Some volcanoes are active because they are located on subduction zones where one tectonic plate sinks beneath another, causing rock to melt and magma to form. Dormant volcanoes are often found in more stable areas, where geological activity is less intense.
Magma, this burning liquid rock beneath the Earth's surface, largely determines a volcano's ability to erupt. Its texture, called viscosity, directly influences its activity: fluid magma, which is less thick, rises quickly and allows the gases it contains to escape easily, resulting in rather calm eruptions, like gentle lava flows. In contrast, very viscous and thick magma with a lot of silica traps the gases. The result: these gases accumulate, create enormous pressure, and eventually explode violently. The chemical composition, particularly the richness in silica, directly determines whether a volcano remains active or prefers to take a long nap.
Tectonic plates, like a giant puzzle, float on the Earth's mantle. When they pull apart, magma rises to the surface, creating active volcanoes along oceanic ridges, such as in the middle of the Atlantic with Iceland. Conversely, where two plates meet, one sometimes slides beneath the other (a phenomenon called subduction), causing partial melting of rocks and increased pressure. The result: intense explosive eruptions, like those in the Pacific "Ring of Fire." In some places, the plates move little, leading to less volcanic activity... hence dormant or extinct volcanoes.
Beneath our feet, the heat from the Earth's mantle circulates hot water through the rocks: this is known as geothermal activity. Where it is strong, this heat can gradually awaken a volcano by warming its dormant magma, making it more fluid and mobile. Hot springs or geysers that appear near a volcano are often a sign that something is happening below. This slowly rising heat makes the magma capable of circulating, moving, and rising to the surface again. In short, geothermal activity acts a bit like a discreet alarm: when it starts to heat up a little too much, the volcano needs to be monitored.
Active volcanoes often release large amounts of volcanic gases, particularly sulfur dioxide, carbon dioxide, and water vapor. These gases come directly from the magma rising to the surface. The greater the pressure exerted by these gases, the faster the magma rises, increasing the chances of an imminent eruption. It's a bit like a shaken soda bottle: the more gas that accumulates, the more likely it is to burst forcefully. In contrast, dormant or inactive volcanoes emit much less gas, indicating that there is little or no magma moving close to the surface. Analyzing the volume and chemical composition of these gases provides volcanologists with valuable clues about the degree of activity or rest of a specific volcano.
The history of eruptions allows scientists to determine the temperament of a volcano. A volcano that has regularly erupted its lava over the past few decades is likely to do so again; whereas a volcano that has been silent for several centuries may be quietly slumbering. But beware, some volcanoes known for their calmness have surprised us: a few awaken abruptly after years of sleep, which highlights the importance of closely observing their past. By studying ash deposits, layers of solidified lava, and accounts of ancient eruptions, we can more reliably predict whether a volcano is likely to return to the forefront or remain comfortably asleep.
The most active volcano in the world, Kīlauea in Hawaii, has been almost continuously erupting since 1983! Its relentless activity is related to the fluid composition of its magma and its location above a hot spot in the Earth's mantle.
The distinction between a 'dormant' volcano and an 'extinct' volcano can sometimes be tricky: a volcano may appear to be extinct for thousands of years before suddenly awakening, as the Chaitén volcano in Chile did in 2008 after 9,000 years of dormancy!
Submarine volcanoes make up an unknown majority of the planet's active volcanoes. They form new islands, especially when they emerge to the surface, as was the case with the Japanese island of Nishinoshima, which appeared suddenly in 2013.
Some volcanoes, known as 'supervolcanoes', have an exceptional eruptive capacity that can temporarily alter the global climate. This is particularly true for Yellowstone in the United States, which last experienced a major eruption about 640,000 years ago.
Unpredictable volcanoes can pose various major risks such as sudden lava flows, devastating pyroclastic flows, rock falls, or toxic clouds of ash and gas that may cause long-term disturbances in the surrounding areas.
Despite numerous scientific advancements, it is impossible to predict the exact moment when a dormant volcano will reawaken. However, monitoring instruments can detect warning signs such as earthquakes, magma uplift, and volcanic gas emissions.
Several signs may indicate that a volcano could become active, such as an increase in earthquakes, physicochemical changes in nearby water sources, or a significant rise in smoke and gas emissions.
An active volcano is a volcano that regularly shows signs of activity such as frequent eruptions, smoke, or gas emissions. A dormant volcano does not show immediate activity but could come back to life in the future. Finally, an extinct volcano has no more sources of magma and can no longer awaken.
Scientists use various monitoring tools such as seismographs to detect underground movements, gas sensors to identify unusual emissions, satellites to monitor topographical changes, and thermal cameras to detect temperature variations on the surface of volcanoes.
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