As you read this sentence, chances are that molten rock is bleeding from Italy's Mount Etna in Sicily, volcanic bombs are exploding out of Arenal volcano in Costa Rica, and lava rivers are pouring from Hawaii's Kilauea volcano. Volcanologists, the men and women who study these living mountains, expect such behavior. In fact, right now about 10 volcanoes are erupting somewhere on Earth. It's a natural way for our planet to release pent-up energy.

About 1,300 are potentially active (meaning they've erupted in the past 10,000 years), and most lie beneath the world's oceans. It's the volcanologists' job to monitor these “windows” to Earth's interior. Their biggest challenges: to predict when a volcano will erupt, to educate local officials about potential hazards, and to set up early-warning systems. These measures can save (and have saved) many lives.

All Volcanoes Are Not Alike

Like people, each volcano looks and behaves differently. Some, like conical Mount St. Helens in Washington state, blow gas and ash high into the atmosphere. Others, like long and smooth Kilauea, gush molten lava. And those like Arenal and Etna shoot out mixtures of both. Generally, a volcano's location and composition reveal a lot about how it works.

The Earth's crust is broken into a dozen sections, called tectonic plates, that fit together like a jigsaw puzzle. Some sections move as much as 8 inches per year, and most volcanic activity occurs along their boundaries. If two plates separate, fluid volcanoes fill the gap. The islands of Iceland in the Mid-Atlantic Ridge are examples of this. They belong to a submarine volcanic ridge that runs down the middle of the Atlantic Ocean.

The famous Ring of Fire—the world's largest volcanic “necklace”—marks where the Pacific Ocean's floor plunges underneath the continents surrounding it. The friction between the rubbing plates there melts rock and dissolves seawater, creating a gaseous magma that rises and explodes to the surface as a volcano. The violent eruption of Mount Pinatubo in the Philippines occurred along the Ring of Fire.

The fluid lava that flows across the islands of Hawaii, however, might originate in the Earth's liquid outer core. Large teardrops of molten rock rise through the mantle and puncture the crust at points called “hot spots.” In the case of Hawaii, that event happened in the middle of the Pacific plate, at ocean bottom. The island of Hawaii's Mauna Loa, the world's largest volcano, rises 33,000 feet above the ocean floor—about 4,000 feet higher than Mount Everest. That's a lot of lava!

Because they rarely explode, the Hawaiian volcanoes of Mauna Loa and Kilauea are relatively safe to watch, even up close. Imagine standing near a molten river at night, watching a vent spray lava 1,000 feet skyward. The volcano sounds like a low-flying 747 jet, and the heat is so intense your skin tingles. You dare not venture farther. Beside you, a tired lava flow snaps, crackles, and pops—like a bowl of Rice Krispies in milk.

On the other hand, nothing in nature can match the power of the volcanoes that dot the Ring of Fire. Two out of every three of Earth's land volcanoes are there, and Indonesia alone harbors more than 127 of them. Some are furious. For example, in 1815, Tambora, near Bali, exploded with the energy of 800,000 hydrogen bombs! It poured about 170 billion tons of volcanic debris into the atmosphere, caused 12,000 deaths on the island, and created a great sea wave that killed another 80,000 people on two neighboring islands. By comparison, the energy released by Mount St. Helens in 1980 was equal to 400 hydrogen bombs and volcanic debris claimed 57 lives.

The Good Side of Such Violence

Volcanoes may cause death, but they also sustain life. Ash, for example, is rich in soil nutrients important to plant growth. The livelihood of many developing countries, therefore, depends on agricultural products grown in the fertile soils that surround volcanoes.

The gases making up Earth's early atmosphere were expelled by volcanoes. Today, emissions from Mount Etna alone make up 10 percent of all the carbon dioxide gas that the Earth releases. Etna also might have caused climate changes during the past 100,000 years.

Volcanic ash and gases do affect climate. Tambora caused the “year without a summer.” El Chichon apparently provoked a severe Northern Hemisphere winter in 1984. And now, the Earth's average global temperature has dropped 1.5 degrees Fahrenheit because of Mount Pinatubo's eruption. That drop is enough to offset the slight temperature increase in recent years, which some scientists attribute to global warming.


  • emission: A substance discharged into the air.

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  1. How have volcanoes affected the Earth's atmosphere and climate?
  2. How have volcanoes helped the Earth's surface over the course of time?
  3. How might the Earth's surface, atmosphere, and climate be different if there were no volcanoes? Write a few sentences to explain your answer.