Rivers of Molten Stone: Flood Basalts
Seventeen million years ago, the Earth's crust ripped open in present day Idaho. Immense rivers of molten rock poured out hundreds of miles, often all the way to the Pacific Ocean. These flows could be well over a meter thick, and some may have flowed as fast as 60 miles per hour, though their average speed was probably around 20 miles per hour. The flows continued happening over the next three million years, and much reduced flows continued for another 8 million years after that, eventually covering over 160,000 square miles, largely in northern Oregon and southern Washington.. These flows are known as flood basalts, and they're some of the most extreme volcanic events known to happen on Earth.
To top it all off? The Columbia River Basalts, as the Idaho flow came to be known, are relatively small as far as these events go, and at least five mass extinctions have been linked to these events.
A modern basaltic lava flow in Iceland. While huge, it's tiny by the standards of flood basalts. The surface of flood basalts looked similar- largely covered with a thin black crust with cracks showing the lava beneath. While the top hardens quickly, the lower parts can stay molten for much longer- a 10 foot deep lava lake might take 6 months to harden. [Image source]
Basaltic magma, on top of being what we stereotypically think of when we think of lava, is fairly representative of magma in the Earth's mantle. It's got relatively low silica (under 50%) which makes it much less viscous than most other magmas. This allows it to flow much faster than andesitic, dacitic, or rhyolitic magmas, which don't flow much at all, and tend to just... explode. (Mt. St Helens had mostly andesitic and dacitic lava.)
Under normal circumstances, basaltic magmas tend to form shield volcanoes, like Mauna Kea and the other Hawaiian islands. Flood basalts, however, due to their much, much larger size, tend to retain their heat and liquid status for a lot longer, traveling over the land in immense sheets.
Mauna Kea. [Image source]
Flood basalts- and most other basaltic eruptions- are closely linked to mantle plumes. Mantle plumes are immense upwellings of hot material from the planet's core into the upper mantle. They tend to cut right through the Earth's crust, as despite the crust being solid, it's also quite thin compared to the mantle, as well as much less dense. (The mantle might technically be somewhat liquid, but it's a slow moving liquid with density and strength comparable to steel. Material properties are really, really weird under high heat and pressure.) Where they do, you often see volcanic features. Yellowstone, Hawaii, and others are all examples of mantle plume related volcanics. Mantle plumes tend to stay rather stationary, while tectonic plates move over them- that's how you generate the long chain of islands and seamounts northwest of Hawaii, for instance.
Columnar basalts, a deposit formed in flood basalts. These geometrically regular columns are the crystalline form of basalt, and form when it has time to cool slowly from magma. This often occurs when a second flood basalt rolls in over a still cooling one, insulating it. [Image source]
There are actually two other types of volcanism on Earth, both related to plate tectonics rather than mantle plumes. The most common form of volcanism today is subduction volcanics, often found around the Pacific. This occurs when an oceanic tectonic plate is being subducted under another, usually continental tectonic plate. The actual process is quite complicated, but it serves well enough to think of the subducting plate melting as it sinks, with the melt rising up and punching through the mantle and plate above it. The other type of tectonic volcanism is rifting zone tectonics, which occur where a continental plate is splitting apart. I'll be doing longer posts on both of these other types. Iceland, interestingly, exhibits both rifting and mantle plume volcanics- this is, in part, because plumes are capable of causing rifting as well.
Columbia River Basalt deposits in the Palouse Canyon. Notice how thick and tabular they are. [Image source]
The larger flood basalts have been linked to five different mass extinctions, including the Permian extinction, the greatest mass extinction in the history of the planet (with the exception of the Great Oxygenation Event, but that's complicated.) The Permian extinction is linked to the Siberian Traps, a large basaltic flood deposit in, obviously, Siberia. It killed up to 96% of marine species and 70% of terrestrial vertebrates. While the magma itself would have killed some local species, the ensuing climactic disturbances would have killed a lot more species. By far the deadliest effect is the potential of the flood basalt to turn the oceans anoxic, killing most of its life due to oxygen starvation. It could have also caused a runaway greenhouse effect due to volcanic gases and disturbed methane hydrates from the seafloor, among many other effects.
Other flood basalts are found all over the world, like in India's Deccan Traps. Earth doesn't have a monopoly on flood basalts, though- they're found across the solar system. The Moon's large dark patches, known as maria, or seas, are actually large flood basalt provinces. Venus' flood basalts are by far the largest in the solar system- due to Venus' much hotter atmosphere, they stay fluid far longer than Earth's. Mars has considerable flood basalts as well, largely in its Southern Hemisphere. Mars' awe inspiring Olympus Mons, the largest mountain in the solar system, is also basaltic. Jupiter's moon Io, the most volcanically active body in the solar system, also has flood basalts.
Columnar basalt deposits on Mars. [Image source]
Interestingly, mantle plume caused volcanism seems to be the dominant form of volcanism in the Solar System- which, of course, is likely due to Earth being the only one of these worlds with active plate tectonics that we know of. So before you discount these immense eruptions as things of the past, take into consideration how many of them have occurred and are occurring around the solar system, and then take into account that it hasn't even been 10 million years since we last had flood basalt eruptions on Earth- a blink of a geological eye. With any luck, we'll never see extinction event level eruptions of this sort, but ones that force the evacuation and abandonment of a region? Don't count that out of the picture.
If you're interested in volcanoes, I recently did a post on cryovolcanism throughout our solar system you might enjoy.
My geology classes and education.