Volcanoes. Volcanoes LEVELED READER Z. Visit for thousands of books and materials.

Transcription

1 Volcanoes A Reading A Z Level Z Leveled Reader Word Count: 1,756 LEVELED READER Z Volcanoes Written by Chuck Garofano Visit for thousands of books and materials.

2 Volcanoes Photo Credits: Cover, back cover, title page, pages 8, 11, 12: PhotoDisc; pages 4, 6, 14, 19: 2002 Clipart.com; page 10: NOAA Coastal Services Center; page 15: J.D. Griggs/USGS; page 16 (top): NASA/MOLA Science Team; page 16 (bottom): T.N. Mattox/USGS; page 17: J.P. Lockwood/USGS; page 18 (top): T.P. Miller/USGS; page 18 (bottom): R. Bailey/USGS. Written by Chuck Garofano Volcanoes Level Z Leveled Reader Learning A Z Written by Chuck Garofano Illustrations by Signe Nordin All rights reserved. Correlation LEVEL Z Fountas & Pinnell V Reading Recovery 29 DRA N/A

3 Table of Contents The Eruption of Vesuvius Why and Where Volcanoes Happen Composite Volcanoes Shield Volcanoes Cinder Cones and Lava Domes Conclusion Glossary This volcano shows signs of life. The Eruption of Vesuvius Few people in the city of Pompeii paid much attention to the earthquakes that began shaking the region in AD 79. No one connected them with the stream of smoke that rose from nearby Mount Vesuvius. The slopes of Mount Vesuvius were covered in trees, flowers, and grass. It had been a thousand years since the volcano had erupted, so most people assumed that it was dormant, or even extinct. But everything changed on August 24th at around 1 o clock in the afternoon. Suddenly, a huge explosion shook the area around the mountain. Melted rock, ash, and dust spewed from the top of the volcano, darkening the sky. The dust, ash, and stone began to rain down on the city, covering the ground as far away as 16 kilometers (10 mi) from the mountain. 3 4

4 By 3 o clock, lava, or melted rock, poured from the mouth of the volcano at the rate of 12,000 tons a second, destroying everything in its path. By 6 o clock, the cloud of dust and ash had risen to a height of 32 kilometers (20 mi), and was so powerful that it created its own lightning. The ash continued to fall over the area, covering the ground to a depth of 1 meter (3.3 ft). Buildings collapsed under the weight of the falling debris. People in the cities surrounding Mount Vesuvius tried to flee as the volcano demolished their homes and farms. Some people tried to walk over the thick layers of ash, but the ash was so deep and so hot that many people died. Others were struck and killed by pieces of stone falling from the dark sky. Many choked to death on the ash- and dust-clogged air. Around midnight, the terrible situation became even worse. The massive cloud of ash, poisonous gasses, glowing-hot dust, and smoke had become so tall and heavy that it could no longer stay in the air. The cloud collapsed, falling down the mountain at speeds of up to 500 kilometers per hour (310 mph). 5 This preserved body of an inhabitant of Pompeii is in a museum. Many others still exist at the actual site of the eruption. This was Mount Vesuvius s first surge cloud. Within a few moments, people in the cities of Pompeii and Herculaneum were scorched and buried by the collapsing cloud. As more dust and ash billowed from the volcano, more surge clouds followed. When the eruption was over, two cities of the Roman Empire were completely buried. They would not be found again for over 1,000 years. 6 Do You Know? A city called Pompeii, not far from Mount Vesuvius, was completely buried by a surge cloud. When the remains of those who died at Pompeii were eventually found, their bodies were perfectly preserved. The hardened ash and mud acted as a plaster cast, keeping the exact shape of the bodies inside the cast.

5 What you just read is an account of a real event. But how did it happen? What could cause such a violent explosion? We will look for answers to those questions in this Leveled Reader. But first, we will look at what volcanoes are and why and where they happen. Why and Where Volcanoes Happen The earth s hard surface, or crust, is made of several enormous sheets of rock called plates. Some plates make up the continents, while others make up the ocean floors. Just under the surface of the plates, the earth is extremely hot so hot that rock melts into a liquid called magma. Sometimes, the magma works its way up to the surface and pours out through cracks in the crust. When this happens, we call it a volcano. Glowing-hot magma bubbles up inside a volcano. Volcanoes usually occur at the edges of the plates. The plates are slowly floating on the thick, liquid magma underneath them. But different plates are moving in different directions, and they can crash into, pull away from, or grind past other plates. As these huge plates crash, pull, and grind, the tension and pressure cause large cracks in the crust. This is where the magma usually breaks through, forming a volcano. Volcanic Vents Plate Movement Oceanic crust Trench Volcanic arc Continental crust Vents Magma chamber Mantle Mantle When an ocean plate crashes into a continental plate, the ocean plate moves down, creating an active volcanic region. 7 8

