Deep within the Earth, under tremendous pressure and at extreme temperatures, rock exists as a hot liquid called magma. This molten rock is found in pockets called magma chambers. Magma is constantly moving. In some places magma works its way toward the Earth's surface through cracks in solid rock. In other places, magma works its way toward the surface by melting the solid rock.

When magma reaches the Earth's surface, it is called lava. The place in the Earth's surface through which magma and other materials reach the surface (and the magma becomes lava) is called a volcano. You may have seen photographs of lava flowing down the sides of a volcano. A lava flow is so hot that it incinerates every burnable thing in its path. In some places, lava can build up to form a cone-shaped mountain. Such a landform, which is the result of an accumulation of volcanic material, is often referred to as a volcano as well. The opening from which lava erupts is called a vent. Volcanoes often have more than one vent. If there is more than one vent, lava will sometimes pour from the sides of a volcano as well as from the top.

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Volcanic Eruptions

Not all volcanic eruptions are alike. Some eruptions are quiet, with lava slowly oozing from a vent.

Other eruptions are very violent, with, lava and other materials being hurled hundreds of meters into the air. Gases from within the Earth's interior mix with huge quantities of volcanic dust and ash and rise into the air as great dark clouds that can be seen from many kilometers away. A violent volcanic eruption is truly an awesome sight. Although it seems to be a dangerous endeavor, many scientists spend their working lives studying volcanoes. For volcanoes are "windows" into the interior of the Earth. By analyzing the mineral makeup of lava, geologists can determine the chemical composition of the magma from which the lava formed. Such data provide information about the composition of the part of the Earth that remains unseen. Although you might not wish to touch lava with a "tenfoot pole, " this potentially hazardous activity is all in a day's work for a volcanologist (scientist who studies volcanoes). The long pole contains a special probe that can measure the temperature of the lava. (Right)

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Types of Lava

There are four main types of lava. One type of lava is dark-colored and contains a lot of water. This lava is rich in the elements ironand magnesium. When this type of lava cools, igneous rocks such as basalt are formed.

Another type of lava is light in color. This lava, which contains little water, is rich in the elements silicon and aluminum. Compounds of these elements account for its lighter color. When this type of lava cools, it forms the igneous rock rhyolite, which resembles granite. The third type of lava has a chemical composition similar to that of both the dark-colored type and the light-colored type. Different varieties of igneous rocks in the Earth's crust, such as andesite, are formed from this type of lava. The fourth type of lava contains large amounts of gases such as steam and carbon dioxide. When this lava hardens, it forms rocks with many holes in them. Like the holes trapped in the dough of a loaf of bread, the holes in this type of lava form as gas bubbles are trapped in the molten rock as it hardens. Pumice and scoria are igneous rocks formed from this type of lava. Do you know an unusual property of pumice? Some dark-colored lava is thin and runny, and most tends to flow. The islands of Hawaii and Iceland were formed by many lava flows. But lightcolored lava causes explosive eruptions. Because lightcolored lava is rich in the element silicon, it tends to harden in the vents of a volcano. Explosive eruptions are caused when lava in the vents hardens into rocks.  There are two basic forms of dark-colored lava, both of which get their names from Hawaiian words. Hot, fast-flowing pahoehoe (pah-HOHay-hoh-ay) hardens into rounded swirls and ropy wrinkles (top). Cooler, slowflowing aa (AH-ah) crumbles into large, jagged chunks as it oozes downhill. As a result, aa is rough and blocky (bottom). Can you explain why pahoehoe may change to aa as the lava moves away from the vent?

Steam and new lava build up under the rocks. When the pressure of the steam and new lava becomes great, a violent explosion occurs. As an example, if you place a cork in a bottle of seltzer water and shake the bottle, what do you think will happen? The cork will be pushed out of the bottle. The increased pressure exerted by the gas in the seltzer as a result of shaking the bottle causes the cork's ejection. This model illustrates what happens to a hardened lava plug in a vent as pressure builds up beneath it.

During volcanic eruptions, many rock fragments are blown into the air. The smallest particles are called volcanic dust. Particles of volcanic dust are very fine, less than 0.25 millimeter in diameter, or as tiny as grains of flour.

This car is covered with volcanic dust spewed from Mt. Pinatubo, in the Phillippines. The effects of this natural disaster prove and irresistible attraction.

