CHAPTER 13 2 SECTION Volcanoes Volcanic Eruptions KEY IDEAS As you read this section, keep these questions in mind: How does the composition of magma affect volcanic eruptions and lava flow? What are the five major types of pyroclastic material? What are the three main types of volcanic cones? How does a caldera form? What are three events that may signal a volcanic eruption? What Are the Two Types of Magma? When magma cools, it hardens into volcanic rock. Scientists study the composition of volcanic rocks to understand the material in Earth s crust and mantle. Scientists classify magma based on its composition. There are two main types of magma: mafic and felsic. Type of Magma Mafic Composition Color Where It Is Commonly Found high in magnesium and iron generally dark oceanic crust Felsic high in silicate minerals generally light continental crust Felsic magma is also more viscous than mafic magma. A viscous fluid is thick, so it does not flow easily. Magma s viscosity affects what happens when a volcano erupts. What Are the Two Main Types of Eruptions? Volcanic eruptions can be quiet or explosive. Mafic lava usually causes quiet eruptions. Felsic lava usually causes explosive eruptions. QUIET ERUPTIONS Most eruptions on the ocean floor form from mafic magma. Therefore, eruptions on the ocean floor are generally quiet. During a quiet eruption, magma flows down the sides of the volcano like a river. Geologists classify the magma from quiet eruptions by the shapes it forms when it cools. The table on the next page shows a few of these shapes. READING TOOLBOX Summarize When you finish reading this section, create a concept map using the following terms: mafic lava, volcanic ash, lapilli, pahoehoe, felsic lava, aa, volcanic bomb, pyroclastic material, shield volcano, volcanic dust, and volcanic block. Critical Thinking 1. Contrast How could you tell the difference between a mafic rock and a felsic rock by looking at them? Critical Thinking 2. Explain What is the most likely reason that felsic magma usually causes explosive eruptions? Holt McDougal Earth Science 195 Volcanoes
3. Describe What does pahoehoe look like when it cools? Type of Lava What It Forms From What Happens to It When It Cools Pahoehoe hot, fluid, mafic lava It has a smooth, ropelike texture. Aa Blocky lava the same material as pahoehoe, but with more gas in it mafic lava that is more viscous than the lava that forms pahoehoe The crust breaks into sharp pieces. It breaks into large chunks. READING CHECK 4. Identify Which type of lava is pyroclastic material usually made from? EXPLOSIVE ERUPTIONS Explosive eruptions are much less common than quiet eruptions. However, explosive eruptions can be much more destructive. During an explosive eruption, hot ash, gas, and rocks shoot out of the volcano. The lavas that produce explosive eruptions are generally felsic lavas. Felsic lavas often contain a lot of trapped gases, such as water vapor and carbon dioxide. The gas in the lava creates pressure inside the volcano, like the bubbles in a can of soda. The pressure can make the volcano erupt explosively. In an explosive eruption, chunks of lava fly through the air. These chunks are called pyroclastic material. How Do Scientists Classify Pyroclastic Material? Some kinds of pyroclastic material form when solid lava breaks apart and is thrown into the air. Other kinds of pyroclastic material form when liquid lava flies through the air and hardens. Scientists classify pyroclastic material based on its size. The table below shows the five main types of pyroclastic material. Talk About It Apply Concepts Use a meterstick to identify how large the four types of pyroclastic material are. As a class, discuss what each of the materials would look like. Find objects in the classroom that are as big as one of the types of material. Type of Material Volcanic dust Volcanic ash Lapilli Volcanic blocks Volcanic bombs Description Volcanic dust is made of pieces of solid rock that are less than 0.25 mm in diameter. Volcanic ash is made of pieces of solid rock that are less than 2 mm in diameter. Volcanic dust is a type of volcanic ash. Lapilli are pieces of solid rock that are between 2 mm and 64 mm in diameter. They generally fall to the ground near the vent. Volcanic blocks are pieces of solid rock that are more than 64 mm in diameter. They are the largest type of pyroclastic material. Volcanic bombs form when large chunks of lava fly out of a volcano and cool in the air. They can have many different shapes. Holt McDougal Earth Science 196 Volcanoes
