Volcanoes A volcano is an opening in Earth s surface that erupts gases, ash, and lava. These materials pile up in layers around the opening, forming volcanic mountains. Earth has more than 600 active volcanoes. An active volcano is one that has erupted within recorded history. Active Volcanoes The most active volcano on Earth is in Hawaii. It is called Kilauea (kee low AY ah). This volcano has been erupting for hundreds of years, but its eruptions are slow, not explosive. Iceland is also famous for its active volcanoes. This island country is located in an area where Earth s plates move apart. Because of its northern location and active volcanoes, it is known as the land of fire and ice. Eruptions Pyroclastic Flow Environment Volcanic eruptions have a serious impact on people who live nearby. Their lives may be in danger. Even if people are able to evacuate or escape, their property is often damaged or destroyed. The lava flowing from a volcano destroys everything in its path. Volcanic ash and dust falling from the sky can collapse buildings and block roads. Ash can cause lung diseases in people and animals. Pyroclastic flow is volcanic ash and other matter that rush down the side of a volcano. Temperatures inside a pyroclastic flow can be hot enough to catch wood on fire. If the flow is heavy, people in nearby towns are forced to abandon their homes. Buildings, roads and crops may be destroyed by the pyroclastic flow with its falling debris. Geologists knew the Soufrière Hills volcano in Montserrat was about to erupt. They warned people living nearby to evacuate. Two years after the eruption began, large pyroclastic flows swept down the sides of the volcano. Cities and towns were buried. Plant life was destroyed. Twenty people who didn t evacuate were killed. This eruption was one of the largest recent volcanic eruptions near North America (July 1995). Sulfurous gas is released during volcanic eruptions. When these gases mix with water vapor in the atmosphere, acid rain forms. Acid rain can destroy the vegetation. Acid rain can fall into the lakes and streams and kill fish. If the vegetation died, the animals living in the forests may leave or die. When a volcano erupts, it is a danger to all living organisms and to the environment. Formation Deep inside Earth, heat and pressure melt rocks. The liquid rock is called magma. Some rocks deep in Earth already are melted. Other rocks are so hot, the smallest rise in temperature or drop in pressure melts them into magma.
Magma Lava Boundaries Divergent Rifts Convergent Magma is not as dense as the rock around it. This difference in density forces the magma to rise toward Earth s surface. After many thousands or even millions of years, magma reaches Earth s surface. Magma flows out through an opening called a vent Once magma reaches Earth s surface, it is called lava. As lava flows out, it cools and becomes solid, forming layers of igneous rock around the vent. Often the area around the vent is bowlshaped. The steep, bowl-shaped area around a volcano s vent is the crater. Volcanoes often form where plates are moving apart (divergent) or where plates are moving together (convergent). Plate boundaries are the areas where there is movement of plates. A divergent plate boundary is an area where Earth s plates are moving apart. The Mid-Atlantic Ridge is a divergent plate boundary. Iceland (island in the North Atlantic ocean) has volcanic activity because it is part of the Mid-Atlantic Ridge. Rifts are long deep cracks on the crust/ seafloor. Rifts form when plates move apart. Lava flows from these rifts and is cooled quickly by the seawater. As more lava flows and hardens, it builds up on the seafloor. Sometimes, the volcanoes and rift eruptions rise above sea level. Islands such as Iceland formed in this way. Areas where Earth s plates are pushing together are convergent plate boundaries. The Andes mountain range in South America began forming when an oceanic plate started sliding under a continental plate. Magma
Hot Spots forms when the plate sliding below another plate gets deep enough and hot enough to melt partially. The magma is forced to rise slowly to Earth s surface, forming volcanoes. Volcanoes that form on convergent plate boundaries usually have more explosive eruptions than other volcanoes. Hot spots can also for volcanoes. Scientists think there are areas between Earth s core and mantle that are unusually hot. Hot rock in these areas is forced toward the crust where it partly melts to form a hot spot. The Hawaiian Islands sit on top of a hot spot under the Pacific Plate. Magma has broken through the crust to form several volcanoes. The volcanoes that rise above the ocean form the Hawaiian Islands. DISCUSSION QUESTION: Where do volcanoes tend to form? Where plates are moving together (convergent) or moving apart (divergent) or at hot spots Outline A. Volcano opening in Earth that erupts gases, ash, and lava B. Volcanoes can kill people, destroy property, and disrupt the environment. 1. Lava and pyroclastic flows can bury cities and towns in their paths. 2. Sulfurous gases from volcanoes can create acid rain, which can kill organisms and pollute water. C. Volcanoes form when magma flows out of a surface opening called a vent; a steep-walled depression around the vent is called a crater. D. Volcanoes often form where plates are moving together or moving apart. 1. The Mid-Atlantic Ridge is a divergent plate boundary that forms rifts through which lava can flow. 2. At convergent plate boundaries, volcanoes tend to erupt more violently than they do in other areas. 3. At the boundary between Earth s mantle and core, unusually hot areas form hot spots, such as at the Hawaiian Islands.
