Volcanoes, powerful geological formations, shape our planet’s surface through eruptions that can be both destructive and creative. While some volcanoes frequently erupt, others remain quiet for extended periods. Among these, dormant volcanoes represent a distinct category, characterized by their prolonged inactivity yet retaining the potential for future eruptions. Understanding their definition and how they contrast with other volcanic types is important for assessing their potential impact.
What Defines a Dormant Volcano
A dormant volcano is defined as one that has not erupted for approximately 10,000 years but still possesses the underlying magma supply necessary for future activity. This classification implies a state of temporary rest, not a permanent cessation of volcanic processes. Geologists consider a volcano dormant if it has not erupted since the last ice age, but is still expected to erupt again. The continued presence of a magma chamber beneath the surface is a key indicator that a volcano is merely inactive rather than entirely spent. Volcanologists use various criteria to classify a volcano as dormant, focusing on historical eruption records and current geological observations. A dormant volcano could reawaken, potentially with little warning, posing a risk to nearby populations. For instance, Four-Peaked Mountain in Alaska, once thought extinct, began showing activity in 2006 and is now considered dormant.
How Dormant Volcanoes Differ
Differentiating dormant volcanoes from active and extinct ones involves important distinctions based on their eruptive history and internal state. An active volcano is one that has erupted within recent historical memory, typically within the last 10,000 years, or is currently showing signs of unrest such as gas emissions or seismic activity. The Global Volcanism Program, for example, considers a volcano active if it has erupted during the Holocene Epoch, which began around 11,700 years ago.
In contrast, an extinct volcano is one that has not erupted for a very long time, often exceeding 10,000 years, and is no longer expected to erupt again due to a lack of an active magma supply. The magma chamber and plumbing system within an extinct volcano are believed to have solidified, making future eruptions highly improbable. The classification can sometimes be challenging, as volcanoes can remain inactive for thousands of years before reawakening, blurring the lines between dormant and extinct.
Signs of Reawakening
Volcanologists continuously monitor dormant volcanoes for signs of reawakening. One indicator is an increase in seismic activity, which manifests as more frequent or stronger earthquakes and volcanic tremors beneath the volcano. These seismic events can signal the movement of magma or the fracturing of rock as pressure builds. Scientists use seismometers to detect these changes, and a migration of earthquake locations towards the surface can suggest magma ascent.
Ground deformation is another sign, where the volcano’s surface swells, tilts, or develops new fissures due to the intrusion of magma. This swelling, or inflation, can be measured with precise instruments, revealing even subtle changes in the ground’s shape. Changes in gas emissions, such as an increase in the release of sulfur dioxide or carbon dioxide, or alterations in steam color, also serve as clues. These chemical shifts can indicate that magma is rising closer to the surface, causing gases to escape. Changes in heat flow, like increased temperatures of hot springs or the appearance of new thermal areas, can point to renewed heat from an ascending magma body.
Examples Around the World
Numerous dormant volcanoes exist globally, each with its unique history of inactivity and potential for future eruptions. Mount Kilimanjaro in Tanzania, Africa’s highest peak, is considered dormant, with its last major eruption occurring approximately 360,000 years ago. While one of its three cones, Kibo, shows some signs of recent activity, it is still classified as dormant. Similarly, Mount Fuji in Japan, a prominent peak, has been dormant since its last eruption in 1707. Despite its long silence, it is still considered capable of erupting again, especially given its proximity to major population centers.
Mauna Kea in Hawaii, one of the five volcanoes forming the Big Island, is another notable example of a dormant volcano. Its last eruption occurred about 4,500 years ago, but volcanologists believe it retains the potential for future activity. Mount Rainier in the United States, although not having erupted for over 500 years, is also classified as dormant and is closely monitored due to its proximity to populated areas.