With volcanic activity in Iceland catching the attention of the world (and the thousands of people who gathered to watch it), the first thing many people are thinking is 'wow, what a sight' with the second being 'is that even safe?'. With the eruption that occurred near Fagradalsfjall, many people saw the sheer number of people gathered at volcanic hotspots with lava exuding from fissures, appearing like cracks in the earth, and wondered how dangerous it really was. While some tourists were evacuated due to safety concerns, much of the lava-viewing took place a good distance away from the volcanic activity where the hiking trails around the volcano did not cross the path of lava beds.
With that being said, it doesn't mean there's no threat in a volcanic eruption no matter how big or how small. There are threats released into the atmosphere that are micrscopic and may not even be accomplanied by lava and things we typically associated with an eruption. According to the Government of Canada website, there are a total of ten hazards that can be caused by volcanic activity and in some cases, they can go on for miles surrounding the volcano itself.
The interesting thing about lava is that while it's the one thing most commonly associated with volcanoes, it's also the one thing that holds the least threat to humans. With that being said, caution should always be excercised when there's an active lava flow because while the slow-moving, viscous liquid might not reach a person quickly, it can still be the cause of landslides, infrastructure damage, and can start a domino effect that isn't realized until it's too late. Between the significant amount of heat lava brings with it and the sheer force of molten rock behind it, it's an unstoppable force.
These are explosions that are usually seen from a distance as they happen when lava connects with water to cause a reaction. When the lava, which is far above the temperature of boiling, hits any surrounding water - especially if the water is a lower temperature - it can cause an upheaval of smoke, debris, and liquid in the form of an explosion.
As opposed to just a typical lava flow, pyroclastic flows occur when debris cascade down the sides of a volcano instead. Obviously, these are incredibly dangerous to anyone spectators or residents who might be at the base of the volcano and, since these flows contain things such as rock and ash, they can carry more of an immediate threat than a slow-moving lava flow.
The threat of lahars is mainly to those who might be residing close to the volcano and its base. These occur when streams or rivers of water carry down debris from the explosion in a similar manner to a landslide but with less force. They can be dangerous if larger items are picked up by the moving current, such as boulders, trees, and anything else that could cause heavy damage to buildings or people.
The only time that jökulhlaups ever really become a concern is when a volcanic eruption occurs under a glacier. When this happens, the immediate threat is flooding due ot the displacement of the water during the explosion, leading to flash flooding.
The common thought when it comes to landslides is that they're the most dangerous when volcanic activity is happening presently but the reality is actually very different. Even a dormant volcano can cause landslides and they could be triggered by something like an earthquake related to a potential eruption. With an active volcano, these can be caused by any number of things including lava, lahars, or flows.
When it comes to tectonic plates, all of these natural events are usually interconnected. Tsunamis can occur just as earthquakes can, and usually come to fruition during underwater eruptions. Rarely, they can also be triggered by land eruptions, when massive amounts of debris and lava enter a body of water and cause a chain reaction when that water is displaced.
These specific tremors are called volcanic-tectonic earthquakes and are usually miniscule in size, but are still measured and recorded as earthquakes. Rather than causing damage themselves, these smaller earthquakes are usually a warning sign - or precursor - to a bigger event, such as a volcanic eruption.
Volcanic gases pose a serious threat to anyone who happens to live in the cloud cover zone, as these clouds of noxious gases often contain carbon dioxide, water vapor, sulphur dioxide, hydrogen sulfide, hydrogen, and carbon monoxide. Long-term exposure to these vapors can cause lifelong problems but even short-term exposure can cause lung irritation, breathing problems, and can be irritating to the eyes, nose, and mouth. Significant amounts of gases can eventually alter the climate.
By definition, tephra means small rock fragments that are sent rocketing into the air during an eruption. Volcanic ash is also technically considered to be tephra but on a much smaller scale, and both pose health risks. Being within the range of tephra can be dangerous for onlookers and this debris has been known to travel up to thousands of kilometers away from a volcano.