**this is sometimes called a Plinian Column, after Pliny the Elder who witnessed the eruption of Mt. Vesuvius.
Actually, the namesake would be his nephew, the younger Pliny. Both of them were witnesses of the eruption. but the Elder died during the events, collapsing dead on the beach of Stabiae the morning after he was forced to abort an evacuation attempt (he was not just a naturalist, but also a navy commander), although it is unknown if that was the result of volcanic poisoning, asphyxiation or exhaustion (although he could spend the night at the villa of a friend, the night was uneasy and the building eventually had to be evacuated. Meanwhile, the then-teenaged Younger witnessed the events from the relative safety of his uncle home (where he and his mother, the Elder’s sister, had lived since his father’s young death; posthumously, the Elder, a confirmed bachelor, adopted him) and lived to describe the events in dramatic details in a series of letters to Tacitus, some of which, such as the first description of pyroclastic flows, were considered dubious at best for nearly two thousand years, but were eventually confirmed by archaeology and the observation of other volcanos.
Also, pyroclastic flows do not contain magma, but are a dispersion of hot ashes, dust and debris in air that results from the collapse of a cavity that can form when the front of a magma flow freetes and dams the rest of the lava. They are hot to the point of instant lethality and extremely fast. Both Heraculaneum and Pompeii were hit by pyroclastic flows.
As to basalts, basaltic eruptions can last very long - indeed, there are several lava lakes that remain active for years - and they can release massive amounts of gasses during these times. Notably, the Laki eruptions in Iceland that span the years 1783 to 1794 not only killed one fourth of the human population of Iceland, half the large kattle and eighty percent of sheep (as a result of acidification and soil poisoning continuing to effect catastrophic cropn failures for years afterwards), but also sent smog down the jetstream as far south-east as Prague and caused two years of worldwide volcanic winter that caused famines that would ultimately be a major cause of the French Revolution a few years later.
Volcanic hotspots far from any tectonic zones, such as Hawai’i, are caused by so-called plumes in the mantle, hot and light regions that can ascend through the crust. They can exist for hundreds of millions of years, with the plates migrating ober them, resulting in island chains. If you track back the course, you will generally eventually find a Large Igneous Province, a region where a stratum is covered by flood basalts, bearing witness a sustained period of intense volcanism on an unfathomable scale, where basaltic eruptions that dwarf the 18th century Laki catastrophe occured within decades to years as opposed to every few centuries, for hundreds of thousands of years. Most of the major mass extinction events occured during such a time. The largest LIP are the Sibirian Traps, comprising two million square kilometres of flood basalts and tuff covering the rest of Western and Central Siberia. This was the greatest natural catastrophe - not only due to its volume and its duration of nearly one million years, but also the circumstance of occuring in sulphor-rich oily slate, causing a fatal combination of global cooling followed by even worse global warming, as well as sustained acidic rains and ocean acidification -, causing the Great Dying, marking the extinction of up to ninety-five percent of oceanic life, the divide not just between Permian and Triassic but also between Palaeozoic and Mesozoic, and the one time where higher life came close to ending altogether.
Finally, the asthenosphere - the “flowing” zone comprising the lower parts of the crust and the mantle where tectonics and volcanism originate - is indeed not “a sea of molten rock flowing underneath the earth’s crust […] leftover from some alleged accretion disk formation processes”. The rock down there is less a free-flowing liquid like water or molten metal, but rather extremely viscous, heated and mollified by extreme pressure. The heat within the Earth is not just left over from planet formation, but also comes from the pressure and most importantly from radioactive decay in the core (due to centrifugal effects, heavy elements are more concentrated near the centre of a planet, while the lighter elements dominate the outer layers - Earth’s crust and mantle consist overwhelmingly of silicates while the inner core is believed to consist primarily of a crystalline high-pressure phase of iron).