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Issue 6 - Greenhouse Gases, Global...
Analysis 1

The Eruption of Mt. Pinatubo: Volcanoes and the Oceans

There are a number of anti-environmentalists who assert, among other things, that the impacts of climate change and global warming are fabrications of extreme environmentalism. Media personality Rush Limbaugh repeatedly asserts that natural phenomena like volcanic eruptions emit pollutants in volumes such as to dwarf those produced by human activity, thus "proving" that fears of global climate change are fabricated.

One way to evaluate the basis for that claim is to assess the impact on the earth's atmosphere of the eruption of a large volcano.

Although there are hundreds of active volcanoes worldwide, most erupt only intermittently. A few massive eruptions have occurred during the past several thousand years, like Mt. Mazama in Oregon, whose remains make up Crater Lake National Park. Once every few hundred thousand years, a truly gigantic eruption occurs, producing deposits like California’s Bishop Tuff, with profound local and regional effects on climate and life.

Our concern here is with the potential long-term impact of volcanic eruptions. Do they in fact emit greenhouse gases and SO₂ in amounts to swamp those from human activities?

The eruption of Mt. Pinatubo in the Philippines was the second largest volcanic eruption of the twentieth century, dwarfing Mt. St. Helens in mass of emitted material. The 1980 eruption of Mount St. Helens had a Volcano Explosivity Index (VEI) of 5. (There have been 84 known eruptions of VEI=5 in the last 10,000 years). The eruption produced 1 billion cubic meters of ash and 10 million cubic meters of lava. The 1991 eruption of Pinatubo had a Volcano Explosivity Index of 6. (There have been 39 known eruptions of VEI=6 in the last 10,000 years). The eruption produced 10 billion cubic meters of ash. If any natural event could be targeted as a major source of CO₂ Pinatubo should be it.

Things you need to know:

Important conversions
1 cubic yard = 0.76 m³
1 ton = 2000 lb
1 metric ton (tonne) = 1000 kg approximately 2200 lbs
1 kg = 2.2 lb = 1,000 g
454 g = 1 lb.

The density (mass per unit volume) of volcanic material ranges from less than 1,000 kg/m³ for pumice and ash to nearly 3,000 kg/m³ for some basalts. A high average for Mt. Pinatubo is 2,000 kg/m³, since most of it was ash.

1 .
Assuming an average density of 2,000 kg/m³, how much did the entire eruptive mass weigh?

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2 .
Now, typically, gas content of volcanic magma ranges from less than 1/2% by weight to around 3% (Mason and Moore, 1982), so let's take 1% as a good conservative estimate for Pinatubo.

What is the weight of all the gas emitted by Pinatubo?

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3 .
The dominant volcanic gas the world over is H₂O, and usually the second most abundant is CO₂, with a ratio of about 95-99% H₂O to 1-5% CO₂. For Pinatubo, the U.S. Geological Survey estimates CO₂ content at 15% (Source: Gerlach, T.M., H.R. Westrich, R.B. Symonds. 1996. Pre-eruption Vapor in Magma of the Climactic Mt. Pinatubo Eruption: Source of the Giant Stratospheric Sulfur Dioxide Cloud in Newhall, C.G. and R.S. Punongbayan (eds.) 1996. Fire and Mud: Eruptions and Lahars of Mt. Pinatubo, Philippines. Philippine Inst. Vol. and Seismol., Quezon City and U. of Washington Press.et al., 1996).

What is the approximate weight of CO₂ from the Pinatubo eruption?

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4 .
The total anthropogenic (human-produced) CO₂ is approximately 7,700 million tons per year.
What percentage of this number is the calculated CO₂ content of the Pinatubo eruption?

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5 . On the basis of these calculations, do you conclude that volcanic eruptions dwarf human sources of CO₂? Discuss.

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6 . Go to http://pubs.usgs.gov/pinatubo/daag1/fig2.gif, and estimate the mass of SO₂ emitted by Pinatubo during the period covered by the graph.

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7 . Go to http://pubs.usgs.gov/pinatubo/daag1/fig4.jpg, and assess the amount of SO₂ emitted for the entire eruption.

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8 . Total U.S. anthropogenic SO₂ emissions are around 13 million tonnes a year. Compare the output of Pinatubo to America's human-generated output.

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9 . If volcanoes can put as much of some pollutants into the atmosphere as humans, is this an argument for ending our attempts to control pollution?

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10 . Go to http://www.giss.nasa.gov/research/intro/hansen_09/, and describe how volcanoes have affected terrestrial climate.

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11 . Go to http://www.geophys.washington.edu/tsunami/general/physics/other.html and http://www.pbs.org/wnet/savageearth/tsunami/index.html, and explain how volcanic eruptions can impact the oceans.

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