Pyroclastic Flows and Surges in Volcanic Eruptions

DIFFERENCES Pyroclastic flows and surges - destroyed and flattened thousands of trees Volcanic ash and rock deposits - covered a total of at least 4 square kilometres of land Note: - Pyroclastic flows are extremely fast-moving flows of lava, hot rocks and gas that flow down the side of the volcano - Pyroclastic surges are fastmoving clouds of hot ash that travel with the pyroclastic flow. - During the eruption, between 4 – 5 million cubic metres of lava was unloaded in the form of pyroclastic flows and surges. P h y s i c a l Effect on the natural landscape The mountain exploded sideways, sending a colossal landslide downslope. St. Helen’s summit elevation was approximately 2,950 m and after, declined to about 2,600 m. A super-heated pyroclastic flow ploughed down the mountainside, levelling millions of trees. A pillar of ash and gas rose high into the sky, blocking the sun and turning daylight into darkness. Houses were buried or burned by the intense heat of the pyroclastic flow and surges. Many villages were affected by ash deposits such as the villages of Farms and Trant’s were completely buried by ash deposits. Other houses in the Exclusion Zone were destroyed by direct impact of rocks up to 5 metres in size. The capital city, Plymouth was gradually being buried by lahars (rapidly moving river of ash and water which buried properties, vegetation and crops) and destroyed (It was directly in the path of the main pyroclastic flow) The closure of the island’s airport halted much economic activity Decline in the economy of Montserrat as tourist arrival dropped. Destruction of properties, factories and farms negatively affected the livelihood of the people. E c o n o m i c Damage/Destruction of property and physical infrastructure Monetary/ Financial Loss Effect on Employment The enormous landslide and pyroclastic flow destroyed more hundreds of homes, buildings and infrastructures such as road and railways. The lahars of St. Helens flowed into the Toutle and Cowlitz rivers, travelling many miles down the two, extensively damaging around 35 bridges in the process. 3 million m3 of material was transported 17 miles (27 km) into the Columbia River by these mudflows. Several airports had to be shut down also, and over 1,000 flights were cancelled because of ash accumulation and poor visibility, causing unsettlement for staff and people awaiting flights. The economic cost of the eruption was estimated at US$860 million. Officials closed highways for a week, and airlines cancelled more than 1,000 flights. Inevitably, unemployment around St. Helens immediately followed the eruption but was back to nearly normal once the timber salvaging and ash clean-up operations were underway. Only an incredibly small percentage of residents left the region because of lost jobs due to the event. 19 people were killed by the pyroclastic floes and surges in the Exclusion Zone. (They had ventured back into the high-risk areas for different reasons) Seven people were killed by the surge in the Streatham and Windy Hill area. Six of the victims were found outside houses, they had been attempting to seek shelter. Two other bodies were recovered from the pyroclastic flow deposits near Trant's. Most of the remaining missing persons are thought to have been in the village of Farm, which was buried by deposits several metres thick, and were never recovered. Other survivors suffered burns to various parts of their bodies, including inhalation injuries and burns to the nostrils and mouth. Half the island’s population was displaced. S o c i a l Residents’ Attitude/Perception An immediate response to the eruption was evacuation. Death Toll Emotionally attached with some refused to leave. The eruption killed 57 people, in the lateral blast, ashfall, and lahars Extent of Injury Extent of Emotional Trauma Effects of Relocation Some people living near the eruption suffered from posttraumatic stress syndrome: depression, troubled sleep, irritability, and a sense of powerlessness.