There’s something beautiful about a woman’s rage (not counting the tarts from Geordie Shore) and in no better way is this sentiment illustrated than by Mother Nature’s ire. As terrifying as it is to be at ground zero, from a safe distance, natural disasters are incredibly awe-inspiring and angry volcanoes deserve a top spot for making people go “ooooh” and “aaaaah” and “oh shit…”
Volcanoes are literal pathways from the Earth’s fiery guts to its crusty exterior. But the channels available for the molten rock and gas that spew forth are far too narrow to satisfy the sheer volume of indigestion within and the result is an immense build-up of pressure. The release of this pressure includes, but is not limited to, violent sprays of lava, devastating pyroclastic flows, stratospheric columns of volcanic ash, electrical storms, scalding gas and dust and Hiroshima-type explosions that not only dislocate millions of tonnes of solid rock, but have been reported to be audible many thousands of kilometres away from the point of origin.
Volcanoes have the potential to send species to extinction, yet at the very same time, they nourish the biosphere in an appreciable radius around them (volcanic ash is highly fertile). Volcanoes are magnificent and a wonderful example of how the surface of our planet is in a constant state of dynamism.
Where Not To Go On Summer Vacation
Volcanoes typically form at the convergent and divergent boundaries between the enormous shifting tectonic plates that comprise the Earth’s crust (see gorgeous image above). It is here that the seams of the Earth permit plumes of its molten interior to travel towards the surface. But as it was mentioned, the surface-bound transport of this material is anything but a six-lane highway. It’s more like a gravelly, pothole-ridden country road. The gas and molten rock that are trying to get from A to B encounter rigid rock and the cracks they exploit along their journey are incredibly narrow. A build-up of pressure results in a potentially explosive situation, so that when something finally gives, the results are disastrous for the local biology: human habitation included.
Volcanoes also form over features called “hot spots”, which don’t necessarily occur near plate tectonic boundaries (see diagram below). The Hawaiian Islands – all of them formed by volcanic activity in the middle of the Pacific Plate – are a prime example of this.
There are several scientific theories that seek to explain what hot spots are and a popular one is that they are upwelling intrusions of molten material (mantle plumes) that originate at the boundary between the Earth’s core and mantle. The exact depth of this varies, but the Hawaiian hot spot is estimated to be 3,000 km deep. That’s 9,842,520 ft. for those of you in ‘Merica.
There’s more to volcanology than your stock standard angry Earth pimple. Volcanoes come in many shapes, sizes and compositions. What happens at the surface – what we see and experience when volcanoes awake from their slumber – is dependent on a suite of factors and an especially important one is the composition of the magma that is trying to escape the lithified constraints of the crust.
Rock that is rich in silicates tends to form chunky, viscous slow-moving magma. This subset of liquid rock is in no hurry to go anywhere and tends to contribute to terrible congestion. It also has the particularly nasty habit of trapping gas, which is why things can get explosive. Since Hawaii is no stranger to seismic activity, its inhabitants have coined a word for this particular magma and it’s pāhoehoe.
At the other end of the spectrum, you get magma that doesn’t contain a lot of silicates, but is rather rich in ferrous (iron) compounds. This magma – ʻAʻa, pronounced “ah ah” – get’s extremely hot and tends to flow hard and fast. If you’ll excuse the crass analogy, the difference between pāhoehoe and ʻAʻa is much like the difference between constipation and Delhi belly.
Both, however, are extremely uncomfortable.
Magma isn’t, of course, one or the other. There is a vast spectrum of mineral compositions between, but by understanding the difference between one extreme and the other, we can begin to understand how different kinds of volcanoes are formed.
Cone, Shield and Stratovolcanoes
If there’s one thing to be said for geologists, it’s that they don’t mess around with terminology. The name bestowed upon a volcano is as transparent as a wet T-shirt.
Cone (Cinder) Volcanoes
Cone volcanoes, also known as cinder cones, generally consist of a hill that can be anywhere from 30 meters (98 ft.) to 400 (1,312 ft.) meters in height. Formed from the eruption of materials that are riddled with gas, crystals and a hodgepodge of fragmented rock. To see an example of this kind of volcano, put on your sombrero, crack open the tequila and get on a plane to New Mexico. There, you will find a spectacular volcanic field called Caja Del Rio, which comprises more than 60 cone volcanoes. If the prospect of New Mexico doesn’t appeal, you can always bum a lift on the next scientific mission to Mars or the moon, both of which are believed to feature this type of volcano.
