A volcano is an opening in the crust of the earth from which steam, hot gases, and ashes are expelled. It is also a conical hill, or mountain with a central crater. A volcano can erupt when the low density of magma causes it to seep through cracks into places of lower pressure, where it dissolves gases, expands, and rises to the surface. The eruption occurs, depending on the temperature of the magma, gas content, and composition, and if the vent is on land, in water, or under ice.

An eruption is the explosion of volcanic materials onto the surface, either from a central vent, a fissure, or a group of fissures. Volcanoes can erupt in any type of season. A volcano can even erupt in winter, but it usually is a dry eruption. Magma that has erupted is called lava; magma is molten rock within the earth's crust that is capable of intrusion into adjacent crustal rocks or extrusion onto the surface. It depends on how thick, runny, or sticky the lava is to see if there would be an explosion or not; if the magma is thick and sticky, pressure will build up and cause an explosion. Eruptions can come from the side or top of the volcano. An eruption is caused when pressure builds in a magma chamber, and magma gets forced through the conduit and out of the vents. The pressure of magma inside builds up so much that the volcano splits and the magma is forced out.

The Inside of a Volcano

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Types and Effects of Volcano Hazards

Volcano Landslides

Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. These mixtures of debris move in a wet or dry state, or both. Landslides commonly originate as massive rockslides or avalanches which disintegrate during movement into fragments ranging in size from small particles to enormous blocks hundreds of meters across.

Explosions (red) begin to rip through the landslide (green)

Exploded rock debris (red) forms a pyroclastic surge that quickly overtakes the landslide (green)

Explosive Eruptions of Magma

Mount St. Helens erupts a pyroclastic flow on July 22, 1980. Explosive eruption of magma and solid-rock fragments or the collapse of a vertical eruption column of ash and larger rock fragments may generate pyroclastic flows. A series of photographs of an eruption at Mount St. Helens on July 22, 1980 shows the development of a pyroclastic flow.

Pyroclastic flows descend on the south-eastern flank of Mayon Volcano, Philippines. Maximum height of the eruption column was 15 km above sea level, and volcanic ash fell within about 50 km toward the west. There were no casualties from the 1984 eruption because more than 73,000 people evacuated the danger zones as recommended by scientists of the Philippine Institute of Volcanology and Seismology.(Photograph by C.G. Newhall on September 23, 1984)

"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994

Ancient volcanic rocks are preserved in many parts of Maine, but there have not been any active volcanoes since the Mesozoic Era. An interesting aspect of Maine's bedrock is that it records a wide array of geologic environments that have been present. All three major rock types, igneous, sedimentary, and metamorphic, are represented.

Visit the geological time scale.

Maine's Volcanic Rock

Maine Granite
Maine granite comes in many colors and textures, with each quarry yielding its own variety of stone. Activity and competition in the granite industry were highest in the late 1800's, reaching a peak in 1901. Many large public buildings such as libraries, post offices, customs houses, and museums built at that time in the eastern U. S., including New York City, Washington, DC, and Chicago are made of Maine granite. Although they are almost all inactive, the old quarries still dot the landscape, mainly in the coastal region from Penobscot Bay to Washington County.

Volcanic Islands
During the protracted geologic history that has brought Maine to the current stage, the movement of continents by the mechanism of plate tectonics has caused the earth's surface to evolve. Volcanic rocks and distinctive fossil shells show that some of Maine's rocks formed on volcanic islands in a wide ocean that no longer exists. Geologic similarities between parts of the Maine coast and parts of Newfoundland and Wales suggest that these areas formed together on a small continent far away from North America.

Look at the volcano diagram.

   http://www.volcano.si.edu/gvp/usgs/index.htm

Weekly report of worldwide volcanic activity prepared by the USGS Volcano Hazards Program and Smithsonian Institution's Global Volcanism Program.