earth sciences and ecology question #770



James McCarty, a 80 year old male from Frankfort, Kentucky asks on April 28, 2002,

Q:

Evacuations of the earth's core through volcano eruptions, the pumping of oil, gas, etc., must leave voids. If this is so, what will eventually happen to the earth's crust? Will it collapse?

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the answer

Ryan Ickert, MSc student, Simon Fraser University answered on December 4, 2004, A:

You are partially correct. The extraction of groundwater from shallow depths by humans and of magma from much greater depths by natural processes does indeed result in the “collapse” of the earths crust…on a small scale. In the subsurface, fluids like oil and water are contained within porous rock or sediment, and therefore do not leave cavernous void spaces once they have been pumped out. When pumping fluids from a strong material such as porous rock, it is the rock that is providing the strength to “hold up” the overlying mass of rock and sediment so it will typically not collapse. The pumping of fluid--such as groundwater--from the pore spaces within unconsolidated sediment, however, can (and commonly does) result in the “compaction” or subsidence of the ground. For example, long-term groundwater pumping in Mexico City has resulted in as much as 27 feet of subsidence in 20 years. Similarly, Las Vegas has experienced as much as 20 feet of subsidence due to groundwater pumping. Volcanic processes can have a similar but more catastrophic effect. The sudden evacuation of magma chambers in the crust during volcanic eruptions CAN result in the generation of “cavernous” void spaces and will result in the nearly instantaneous collapse of the overlying ground. Volcanologists refer to these collapse features as “calderas”. In the United States, some pre-historic calderas occur at Crater Lake in Oregon and at Yellowstone National Park. The caldera-generation eruption at Yellowstone was particularly large and the associated caldera, or depression, was 60km in diameter! A modern caldera forming eruption occurred at Mount Pinatubo in the Philippines in 1991 and resulted in a 2.5km diameter feature.

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