Volcanoes Help

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By — McGraw-Hill Professional
Updated on Sep 4, 2011

Introduction to Volcanoes

Suddenly the ground jerks and a roar like a thousand jet engines is heard. The air becomes a searing furnace with smoke and ash raining down from the sky. Is this something from a science fiction novel or “movie magic?” No, it’s just another of Mother Nature’s temper tantrums, known commonly as a volcanic eruption .

Volcanoes have erupted with hot magma, from pressure buildup in the Earth’s interior, since the planet was formed. Magma from deep within the earth is sent out onto the surface in many different forms and intensities.

A volcano is a mound, hill, or mountain formed from hot magma exiting the Earth’s crust and piling up on the land or beneath the seas.

Ancient humans thought volcanic eruptions were caused by their bad behavior and the gods’ resulting displeasure. Some cultures even thought that if they threw some unlucky sacrifice into the crater, that the gods would get happy again. Unfortunately for the sacrifice and any villages in the path of an erupting volcano, this tactic didn’t work very well.

Native Americans in the Pacific Northwest told early explorers a legend about the active Mount St. Helens. In fact, the Indian name for the mountain, Louwala-Clough, means “smoking mountain.” According to one legend, the snow-capped mountain was once a beautiful maiden. When two sons of the Great Spirit fell in love with her, she couldn’t choose which one she liked best. The two braves fought over her, burying villages and forests in the process. The Great Spirit was not pleased. He smote the three and erected a mighty mountain peak where each fell. Because the maiden was beautiful, her mountain (Mount St. Helens) was a beautiful, symmetrical cone of dazzling white. One brave (Mount Hood) lifts his head in pride, but the other (Mount Adams) wept to see the beautiful maiden wrapped in snow, so he looks downward.

In the days of the Romans, Vulcan was not the home planet of a race of pointy-eared aliens of television’s and movies’ “Star Trek,” but instead the name of the Roman god of fire and the blacksmith to the gods. Long ago, it was thought that an ancient island off the coast of Sicily was the location of Vulcan’s home and smithy. Local people thought whenever neighboring volcanoes smoked, Vulcan was heating his forge. When noise and vibrations were felt in nearby villages, people thought Vulcan was hard at work hammering on his anvil. The only problem was that sometimes he got carried away and caused an eruption. The word volcano comes from the Latin word vulcanus which means “fire breathing.”

The Romans experienced this in a big way when the city of Pompeii was destroyed by the eruption of Mount Vesuvius in AD 79. The inhabitants are thought to have had notice of the explosion, but chose to stay in the city, only to be buried by the unbreathable, searing ash.

When referring to volcanic materials ejected during an eruption, most volcanologists talk about volcanic dust , ash , cinders , lapilli , scoria , pumice , bombs , and blocks . These will be explained separately as different eruption types and volcanoes are described. If you want to talk about all the things that shoot out of a volcano, the word tephra is used. Table 11-1 shows the size differences of single and combined tephra.

Table 11-1 Tephra rocks and tephra clumps have different names depending on size.

Particle size (mm diameter)

Tephra (single)

Pyroclastic rock (combined material)

< 2 mm


ash tuff

2–64 mm


lapilli tuff

> 64 mm




Tephra includes all the different types of matter that is sent blasting out of a volcano compared to bubbling, flowing lava.

In 1751, French geologist, Jean Guettard discovered evidence of ancient volcanic activity in the south of France near Auvergne. The highest peak in that chain of 50 extinct volcanoes is Puy de Dome at 1465 m. No one thought volcanoes had ever existed in Western Europe and scientists were stunned at the time of Guettard’s news. Ten years later, Nicholas Desmarest became so interested in Guettard’s discovery that he began mapping newly recognized lava flows and volcanic mounds in the area. Since they had been worn mostly away by erosion, people had not previously realized what they were.

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