Pliny's pine-tree cloud gave the most explosive volcanoes their name
On 24 August in AD 79, a woman urged Pliny the Elder to look at a strange cloud rising across the Bay of Naples. Her son later compared it to a pine tree, a tall trunk spreading into branches. That letter about Vesuvius is why violent, towering eruptions are still called Plinian.
A Plinian eruption hurls a column of ash, pumice and hot gas high into the stratosphere, the second layer of the atmosphere, driven by a powerful and sustained blast of gas. Some are over within a day; others rumble on for months, starting with ash clouds and sometimes ground-hugging pyroclastic surges. They are loud, and the rising column builds up so much static charge that lightning often crackles through it, something the geologist George Scrope painted in 1822 and observers saw again during the Hunga Tonga eruption in 2022.
So much magma can pour out that the chamber beneath empties and the summit collapses, leaving a huge basin called a caldera. Crater Lake in Oregon formed this way around 4860 BC. The magma involved is usually rich in silica, which makes it thick and gassy. Runny, low-silica basalt seldom behaves like this, although New Zealand's Mount Tarawera managed it in 1886 under unusual conditions.
The name honours a tragedy. Pliny the Elder, a man of great learning, launched his galleys across the bay towards Stabiae to rescue people trapped on the shore. He collapsed there, and his body was found under the ash on 26 August with no visible injuries. His nephew, Pliny the Younger, thought poisonous fumes killed him, though a heart attack, asthma attack or stroke would fit the evidence too. The same eruption buried Pompeii and Herculaneum.
Later examples span the globe: Thera in the Aegean around 1645 BC, Tambora in 1815, Krakatoa in 1883 and Mount St. Helens in 1980. The 1912 blast of Novarupta in Alaska was the largest eruption of the 20th century, and Pinatubo in the Philippines followed in 1991.
Source: Plinian eruption
