This article is a travel topic
Volcanoes come in all sorts of shapes and sizes. Although the name evokes images of the conical mountain spewing ash and lava, such events are rare though often newsworthy, and are probably best seen on the TV news rather than witnessed up close and in person. However, many volcanoes around the world are tourist attractions in their own right and are worth a visit, so one can appreciate the awesome power of nature in person.
Some of the largest volcanoes on earth are not recognisable as such. For example, Yellowstone in the United States, Toba in Indonesia, and Taupo in New Zealand are known as super-volcanoes. They are generally in the form of large calderas, which are giant volcanic depressions formed either by large explosive eruptions or quiet long-term drainage of magma, and they often have associated lava flows or domes. Most supervolcanoes are explosive in origin and have undergone unimaginably huge eruptions in the geologic past. However, it is important to note that such volcanoes have erupted violently very infrequently, usually only once every 100,000-800,000 years. Hence, there is no reason to worry that Yellowstone, for example, will explode within your lifetime! Many are so big that, for many years, geologists did not appreciate that these features were volcanoes in their own rights. Compared to these, the currently active volcanoes on earth are relatively quite small!
Although there are literally hundreds of active volcanoes around the world, there are many more that are extinct. One can still appreciate the awesome power of nature from such places without the hazards that go with active volcanoes. A volcano is defined as active if it has erupted within the last 10,000 years (less than a second in geological time!), according to the Smithsonian Global Volcanism Program, the major volcano monitoring organization based in Washington, DC. One that is active but not currently erupting is considered dormant. Any volcano that has not erupted for more than 10,000 years and often, but not always, lacking geothermal heat and activity, may be considered extinct. (But this is problematic when dealing with supervolcanoes because of their long and slow cycles of activity, and not to mention that some supposedly extinct volcanoes can come back to life).
Throughout volcanic areas of the world, one may also encounter geothermal areas. These places are often, but not always, associated with volcanic activity. Hot springs, geysers, mud pools and fumaroles (steam/gas vents) are common features in geothermal areas, and can be scenic and hot springs can be great places to take a dip in. Countries such as Japan and Iceland are especially famous for hot spring baths. Geothermal areas are also an environmentally friendly source of energy, and Iceland takes advantage of this very well.
Yellowstone National Park is probably the best known example of a geothermal area, as molten magma lies not far beneath its 640,000 year-old caldera. New Zealand, with its volcanoes in the North Island, is also known for extensive geothermal areas.
If you are intending to venture into an area containing a highly active volcano, be aware that you are entering a potential hazard zone. Depending on the level of activity, the hazard zone may extend for several dozen kilometres, and there is the serious potential for pyroclastic flows and flying volcanic bombs, which are larger pieces of rocks thrown up by eruptions. Other associated hazards like volcanic ashfall and lahars (volcanic mudflows) can extend for hundreds of kilometres more.
Volcanic bombs can be up to the size of a soccer ball or even larger - enough to kill an unprotected person. Pyroclastic flows are clouds of red-hot ash and other volcanic debris that rush downslope from their source vents. They are some of the most dangerous parts of volcanic eruptions, as the temperature inside a pyroclastic flow can reach 400-700°C (~750-1300°F) and can reach speeds of up to 150 km/h (93 mph). Obviously, this means they can literally incinerate everything flammable in their path and then bury the area in hot ash. Also, volcanic ash tends to stop aircraft and vehicle engines, so transportation in the area may be restricted or disrupted. Public safety authorities may order hazard areas evacuated and would normally prefer that the casual tourist stay well away, but authorities are slow to act in much of the world. This does not eliminate dangers. Unless you have some very genuine reasons for going to these places, the best advice is to stay away and watch it from the safety of your home on the TV news.
When active volcanoes are not erupting (in other words, simply dormant), many of them can be approached reasonably closely in safety. However, when visiting active volcanic areas, there are still plenty of hazards even in dormant volcanic areas. New lava flows may still be very hot for months or even years afterwards and may be only covered with a thin crust of solid rock. Old lava flows can be as sharp as broken glass, so wearing a good pair of shoes or hiking boots are a must. Gases may be seeping out of vents, poisonous enough to kill. Towns many kilometers away may be prone to a lahar, basically old ash leftover from an eruption that acts like an avalanche or raging flow (that debris and ash may be many years old as the event that generated it forgotten) that becomes mobile by rainfall or earthquake. Lahars depending on size and speed, can be as devastating as an eruption itself, bury victims alive, and can kill thousands of people and bury and bulldoze homes, but unlike eruptions which give more obvious warning signs, lahars happen with zero warning and with often zero notice from authorities. Keep in mind lahars hazards during periods of heavy rainfall.
Even geothermal areas can be hazardous. Many of the hazards encountered in geothermal areas are similar to volcanic hazards, because both arise from similar geological mechanisms. Hot springs and mud pools can be boiling hot, acidic or downright poisonous, hence do not even try to take a dip or approach unless it is absolutely safe to do so. Geysers are a common feature of major geothermal areas, and can erupt hot water or mud unexpectedly. Landslides are also common, as even volcanic rock can become weakened over time with acidic fumes seeping out of fumaroles (steam/gas vents) or hot springs. Noxious gases may come out of vents or similar holes in the ground and may be concentrated in enclosed low-lying spaces such as caves, manholes, and pool enclosures. Carbon dioxide, a byproduct of volcanic activity, is notorious for this and can kill swiftly with little or no warning.
A selection of some of the more active and/or better known volcanoes in the world today. Some of these can still be explored from up close.
With 167 known active volcanoes, Indonesia is the world's most volcanic country by far. Volcano Adventure Indonesia, Sukapura, tel. +62-335-581439 , +62-81319090225,  runs specialist volcano tours all around the country.
North America and Caribbean
South and Central America
Papua New Guinea
The following travel agencies specialize in volcano tourism.