Some volcanoes have effusive eruptions because their magma is more fluid and less viscous, while others have explosive eruptions due to the high content of dissolved gases in the magma, creating significant pressure.
Viscosity is basically the ease with which magma flows or doesn't. A very viscous magma, like a paste, struggles to release the gases within it. This creates a buildup of internal pressure until BAM! It explodes violently: that's the explosive eruption. In contrast, fluid magma, which is less viscous, allows gases to escape easily without much obstruction. So it flows calmly, forming flows that gently slide down the sides of the volcano. This is the effusive eruption, which is quite cool in terms of spectacle but less impressive than the other. Magmas rich in silica, like rhyolite, are often highly viscous, while magmas low in silica, like basalt, flow easily. That's why Hawaii, which produces a lot of low-viscosity basaltic lavas, offers spectacular flows but without major explosions. Conversely, a volcano like Mount St. Helens, with its pasty magma, is capable of extremely violent and destructive eruptions.
When the amount of dissolved gas in the magma is high, it changes everything. The higher the magma rises to the surface, the lower the pressure becomes, and that's when the gases begin to escape very quickly: a bit like opening a vigorously shaken soda bottle. The result: a violent release that propels materials into the air, giving rise to an explosive eruption. Conversely, if the magma contains little gas, it escapes quietly without much agitation and flows slowly in the form of a lava flow: this is what we call an effusive eruption, much calmer and more regular.
Volcanoes located in subduction zones, where one tectonic plate dives beneath another, tend to produce rather explosive eruptions. The reason? The magma generated in these contexts is often thick, viscous, and gas-laden, creating significant pressure in the conduit until it violently erupts. This is typically the case with Mount St. Helens or Japanese volcanoes.
In contrast, volcanoes located in hot spot contexts (like in Hawaii) or on mid-ocean ridges (like in Iceland) release fluid magma from deeper origins. This magma is less viscous, allowing it to flow gently to the surface, enabling gases to escape without major violence. The result: peaceful lava fountains and flows instead of cataclysmic explosions.
When magma is very hot, like that of Hawaiian volcanoes, it becomes more fluid and flows easily, resulting in rather effusive eruptions. In contrast, cooler magma will be viscous and will struggle to flow gently, causing a buildup of pressure and more violent explosions. Chemical composition also plays a role: magmas rich in silica (such as rhyolite or andesite) are thicker and more viscous, trapping gases and leading to highly explosive eruptions. Conversely, magmas low in silica, typically basaltic, easily release their gases, flowing quietly down the sides of the volcano in beautiful flows of fluid lava.
The shape of the volcanic conduit directly affects the style of the eruption. A narrow or rugged conduit hinders the ascent of magma, leading to pressure buildup. As a result, gases accumulate, and when it erupts, it's seriously explosive. In contrast, a wide, smooth, unobstructed conduit allows magma to escape easily without too many obstacles, resulting in rather fluid and calm flows (effusive eruptions). Similarly, a branched or winding conduit can trap more gas, increasing the risk of spectacular and highly explosive big bangs.
Effusive volcanoes can form shield volcanoes with gentle slopes, like Mauna Loa in Hawaii. In contrast, explosive eruptions often create stratovolcanoes with steep slopes, such as Mount Fuji in Japan.
The famous eruption of Krakatoa in 1883 produced a sound that was audible from nearly 5,000 kilometers away, making it one of the loudest sounds in recent history. This extremely explosive eruption demonstrates the immense energy potential of explosive volcanoes.
In Iceland, located on a divergent tectonic plate boundary, volcanic eruptions are generally effusive, slowly releasing very fluid basaltic lava flows that often attract tourists and scientists from around the world.
The viscosity of magma, closely linked to its silica content, plays a crucial role in the type of eruption. The higher the silica content in the magma, the more viscous it becomes, thereby promoting explosive eruptions due to the increased trapping of volcanic gases.
Before an eruption, an increase in seismic activity, changes in composition, and a rise in gas emissions are often observed, along with a gradual deformation of the volcano caused by the pressure of magma accumulating underground.
In general, explosive eruptions are more dangerous in the short term as they violently eject gases, hot ash, and fragmented materials that can quickly impact large areas. However, effusive eruptions, with their slow lava flows, can also pose a serious long-term threat to the environment and infrastructure.
Scientists use complementary analyses such as the chemical composition of magma, its temperature, as well as monitoring volcanic gases and earthquakes to predict the style of eruption. Magma rich in silica and gas is more likely to cause an explosive eruption, while fluid magma low in gas typically leads to an effusive eruption.
A classic example of a volcano with effusive eruptions is Kilauea in Hawaii, known for its frequent and relatively harmless fluid lava flows. In contrast, Mount St. Helens in the United States and Vesuvius in Italy perfectly illustrate explosive eruptions, which are violent and potentially devastating.
It mainly depends on the geological context and the rate at which magma is produced and accumulated beneath the volcano. For example, volcanoes located at hot spots or at the boundaries of active tectonic plates may have their magma chamber replenished frequently, while others, situated in more stable geological contexts, slowly accumulate magma over long periods before a new eruption.
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