In 2009, the International Space Station flew over the Sarychev Volcano on the Kamchatka Peninsula just as it was erupting and punching a spectacular hole in the clouds. The photos and videos of it are some of the best we’ve ever seen of an erupting volcano from above. Take a look at the pyroclastic flows streaming down the sides of the peak as the station passes (shaking is the camera position adjusting as the ISS moves).
Filming a pyroclastic flow in Japan, 1991. A pyroclastic flow is a fast-moving current of hot gas and rock which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C (1,830 °F).
Now this is the real horrors of our world. Imagine being caught in that!
Yellowstone National Park has been hastily evacuated as fear of the Yellowstone Caldera’s eruption is deemed to be approaching sooner than previously expected. Researchers on-site claim that the 640,000 year-old super volcano has exhibited a sudden spike of activity which indicates that it could erupt in as little as two weeks. The explosion caused by the volcano would very well throw all of United States into a 200 year long volcanic winter, with ash blotting out the sun, and pyroclastic flow irreparably damaging the surrounding ecosystem.
Saturday morning Japan’s Mount Ontake erupted unexpectedly, sending a pyroclastic flow streaming down the mountain. Many, though sadly not all, of the volcano’s hikers and visitors survived the eruption. Pyroclastic flows are fast-moving turbulent and often super-heated clouds filled with ash and poisonous gases. They can reach speeds of 700 kph and temperatures of 1000 degrees C. The usual gases released in a pyroclastic flow are denser than air, causing the cloud to remain near the ground. This is problematic for those trying to escape because the poisonous gases can fill the same low-lying areas in which survivors shelter. Heavy ashfall from the flow can destroy buildings or cause mudslides, and the fine volcanic glass particles in the ash are dangerous to inhale. The sheer power and scale of these geophysical flows is stunning to behold. Those who have witnessed it firsthand and survived are incredibly fortunate. For more on the science and history of Mount Ontake, see this detailed write-up at io9. (Image credits: A. Shimbun, source video; K. Terutoshi, source video; via io9)
Major volcanic eruptions can be accompanied by pyroclastic flows, a mixture of rock and hot gases capable of burying entire cities, as happened in Pompeii when Mt. Vesuvius erupted in 79 C.E. For even larger eruptions, such as the one at Peach Spring Caldera some 18.8 million years ago, the pyroclastic flow can be powerful enough to move half-meter-sized blocks of rock more than 150 km from the epicenter. Through observations of these deposits, experiments like the one above, and modeling, researchers were able to deduce that the Peach Spring pyroclastic flow must have been quite dense and flowed at speeds between 5 - 20 m/s for 2.5 - 10 hours! Dense, relatively slow-moving pyroclastic flows can pick up large rocks (simulated in the experiment with large metal beads) both through shear and because their speed generates low pressure that lifts the rocks so that they get swept along by the current. (Image credit: O. Roche et al., source)
Looking at the dark blue waters and steep caldera walls of Crater Lake in Oregon, it is not hard to imagine the violence in which it was created. About 7700 years ago 3700m high Mount Mazama literally blew its top off reducing its height by 1600m and throwing 50 cubic km of lava into the sky. The eruptions column reached a height of 50km and pyroclastic flows filled the surrounding valleys with a 100m thick blanket of pumice and ash. The devastating eruption left a 650m deep gaping hole in the landscape. Today the dark waters (now 594m deep) of this filled hole are the deepest of the US.
This is what geological history tells us. However, there is also an exceptionally well preserved oral tradition about the event left behind by indigenous peoples. These so-called geomyths actually mark the relation between myth and geology and tell intriguing stories.
Erupting suddenly and violently on the 27th
August 1883, the volcanic island Krakatoa,
located between Java and Sumatra, threw out an estimated 17 cubic kilometres of
rock, ash, and pumice high into the atmosphere and triggered a deadly
pyroclastic flow of molten ash and gas that killed 1000 people on the coast
of Sumatra instantly. In the loudest bang ever-recorded in human history, the
explosion triggered a catastrophic tsunami that swept coastal towns and
villages and killed at least 36,000 people (though some estimates reach as high
as 120,000). Heard some 3000 miles away in Sri Lanka and Perth, with pressure
waves enough to travel round the world four times over, the explosion caused
global temperatures to fall 1.2° for the next five years, with the sun in the
area blacked out completely for three days.
Image credit: An 1888 lithograph of the 1883 eruption of Krakatoa. Image published as Plate 1 in The eruption of Krakatoa, and subsequent phenomena. Report of the Krakatoa Committee of the Royal Society (London, Trubner & Co., 1888). Lithograph via Parker & Coward, Britain. Public domain via Wikimedia
This photo was taken on the slopes of the volcano that makes up the heart of the island of Tenerife, the largest of the Canary Islands. Each of these distinct layers likely represents one eruption of the volcanic system that built the island.