Case Studies

 

 

Left:  Mt. Pinatubo eruption plume, July 1991, from Clark Air Base control tower.  Photo by J.N. Marso, U.S. Geological Survey.

The effects of several historic eruptions have been observed and the impacts of larger, prehistoric eruptions can be estimated.

 

Estimates of the fraction of sunlight transmitted through stratigraphic aerosols after major eruptions. Roza refers to a flood basalt eruption in the northwestern United States. Graph from Rampino and others (1988).

 

The pages in this section explore the following case studies for their impact on global climate

Toba, 75,000 years ago
Laki, Iceland, 1783
Tambora, Indonesia, 1815
Krakatau, 1886
El Chichon, Mexico, 1982
Mt. Pinatubo, Philippines, 1991

 

Impact of some major historic eruptions.

Eruption

VEI  (Explosivity Index)

Magma Volume (km3)

Column height (km)

H2SO4 aerosols (kg)

Northern Hemisphere temperature decrease

Laki, 1783

4

14-15

 

<1 x10 11

about 1.0

Tambora, 1815

7

>50

>40

2x10 11

0.4-0.7

Krakatau, 1883

6

>10

>40

5x10 10

0.3

Santa Maria, 1902

6

about 9

>30

<2x10 10

0.4

Katmai, 1912

6

15

>27

<2x10 10

0.2

St.Helens, 1980

5

0.35

22

3x10 8

0-0.1

Agung, 1963

4

0.3-0.6

18

1-2x10 10

0.3

El Chichon, 1982

4

0.3-0.35

26

1-2x10 10

0.4-0.6

Data from Rampino and Self, 1984.

 


Mt. Pinatubo, Philippines - 1991

Photo by R.S. Culbreth, U.S. Air Force.

Mt. Pinatubo eruption of June 12, 1991. Height of ash cloud about 12 miles (20 km).

The volume of the Mt. Pinatubo eruption was about 5 cubic km of dacite. It was the third largest eruption of this century. It produced the greatest volume of SO2 ever measured, 20 Mt, about three times more than El Chichon (McCormick, 1992). This gas reached the stratosphere and circled the globe in three weeks (Bluth and others, 1992).

SAGE II map of the distribution of stratospheric aerosols from the Pinatubo eruption between June 14 and July 26, 1991 (i.e., the period of the major eruptions). Note the nearly two orders of magnitude increase in the optical depth in the tropics. Image by Pat McCormick, Langley Research Center. Slides from EOS Slide Set #1.

Most of the early aerosol was limited to tropical latitudes of 30 degrees N to 20 degrees S. Temperatures in the stratosphere increased as much as 3.5 degrees C at some latitudes (Labitzke and McCormick, 1992). It is estimated that the gases caused a global temperature decrease of 0.5 degree C for about 2 to 4 years after the eruption.

 

El Chichon, Mexico - 1982

 

El Chichon was the first major eruption to have its atmospheric effects studied in detailed by modern instruments. Although the volume of the eruption was small (< 1 cubic km of alkalic trachyandesite tephra, similar in volume to Mount St. Helens), El Chichon was also notable because it released an unusually large volume of aerosols (7 Mt of SO2 compared to 1.0 for Mount St. Helens). The ash contained up to 2 weight percent sulfate. Anhydrite (made of CaSO4) crystals were also in the deposits. Anhydrite is rare in volcanic rocks.

El Chichon had three Plinian eruptions, each sending gas and dust to the stratosphere. The eruption cloud was carried west, reaching the Philippines in 10 days and circling the globe at returning to Mexico in 20 days. Atmospheric circulation cells kept the cloud at about 30 degrees north latitude for more than six months after the eruption. All of the gaseous SO2 ejected into the stratosphere had been converted to sulfuric acid aerosol within six months.

LIDAR measurements showed that the atmosphere was 140 times more dense than after the Mount St. Helens

Balloon measurements discovered that a month after the eruption 20 million tonnes of sulfuric acid remained in the atmosphere. After one year, less than 8 million tonnes remained.

El Chichon produced some climate effects. The temperature of the stratosphere increased by 4 degrees C. This was caused by the absorption of some of the incoming solar radiation. This was the greatest increase since measurements began in 1958. Impact on ground temperatures is harder to quantify but temperatures in the Northern hemisphere may have been 0.2 degree C less about 2 months after the eruption.

Krakatau - 1883

 

 

Krakatau erupted more than 10 cubic km of magma releasing 30-38 Tg of stratospheric aerosol in the Southern Hemisphere and 55 Tg of stratospheric aerosol in the Northern Hemisphere. Although Krakatau erupted a large volume, the magma was relative poor in sulfur, and the eruption had less climate impact compared to some small volume eruptions that were sulfur rich (e.g., Agung in Indonesia). T

 

 

he abundance of sulfur in a magma is inversely proportional to silica content. The basaltic andesite magma of Agung contained more sulfur than the higher silica magma of Krakatau. Rampino and Self (1982) estimated that the temperature in the Northern Hemisphere decreased 0.3 C due to the eruption. Drawing of the ash cloud from the 1883 eruption. Photo credit: National Geophysical Data Center (P. Hedervari).

Tambora, Indonesia, 1815

 

The eruption of Tambora produced a sulfuric acid aerosol mass of 1.75x10E11 kg. The temperature decline associated with the eruption is about 0.7 degree C. Full impact of the eruption is hard to identify because global temperatures were already declining , possibly as a result of the sunspot minimum (Sigurdsson, 1

Laki, Iceland - 1783

The Laki eruption lasted eight months during which time about 14 cubic km of basaltic lava and some tephra were erupted. Haze from the eruption was reported from Iceland to Syria. In Iceland, the haze lead to the loss of most of the island's livestock (by eating fluorine contaminated grass), crop failure (by acid rain), and the death of one-quarter of the human residents (by famine). Ben Franklin noted the atmospheric effects of the eruption (Wood, 1992). Photo of main fissure at Laki by Thor Thordarson.

It is estimated that 80 Mt of sulfuric acid aerosol was released by the eruption (4 times more than El Chichon and 80 times more than Mount St. Helens).

 

The climatic effects of the Laki eruption are impressive. In the eastern United States, the winter average temperature was 4.8 degrees C below the 225 year average. The estimate for the temperature decrease of the entire Northern Hemisphere is about 1 degree C. The top graph shows change in acidity in micro equivalents H+ per kg in the Greenland icecap. The bottom graph represents the winter temperature records in the eastern United States. From Sigurdsson (1982).

The Laki eruption illustrates that low energy, large volume, long duration basaltic eruptions can have climatic impacts greater than large volume explosive silica-rich eruptions. The sulfur contents of basaltic magmas are 10-100 times higher than silica-rich magmas (Palais and Sigurdsson, 1989)

Toba, Indonesia, 75,000 years ago

 

The eruption of 2,800 cubic km of magma at Toba caldera 75,000 years ago was the largest eruption in the last 2 million years. The eruption may have release as much as 10E12 kg of sulfuric acid, an order of magnitude more than Laki in 1783 and Tambora in 1815, two of the greatest Holocene eruptions. The Toba eruption may have caused about 3 to 4 degree C cooling at the surface but this impact is hard to detect because of concurrent glacial conditions (Sigurdsson, 1990).