VOLCANOES The intense heat and pressures in the mantle turn solid rock into liquid magma. Magma comes out through the surface of the crust to form volcanoes. The type of volcano formed and the style of eruption depends on the silica content, and therefore, the viscosity of the magma. The “Ring of Fire” is a series of volcanoes that indicates tectonic plate boundaries and contains about 75% of the world’s active volcanoes. The “Ring of Fire” marks the boundary of the Pacific Ocean basin. Volcanoes and Plate Boundaries There are two distinct types of volcanic eruptions. Explosive eruptions occur when highly viscous magma (that is, magma with a very high silica content) erupts through continental crust. Explosive eruptions are truly explosive and extremely destructive. They throw hot ash, particles and chunks of rock, and gases into the air. Ash from explosive eruptions can travel into the stratosphere and encircle the entire Earth for many years. This ash layer can lower global temperatures. Explosive eruptions are typical of tectonic plate boundaries where an oceanic plate is subducted under a continental plate. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
The ash and rock that is extruded by explosive volcanoes is called pyroclastic material. A mudflow of mixed water and volcanic ash can travel very, very quickly down a volcano and cause widespread destruction. This volcanic mudflow is called a lahar. The second type of volcanic eruption is called non-explosive. Liquid, less viscous lava pours out of the crust and flows onto the surface in non-explosive eruptions. (When liquid rock is in the crust it is called magma. When it flows or explodes onto the surface it is then called lava.) Non-explosive eruptions occur at mid-ocean ridges and other places where tectonic plates pull away from each other. They can also occur in the middle of oceanic plates at hot spots. A hot spot is an extremely thin portion of the oceanic plate that has a magma body very close to the surface. The Hawaiian Islands are non-explosive volcanoes that formed at a hot spot in the Pacific Ocean basin. Sometimes lava flows out of a long crack in the crust rather than through a hole. When this happens, extremely thick layers of lava can cover thousands of square miles. These thick, extensive layers of lava are called lava plateaus. Lesson Checkpoint: How did the Hawaiian Islands form? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Volcanic Eruptions and Properties of Magma The type of a volcanic eruption is determined by the physical properties of the magma extruded by a particular volcano. In short, non-explosive volcanoes extrude lava that has lower viscosity (in other words, it is runnier). By comparison explosive volcanoes extrude pieces of hot rock because the lava has a high viscosity (in other words, it is very, very sticky). The igneous rock type that flows from non-explosive volcanoes is basalt. Different names are given to the types of lava that pours from a non-explosive volcano. Pahoehoe lava is ropy-looking because the outer crust hardens but underneath the liquid lava continues to flow. Here is a picture of pahoehoe lava. Aa lava is blocky lava that cooled into irregular, sharp-edged chunks. Lava that cools instantly into volcanic glass is called obsidian. Small and large tear-shaped blobs of lava can be thrown from the volcano. These are called volcanic bombs. Obsidian When lava pours out of mid-ocean ridges onto the ocean floor, it cools rapidly from contact with the very cold water of the deep ocean. The result is the accumulation of rounded masses of basalt. This is called pillow basalt. Pillow basalts found on dry land indicate that that land was, at one time in geologic history, under an ocean and volcanically active. Lesson Checkpoint: What is obsidian? © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Three Types of Volcanoes Volcanoes can also be classified based on their shape. The three categories are cinder cones, shield volcanoes and stratovolcanoes. Cinder cones are relatively small volcanoes formed by the accumulation of pyroclastic material. The sides of cinder cones are steep. Shield volcanoes are wide, flat volcanoes formed by the non- explosive flow of lava onto the surface. The largest shield volcano in our solar system is Olympus Mons on Mars. Stratovolcanoes are the most common type of volcanoes. They are formed by the alternating of layers of pyroclastic material and lava flows. Below are pictures of each type of volcano. Cinder cone Mauna Loa, a shield volcano, in Hawaii © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
Mt. St. Helens, a stratovolcano, erupted in 1980 in Washington State Lava makes its way from deep in the crust to the surface through a tube called the vent. At the top of many volcanic vents is a cone- shaped depression formed when pyroclastic material erupts from the volcano. Geologists call this depression a crater. After a volcano has erupted, the magma chamber that fed magma to the volcano can become partially emptied. In time the volcano can collapse into this chamber, forming a large depression on the surface. This depression is called a caldera. Lesson Checkpoint: Name the three categories of volcanoes. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.
What Causes Hot Springs and Geysers? In special situations, magma reservoirs occur very near the surface. The intense heat from the magma heats the groundwater. Hot groundwater can come to the surface in a number of interesting and spectacular ways. Geysers are fountains of hot water that spray out of the ground at regular intervals. The water is heated and when it reaches a certain pressure it shoots out of the ground until the pressure is relieved. Then new water percolates through the rock, is heated and will eventually spray out of the ground. Old Faithful is a famous geyser in Yellowstone National Park. When the water is not under pressure, it flows out of the ground. This is called a hot spring. Mud pots form when heated water mixes with sediments and collects in pools. These pools typically bubble like thick stew in a pot. In some situations this naturally heated water can be collected and used as an alternative form of energy. This is called geothermal energy (literally, “earth heat energy”). The hot water is collected and piped directly into buildings as a source of heat. It can sometimes be used to drive turbines to generate electricity. © Copyright NewPath Learning. All Rights Reserved. Permission is granted for the purchaser to print copies for non-commercial educational purposes only. Visit us at www.NewPathLearning.com.