Even more surprisingly, it isn’t the lava that is the major source of death and destruction.

There are a probably more volcanoes on earth than you realize — more than 500 “active” volcanoes in the world, about as many “dormant” volcanoes, and many volcanoes that have been deemed “extinct.” In fact, f the 500 or so active volcanoes, around 10 are erupting on any given day. For the most part, these eruptions are small and well contained and may even go unnoticed.

Every now and then, though, a volcano eruption wreaks its destructive powers upon thousands of victims, devastates the environment and may even be powerful enough to affect the climate of the entire world. There have been, in recorded history, dozens of extremely catastrophic volcanic eruptions — one may even have wiped out an entire civilization.

Historic Volcanic Eruptions

Mount Vesuvius, Italy (79 A.D.)

Pompeii was an eruption that occurred in ancient Rome. This eruption was not a major lava eruption. The cause of destruction was a mixture of toxic gases, hot ash and falling rock. When Mount Vesuvius first erupted, a huge plume of smoke, ashes, gases, and rocks shot straight up for 18 miles in the sky. This plume is called a “pyroclastic surge.” But when the column collapsed, the first “pyroclastic flow” of the eruption came barreling down the slopes of the volcano and destroyed everything in its path – including all the residents of the four towns at its base, including Pompeii.

A pyroclastic flow is a mixture of searing gas and rock fragments that takes the form of a great lethal avalanche. The temperature of a pyroclastic flow can reach up to 1000 degrees Fahrenheit and it can race down the volcano and surrounding lands from 60 to 100 miles per hour. This mixture moves easily up and over ridges and obstacles, and its effects are far-reaching in their devastation. In the town of Herculaneum, on the northern edge of the area affected by the volcano, the surge burned away the victims’ skin and muscle tissue, leaving fairly well preserved skeletons.

Tambora, Indonesia (1902)

The 1815 eruption of Tambora was the largest eruption in historic time. About 150 cubic kilometers of ash were erupted (about 150 times more than the 1980 eruption of Mount St. Helens). Ash fell as far as 800 miles from the volcano. In central Java and Kalimantan, 550 miles from the eruption, one centimeter of ash fell. The eruption column reached a height of about 28 miles . The collapse of the eruption column produced numerous pyroclastic flows. As these hot pyroclastic flows reached the ocean where they caused additional explosions. During these explosions, most of the fine-fraction of the ash was removed. The eruption formed a caldera. An estimated 92,000 people were killed by the eruption. About 10,000 direct deaths were caused by bomb impacts, tephra fall, and pyroclastic flows. An estimated 82,000 were killed indirectly by the eruption by starvation, disease, and hunger.

Krakatau, Indonesia (1883)

36,000 people were killed when Mt. Krakatau erupted in Indonesia. The initial blast generated an ear-shattering fusillade accompanied by a black churning cloud of volcanic debris that rose miles above the island. Later in the day, these villages would be battered by a series of devastating tsunamis generated by pyroclastic flows plunging into the sea.

Krakatau’s eruptions generated mountainous tsunamis, up to 40 m tall, that ravaged coastlines across the Sunda Straits. Many of the closest islands were completely submerged. After first being overwhelmed by massive pyroclastic flows, Sebesi Island northeast of Krakatau was inundated by mammoth sea waves. These tsunamis stripped away all vegetation, washed approximately 3000 people out to sea, and destroyed all signs of human occupation. Although located at seemingly safe distance, 80 km east of the Sunda Straits, the low-lying Thousand Islands were buried by at least 2 m of seawater and their inhabitants had to save themselves by climbing trees.

What are Volcanoes?

A volcano is an opening in the earth’s surface through which lava, hot gases, and rock fragments burst forth . Such an opening forms when melted rock from deep within the earth blasts through the surface. Most volcanoes are mountains, particularly cone-shaped ones, which were built up around the opening by lava and other materials thrown out during eruptions.