6 Ring of Fire Pacific Ocean Equator More than half of the active volcanoes in the world occur in the Ring of Fire. There are at least 500 active volcanoes in the world, and most of them lie near the edges of the plates. The edge of the Pacific Ocean plate is a particularly active volcanic region known as the Ring of Fire. Another very active volcanic region is along the Mid- Atlantic Ridge, which is a mountain chain in the Atlantic Ocean. Two plates are pulling apart here, allowing magma to pour into the ocean bottom, where it piles up. Over time, the magma can pile up into mountains so tall that they stick up out of the water. The country of Iceland is made of the tops of large volcanoes in the Mid-Atlantic Ridge. 9 Some volcanoes are located in areas away from the edges of the plates. The islands of Hawaii are in the center of the Pacific plate. They are above a hot spot, a place where hot magma sits very close to the surface of the Earth s crust. In Yellowstone National Park, in the middle of the North American plate, an underground hot spot sits very close to underground lakes and waterways. The magma heats up the ground and the water, causing geysers and hot springs. Do You Know? A hot spot created the volcanoes of Hawaii. As the Pacific Plate moves over the top of the hot spot, old volcanoes go extinct and new ones form. It is almost like moving a piece of paper over a burning candle the candle makes a row of holes, much like the hot spot makes a row of volcanoes. 10 A satellite view of the Big Island of Hawaii

7 Different volcanoes erupt in different ways, depending on where and why the volcano forms. Some volcanoes erupt peacefully and slowly, while others suddenly explode with the force of many atomic bombs. In the next few chapters, we will take a look at different kinds of volcanoes and how they erupt. Smoke pours from this huge volcanic crater. Composite Volcanoes Mount Vesuvius, which you read about earlier in this book, is a composite volcano on the west coast of southern Italy. Composite volcanoes are sometimes called explosive volcanoes, because their eruptions can be extremely violent and destructive. They are usually very large mountains with steep sides and evenly shaped peaks, often with a bowlshaped crater at the top. The crater is a hollow area where the magma, hot gasses, and ash come out. 11 Most composite volcanoes occur where the magma near the surface is very thick and slow moving. Magma moves toward the surface through rocky pipes to openings called vents, but when the magma is very thick, it can cool and harden inside the pipe, plugging the vent. The magma below becomes trapped. Heat and gasses within the magma have nowhere to go, and the pressure slowly begins to build. Eventually, the pressure becomes so great that the volcano explodes. Sometimes, the explosion is so violent that the entire mountain can be destroyed. When Mount St. Helens erupted in 1980, the entire northern side of the mountain was blown away. Do You Know? When Mount St. Helens, in Washington State, erupted on May 18, 1980, 63 people died and 311 square kilometers (193 sq. mi) of forest were flattened by hot winds that blew down the mountainside. Mount St. Helens erupting 12

8 Lava flow Eruption column Eruption cloud Ash-cloud surge Dust, ash, and liquid rock pour out of a composite volcano during an eruption. When composite volcanoes erupt, they send huge clouds of ash, dust, smoke, hot gas, and rock into the air. These clouds rise for many kilometers, turning the sky black and raining ash onto the ground. When the cloud of debris becomes too heavy to stay in the air, it collapses in a violent surge cloud. Many composite volcanoes are very tall, and they have ice and snow on top. In an instant, the hot explosion can melt all of the snow on the top of the volcano, sending a flood of water, mud, and rock down the mountain at up to 100 kilometers per hour (62 mph). 13 Some volcanic ash clouds are so large that the ash and dust can travel around the world, blocking out sunlight and cooling down the whole earth. After the eruption of the volcano Tambora in Indonesia in 1815, people as far away as North America had a cold, snowy summer. Sometimes, an entire underground lake of magma can pour out of a composite volcano, leaving a huge empty space below. The surface collapses into the empty chamber, leaving a large bowl-shaped basin called a caldera. Calderas can be as much as 100 kilometers (62 mi) across. Over time, they often fill with water, creating large lakes. Crater Lake, in the state of Oregon, is a well-known caldera. The small island at the top center of the lake is a cinder cone. 14 Do You Know? The Klamath Indians in southern Oregon used to tell a story about the origin of Crater Lake. In the story, the first coyote fell in love with a star and found a way to join her in the sky. When he fell back to Earth, his impact made a hole that became Crater Lake.

