Volcanoes are geological features that shape Earth’s surface. Classifying them by activity levels is crucial for scientists to assess potential hazards and monitor Earth’s crust. This classification goes beyond whether a volcano is currently erupting, delving into its long-term potential for activity.
Defining an Active Volcano
An active volcano is defined as one currently erupting or with potential to erupt. Volcanologists commonly classify a volcano as active if it has erupted at least once within the Holocene Epoch, which began approximately 11,700 years ago at the end of the last ice age. This timeframe is a widely accepted criterion. This definition does not mean the volcano is erupting at this moment, but signifies a youthful magmatic system capable of future eruptions.
Some interpretations extend the definition to include volcanoes that show signs of unrest, such as unusual earthquake activity or significant gas emissions, even without a recent eruption. This broader view recognizes activity from ongoing geological processes beneath the surface, not just visible eruptions. For instance, a volcano might not have erupted for thousands of years but still exhibits magma movement or heating, indicating its active status. A magma chamber retaining liquid material supports its potential for future activity.
Key Indicators of Activity
Scientists use several phenomena to determine if a volcano is active and assess its eruption potential. One primary indicator is increased seismic activity, including more frequent and intense earthquakes near the volcano. These tremors often result from magma moving underground, fracturing surrounding rock as it ascends or shifts. Volcanic tremors, continuous ground shaking, also signal magma movement and can precede an eruption.
Changes in gas emissions serve as another significant indicator of volcanic activity. As magma rises and decompresses, dissolved gases like water vapor, carbon dioxide, and sulfur dioxide are released through cracks and vents. An increase in the volume or a change in the composition of these gases, particularly sulfur dioxide, can signal that magma is nearing the surface.
Ground deformation, involving swelling or sinking of the surface, also points to an active state. This deformation occurs as magma accumulates beneath the volcano, causing the surface to inflate, or as it withdraws, leading to subsidence.
Active, Dormant, and Extinct
Volcanoes are commonly categorized into active, dormant, and extinct states, though the boundaries between these classifications can sometimes be nuanced. This category includes volcanoes that are actively erupting, like Kīlauea in Hawaii, or those that have erupted recently and are expected to erupt again.
A dormant volcano is one that has not erupted for a considerable period but retains the potential for future eruptions. This term is often used colloquially for volcanoes that are not currently erupting but are still considered active because they could erupt again. Mount St. Helens, for example, had a period of quiet between 1857 and its dramatic 1980 eruption, demonstrating a dormant phase within its active lifespan.
In contrast, an extinct volcano is one scientists believe will not erupt again. This typically applies to volcanoes that have not erupted for more than 11,000 years, with the assumption that their magma chambers have solidified. However, instances of “extinct” volcanoes showing renewed activity demonstrate the inherent challenges in definitive classification.
Monitoring Volcanic Activity
Scientists monitor active volcanoes using various techniques to detect changes that might precede an eruption. Seismographs detect and analyze earthquakes and ground tremors caused by magma movement beneath the surface. Precise ground deformation measurements, utilizing Global Navigation Satellite Systems (GNSS) like GPS and tiltmeters, track swelling or subsidence of the volcano’s flanks. These instruments can detect changes as small as a few millimeters.
Gas sensors and aerial surveys measure the composition and volume of gases emitted from fumaroles and vents, providing insights into the magma’s depth and activity. Changes in heat flow, detected through infrared cameras and satellite imagery, can also indicate magma rising closer to the surface. By integrating data from these various monitoring systems, volcanologists can establish baseline behaviors and identify deviations that signal increasing unrest, aiding in the assessment of eruption probability.