Types of Volcanoes

Based on the shape of the volcanoes and the type of material they are built of, scientists divide volcanoes into three main groups:

• Shield volcanoes are formed when a large amount of free-flowing lava spills from a vent and spreads widely. The lava gradually builds up a low, broad, dome-shaped mountain. The famous Mauna Loa in Hawaii is a shield volcano. Thousands of separate, overlapping lava flows, each less than 50 feet thick, formed Mauna Loa.
• Cinder cones build up when mostly tephra erupts from a vent and falls back to earth around the vent. The accumulated tephra, which is generally cinders, forms a cone-shaped mountain. Paricutin in western Mexico is a well-known cinder cone. It began in 1943, when a crack opened in the ground of a cornfield. When the eruptions ended in 1952, the top of the cone was 1,345 feet above its base.
• Composite volcanoes are formed when both lava and tephra erupt from a central vent. The materials pile up in alternate layers around the vent and form a towering, cone-shaped mountain. Mount St. Helens, which erupted frequently between 1980 and 1986, is one of the most active composite volcanoes in the United States.

Occasionally, the magma chamber of a shield volcano, cinder cone, or composite volcano may become nearly empty. This happens when most of a volcano’s magma erupts onto the surface. Because the chamber is empty, it can no longer support the volcano above. As a result, a large part of the volcano collapses, forming a huge crater called a caldera. Crater Lake in Oregon is a caldera that has filled with water. It is about 6 miles across at its widest point and 1,932 feet deep.

Types of Volcanic Eruptions

Scientists examine the violence of the eruption and the type of material that erupts in order to divide volcanic eruptions into four basic groups:

• Hawaiian eruptions are named after the volcanoes in Hawaii and are the least violent type. In such eruptions, highly fluid lava flows quietly from several vents and gradually builds up a shield volcano.
• Strombolian eruptions are named after Stromboli. Such eruptions result from the continuous escape of gas from the magma. As the gas escapes, it produces tephra that piles up into a cinder cone.
• Vulcanian eruptions get their name from Vulcano, a volcanic island off the Italian coast. These eruptions occur when sticky magma plugs up the central vent. The magmatic gas gradually builds up pressure until it blasts the magma into volcanic dust and bombs.
• Peleean eruptions are the most violent. Their name comes from the eruption in 1902 of Mount Pelee on Martinique, an island in the West Indies. The eruption killed about 38,000 people. A Peleean eruption occurs when the gas in highly sticky magma builds up tremendous pressure. This pressure causes violent explosions that produce glowing clouds of hot ash and dust. In a Peleean eruption, much of the volcano blows apart.

Volcanism

Volcanism refers to any volcanic activity. It may be spectacular with red-hot fountains of lava or tremendous explosions of ash and rock. Volcanism has significantly influenced the global climate. Volcanic eruptions spew ash high up into the sky where it is spread by the wind until it may cover much of the earth. This layer of ash blocks the sunlight from reaching the earth’s surface and lowers the earth’s temperature – creating a cold, winter-like climate that affects all the plant and animal life on earth.

Volcanism has positive and negative economic effects, as well. The negative effects are the destruction of homes, industrial facilities, communication and other aspects of a community. The positive are fertile soil, great farming, and other agricultural gains. [The positive effects include the long-term benefits of having the ash produce] more fertile soil that improves farming and other agricultural gains.

Volcanic Material (Pyroclastic)

The material that erupts depends chiefly on how sticky or fluid a volcano’s magma is. Three basic kinds of materials may erupt from a volcano:

