions documented by man; or having erupted within the last 10,000 years (holocene) based on analyses of datable materials. However, there is no consensus among volcanologists on how to define an "active" volcano. As of 2012, PHIVOLCS lists 23 volcanoes as active in the Philippines, 21 of which have historical eruptions and two strongly fumarolic volcanoes - Cabalian and Leonard Kniaseff.[1][not in citation given]; the Smithsonian Institution's Global Volcanism Program categories 20 Philippine volcanoes as "historical" and 59 as "holocene".[2] The Smithsonian Institution's Global Volcanism Program (GVP) list volcanoes with historical, Holocene eruptions, or possibly older if strong signs of volcanism are still evident through thermal features like fumaroles, hot springs, mud pots, etc.[3] GVP currently lists 50 Philippines volcanoes.[2] The eruptions from the table below were based more on GVP which gives a more detailed eruption history for a particular mountain. The frequency of Historical Eruptions are based on definite historical eruptions and excludes questionable or Uncertain accounts based on the two sources mentioned. The list below shows 25 active volcanoes in the Philippines was based more on the PHIVOLCS list with some included from the GVP list. The number is not a definite number and could depend on someone's definition of "active" or historical timeframe. Also, volcanoes listed as inactive or potentially active could renew activity after long periods of dormancy such as Mount Pinatubo in 1991.