9 Composite volcanoes can destroy huge forests, bury entire cities, and kill thousands of people. These volcanoes are very dangerous because they often remain quiet for hundreds of years before an eruption. People forget that the volcano might explode, and they build their homes dangerously close to the mountain. Mauna Loa Volcano, a shield volcano in Hawaii Shield volcano eruptions are usually gentle rather than explosive, although the lava flows can pour over roads, homes, and forests. Sometimes, the lava in a shield volcano contains hot gasses or steam that sprays from the crater, creating a bright lava fountain. Shield volcanoes can remain quietly active for a very long time and grow extremely large. Mauna Loa on Hawaii is the tallest volcano in the world, rising 9,170 meters (30,080 ft) from its base on the sea floor, making it taller than Mount Everest. The largest known volcano in the solar system is Olympus Mons on Mars. It is a shield volcano that measures 27 kilometers (17 mi) in height. Shield Volcanoes You have probably seen video of bright orange lava flowing or spraying from the top of a volcano. The lava is thin and it flows smoothly and quickly. The lava spreads out over the land before slowly hardening and building up. This kind of volcano is called a shield volcano because it often creates a gentle, even mountain slope in the shape of a shield. 15 Olympus Mons, on the surface of Mars, is the largest volcano in the solar system. It would cover the entire state of Arizona. 16

10 A cinder cone on the flank of Mauna Kea Volcano, Hawaii Lava domes also come from small, brief eruptions, but the lava that forms these structures is a thick, pasty liquid that oozes from the vent and quickly hardens. Sometimes, more lava can push up and expand through the center of the dome, cracking the outside. These domes often form in areas where there is other volcanic activity. They can be found in the craters and calderas of larger volcanoes. Cinder Cones and Lava Domes Sometimes, volcanoes are not active long enough to form entire mountains. Some volcanoes spray small bits of lava into the air for a short time. As the tiny lava chunks and bits of ash fly through the air, they harden into light, black-colored rocks called cinders. The cinders pile up around the vent into a cone-shaped hill with a bowl-shaped crater in the top. These are called cinder cones. Often, when a brand-new volcano suddenly appears, it forms a cinder cone. Other cinder cones pile up inside the craters or calderas of larger, older volcanoes. Because cinder cones are made of very loose material, they usually erode quickly. Lava dome atop Novarupta vent, Katmai National Park and Preserve, Alaska (above); Wilson Butte lava dome, Long Valley, California (left) 17 18

11 Mount Rainier, a dormant volcano, can be seen from the nearby cities of Seattle and Tacoma in the state of Washington. Conclusion Volcanoes are the most spectacular evidence we have that the earth is a changing planet. Lava pouring from a volcanic vent creates new rock and new land. Volcanic ash makes extremely fertile soil that is wonderful for farming. In some places, people use the heat from volcanoes to run power plants and make electricity. And volcanic mountains, including Mount Fuji in Japan, Mount Rainier in Washington State, and Mauna Loa in Hawaii, are some of the most recognizable and beautiful mountains in the world. It is no wonder that humans often choose to live near volcanoes. But we should always remember that volcanoes can be violent, dangerous places. Volcanoes unleash some of the most powerful forces from within the earth Glossary caldera (n.) a large, bowl-shaped basin where the land has collapsed into an empty magma chamber (p. 14) cinders (n.) light, black-colored rocks or pebbles formed by small pieces of flying lava and ash (p. 17) composite an explosive volcano made of volcano (n.) different layers of ash and lava (p. 11) crater (n.) a bowl-shaped hollow area in a volcano where lava, ash, and gasses come out (p. 11) dormant (adj.) quiet for many hundreds of years (p. 4) extinct (adj.) has not erupted in thousands of years and shows no sign of future eruptions (p. 4) lava (n.) melted, liquid rock on the surface of the earth (p. 5) magma (n.) melted, liquid rock beneath the surface of the earth (p. 7) plates (n.) the large sheets of rock that make up the earth s crust (p. 7) shield a non-explosive volcano made of volcano (n.) hardened lava (p. 15) surge a fast, superheated cloud of ash, gas, cloud (n.) dust, and rock that moves along the ground (p. 6) vents (n.) openings in Earth's crust through which magma and gasses emerge (p. 12)