• Lava is the name for magma that has escaped onto the earth’s surface. When lava comes to the surface, it is red hot and may have a temperature of more than 2012 °F. (1100° C). Highly fluid lava flows rapidly down a volcano’s slopes. Sticky lava flows more slowly. As the lava cools, it hardens into many different formations. Highly fluid lava hardens into smooth, folded sheets of rock called pahoehoe. Stickier lava cools into rough, jagged sheets of rock called aa. Pahoehoe and aa cover large areas of Hawaii, where the terms originated. The stickiest lava forms flows of boulders and rubble called block flows. It may also form mounds of lava called domes.
• Other lava formations include spatter cones and lava tubes. Spatter cones are steep hills up to 100 feet (30 meters) high. They build up from the spatter of fountain like eruptions of thick lava. Lava tubes are tunnels formed from fluid lava. As the lava flows, its outer surface cools and hardens. But the lava underneath continues to flow. After the flowing lava drains away, it leaves a tunnel.
• Rock fragments, generally called tephra, are formed from sticky magma. Such magma is so sticky that its gas cannot easily escape when the magma approaches the surface or central vent. Finally, the trapped gas builds up so much pressure that it blasts the magma into fragments. Tephra includes, from smallest to largest, volcanic dust, volcanic ash, and volcanic bombs.
• Volcanic dust consists of particles less than 0.0625 millimeters in diameter. Volcanic dust can be carried great distances. In 1883, the eruption of Krakatau in Indonesia shot dust 17 miles into the air.
• Volcanic ash is made up of fragments less than 0.5 centimeters in diameter. Most volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may reach speeds of 97 kilometers per hour and can be highly destructive.
• Volcanic bombs are large fragments. Most of them range from the size of a baseball to that of a basketball. The largest bombs may measure more than 1.2 meters across and weigh up to 91 metric tons. Small volcanic bombs are generally called cinders.
• Gas pours out of volcanoes in large quantities during most eruptions. The gas is made up chiefly of steam. But it includes carbon dioxide, nitrogen, sulfur dioxide, and other gases. Most of the steam comes from a volcano’s magma. But some may also be produced when rising magma heats water in the ground. Volcanic gas carries a large amount of volcanic dust. This combination of gas and dust looks like black smoke.

Ring of Fire

The Pacific Ring of Fire is a ring of active volcanoes in the Pacific Ocean. The Ring of Fire is an area of frequent earthquakes and volcanic eruptions encircling the basin of the Pacific Ocean. In a 40,000 km horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. The Ring of Fire has 452 volcanoes and is home to over 75% of the world’s active and dormant volcanoes

Two of the largest, most devastating volcanic eruptions, Mt. Tombar and Mt. Krakatau, are located within the Pacific Ring of Fire.

What’s GOOD about Volcanoes?

Volcanoes are among the most destructive natural forces on the earth. Since the 1400’s, they have killed almost 200,000 people. But volcanoes also produce benefits.

Industrial and Chemical Uses

Rock formed from lava is commonly used in building roads

Pumice, a natural glass that comes from lava, is widely used for grinding and polishing stones, metals, and other materials

Sulfur deposits from volcanoes are used in making chemicals.

Agriculture

Weathered volcanic ash greatly improves soil fertility. Areas recovering from volcanic devastation have become grassy, fertile lands. Volcanic soil is the most fertile soil ever. It gives the land a massive amount of nutrients to grow plants.

Geothermal Energy

In many volcanic regions, people use underground steam as a source of energy. This geothermal energy is used to produce electric power in such countries as Italy, Mexico, New Zealand, and the United States. In Reykjavik, Iceland, most people heat their homes with water piped from volcanic hot springs.

Land Maker

Volcanoes also create more land for living. For example, look at Hawaii: its borders will keep expanding until it becomes dormant.

Historical Study

The hardened ash that immersed Pompeii and Tambora perfectly preserved the everyday objects, art and structures that it immersed. Pompeii is a museum of an ancient city and its cultural artifacts. Information on how the people lived could be deduced through their possessions and also by recreating “statues” of the actual people. By filling the hollowed area inside the ash where the bodies of the victims inside had deteriorated, casts were made of the people in the exact positions in which they died.

Scientific Study

Finally, volcanoes serve as “windows” to the earth’s interior. The materials they erupt help scientists learn about conditions within the earth.

Conclusion

As destructive as it is, volcanic activity is one of the most important, constructive geological processes on Earth. Volcanoes are constantly rebuilding the ocean floor. As with most natural forces, volcanoes have a dual nature. They can wreak horrible devastation and loss of life, but they are also a crucial element of the earth’s ongoing regeneration