Chapter 12 Vocabulary and Study Guide Volcanoes 1) acid rain Moisture with a PH below 5.6 that falls to Earth as rain or snow and can damage forests, harm organisms, and corrode structures. Sulfurous gases emitted by volcanoes can mix with water vapor and form acid rain 2) tephra Bits of rock or solidified lava dropped from the air during an explosive volcanic eruption. These range in size from volcanic ash to volcanic bombs and blocks. 3) pyroclastic flow A ground-hugging avalanche of hot ash, pumice, and rock fragments that rushes down the side of a volcano during an eruption. The flow travels down the slope of the volcano at speeds up to 150 miles per hour. The temperature inside the flow of hot gases and rock can reach 1500 degrees Fahrenheit. These "stone winds" traveling at hurricane speeds kill or destroy everything in their path. The flows usually follow the curvature of the land to the valleys below the mountain. Sometimes a pyroclastic surge will jump ridges and flow down nearby valleys spreading the destruction into new areas. 4) intrusive A type of igneous rock feature that generally contains large crystals and forms when magma ( not lava!) cools slowly beneath Earth’s surface. Magma underground cools very slowly, over thousands to millions of years. As it cools, elements combine to form common silicate minerals, the building blocks of igneous rocks. These mineral crystals can grow quite large if space allows. The mineral crystals within this type of rock are large enough to see without a microscope. There are many different types of intrusive igneous rocks but granite is the most common type. Intrusive igneous rock bodies such as batholiths, dikes, and sills form when magma solidifies underground. 5) extrusive This describes fine-grained igneous rock that forms when magma cools quickly at or near the Earth’s surface. When lava erupts onto the Earth's surface, it cools quickly. If the lava cools in less than a day or two, there is no time for elements to form minerals. Instead, elements are frozen in place within volcanic glass. Often, lava cools over a few days to weeks and minerals have enough time to form but not time to grow into large crystals. Basalt is the most common type of extrusive igneous rock and the most common rock type at the Earth's surface. 6) lava Molten rock that flows from volcanoes onto Earth’s surface. After magma reaches the surface it's called lava. 7) magma Hot, melted rock material beneath Earth’s surface. Magma originates in the mantle. There, high temperatures and pressure cause some rocks to melt and form magma. After magma reaches the surface it's called lava. Magma rises to the surface because magma is less dense than rock. The types of magma are determined by the chemical composition and the physical properties of the magma. Three general types are recognized: basaltic magma, andesitic magma, and granitic magma. Basaltic magma has a high temperature and due to its flowing nature is the type of magma most commonly thought of when talking about volcanoes. Andesite magma has a moderate temperature and is moderately explosive. Granitic magma has a low temperature and is highly explosive. The way a volcano erupts is largely determined by the viscosity, or ‘gooeyness’ of the magma. Viscosity is the resistance to flow (opposite of fluidity) and depends primarily on the composition of the magma, and its temperature. Water has low viscosity (flows easily) while syrup and honey have greater viscosity. High viscosity lavas flow slowly and typically cover small areas. Low viscosity magmas flow more rapidly and form lava flows that cover thousands of square kilometers. Low viscosity magmas allow gases to escape easily whereas gas pressures can build up in high viscosity magmas resulting in violent eruptions. Granitic Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma). The silica content is another factor in determining how viscous magma is. Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma). 8) caldera A large, circular shaped opening formed after an eruption of a volcano when the top of the volcano collapses. A collapse is triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a very large volcanic eruption. If enough magma is ejected, the emptied chamber is unable to support the weight of the rock above it and it collapses. Many times, caldera will fill with water and become mountaintop lakes. Most calderas are massive, crater-like depressions that can cover many tens of square miles. 9) cinder cone volcano A steep-sided, loosely packed volcano formed when tephra falls to the ground. These generally do not have a lava flow, but when they erupt they are explosive and throw lava high into the air. Paricutin, in Mexico, is a cinder cone volcano. Cinder cones are made up of small fragments of lava from a single vent that have been blown into the air, cooled and fallen around the vent. Cinder cone volcanoes are the simplest and the most common type of volcano. They are steep sided circular or oval cone shapes of basaltic fragments and are much smaller and much simpler than composite volcanoes. 10) composite volcano Composite volcanoes, also known as a stratovolcanos, are found mostly where Earth's plates come together and one plate sinks beneath the other. Composite volcanoes are steep-sided volcanoes composed of many alternating layers of volcanic rocks, lava, volcanic ash, tephra, and pumice. Mt. Rainier and Mount St. Helens are examples of this type of volcano. The eruptions of Krakatoa in 1883, Mount Saint Helens in 1980 and Mount Pinatubo in 1992 were examples of large volcanic eruptions by composite volcanoes. Often, the large-volume explosions rapidly drain the lava beneath these mountains and cause the top to collapse to form large depressions called calderas. Later, these depressions fill with water and form beautiful lakes, like Crater Lake in Oregon. 11) shield volcano Broad, gently sloping volcano formed by quiet eruptions of basaltic lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of a flat, rounded shape, with a profile much like that of a warrior's shield. Shield volcanoes are the least explosive. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 miles and heights of 1,500 to 2,000 feet. The Hawaiian Islands are composed of linear chains of shield volcanoes including Kilauea and Mauna Loa on the island of Hawaii, two of the world's most active volcanoes. 12) volcanic mountain A volcanic mountain is formed when molten material and ash reaches Earth’s surface through a weak crustal area and piles up into a cone-shaped structure. The term ‘volcano’ is only the rupture on the crust of Earth which allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. It is not a volcanic mountain until enough material has piled up to make it a mountain. 13) volcano Opening in Earth’s surface that erupts allowing sulfurous gases, ash, and lava to escape from a magma chamber below the surface. They can form at Earth’s plate boundaries, where plates move apart or together, and at hot spots. Volcanoes often are found in areas on Earth where plates collide and at these convergent plate boundaries, volcanoes tend to erupt more violently than they do in other areas. A volcano often forms a mountain when layers of lava and volcanic ash erupt and build up. Sulfurous gases emitted by volcanoes can mix with water vapor and form acid rain. More than 600 of Earth’s volcanoes are currently active but Kilauea in Hawaii is the world’s most active volcano. The Pacific Ring of Fire is an area around the Pacific Ocean where over 75% of the volcanoes on Earth are found. The Ring of Fire outlines the borders of the Pacific Plate and other major tectonic plates and includes the western coast of the United States. 14) hot spot The result of an unusually hot area at the boundary between Earth’s mantle and core that forms volcanoes when melted rock is forced upwards and breaks through the crust. Mantle plumes are these areas of hot, upwelling mantle. A hot spot develops above the mantle plume. Hotspots may be far from tectonic plate boundaries. A volcanic hotspot is where lava pushes up from under the mantle and creates a volcano. The earth's plates move along and another volcano is created later. This creates a chain of volcanoes, such as in Hawaii. The Hawaiian Islands are formed because of hot spots. The Hawaiian hot spot has been active at least 70 million years, producing a volcanic chain that extends 3,750 miles (6,000 km) across the northwest Pacific Ocean. 15) volcanic neck The solid igneous core of a volcano left behind after the softer portions of the cone have been eroded. A volcanic neck is the remnant of an old eroded volcano. Differences in resistance to erosion cause a volcanic neck to form. As the volcano died, the last bit of lava inside of the volcanoes opening, or neck, cooled and hardened. Over many hundreds of thousands of years the material around the neck is removed by erosion, leaving only the harder neck behind.