News reports have drawn attention to recent volcanic eruptions in Hawaii (Kilauea) and Guatemala (Fuego). These two volcanos are quite different in origin, size, shape, explosive behavior and potential hazard. Dr. Tim Chowns’ talk helped us understand the difference. Hawaii is the largest volcano on Earth rising almost 14,000 feet above sea level and twice as high above the floor of the Pacific ocean (for a total height of about 28,000, compared to Mt Everest at 29,000 feet) In spite of its huge size, average slopes are quite gentle (less than 5 degrees ), because the island’s basalt lava is relatively fluid and flows freely away from fissures. The gas content of the lava is low and the greatest hazard is not from explosion but from creeping lava flows and resulting fires. Kilauea is located over a ‘hot spot’ in the earth’s mantle with magma rising some 450 miles to burn a hole in the overlying oceanic crust. Because the crust is moving (plate tectonics) eruptions spanning some 85 million years have left a chain of submerged volcanoes stretching 3600 miles northwest across the floor of the Pacific (past Midway Island all the way to Alaska). Dividing the age of Midway volcano (which originated over the Hawaiian ‘hot spot’) by its distance from Hawaii gives an estimate of the rate that the crust is drifting over the ‘hot spot’ (1500 miles/ 27,000,000 years = 3.5 inches/year).
Compared to Hawaii, Fuego has a much more typical volcano shape with slopes of around 30 degrees. Although much smaller it is highly explosive. This is because its andesite lava is more viscous and contains a higher proportion of gas. As a result the lava does not flow freely, the vent tends to clog and pressure builds up to horrendous eruptions. The resulting volcano is a mixture of lava and ash with steep sides. During violent eruptions clouds of burning hot gas and ash may collapse though gravity and pour down the sides of the volcano at phenomenal speeds (100 MPH). While most of us could outrun a lava flow on Kilauea, a glowing avalanche on Fuego would be impossible to escape. This was the lesson learned in Pompeii at the foot of Mt Vesuvius in 79 AD.