Simply said, the most common way for categorizing volcanoes is based on the frequency of their eruptions. Those that erupt on a regular basis are referred to as "active," whereas those have erupted in the past but are now dormant are referred to as "dormant" (or "inactive"). Dormant volcanoes may become active at any time due directly to changes in their internal structure or indirectly due to changes in the external environment. However, they do not exhibit all the signs of activity observed at actively venting volcanoes.
Other factors used to classify volcanoes include the type of rock they consist of, the position they occupy with respect to other rocks in the Earth's crust, and evidence of recent activity such as lava flows or volcanic vents. Active volcanoes are usually high in the atmosphere while dormant ones tend to be low on the ground level. The term "volcano" is applied to these types of mountains because of the large amount of water vapor they emit into the atmosphere. These features are useful tools for determining how long ago a volcano last erupted by looking at geological samples returned by aircraft pilots during aerial surveys called "geo-logs."
Volcanologists use technology such as seismometers and thermal sensors to detect signs of activity at distant locations where conditions might not be favorable for direct observation. They also study the composition of volcanic gases released into the atmosphere over time to determine if the source material for an eruption is still present within the volcano itself.
One of the most basic classifications of volcanoes is based on their recent eruptive history and potential for future eruptions. Scientists use the phrases "active," "dormant," and "extinct" to describe this. It's difficult to tell if a volcano is active, inactive, or extinct, and volcanologists don't always get it right. But they do know how to identify major volcanic events - past, present, and future.
Volcanologists classify volcanoes according to how long it has been since their last eruption. If a volcano has not erupted in anything like an average lifetime of tens of thousands of years then it is called "extinct" or "dead." Volcanoes that have had recent activity but not within the last few hundred years are called "dormant." Finally, if a volcano has recently erupted then it is called "active".
All volcanoes are dangerous, but some are more so than others. The power of a volcano is measured by its volume of lava that flows from its mouth over time. Larger volumes of lava flow faster, producing larger mountains. Smaller volumes of lava flow slower, producing smaller mountains. The size of a volcano is one factor used to predict how likely it is to burst open with catastrophic results.
Volcanoes that have erupted in the past might be categorised as active or dormant. Volcanoes that have not erupted in their known history, on the other hand, are categorized as inactive or extinct. A few large volcanoes have been observed to change state from active to inactive or vice versa, so they can't be correctly classified based on current knowledge.
Some large volcanoes such as Mauna Loa on Hawaii have been observed to change state more than once during their histories. Based on this fact, it is possible that some smaller volcanoes may have gone through a similar process and are currently hidden under water. For example, the now-drowned island of Surtsey in Iceland was once an active volcano that recently collapsed due to tectonic activity.
Other large volcanoes have completely disappeared under ground level or have done so at one time but later re-emerged. Examples include Heimaey in Iceland, which was once one side of a volcanic island but has since been submerged by lava flows; and Krakatau, which was once the size of Rhode Island but is now only a small fraction of its original volume.
Smaller volcanoes often go undetected until after they have erupted. The islands of Vanuatu are made up of hundreds of small volcanoes that have all merged together over time into one big island group.
Dormant volcanoes are those that have not erupted in a long time but are projected to do so in the future. Mount Kilimanjaro in Tanzania, Africa, and Mount Fuji in Japan are two examples of dormant volcanoes. These volcanoes are not considered dangerous because they are not actively erupting at this time.
The term "sleeping volcano" is used to describe a volcano that has not emitted lava in a long time but is expected to do so again sometime in the future. Because these volcanoes are not emitting lava, there is no risk of them exploding. Sleeping volcanoes can be found in many places around the world, including Italy, Japan, Mexico, and Iceland. They can be very large; for example, Mt. Etna in Sicily is a sleeping volcano. It has not emitted lava in about 300 years but it is expected to do so again soon.
Mt. St. Helens in Washington was not actually dormant when it blew its top in 1980; rather, it had just finished an eruption several months before. But because it was not working hard at cleaning itself up after the last eruption, scientists think that it could be considered a sleeping mountain again now.
People live near both active and dormant volcanoes. When a volcano starts to emit gas, water vapor, and ash, it can signal that an eruption is coming.