Eruption strength Some volcanic eruptions are explosive; These eruptions are rapid, powerful, and destructive. Other volcanic eruptions are quiet; The lava flows slowly from the vent. Two factors determine whether an eruption will be explosive or quiet. One factor is the amount of water vapor and other gases trapped in the magma. The other factor is how much silica is in the magma. Silica is a compound made of the elements silicon and oxygen Gas and H 2 0 vapor Gases, like water vapor and carbon dioxide, are trapped in magma by the pressure of the surrounding magma and rock. As magma nears the surface, it is under less pressure. This allows the gas to escape from the magma. Gas escapes easily from some magma during quiet eruptions. However, gas that builds up to high pressures causes explosive eruptions. The magma at some convergent plate boundaries contains a lot of water vapor. This happens because an oceanic plate and some of its water slides under other plate material at some convergent plate boundaries. The trapped water vapor in the magma can cause explosive eruptions. Silica content Basaltic The second major factor that affects the nature of eruptions is the composition of magma. Magma can be divided into two major types silica poor and silica rich. Magma that is low in silica is called basaltic magma. It is fluid and produces quiet eruptions. This type of lava pours from the volcanic vents and runs down the sides of a volcano. This is called pahoehoe lava. When pahoehoe lava cools, it forms a ropelike structure. If the same lava flows at a lower temperature, it forms stiffer, slower moving lava called aa lava. Quiet eruptions form volcanoes over hot spots, which is how the Hawaiian volcanoes formed. Basaltic magma also flows from rift zones, which are long, deep cracks in Earth s surface. Many lava flows in Iceland are from rift zones. Basaltic magma is so fluid that when it is forced upward in a vent, the trapped gases can escape easily. As a result, the explosion is quieter, sometimes forming lava fountains. Lava that flows underwater forms pillow lava formations. Just as their name suggests, they are shaped like pillows. Basaltic lava = quiet eruptions. Granitic Magma that contains a lot of silica, or granitic magma, produces explosive eruptions. Granitic magma sometimes forms in areas where Earth s plates are moving together and one plate slides under the other. As the sinking plate goes deeper, some rock melts. The magma is forced upward because it is less dense than the rock around it. As it moves up, it comes in contact with Earth s crust, and becomes enriched in silica. Silicarich granitic magma is thick. As a result, gas gets trapped inside, causing pressure to build up. When an
Andesitic Volcanoes Shield Flood Basalts explosive eruption occurs, the gases expand quickly, often carrying pieces of lava in the explosion. Granitic lava = explosive eruptions. There is another type of magma andesitic magma. Andesitic magma contains more silica than basaltic magma, but less than granitic magma. It often forms at convergent plate boundaries where one plate slides under the other. Because of the higher silica content, andesitic magmas erupt more violently than basaltic magmas. The word andesitic comes from the Andes mountain range located in South America. These mountains contain many andesite rocks. Many of the volcanoes that circle the Pacific Ocean also are made up of andesite. All volcanoes do not look alike. The shape of a volcano depends on whether it was formed by a quiet or explosive eruption. The shape also depends on what type of lava it is made of basaltic, granitic, or andesitic. The three basic types of volcanoes are shield volcanoes, cinder cone volcanoes, and composite volcanoes. A broad, gently sloping volcano formed by quiet eruptions of basaltic lava is a shield volcano. Hawaiian islands The same basaltic lava that forms shield volcanoes also forms flood basalts. Basaltic lava can flow out of Earth s surface through large cracks called fissures. This type of eruption does not form volcanoes. It forms flood basalts and accounts for the greatest amount of erupted volcanic material. The basaltic lava flows over Earth s surface, covering large areas with thick deposits of basaltic igneous rock. The Columbia Plateau, located in the northwestern United States, is a flood basalt. Much of the new seafloor that begins at mid-ocean ridges forms as underwater flood basalts. Tephra Cinder Cone Composite Volcano Explosive eruptions throw lava and rock high into the air. Bits of rock and solidified lava dropped from the air during an explosive volcanic eruption are called tephra. Tephra comes in different sizes from small pieces of volcanic ash to large rocks called bombs and blocks. Cinder cone volcanoes are steep-sided, loosely packed volcanoes formed when tephra falls to the ground. The alternating eruptions of lava and tephra produced the composite volcano
