What Happens When a Volcano Erupts
by Spencer Christian

  Now that we have built our own erupting volcano, let's try to understand what happens when a "real" volcano erupts. First, it's important to understand that not all volcanoes look like tall, smooth mountains. Anywhere that lava flows from an opening in the earth is a volcano, even if it's just a hole in the earth's crust. However, since most of the volcanoes we've seen in the movies or on the news are the ones that look like mountains of fire, we'll discuss that type.

  The erupting mountains, scientists believe, were built up over a period of hundreds of thousands of years. They were formed from the many layers of lava flowing out in a sereis of eruptions. Over the ages, the lava cooled and hardened and formed cone-shaped mountains around the openings through which volcanic materials come to the surface. Those openings are called "vents". But what happens beneath the earth's surface that causes those eruptions in the first place?

  Well, let's take a look at the structure of this big ball called the earth. Just below the surface is the outer layer of the earth, called the "crust". Oceanic crust can be at thin as 4 or 5 miles, while continental crust can be as thick as 15 to 50 miles. Beneath the crust is a layer of extremely hot rock called the "mantle", which flows like a thick liquid. And below the mantle lies the earth's "core". Temperatures in the mantle are unimagineably extreme, 950 degrees to 7,500 degrees Fahrenheit. When oceanic crust reaches the mantle, it melts and becomes molten rock, or "magma". And, NOW, we get to the exciting part.

  The magma gathers in an underground chamber called a "magma chamber". Water and gases mix with magma in the magma chamber; and, because of the intense heat and pressure, the magma mixture is carried toward the surface through narrow tunnels called "feeder pipes". These feeder pipes lead to the "vents" I described earlier; and the result is an explosion, or "eruption", more powerful than a nuclear bomb--blasting a hole through the surface of the volcano. Bright red lava shoots into the sky, cooling and becoming solid as soon as it hits the air. Lava that cools into bits of dust is called "ash". Lava that hardens into very lightweight rocks and boulders is called "pumice". The water from the magma chamber shoots up as steam, building a giant cloud over the volcano. Ash and pumice pile up in layers around the fountain of lava, giving it a cone shape. With each eruption, the volcano builds up thicker slopes from new layers of lava and can grow to great heights.

  Lava can shoot out from a volcano for two days during a violent eruption. The magma chamber, which may have been growing for hundreds of thousands of years, empties out completely. When the eruption is over, the volcano is quiet. Beneath the ground, more oceanic crust is melting into magma at the same spot. New molten rock begins to rise into the magma chamber. One day, it will blast to the surface--and the volcano will blow its top again. When there is no more magma to fill up the chamber, the volcano will shrink. Over millions of years, wind and rain will smooth the mountain down until the volcano's life cycle is over.