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Pyroclastic flows move fast and destroy everything in their path
Heed evacuation warnings if a volcano is known to be active. If you witness a pyroclastic flow, run in the opposite direction as quickly as possible.
Pyroclastic flows contain a high-density mix of hot lava blocks, pumice, ash and volcanic gas. They move at very high speed down volcanic slopes, typically following valleys. Most pyroclastic flows consist of two parts: a lower (basal) flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.
Pyroclastic flows form in different ways:
- Collapse of eruption column: during a highly explosive eruption, the column ejected upwards into the atmosphere cools and can become too cool and dense to maintain upward momentum.
- “Boiling over” from eruptive vent: during an explosive eruption, material is erupted without forming a high plume and rapidly moves down slope.
- Collapse of lava domes or flows: The fronts of lava flows or domes can become so steep that they collapse due to gravitational force.
Pyroclastic flows destroy nearly everything in their path
With rock fragments ranging in size from ash to boulders that travel across the ground at speeds typically greater than 80 km per hour (50 mph), pyroclastic flowsknock down, shatter, bury or carry away nearly all objects and structures in their path. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C (390-1300°F), can ignite fires and melt snow and ice.
Pyroclastic flows vary considerably in size and speed, but even relatively small flows that move less than 5 km (3 mi) from a volcano can destroy buildings, forests, and farmland. On the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hot ash and gases.
Pyroclastic flows generally follow valleys or other low-lying areas and, depending on the volume of rock debris carried by the flow, they can deposit layers of loose rock fragments to depths ranging from less than one meter to more than 200 m (up to about 700 ft).
Pyroclastic flows can also lead to secondary hazards, especially flooding and lahars by:
- Eroding, melting and mixing with snow and ice, thereby sending a sudden torrent downstream.
- Damming or blocking streams in volcanic valleys, which may create lakes behind the blockage that eventually overtop and erode the blockage producing a rush of water and volcanic material downstream.
- Increasing the rate of stream runoff and erosion during rainstorms due to the creation of an easily eroded landscape with sparse vegetation.
Building remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico 1982. Reinforcement rods in concrete bend in direction of flow.
Published scientific estimates of the global CO2 emission rate for all degassing subaerial (on land) and submarine volcanoes lie in a range from 0.13 gigaton to 0.44 gigaton per year. The 35-gigaton projected anthropogenic CO2 emission for 2010 is about 80 to 270 times larger than the respective maximum and minimum annual global volcanic CO2 emission estimates.
There is no question that very large volcanic eruptions can inject significant amounts of carbon dioxide into the atmosphere. The 1980 eruption of Mount St. Helens vented approximately 10 million tons of CO2 into the atmosphere in only 9 hours. However, it currently takes humanity only 2.5 hours to put out the same amount. While large explosive eruptions like this are rare and only occur globally every 10 years or so, humanity’s emissions are ceaseless and increasing every year.
There continues to be efforts to reduce uncertainties and improve estimates of present-day global volcanic CO2 emissions, but there is little doubt among volcanic gas scientists that the anthropogenic CO2 emissions dwarf global volcanic CO2 emissions.
For additional information about this subject, please read the American Geophysical Union’s Eos article “Volcanic Versus Anthropogenic Carbon Dioxide” written by USGS scientist Terrence M. Gerlach.
| Yearly CO2 emitters | Billion metric tons per year (Gt/y) |
| Global volcanic emissions (highest preferred estimate) | 0.26 |
| Anthropogenic CO2 from fuel combustion 2015+ | 32.3 |
| Worldwide Road Transportation 2015+ | 5.8 |
| Approximately 24 1000-megawatt coal-fired power stations * | 0.22 |
| Argentina 2015+ | 0.19 |
| Poland 2015+ | 0.28 |
| United States 2015+ | 4.99 |
| CO2 emission events | |
| Mount St. Helens, 18 May 1980 | 0.01 Gt |
| Mount Pinatubo, 15 June 1991 | 0.05 Gt |
| Number of Pinatubo-equivalent eruptions equal to 2010 global anthropogenic CO2 | 700 |
| Number of Mount St. Helens-equivalent eruptions equal to 2010 global anthropogenic CO2 | 3500 |
| 2010 global anthropogenic CO2 multiplier (ACM)** | 135 |
| 1950 ACM | 38 |
| 1900 ACM | 18 |
| Number of days for anthropogenic CO2 to equal a year’s worth of global volcanism | 2.7 |
* Equal to 2% of the world’s coal-fired electricity-generating capacity.
**Ratio of annual anthropogenic CO2 (approximately 35 Gt) to maximum preferred estimate for annual volcanic CO2.
+2015 data from IEA CO2 Emissions From Fuel Combustion 2017 edition.
Since 2021 VOLCANO ACTIVE FOUNDATION has been committed to the UN Global Compact corporate responsibility initiative and its principles in the areas of human rights, labour, the environment and anti-corruption.
