What is a volcano?
A volcano is simply a hole or vent in Earth's crust through which molten rock, steam and other gases come forth. Scientists group volcanoes into four main kinds - cinder cones, strato or composite volcanoes, shield volcanoes, and lava domes. Volcanoes take their name from the island of Vulcano in the Mediterranean Sea. Long ago people thought this island mountain was the chimney of the blacksmith forge of the Roman God Vulcan. The steam and ash that came out of the vent was a sign that Vulcan was working at his forge making weapons for Jupiter and Mars. In Hawaii and other Polynesian islands, local people once attributed volcanic eruptions to the Goddess Pele. They believed Pele was moving from island to island as she sought to escape her evil sister, Na Maka O Kaha'i, the goddess of the sea. Today scientists understand that volcanic eruptions are surface reminders of Earth's still hot interior.
Where do volcanoes occur?
Volcanic eruptions do not occur just anywhere. Sixty percent of all active volcanoes are found at crustal plate boundaries such as the Pacific Plate, which has become known as the Ring of Fire because of the active volcanoes on its perimeter. Earth's crust, like the cracked shell of a hard boiled egg, is broken into a number of "plates." These floating pieces of the crust are moving about very slowly on the hotter interior. Where the plates are moving apart or colliding with one another, volcanoes may form. Many volcanoes form oceanic islands in the Pacific Ocean or Mediterranean Sea. These volcanoes formed over "hot spots" in the crust and mantle. The Northern Hemisphere has approximately two thirds of the land-based volcanoes.
Are all volcanoes dangerous?
Not all volcanoes erupt and not all eruptions are explosive. Volcanologists label volcanoes active, dormant, and extinct, depending on the likelihood of an eruption occurring. Volcanoes that will never erupt again are considered extinct. Active volcanoes come in two classes - volcanoes which are active, either erupting now or having recently erupted and dormant volcanoes, volcanoes which are currently quiet but are expected to erupt in the future. About sixty volcanoes are actively erupting each year.
Not all volcanoes erupt explosively. The style of eruption (quiet lava flows versus violent explosions of gases, ash, and debris) and frequency of eruption are related to the viscosity and amount of dissolved gas in the magma. Hot, runny magmas with little dissolved gas tend to flow smoothly out of vents and produce broad gentle volcanoes. The shield volcanoes of Hawaii are examples of this type of eruption. While such flows are not especially dangerous to humans, they can destroy buildings and agricultural land.
Somewhat cooler magma with more dissolved gas is more viscous. The magma does not run smoothly, but rather oozes out like toothpaste, clogging the vent. As the magma rises closer to Earth's surface, the pressure decreases and gases dissolved in magma separate from the liquid. If the gases cannot escape, pressure builds. When the pressure from the trapped gases exceeds the pressure of the overlying rock, an eruption occurs. This is typically a sudden violent blast sending particles as far as 20 miles high and many miles away from the volcano. The erupted material can range in size from tiny particles of ash to house-size boulders. Commonly there is little, if any, lava extruded. Such eruptions can be very dangerous and even deadly. It is difficult to predict when a long dormant volcano will become active. Because they erupt infrequently, unpredictably, and violently, and because they occur in populated areas, these explosive volcanoes pose the greatest danger to humans.
What are Stratovolcanoes?
Stratovolcanoes, also called composite volcanoes, are typically steep-sided, symmetrical cones built of alternating layers of lava flows, volcanic ash, and other eruptive products. Most stratovolcanoes have a crater at the summit containing a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. Their stable construction allows stratovolcanoes to rise as much as 2,400 meters above their bases. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in Earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc., are added to its slopes. These volcanoes are most often found at crustal plate boundaries.