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Arenal volcano at night.
We all know what volcanoes are capable of doing. Whether it’s hot, rapidly-moving lava or falling ash, the effects of volcanoes are downright dangerous — killing people, animals, and plants, destroying houses and buildings, drying up fields, and in general, causing large-scale havoc. On a broader extent, volcanoes can even cause changes in the climate. But before we go on and on about what a volcano can do, let’s first answer the basic question “What is a volcano?”
Deriving its name from the Roman god of fire Vulcan, a volcano is an opening or vent in a planet’s surface that allows the release of liquid rock known as magma, hot ash, and gases from out of the most inner depths of that planet. As magma erupts through the Earth’s surface, it is already known as lava.
Lava is the substance that builds up and around the surface opening, usually a mountain landform, reinforcing its cone shape. But a conical, mountain-shaped volcano is only one of the many volcano shapes developed, depending on the size, style, and frequency of eruptions.
Shield volcano. Don’t expect to see any catastrophic explosions from this type, as it is characterized by low-viscosity lava flowing out of the vent and running far distances, thereby forming expansive, shield-like profiles. Shield cone volcanoes are commonly found in continental settings like Hawaii and Iceland.
Composite volcano. Also known as stratovolcanoes, composite volcanoes are the tall, conical mountain volcanoes formed from layers of lava, ash and cinders. Because the magma in this type of volcano is cooler and therefore viscous, gas bubbles find it difficult to expand and break out, causing tremendous pressure buildup within the volcano. What results is an explosive eruption from which lava flows and volcanic ash are released, forming the steep cones.
Caldera volcano. An explosion from a caldera volcano is of such great proportions that it can partly or completely empty the volcano’s magma chamber. With the area around the opening left unsupported, the volcano caves in, unable to carry its own weight. This leaves a large, circular depression, the diameter of which can measure up to several kilometers. While a caldera volcano is a rarity, its eruption is one of the most dangerous and the effects, long-lasting.
While not as widely-known as the above types of volcanoes, there are also submarine volcanoes which are found on the ocean floor, and subglacial volcanoes that develop underneath icecaps.
Other than their style and extent of eruptions, a volcano can also be classified as being active (erupting or likely to erupt), dormant (one that has not erupted for a long time), or extinct (unlikely to erupt ever again because of the absence of lava).
So, what is a volcano? Obviously, we’ve only touched the surface of a volcano’s features and the subsequent effects of an eruption, but at least for now, we’ve got the basics covered.
Plate tectonics is the reason why we have volcanoes. You can read more about plate boundaries here in Universe Today. Here are the links:
Here are the links of two more articles from USGS:
Main Types of Plate Boundaries
Understanding Plate Motions
Here are two episodes at Astronomy Cast that you might want to check out as well:
Plate Tectonics
Volcanoes, Hot and Cold
Reference:
USGS Volcanoes Page
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