The study of volcanos is called volcanology (or vulcanology in some spellings).
Mount Saint Helens is found on the margin between the oceanic Juan de Fuca Plate and the continental North American Plate.
A volcano generally presents itself to the imagination as a mountain sending forth from its summit great clouds of smoke with vast sheets of flame, and it is not unfrequently so described. The truth is, however, that a real volcano seldom emits either true smoke or true flame. What is mistaken for smoke consists merely of vast volumes of fine dust, mingled with much steam and other vapours -- chiefly sulphurous. What appears like flames is simply the glare from the glowing materials which are thrown up towards the top of the mountain--this glare being reflected from the clouds of dust and steam.
The most essential part of a volcano is the crater, a hollow[?] basin, generally of a circular form. It is often of large dimensions, and sometimes of vast depth. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on the top of a mountain, which in some instances towers to an enormous height. The part of the mountain which terminates in the principal crater is usually of a conical form -- much like a glass-house chimney, and is therefore named the cone. It is generally composed of loose ashes and cinders, with here and there masses of stone, which have been tossed into the air by the volcanic forces. In some mountains the cone rises out of a hollow at a considerable height from the base. A hollow of this kind is generally regarded as having been a former crater, which had become extinct before the existing cone was raised. There are sometimes formed lower down the mountain subordinate craters, smaller than that which occupies the summit of the cone. Within the crater itself there are frequently numerous little cones, from which vapours are continually issuing, with occasional volleys of ashes and stones.
One way of classifying volcanoes is by the type of material erupted, which also affects the shape of the volcano:
Volcanoes are usually situated either at the boundaries between tectonic plates or over hot spots. Volcanoes may be either dormant (having no activity) or active (near constant expulsion and occasional eruptions), and change state unpredictably.
Volcanoes on land often take the form of flat cones, as the expulsions build up over the years. Under water, volcanoes often form rather steep pillars and in due time break the ocean surface in new islands.
Science has not yet been able to predict with absolute certainty when a volcanic eruption will take place, but significant progress in judging when one is probable has been made in recent time. Volcanic activity is often accompanied by earthquakes, hot springs, fumaroles, solfatare and geysers. Low-magnitude earthquakes often precede eruptions.
Surprisingly, there is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of activity. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless but do not actually erupt. Are these volcanoes active?
Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years.
Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again.
Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. For example, since calderas have lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Yellowstone caldera[?] in Yellowstone National Park is at least 2 million years old and hasn't erupted for 70,000 years, yet scientists do not consider Yellowstone as extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system, and rapid rates of ground uplift, many scientists consider it to be a very active volcano.
Public domain picture from NASA of Tambora, Indonesia.