What Are the Three Main Types of Volcanoes? Quiet and explosive eruptions produce different types of volcanoes. During quiet eruptions, lava flows out of the vent to create a volcanic cone. During explosive eruptions, pyroclastic material piles up around the vent. There are three main types of volcanoes: shield volcanoes, cinder cones, and composite volcanoes. SHIELD VOLCANOES Shield volcanoes form from quiet eruptions of hot, mafic lava. The lava spreads out over a wide area, and layers of lava build up. Therefore, a shield volcano has a wide base, and its sides have a very gentle slope. Although shield volcanoes have a gentle slope, they can still be very large. The volcanoes in the Hawaiian Islands are shield volcanoes. Lava flow READING CHECK 5. Identify What are the three main kinds of volcanoes? Critical Thinking 6. Describe From where do you think the shield volcano got its name? Quiet eruptions of thin, fluid lava form shield volcanoes. CINDER CONES Cinder cones form from small, explosive eruptions. They have very steep slopes, and they are made of pyroclastic material. Cinder cones are generally very small. Parícutin volcano in Mexico is an example of a cinder cone volcano. Pyroclastic material 7. Explain Which type of volcanic eruption produces cinder cone volcanoes? Pyroclastic material that builds up during explosive eruptions can form a cinder cone. Holt McDougal Earth Science 197 Volcanoes
COMPOSITE VOLCANOES Composite volcanoes form when a volcano has both explosive eruptions and quiet eruptions. During a quiet eruption, lava flows down the sides of the cone. When an explosive eruption occurs, pyroclastic material falls all around the vent. Therefore, composite volcanoes contain layers of hardened lava and layers of pyroclastic material. Composite volcanoes generally have a gentle slope near the base, but a steeper slope near the vent. Mount St. Helens is an example of a composite volcano. Pyroclastic material 8. Identify What two materials are composite volcanoes made of? Lava flow Composite volcanoes contain both lava flows and pyroclastic material. How Are Calderas and Craters Different? Many explosive volcanoes have circular, funnel-shaped pits around their vents. These pits are called craters. They form when pyroclastic material builds up around the vent. As wind and water wear away the rock around the edge of the crater, the crater becomes larger. Some volcanoes contain large, deep depressions called calderas. A caldera is different from a crater. When the magma chamber beneath a volcano empties, the volcano may collapse. A deep pit forms where the volcanic cone was. This pit is the caldera. Talk About It Describe Processes Talk with a partner about the process shown in the figures. Together, discuss what is shown in each part of the figure. FORMATION OF A CALDERA Magma Chamber Ground surface sinks. Caldera Holt McDougal Earth Science 198 Volcanoes
Can Scientists Predict Volcanic Eruptions? Scientists cannot always predict where or when a volcano will erupt. However, by studying volcanoes, scientists have been able to identify some clues about when an eruption may happen. One way scientists predict volcanic eruptions is by studying the earthquakes that happen near a volcano. Just before an eruption, the earthquake activity around a volcano may change. This can happen for any of the following reasons: Magma moving upward puts increasing pressure on the surrounding rock. The temperature inside the rocks can change. Rocks near the volcano can break as magma moves through them. Scientists also study the gases that rise from the volcano. Just before an eruption, volcanoes often give off more gas. The composition of the gas may also change before an eruption. These scientists are taking samples of gases that are coming from a volcano. 9. Infer Why are these scientists probably wearing protective clothing? Scientists also study the shape and size of a volcano to figure out whether it may erupt. Before an eruption, rising magma can cause the volcano s sides to bulge. The movement of magma can also cause the ground to tilt slightly. Scientists can use special instruments to measure these changes. Scientists need to identify patterns in earthquake activity, released gases, and land tilt to be able to make these predictions. Unfortunately, only a few active volcanoes have been studied for long enough for these methods to work. In addition, volcanoes that have been inactive for years may erupt without warning. Therefore, scientists generally cannot accurately predict volcanic eruptions. READING CHECK 10. Explain Why might the volcano s sides bulge before an eruption? Holt McDougal Earth Science 199 Volcanoes
Section 2 Review SECTION VOCABULARY caldera a large, circular depression that forms when the magma chamber below a volcano partially empties and causes the ground above to sink felsic describes magma or igneous rock that is rich in feldspars and silica and that is generally light in color mafic describes magma or igneous rock that is rich in magnesium and iron and that is generally dark in color pyroclastic material fragments of rock that form during a volcanic eruption 1. Explain Why does felsic lava produce explosive eruptions more often than mafic lava does? 2. Compare Give one similarity and one difference between lapilli and volcanic blocks. 3. Describe How do calderas form? 4. Infer Which would be more likely to increase the steepness of a volcanic cone a quiet eruption or an explosive eruption? Explain your answer. 5. Explain Scientists today are studying many active volcanoes around the world. They are even studying volcanoes that have not erupted for hundreds or thousands of years. What is the most likely reason they are studying these volcanoes? Holt McDougal Earth Science 200 Volcanoes