Major Historic Volcanic Eruptions
Despite almost 23 years of precursor activity, the eruptions on 19 September 1994 at Rabaul took place within only 27 hours of heightened seismic activity. Almost simultaneous eruptions at Tavurvur and Vulcan on opposite sides of the harbour produced an eruption cloud which extended up to 18 kilometres into the air. Pyroclastic flows also travelled up to two kilometres from the vent and a number of tsunami were generated, the largest of which was five metres high. About 30,000 people were evacuated from Rabaul and surrounding villages the night before the eruption and thousands were evacuated from beaches near Rabaul on 20 September. Eruption debris buried the airport, blocked roads and a heavy mud mixture of ash and rain caused roofs to collapse. By 23 September five people had been killed, four from roof collapse and one from lightning, and 53,000 people had been displaced. An important lesson from this eruption was that data from seismic instrumentation useful for monitoring volcanic unrest may be swamped during an eruption, greatly hindering volcanologists and emergency personnel at the most critical time.
The world’s largest eruption in more than half a century took place on 15 June 1991 at Mount Pinatubo. Careful monitoring of the volcanic unrest and successful communication of warnings to authorities and the public led to the evacuation of 60,000 people. Of those, more than 20,000 escaped probable death.
Earthquakes in July 1990 measuring magnitude 7.8 and 4.8 followed by rumbling sounds, ground cracks and a large landslide heralded the onset of volcanic activity. An increase in activity in April 1991 triggered urgent compilation of information on Pinatubo’s geological past by the Philippine Institute of Vulcanology and Seismology and the United States Geological Survey. Meanwhile, surveys of the volcano revealed repeated engulfment by pyroclastic flows 20 kilometres from the summit and lahar deposits in drainages well beyond 20 kilometres. A volcanic hazard map was produced and public awareness activities were initiated.
The climactic eruption on 15 June 1991 blanketed 2,000 square kilometres of the densely settled area around the volcano with 10 to 25 centimetres of tephra. Pyroclastic flows travelled up to 16 kilometres from the vent and cover more than 400 square kilometres. Tropical Storm Yunya passed 75 kilometres north of Pinatubo during the eruption and the heavier, rain-soaked tephra, combined with wind buffeting, caused more roofs to collapse than anticipated. Most of Luzon received ash fall and a total of 189 people were killed while 216,000 families were affected in some way by the eruption. The extensive damage was estimated at US$400 million. Communities continued to be affected long after the eruption ceased when frequent lahars swept down volcano drainages every monsoon season. Many lives and some property were saved by accurate hazard assessments, public education and timely warnings.
Prior to 1980, Mount St. Helens formed a conical, youthful volcano sometimes known as the ‘Fuji-san’ of America. During the 1980 eruption the upper 400 metres of the summit was removed as a consequence of slope failure, leaving a two kilometre by 3.5 kilometre horseshoe-shaped crater which subsequently has been partially filled by a lava dome.
Prior to the catastrophic eruption in 1951, Mt Lamington was not considered to be a volcano by the people living in the vicinity and there was no known native folklore of eruption in the area. Preliminary activity six days prior to the climactic eruption included landslides in the crater area, earthquake swarms and emissions of gas and ash which increased daily. Despite an initial ash eruption on the 18 January, the Territory’s administration in Port Moresby was informed by the District Commissioner that there was no need for alarm. On the following day, the Acting Administrator in Port Moresby announced that there was no immediate danger to human life and no order was given to evacuate settlements closest to the volcano. The eruption on the 21 January produced a pyroclastic flow which travelled 13 kilometres from the vent, resulting in the near-instant deaths of almost 3,000 people and complete devastation of 10 square kilometres of the surrounding countryside.
Mt. Pelee is famous for an eruption on 8 May 1902 which resulted in the deaths of 29,000 people and destroyed the city of St. Pierre. The event was attributed with the largest number of casualties for a volcanic eruption during the 1900’s.
The eruption at Krakatau on 26 and 27 August 1883 unleashed a series of devastating tsunami which claimed most of the 36,000 lives lost during the eruption. The tsunami, which resulted from pyroclastic flows discharging into the sea, inundated the low-lying coasts of the Sunda Straits with reported run-up as high as 36 metres above sea level and flooding as far as 10 kilometres inland. The waves were sufficient to raise large ships 10 metres above sea level and carry them up to three kilometres inland. Reported deaths from the tsunami occurred up to 800 kilometres from the site of the eruption. Pyroclastic flows from the eruption travelled 80 kilometres over the sea surface, resulting in about 2,000 people along the south east coast of Sumatra dying from burns.
The most explosive eruption of the last 10, 000 years was Tambora, located on an island 300 kilometres east of Bali. Only 26 of the island's original 12, 000 inhabitants survived. In all, the eruption, accompanying earthquakes and subsequent starvation caused the deaths of more than 90,000 people. Within an area extending 320 kilometres from the volcano there was three days of total darkness as ash blocked out the sun. Ash fallout from this eruption was widespread with one centimetre recorded as much as 900 kilometres from the vent. The climatic effects from this eruption were dramatic, both in the region and globally. In Europe and parts of the United States, 1815 became known as the year without summer. Overall, the global average temperature is estimated to have fallen by 0.4°C to 0.7°C with the cooler temperatures triggering widespread famine, starvation and disease.
The 79 A.D. eruption of Vesuvius was the first volcanic eruption to be described in detail. Pliny the Younger witnessed the eruption from 30 kilometres west of the volcano and recorded his observations in two letters. He described the earthquakes before the eruption, the eruption column, pyroclastic flows, air fall, and the effects of the eruption on people, as well as the resulting tsunami.
The largest eruption in the last few hundred thousand years took place on the island of Sumatra in Indonesia. The eruption produced a widespread dust cloud and a dense sulphuric acid aerosol cloud which persisted for up to eight years. It triggered abrupt climate cooling similar to that predicted for a nuclear winter and a catastrophic decrease in the human population. Near-instantaneous temperature decreases to near or below freezing in the tropics was followed by an extended global cooling of 3°C to 5°C or more. These extreme climatic changes coincide with a time when the early human population suffered an extreme decline with some estimates suggesting survival of only several thousand individuals worldwide.
Further Reading
Related Websites
- Emergency Management Australia - Volcanoes
- United States Geological Survey - Volcano Hazards Program
- Benfield UCL Hazard Research Centre - Volcanoes
- University of North Dakota - Volcano World
- Smithsonian Institute
- University of Western Australia - Volcanoes and Earthquakes in SE Australia
- Volcano Live - Volcanoes of Australia
- Emergency Management Australia Library