Shield Cinder cone Composite DISCUSSION QUESTION: What two factors account for the varying force of volcanic eruptions? Amount of gas or water vapor present in the magma, and amount of silica in the magma Outline A. The amount of water vapor and other gases present is one factor that determines whether a volcanic eruption will be quiet or explosive. 1. Gases can be trapped in magma by pressure of surrounding magma and rock; eventually they cause an explosive eruption. 2. Magma at convergent plate boundaries can contain a lot of water vapor that can cause explosive eruptions. B. The composition of magma is a second factor affecting the nature of a volcano s eruption. 1. Low-silica magma, called basaltic, is fluid and produces a quiet, nonexplosive eruption. a. Pahoehoe lava runs down the side of a volcano. b. Aa lava is a stiff, slow moving lava. 2. High-silica magma called granitic and intermediate silica magma called andesitic produce explosive eruptions. C. Three types of volcanoes form from the three types of lava. 1. As quiet eruptions of basaltic lava spread out in flat layers, they form a broad volcano with gently sloping sides called a shield volcano. 2. As tephra (bits of rocks or solidified lava) falls to the ground, it forms a steep-sided, loosely packed cinder cone volcano. 3. A composite volcano forms from alternating layers of quiet lava and more explosive tephra.
Intrusive Batholiths Most magma never reaches Earth s surface to form volcanoes or to flow as flood basalts. Most magma cools underground and produces underground rock bodies. Over time, these rock bodies may be seen at Earth s surface if erosion exposes them. These underground rock bodies are called intrusive igneous rock features. There are several different types of intrusive features. The most common types are batholiths, sills, dikes, and volcanic necks. Batholiths are the largest intrusive igneous rock bodies. They can be many hundreds of kilometers wide and long. They can be several kilometers thick. Batholiths form when magma bodies cool slowly and solidify before reaching Earth s surface. Some batholiths have been exposed at Earth s surface after millions of years of erosion. Dikes, sills, and volcanic necks Magma that is squeezed into a crack that cuts across rock layers and hardens underground is a dike. Magma that is squeezed into a crack parallel to rock layers and hardens underground is a sill. A volcanic neck is the solid igneous core of a volcano that is left behind after the softer cone erodes. When a volcano stops erupting, the magma hardens inside the vent. Erosion begins to wear away the volcano. Because the cone is softer than the igneous rock in the vent, it erodes first, leaving behind a volcanic neck
Caldera A caldera is a large depression, or bowl shape, created when a volcano collapses. Crater Lake in Oregon is a caldera that filled with water and is now a lake. Crater Lake formed about 7,000 years ago when Mount Mazama erupted violently and then collapsed. DISCUSSION QUESTION: How do a dike and a sill differ? A dike forms from magma that hardens after being forced into a crack cutting across rock layers; a sill forms from magma that hardens after being forced into a crack parallel to rock layers. Outline A. Many intrusive igneous features form underground and are later exposed. 1. Batholiths rock bodies formed when magma bodies that are being forced upward from inside Earth cool slowly and solidify before reaching the surface 2. Dike magma that hardens after being forced into a crack cutting across rock layers; sill magma that hardens after being forced into a crack parallel to rock layers B. A volcanic neck forms when the cone is eroded away, leaving the solid igneous core. 1. Caldera large depression formed when the top of a volcano collapses 2. Weathering and erosion wear down surface rock and expose igneous rock features.