Shield volcanoes have a much broader profile than cone volcanoes and, as the name suggests, are shaped like shields. Bet you didn’t see that one coming. These beasts are formed from the eruption of very runny lava that tends to escape the Earth’s crust before causing too much mayhem as a result of a build-up of pressure. Shield volcanoes are, by comparison, the placid elderly aunt of volcanoes and are most commonly found at oceanic tectonic boundaries. Oceanic plates aren’t usually rich in silicates, which explains why the magma produced here is more felsic in composition, hence its lower viscosity. Skjaldbreiður in Iceland (say that three times fast) is an example of a shield volcano. The Hawaiian Islands, which have formed almost smack bang in the middle of the Pacific Plate over a “hot spot,” are also shield volcanoes.
Stratovolcanoes, or composite volcanoes, are the tri-polar member of the volcanic family. They look like your typical volcano but actually consist of alternating layers of different kinds of erupted material as the above diagram depicts. Stratovolcanoes produce a range of eruptions depending upon their mood and these include chunky cinders, choking ash and molten rock (lava). One of the best known (and least loved) of these volcanoes is Mount Vesuvius, which is located in Stromboli, Italy. This one was responsible for the notorious levelling of the cities of Pompeii and Herculaneum in AD 79, killing 16,000 people. It is estimated that Mount Vesuvius released 100,000 times the energy liberated by the Hiroshima bomb.
When volcanoes become active, a number of things can happen, none of them good if you’re fond of life. One of the most devastating of these consequences is ash. You wouldn’t think so… ash is soft and white. How on Earth could it possibly inconvenience you the way a searing hot lake of lava might? Stratovolcanoes are especially fond of explosive eruptions, which send voluminous clouds of ash into the atmosphere and cascading down their slopes.
This ash, however, isn’t the kind you find in your barbeque pit after a night of camping, beer and sing-a-longs. It’s mixed with gas that is hot enough to disassociate your atoms. These eruptions send roiling clouds of gas, dust, ash and other debris down the mountain, which devastate anything organic in their path, leaving behind a scene that looks like a bomb went off in a cocaine factory.
Extinct, Dormant and Active Volcanoes: The Good, the Bad and the Ugly
Volcanoes are dangerous creatures. So an apt analogy for the popular classifications of these geological features would be your mother. When she has a gin and tonic in her hand (dormant), you may want to make plans for the evening. When she’s 10 G&T’s down (active), it’s time to execute those plans and get the hell out of the house. When she’s passed out on the couch (extinct), it’s safe to come home, although my recommendation to you would be to move out your childhood home and get yourself an education.
Extinct volcanoes, such as the Netherland’s Zuidwal and Shiprock volcanoes, are no longer considered to be active at all because they don’t have a supply of magma. They also have no documented history of indigestion. Dormant volcanoes, on the other hand, are known to have erupted at some stage in recent history. They may be quiet, but that doesn’t mean they can’t suddenly awaken. Mount Vesuvius (Gulf of Naples) was a purring kitten before it went psycho in AD 79, as was Mount Pinatubo (Philippines) prior to its epic tantrum in 1991. The latter is now considered an active volcano, which is one that has exhibited recent activity and is therefore a potential hazard to all within its vicinity.
If you’ve ever had a fight with Mexican food and lost (who hasn’t?) then integrating “Krakatoa” into your vocabulary is a wonderful idea if you need help explaining exactly what just happened to you to the flat mate who is next in line for the bathroom. You may not be absolved for your sins, but it’ll get you a laugh or two.
Krakatoa is a first class example of what happens when Mother Nature gets really cross and decides to let off a bomb that makes Hiroshima look like a fart. In 1883, the build-up of pressure under the Earth’s crust between the islands of Sumatra and Java in the Sunda Strait was so immense that it caused an apocalyptic-sized explosion, sending a once much bigger island into the stratosphere.
The Krakatoa eruption was reported to have been heard almost 5,000 km away (the loudest sound ever made in recorded history) and the resultant shock waves sent barograph needles oscillating violently off the page. Over 36,000 people were killed by the eruption: if not by the devastating pyroclastic flows and falling debris, then by the tsunamis that followed. The dust catapulted into the atmosphere caused stunning sunsets around the world for months after the eruption.
Too bad colour photography wasn’t in vogue in the 19th Century.
Class Dismissed: Your Take-Home Message
If you ever needed to respect the fact that we are just not in control of our natural environment, then stand next to an active volcano. From lakes of lava and earthquakes that shake the foundations of your stick hut to falling debris and scalding hot pyroclastic flows that choke the biosphere, volcanoes are creatures to be respected, studied and understood. If ever there were an item to put on your bucket list, it would be to stand next to an active volcano and feel the heat of Earth’s exterior lap at your cheeks. Just make sure you’ve ticked off the rest of those bucket list items before you do so…