Science behind volcanic eruptions
Source: By Arjun Sengupta: The Indian Express
Mauna Loa, the world’s largest active volcano, erupted after 38 years on 27 November 2022, spewing ash and debris, and covering the night sky of Hawaii’s Big Island in an incandescent red hue.
Aerial images showed molten lava flowing out of the volcano, whose name translates to “long mountain” in the native Hawaiian language. Mitch Roth, mayor of Hawaii County, was quoted as saying by NPR that the eruption does not appear to be threatening any downslope communities.
We take a closer look at the science behind volcanoes, and some of the largest eruptions in recorded history.
Why do volcanoes erupt?
The deeper one goes under the surface of the Earth towards its core, the hotter it gets. The geothermal gradient, the amount that the Earth’s temperature increases with depth, indicates heat flowing from the Earth’s warm interior to its surface. At a certain depth, the heat is such that it melts rocks and creates what geologists call ‘magma’.
Magma is lighter than solid rock and hence it rises, collecting in magma chambers. Chambers which have the potential to cause volcanic eruptions are found at a relatively shallow depth, between six to ten km under the surface. As magma builds up in these chambers, it forces its way up through cracks and fissures in Earth’s crust. This is what we call a volcanic eruption. The magma that surfaces on the Earth’s crust is referred to as lava.
Why are some volcanic eruptions explosive and some not?
While the typical image of a volcano is that of a fountain of lava spouting high in the air from the mouth of the volcano, eruptions vary in intensity and explosiveness, depending on the composition of the magma.
In simple terms, runny magma makes for less explosive volcanic eruptions that typically are less dangerous. Since the magma is runny, gasses are able to escape, leading to a steady but relatively gentle flow of lava out of the mouth of the volcano. The eruption at Mauna Loa is of this kind. Since the lava flows out at a slow pace, people typically have enough time to move out of the way. Geologists are also able to predict the flow of the lava depending on the incline and exact consistency it has.
If magma is thick and sticky, it makes it harder for gasses to escape on a consistent basis. This leads to a build-up of pressure until a breaking point is reached. At this time, the gasses escape violently, all at once, causing an explosion. Lava blasts into the air, breaking apart into pieces called tephra. These can be extremely dangerous, ranging from the size of tiny particles to massive boulders.
This sort of eruption can be deadly: as thick clouds of tephra race down the side of the volcano, they destroy everything in their path. Ash erupted into the sky and fell back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. Further, when the hot volcanic materials mix with nearby sources of water, they can create mudflows that have been known to bury entire communities alive. Mount Vesuvius, which obliterated the city of Pompeii, is an example of an explosive volcano.
The Volcanic Explosivity Index (VEI) is a scale used to measure the explosivity of a volcano. It has a range of 1 to 8 with a higher VEI indicating more explosivity. While the VEI of the current eruption at Mauna Loa is not known yet, the previous eruption in 1984 was deemed to have a VEI of 0. The highest VEI ever recorded in Mauna Loa has been 2 (in 1854 and 1868).
Some famous volcanoes
Any volcano that has erupted within the Holocene period (in the last 11,650 years) is considered to be “active” by scientists. “Dormant” volcanoes are those active volcanoes which are not in the process of erupting currently, but have the potential to do so in the future. Mauna Loa was a dormant volcano for the last 38 years. “Extinct” volcanoes are ones which scientists predict will never face any further volcanic activity. Ben Nevis, the tallest mountain in the UK, is an extinct volcano.
Here are some famous volcanoes in the world.
Krakatoa, Indonesia
One of the most catastrophic volcanic eruptions ever occurred in Krakatoa in 1883 (VEI 6). The volcano released huge plumes of steam and ash. The explosions were so brutal they were heard 3,100km away in Perth, Western Australia. According to the Dutch colonial authorities, Krakatoa’s eruption and the consequent tsunamis caused 36,417 deaths, though modern estimates peg the number to be much higher.
Mount Vesuvius, Italy
In 79 CE, Mount Vesuvius erupted (VEI 5), in one of the deadliest eruptions in European history, killing as many as 16,000 and destroying the town of Pompeii. According to scientists, the explosion released 100,000 times the thermal energy that was released with the atomic bombings at Hiroshima and Nagasaki. It is said to have instantly boiled the blood of all those who were too close to it. The explosion was described by Greek writer Pliny the Younger, who was present nearby, as being “sometimes bright and sometimes dark and spotted… more or less impregnated with earth and cinders.”
Mount Fuji, Japan
A defining image of Japan, Mount Fuji towers over the countryside with its snow-capped peaks and barren surface. It last erupted in 1707-1708 (VEI 5) and had a devastating effect on the local population. The tephra release led to significant agricultural decline, leading to widespread starvation in the Edo (now Tokyo) area. Although this eruption itself did not directly kill a lot of people, its subsequent impact proved deadly.
Eyjafjallajökull, Iceland
Sometimes referred to as E15, it is one of the many volcanic features of Iceland. In 2010, a relatively small eruption (VEI 4) managed to bring air traffic in Europe to a complete standstill. 20 countries closed their airspace, impacting approximately 10 million travellers.
Kīlauea, Hawaii
Adjacent to the Mauna Loa, this is one of the most active volcanoes on the planet. It has been erupting intermittently since recorded history, with its eruption lasting from 1983 to 2018 being the longest continuous eruption ever recorded. It is a major tourist attraction, with the earliest hotel built at the edge of the volcano in the 1840s.
