Steve Carey (right) and Haraldur Sigurdsson conduct volcanological studies at Anak Krakatau, Indonesia. 

Figure 1 

Figure 2

Figure 3

Volcanic eruptions are fundamental in the exchange of material and energy from the earth's interior to the oceans and atmosphere. Throughout history mankind has been fascinated with the power of volcanoes and the dangers that they pose to the people who live around them. In the last few decades new research has shown that large volcanic eruptions can cause both local and global scale environmental effects. Thus, it is important to understand the processes of volcanism, the role they have played in the past, and what we might expect in the future.
     Many people's view of volcanism has been shaped by popular images of red hot lava flows erupting from Hawaiian volcanoes. Although this constitutes an important type of volcanic activity, a far more dangerous style of activity involves the explosive eruption of magma. In this type of eruption, magma is explosively broken up into small pieces by the tremendous force of rapidly expanding volcanic gases. The eruption of Mount St. Helens in the Pacific Northwest (May 18, 1980) is a good example of this type of event. During such eruptions, large amounts of volcanic particles and gases can be injected up into the air by large mushroom-shaped clouds that rise from the vent. These clouds can reach altitudes of 40 km in the Earth's atmosphere and often resemble the large plumes developed when an atomic bomb is detonated. The volcanic plumes are transported by local wind and can form a global belt of high altitude aerosols. A particularly important aerosol that forms from explosive eruptions are tiny droplets of sulfuric acid that can reside in the atmosphere for several years. These droplets can affect the energy balance of the atmosphere and, in many cases, reduce incoming sunlight and lower surface temperatures.
     How large are potential eruptions? This is an important question about explosive volcanism. Some insight can be derived from historical observations of volcanic eruptions. Volcanologists gauge the size of eruptions by calculating the volume of magma that has been erupted. The unit that is commonly used is km3 of magma (a volume equivalent to a cube of magma 1 km long on each side). The eruption of Mount St. Helens in May 1980 ejected about 0.5 km3 of magma. Although the eruption looked quite spectacular, in reality it was actually a relatively small explosive eruption. The largest historic explosive eruption was the 1815 eruption of Tambora volcano in Indonesia. About 50 km3 of magma was discharged during this event and more than 90,000 people were killed. There is, however, evidence in the geologic record of much larger explosive eruptions. For example, an eruption in Indonesia from the Toba caldera discharged more than 1000 km3 of magma about 75,000 years ago. This raises a variety of interesting questions. If such an event occurred today, what would the environmental consequences be? How would the world's population be impacted? Are these kinds of eruptions likely to occur in the future?
     Recent drilling on Leg 165 of the Ocean Drilling Program in the Caribbean Sea has provided a fascinating new record to evaluate these types of questions. Five sites were drilled in the Caribbean Basin. (see Fig. 1) Four of the sites revealed evidence of extensive explosive volcanism in the circum-Caribbean area. This evidence takes the form of thin bands of volcanic ash that are between layers of sediment which accumulated very slowly in the deep sea. The layers are usually a few to tens of centimeters thick with a very sharp bottom contact and a diffuse upper contact. The ash was formed when magma was cooled very quickly by contact with air (turning to glass) and broken up into very small pieces by the expansion of gases during the eruption. (see Fig. 2) In essence, these are layers of broken glass that extend over hundreds of kilometers. Each layer represents the fallout of material that was injected high into the atmosphere during an explosive eruption and transported downwind. In all, over two thousand layers were recovered, ranging in age from 0 to 67 million years.
     The thickness and lateral extent of the layers are related to the size of the eruption and the strength of the winds that blow the volcanic ash into the ocean. These parameters can be used to calculate the volume of material ejected during individual eruptions. In the western Caribbean the layers cover an enormous area with the thickest total accumulation centered around site 999. Judging from the thickness of individual layers as a function of distance from source, the layers in the Caribbean cores suggest enormous eruptions, probably in the range of 100 to 1,000 km3 of magma.
     What was the source of all of this explosive volcanism? The rims of the Caribbean basin today are volcanically active, with explosive volcanism occurring along the eastern margin, in the Lesser Antilles island arc, and on the western margin, through Mexico and Central America (see Fig. 1). One way to assess the potential source areas of volcanism is to look at the atmospheric circulation, or transporting agent, of volcanic ash in the area. In most of the Caribbean, the surface winds blow from the east to the west, in a belt known as the trade winds. However, starting at aboiut 14 km up to 24 km in the atmosphere, the winds shift direction by 180o ,blowing west to east transecting the boundary between the troposphere and the stratosphere. Most large explosive eruptions can easily inject volcanic ash and gases to stratospheric levels and thus the most significant amount of transport occurs from the west (Central America) to the east (Caribbean Sea). Based on the present day wind patterns, it is apparent that most volcanic ash transported into the Caribbean must be derived from sources to the west. Volcanic ash fallout from explosive eruptions in the Lesser Antilles, on the other hand, mostly ends up in the eastern equatorial Atlantic.
     A computer simulation of the ash fallout has been developed to evaluate the types of conditions necessary to produce the observed layers in the cores. Input to the model consists of the wind strengths and directions in the Caribbean area based on a large meteorological database. The size of the eruption can then be varied to match the size of volcanic ash found at a particular distance downwind from source. Preliminary results indicate that transport was most likely from the west, but with eruption column heights that must have reached well into the stratosphere. (see Fig. 3).
     If, as we suspect, most of the explosive volcanism recorded in the Caribbean deep sea sediments is derived from a western source, then there should be evidence on land to support this hypothesis. As previously mentioned, active volcanism is occurring along Central America. This volcanism is the result of Pacific sea floor being forced under, or subducted, beneath the Central American landmass. As this ocean floor returns deep into the earth, overlying rocks melt supplying the magma necessary for volcanic activity. There is evidence on land of major volcanic deposits produced by explosive volcanism in Honduras, Nicaragua, and Guatemala. These deposits are similar in age to many of the volcanic ash layers in the deep sea cores. Ignimbrites, a distinctive rock preserved on land, are created from pyroclastic flows, formed when an eruption column collapses into a deadly mixture of hot gases and particles that move at hurricane speeds down the slope of a volcano. In Central America, there are abundant, thick ignimbrites that undoubtedly correlate to the widespread ash fall layers found in the deep sea cores.
     Research at the Graduate School of Oceanography will attempt to link the record of volcanism on land with the excellent record of ash layers in the deep sea. By using computer simulations of the volcanic events the size, nature and potential environmental impacts of these eruptions will be evaluated. The Leg 165 deep sea record holds tremendous potential for reconstructing the detailed history of explosive volcanism in the Central American area and contributing to a better understanding of this important type of volcanism.

Recommended Readings
Volcanoes: A planetary perspective, by P. W. Francis. Oxford University Press, Oxford, pp. 209-234, 1993.
Sigurdsson, H, Leckie, M, Acton, G. et al., 1997. Proc. ODP, Inititial reports, 165. College Station, TX (Ocean Drilling Program)