Paradise Lost: Volcanic Eruptions on Montserrat
Steven Carey, Professor
Graduate School of Oceanography
Steven Carey earned a BS in geology from the University of Massachusetts, Amherst, and a PhD in geological oceanography from GSO. His research interests include modeling the dispersal of pyroclastic material from explosive eruptions, generation and deposition of volcaniclastic sediment at subduction zones and convergent continental margins, and petrology and geochemistry of subduction zone magmas. He has conducted field work in Iceland, Indonesia, Nicaragua, and the West Indies.
Montserrat is a small island in the north ern
part of the volcanically active Lesser Antilles island arc. Until 1995, it was
a quiet, idyllic place often referred to as the Emerald Island because of its
lush tropical vegetation and great beauty. The lifestyle of the 11,000 people
on the island centered principally around agriculture and tourism. All that
changed abruptly when the Soufriere Hills volcano, located in the southern part
of the island (see Fig. 1), became active for the first time in recorded history
on July 18, 1995.
During the next five years, repeated eruptions of the volcano led to widespread destruction of a large part of the southern half of the island and forced the evacuation of thousands of people from their homes and businesses. These events have led to severe hardships for the residents of the island, some of whom have emigrated to other countries to begin a new life. The volcano continues to be active, and its behavior has eluded accurate forecasting. Explosive activity at the Soufriere Hills volcano has produced an extreme environment where scientists can observe firsthand the interactions of volcanism and the earth's atmosphere, biosphere, and hydrosphere. A unique aspect of the Montserrat eruptions is the interaction between volcanic processes and the ocean, a topic of particular interest to marine geologists at the Graduate School of Oceanography.
The volcanic activity on Montserrat is a manifestation of the dynamic geological environment of the Lesser Antilles island arc of the West Indies. Here, the seafloor of the Atlantic Ocean is slowly being thrust beneath the seafloor of the Caribbean Sea by subduction. As the Atlantic crust descends deep into the earth, the release of water triggers melting of the Earth's mantle and the generation of magmas that rise and are eventually erupted at the surface. In addition, subduction contributes to the formation of great earthquakes, adding to the hazards associated with volcanism. Eruptions of the Soufriere Hills volcano have been associated with the growth of a dome of viscous magma at the summit. In 1996, the dome grew by 230,000m3 of magma per day. As the dome grew, it periodically became unstable and parts of it collapsed. The collapses produced a turbulent mixture of hot rocks, particles, and gases that swept down the flanks of the volcano at speeds in excess of 100 miles per hour (see Fig. 2). These pyroclastic flows are somewhat analogous to large snow avalanches except they carry more material and are extremely hot (up to 500oC). They are among the most dangerous phenomenon of explosive volcanism and have resulted in the deaths of thousands of people around the world. The flows destroy virtually everything in their path and are difficult to escape because of their great speed. On the Soufriere Hills volcano, these flows most often traveled down the river valleys but occasionally covered large areas of the lower slopes. The capital city of Plymouth, evacuated during the early stages of the eruptions, was eventually inundated and destroyed by pyroclastic flows.
During some of the explosive eruptions of the Soufriere Hills volcano, pyroclastic flows were energetic enough to travel all the way to the coast and into the ocean (see Fig. 3). This occurred mainly at the outlets of the Tar and White River valleys on the east and south coasts respectively (see Fig. 1). The behavior of hot pyroclastic flows when they interact with cold seawater has been a topic of great debate in the field of volcanology, largely because the process has rarely been observed. As a result of the Soufriere Hills eruption, this process was captured on video for the first time. A remarkable feature of this interaction is that part of the hot flow continues over the surface of the water. There is speculation that as a flow travels across the sea surface it is supported by a layer of steam that develops at its base, much like a hovercraft skimming over water at high speed. An extreme example of a pyroclastic flow travelling over water occurred during the great eruption of Krakatau volcano in Indonesia. On August 27, 1883, a large pyroclastic flow was generated during the eruption and part of the flow traveled over 40 kilometers of water to the coast of Sumatra, where the hot gases and particles killed more than 1,000 people.
Another dangerous aspect of pyroclastic flow discharge into the sea is the potential for tsunami generation. Flows carry large amounts of volcanic material and travel at great speeds. Consequently, when the flow initially hits the sea it displaces water and produces a wave. At Montserrat, the discharge of flows into the sea has generated small tsunamis with wave heights up to five meters. The 1883 Krakatau event produced enormous tsunamis that crested along the shore of Java with heights of up to 30 meters. Despite the great hazards posed by pyroclastic flows, their discharge into the ocean is an important process in the growth of oceanic islands such as Montserrat. New land is created as flows enter the sea and the coastline is expanded. Along the east and south coasts of Montserrat, there are two new areas of growth seaward of the Tar and White River valleys (see Fig. 4).
In 1998, GSO Professor of Oceanography Haraldur Sigurdsson and I conducted a marine geological survey of the areas where pyroclastic flows were discharged into the sea. The objectives of the study were to learn more about the dynamic interaction of hot flows and seawater and to examine the submarine extensions of the new coastlines to determine how they were formed. Using acoustical equipment, we collected information about water depth and the deeper structure of the seafloor. By comparing water depths before and after the eruptions, we located the areas on the seafloor where new material was deposited. Our results indicate that the new coastlines extend to depths of 50 meters but then terminate abruptly at the edge of a narrow shelf. Beyond the shelf, the submarine slopes drop off quickly into deeper water. In depths of about 200 meters or more, the slopes decrease sharply and new material from the eruptions appears to have been deposited. This observation suggests that flows that entered the sea were able to travel into relatively deep water but did not deposit material on steep slopes. This is similar to the behavior of hot flows on land. Deep water deposition also suggests that the hot flows are able to mix with seawater to form mixtures of volcanic particles and water that move underwater as a result of their high density.
The marine geological survey also revealed areas along the coast where the seafloor became deeper following the eruptions. Although the exact process for this is not clear, these areas may have suffered slumping where part of the seafloor collapsed into deeper water. These slumps may have been triggered by the frequent earthquakes associated with the volcanic eruptions or they may have resulted from the loading of rapidly accumulating material on the seafloor as flows were discharged into the ocean.
No one knows for sure how long the present activity at Montserrat will last. However, the eruptions provide a unique insight into the growth of oceanic island volcanoes and to the hazards they pose to the people who call Montserrat their home.
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