Rock particles more than 0.25 millimeter but less than 5 millimeters in diameter are called volcanic ash. Particles of volcanic ash are about the size of rice grains. Volcanic ash falls to the Earth's surface and eventually forms small rocks. Both volcanic dust and volcanic ash can be carried away from a volcano by the wind. In this manner, they can fall to the Earth near the volcano, or they can be carried completely around the world. Larger rock particles are called volcanic bombs. Volcanic bombs are a few centimeters to several meters in diameter. Some bombs are the size of boulders and have masses of several metric tons. Small volcanic bombs about the size of golf balls are called cinders. When volcanic bombs are hurled out of a volcano, they are molten. They harden as they travel through the air.

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Types of Volcanoes

Different types of volcanic eruptions form different types of volcanoes. Some volcanoes are built from quiet flows of thin, runny lava that spread over a broad area. Other volcanoes are formed from violent eruptions. Some volcanoes are formed from a combination of quiet flows of lava and violent eruptions.

CINDER CONES

Volcanoes made mostly of cinders and other rock particles that have been blown into the air are called cinder cones. Cinder cones form from explosive eruptions. Because the material in cinder cones is loosely arranged, the cones are not high. But they have a narrow base and steep sides. Paricutin in Mexico is a cinder cone.

SHIELD VOLCANOES

Volcanoes composed of quiet lava flows are called shield volcanoes. Because it is runny, the lava flows over a large area. After several quiet eruptions, a gently sloping, domeshaped mountain is formed. The largest shield volcano is Mauna Loa in the Hawaiian Islands. Mauna Loa rises from the bottom of the Pacific Ocean to a height of 4 kilometers above sea level.

COMPOSITE VOLCANOES

Volcanoes built up of alternating layers of rock particles and lava are called composite volcanoes. During the formation of a composite volcano, a violent eruption first occurs, hurling volcanic bombs, cinders, and ash out of the vent. Then a quiet eruption occurs, producing a lava flow that covers the rock particles. After many alternating eruptions, a large coneshaped mountain forms. The most famous composite volcanoes are Mount Vesuvius near Naples and Mount Etna in Sicily, both in Italy.

There is often a funnel-shaped pit or depression at the top of a volcanic cone. This pit is called a crater. If a crater becomes very large as a result of the collapse of its walls, it is called a caldera. A caldera may also form when the top of a volcano collapses or explodes. You may be familiar with the word caldron, a type of large cooking pot or kettle. The witches in Macbeth, a play written by William Shakespeare, stir their potions in a bubbling caldron. Both these words are derived from the Latin word caldarius, which pertains to warming. As you might guess, a volcano's caldera was at one time quite hot and contained bubbling lava.
Izalco in El Salvador is a cinder cone (left). Kilauea in Hawaii is a shield volcano. The crater within the caldera is
clearly visible (center). Mount Egmont in New Zealand is a composite volcano (right). How does a composite volcano form?

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Volcanic Activity

Volcanoes are rather unpredictable phenomena. Some volcanoes erupt fairly regularly; others have not erupted within modern history. In order to indicate the relative activity of volcanoes, scientists classify them as active, dormant, or extinct.

An active volcano is one that erupts either continually or periodically. There are several active volcanoes in the continental United States: Lassen Peak in Lassen Volcanic National Park (California), Mount St. Helens in the Cascade Range (Washington State), and Mount Katmai (Alaska).

A volcano that has been known to erupt within modern times but is now inactive is classified as a dormant, or "sleeping," volcano. Mount Rainier (Washington State), Mount Hood (Oregon), and Mount Shasta (California) are examples of dormant volcanoes in the continental United States.

A volcano not known to have erupted within modern history is classified as an extinct volcano. Volcanologists, (scientists who study volcanoes) consider truly extinct volcanoes to be only those that have been worn away almost to the level of their magma chamber. But even so-called extinct volcanoes can prove unpredictable. Both Lassen Peak and Mount St. Helens suddenly erupted after long periods of inactivity.

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In May 1980, Mount St. Helens in the state of Washington erupted explosively. These photographs and diagrams show the first few minutes of the eruption. What is the term for the openings from which lava erupts?

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1. What is a volcano?

2. What determines the type of volcano formed?

3. Describe the three types of volcanoes.

4. What is the difference between magma and lava?

5. List in order of increasing size the different kinds of particles blown from a volcano.

6. How are volcanoes classified according to activity?

7. What is the term for the opening from which lava erupts

You will need to to do research to find out the answers to the following questions

8. Find out the dates of the last known eruption of the three examples of dormant volcanoes listed in the reading.

9. What was the name of the city that was destroyed, along with Pompeii, when Mt. Vesuvius erupted. What year did it happen?