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Explore volcanoes, from the fiery explosion of Mount St.

Helens to the slow-flowing lava on Hawaii’s ‘Big Island.’ Learn about how volcanoes form and how they are classified.

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From Cornfield to Volcano

In 1943 in the small village of Paricutin, Mexico, a farmer named Dionisio Pulido noticed that a crack spewing hot steam had suddenly appeared in his cornfield. By the end of what must have been a difficult day for Se;or Pulido, enough ash, rocks and other materials had been expelled from the crack to form a hill 40 meters high (131 feet), about the height of a 13-story building.As more debris spewed from the crack, the bigger the hill grew. Nine years passed, and both Pulido’s farm and the village of Paricutin were taken over by this growing volcano, which eventually measured one-quarter of a mile high and blew ash and smoke as far as 200 miles away.

This photo taken in 1943 of the volcano at Paricutin, Mexico, shows just how bad things got for Pulido and his cornfield that year.
Photo of volcano in Paricutin, Mexico

What are Volcanoes, and How Do They Form?

Not all volcanoes, cracks in the Earth’s crust which allow melted rock to rise to the surface, appear as quickly or dramatically as the one that formed in Dionisio Pulido’s cornfield. In fact, many volcanoes start as mountains which lay dormant, or inactive, until shifts in the Earth’s surface allow melted or partially melted molten rock, called magma, to pour out.

(It’s important to note that molten rock is called magma when it’s beneath the ground and lava when it is expelled to the surface.)The crust of the Earth is broken into plates, much like if you cracked the shell of a hard-boiled egg. The plates, like the pieces of eggshell, can move apart, move against each other and even move under one another. These plate movements create opportunities for magma to break through to the surface and create havoc in the form of volcanoes.Places on Earth where plates crash into one another are called convergent boundaries, and unsurprisingly these boundaries are also places where subduction, or the movement of one plate under another, often occurs. Many of these plates come together deep under the ocean, where we cannot see how they are dramatically altering the Earth.

Burning Ring of Fire

The Ring of Fire, which is less of a ring and more of a horseshoe that encompasses the Pacific Ocean, is a collection of shifting plates that create many volcanoes and other plate-related activities, such as earthquakes and seafloor spreading, in places including Japan, Indonesia and New Zealand and down the west coast of the United States and South America.

The Ring of Fire spans over 40,000 kilometers (25,000 miles) and contains 452 volcanoes.
Mount Tambora in Indonesia is a dome-shaped volcano with a caldera on top.
Photo of a caldera

Geologists classify volcanoes by their shape and how they erupt. Cinder cones are the smallest type but also the most common, and they are formed when debris emitted from the volcano builds up around the volcanic vent.The second type of volcano is the composite cone or stratovolcano.

This type of volcano is responsible for the larger, more noteworthy eruptions that we still talk about years later. Mount St. Helens in Washington state was once a tranquil mountain getaway until May 18, 1980, when the entire north side of the mountain exploded right off, leaving a giant crater behind.

Mount St. Helens is a composite cone or stratovolcano. Notice the crater created on the north side of the mountain from its initial, violent eruption.

Photo of crater at the top of Mount St. Helens

Some volcanoes emit only ash and rock, while others also release gases such as carbon dioxide – which, odorless, led to the death of 1,700 people who lived near Lake Nyos in Cameroon, Africa – and hydrogen sulfide (the gas that smells like rotten eggs).

Other gases released include sulfur dioxide and hydrogen chloride, both of which are acidic enough to burn holes through fabric.

Learning Outcomes

After you are finished, you should be able to:

  • Describe how a volcano is formed
  • Explain the source of The Ring of Fire and recall where it is located
  • Name and describe the different types of volcanoes

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