1. Chapter 12: Volcanoes!

2. Volcanoes and Earth's Moving Plates A volcano is an opening in Earth that erupts gases, ash and lava. Volcanic mountains form when layers of lava, ash and other material build up around these openings. Kilauea (kee low AY ah), located in Hawaii, is the world's most active volcano. The most recent series of eruptions began in 1983 and still continues.

3. How do volcanoes form?? Magma is forced upward because it is less dense than the rock around it. Ex: Air bubbles rise to surface in shampoo/syrup bottles. After many thousands or even millions of years, magma reaches Earth's surfaces and flows out through an opening called a vent. As lava flows out, it cools quickly and becomes solid, forming layers of igneous rock around the vent. The steep walled depression around a volcano's vent is called a crater.

4. Where do volcanoes occur?? Divergent Plate Boundaries: the mid-Atlantic ridge is an area where the plates are moving apart. When plates separate, they form long, deep cracks called rifts. Lava flows from these rifts and is cooled quickly by sea water. Sometimes, the volcanoes and rift eruptions rise above sea level, forming new islands such as Iceland. In 1963, the new island Surtsey was formed during a volcanic eruption.

5. Convergent Plate Boundaries: places where the Earth's plates move together...especially form volcanoes where an oceanic plate slides below a continental plate. Magma forms when the plate sliding below another plate gets deep enough and hot enough to melt partially. The magma is then forced upward to the surface. Volcanoes that form at convergent plate boundaries erupt more violently than other volcanoes do. Hot spots: form when areas between the Earth's mantle and core are unusually hot and rock at these areas is forced toward the crust where it melts partially. Magma breaks through the crust to form several volcanoes. Not at a plate boundary. Formed Hawaiian islands What would happen to the big island of Hawaii if the Pacific plate stopped moving?

6. Types of Volcanoes What controls eruptions? Trapped gases like water vapor and carbon dioxide build up in volcanoes. If the gas escapes easily, it is a quiet eruption. If gas and water vapor build up, eruptions can be explosive. At what plate boundary could water vapor build up? Composition of Magma Quiet eruptions: magma that is low in silica and are basaltic. This type of lava pours from volcanic vents and runs down the sides of a volcano (pg. 338) Explosive eruptions: magma that is high in silica and is andesitic. This type of magma is thick and pressure builds up an explosion occurs...often carrying pieces of lava.

7. Forms of Volcanoes Shield Volcano: broad, gently sloping volcano formed by quiet eruptions of basaltic lava. The basaltic lava flows over Earth's surface covering large areas with this deposits of basaltic igneous rocks when it cools. Basaltic lava can also flow onto Earth's surface through large cracks (not volcanoes) called fissures and creates flood basalts. Accounts for the greatest volume of erupted volcanic material. Much of the new seafloor that originates at the mid-ocean ridge forms as underwater flood basalts.

8. Cinder Cone Volcano: steep- sided, loosely packed volcano formed when tephra falls to the ground. Tephra: bits of rock or solidified lava dropped from the air after an explosive eruption. Vary in sizes from volcanic ash, cinders, large rocks and blocks. Composite Volcano: volcano built by alternating explosive and quiet eruptions that produce layers of tephra and lava. Found mostly where Earth's plates come together and one plate sinks below the other. (What plate boundary is this?)

9. Igneous Rock Features Intrusive Features Most of the magma never reaches Earth's surface...as magma cools underground it produces underground rock bodies that could become exposed later at the surface by erosion. