The Changing Earth
Chapter Twelve: Earthquakes 12.1 Earthquakes 12.2 Volcanoes
Investigation 12B Volcanoes How are volcanoes and plate boundaries related?
12.2 Looking inside a volcano A volcano is where melted rock and other materials from Earth s mantle are released. Mount St. Helens is a type of volcano called a composite volcano (also known as a stratovolcano).
12.2 Looking inside a volcano Melted rock called magma leaves the magma chamber (where magma is stored) and moves up the conduit during an eruption. The magma leaves through the vent either gently or with violent force. Magma may move upward in a sheet, which is called a dike. Magma is called lava after it leaves the vent.
12.2 Volcano features after an eruption The bowl-shaped depression formed by the collapse of a volcano after eruption is known as a caldera.
12.2 Volcano features after an eruption If magma flows back up the conduit, a mound called a resurgent dome may form on the caldera floor.
12.2 Volcano features after an eruption If water fills the caldera, or magma doesn t drain completely, a lava lake remains.
12.2 The life of a volcano Active volcanoes are erupting or have erupted recently, and are expected to erupt again in the near future. Dormant volcanoes are not active now, but may become active again in the future. Extinct volcanoes are at the end of their life and are no longer able to erupt.
12.2 The life of a volcano Devil s Tower and Ship Rock are examples of extinct volcanic necks. As the volcano erodes once it becomes extinct, a core of solid magma gets exposed.
12.2 Where does magma come from? There are two ways to make rock melt. One way is to reduce the pressure. Rock under low pressure melts at a lower temperature.
12.2 Where does magma come from? The other way is to mix water with the hot rock. Rock that contains water melts at a lower temperature.
12.2 Types of magma Magma with low amounts of silica is runny. Magma with a lot of silica is thick and sticky. Volcanoes near subduction zones tend to form magma high in silica.
12.2 Types of magma Another important property of magma is how much gas is dissolved in it. High gas content in magma results in more explosive eruptions.
12.2 Volcanoes with low silica magma Low silica (runny) magma can t build up tall sides and produces a shield volcano.
12.2 Volcanoes with low silica magma Because low-silica magma is runny, it can t build up a tall, cone-shaped volcano.
12.2 Volcanoes with low silica magma When low silica magma has high levels of dissolved gas, gas bubbles out as it reaches the volcano vent. A fire fountain is produced. Imagine shaking a soda bottle to produce a shower of soda.
12.2 Cinder cones The lava cinders form a cone around the vent called a cinder cone. Cinder cones are a common form of volcano. Cinders are loose clumps of particles of lava.
12.2 Volcanoes with high silica magma A tall composite volcano forms from thick, sticky magma that piles up near the vent because it is silicarich.
12.2 Volcanoes with high silica magma Before volcano eruption, the magma may be under so much gas pressure that the volcano cone bulges.
12.2 Volcanoes with high silica magma The cone may explode near the vent, throwing a column of gas and lava bits high into the atmosphere. The lava bits puff up and rip apart as the dissolved gas expands inside each bit.
12.2 Explosive eruptions When a column of exploding material collapses, it races down the side of a composite volcano as a pyroclastic flow.
12.2 Explosive eruptions If water is present in the ground, mudflows called lahars may accompany a composite volcano eruption. Lahars can destroy forests and property and add to the death toll.
12.2 Volcanoes divergent boundaries What causes lava to melt? Mid-ocean ridges occur underwater at diverging plate boundaries. When lava oozes out at a mid-ocean ridge, it immediately hits cold seawater, forming a crust.
12.2 Volcanoes at mid-ocean ridges Basalt magma is runny because of its low silica content. The seawater cools the lava, forming a crust. The crust cracks and another blob of basalt magma oozes out. When this cycle repeats over and over, the lava looks like a pile of pillows.
12.2 Volcanoes at convergent boundaries About half of the active surface volcanoes on Earth occur along the shores of the Pacific Ocean. This region is called the Ring of Fire. What is the Ring of Fire?
12.2 Volcanoes at convergent boundaries The Ring of Fire is found where the oceanic crust of the Pacific Plate is subducting under nearby plates. Most volcanoes are located along plate boundaries.
12.2 Volcanoes on continents The famous granite domes of Yosemite National Park in California were formed as silica-rich magma rose through the edge of the North American Plate. The surrounding land later eroded away, exposing granite domes.
12.2 Volcanic islands away from plate boundaries Volcanic island chains are formed as lithospheric plates move over the top of plumes that are known as hot spots. (such as Hawaii)
12.2 Volcanic chains 1. As the plate moves, it carries the volcanic island away from the active hot spot. 2. Without the hot spot to supply magma, the volcano becomes extinct. 3. The hot spot begins to form a new volcano beside the old one. 4. The result is a volcanic island chain.
12.2 Volcanic chains
12.2 Volcanic chains A hot spot is making Loihi bigger, so Hawaii is growing. The island of Hawaii sits on top of a hot spot. The hot spot has formed the Mauna Loa and Kilauea volcanoes on the island. Currently, the hot spot is making the undersea volcano Loihi to the southeast of the island.
12.2 Measuring motion of a plate By studying a volcanic island chain, scientists can determine the direction and speed that a plate is moving.
Geology Connection Indian Ocean Earthquake and Tsunami Tilly Smith probably never imagined what she learned in geography class would help save lives.
Activity The Dragon and Toad Mystery In this activity, you will learn something about the history of using seismographs.