A volcano is a rupture in the Earth’s crust allowing molten rock, ash, and gases to escape from a magma reservoir below the surface. Scientists use the activity level of these structures to evaluate risk and plan for hazard mitigation. Volcanologists classify a volcano’s status into three categories based on its current state and potential for future eruption. This classification determines the necessary level of monitoring and preparedness for nearby communities.
Active Volcanoes
An active volcano is currently erupting or shows measurable signs that an eruption is highly probable. The standard definition includes any volcano that has erupted within the Holocene epoch (the last 11,700 years). This indicates a connected magmatic system capable of future eruptions. Classic examples include Mount Etna in Italy and Kīlauea in Hawaii.
Even when quiet, active volcanoes exhibit distinct precursors signaling magma movement beneath the surface. Scientists track these signals using continuous monitoring to forecast potential activity. Seismometers detect volcanic earthquakes caused by rock fracturing as magma forces its way upward. Ground deformation is measured using GPS and satellite radar, detecting swelling due to increasing pressure.
Gas sensors measure the composition and flux of volcanic gases, such as sulfur dioxide and carbon dioxide, which increase as magma rises closer to the surface. Continuous observation allows for real-time analysis and provides early warnings. A volcano remains active, even if quiet, as long as subsurface magmatic and hydrothermal activity persists.
Dormant Volcanoes
A dormant volcano is an active volcano not currently erupting but expected to erupt again in the future. This classification applies to volcanoes that may not have erupted for hundreds or thousands of years. The distinction between dormant and “resting” is ambiguous, relying on subtle geological evidence. Dormant systems still possess a functioning magma reservoir, but the supply is temporarily sealed or pressure is insufficient to force an eruption.
The risk associated with a long-dormant system can be high because the repose period allows pressure to build, potentially leading to a more violent eruption. Mount Vesuvius in Italy, which destroyed Pompeii in 79 AD, is a prime example; its underground system remains active despite no major eruption since 1944. Classifying a volcano as dormant requires continuous, long-term monitoring to detect residual activity, such as geothermal vents or subtle ground movement.
Volcanoes can remain silent for vast periods, leading to misclassification as extinct before they unexpectedly erupt. Mount Pinatubo in the Philippines was dormant for over 400 years before its massive 1991 eruption, demonstrating the difficulty in assessing these sleeping giants.
Extinct Volcanoes
An extinct volcano is one that scientists believe will never erupt again because its magma supply has been permanently cut off or the heat source has solidified. Classification requires evidence of no activity for tens of thousands of years, extending beyond the 11,700-year Holocene threshold. This time frame is considered long enough for the magma chamber to crystallize into solid rock, making future eruption impossible.
The appearance of extinct volcanoes often differs dramatically from active or dormant ones due to extensive weathering and erosion. Over immense geological timescales, wind, water, and ice wear down the cone, sometimes leaving behind only the solidified magma conduit, known as a volcanic neck. While considered safe, the classification is not entirely absolute, as extreme geological processes could theoretically reactivate an ancient region.
The lack of recent eruptions, seismic activity, or gas emissions, combined with geological analysis showing significant erosion, provides the scientific basis for this classification. Extinct volcanoes offer valuable insight into Earth’s history, allowing geologists to study past conditions and the long-term impact of volcanic activity.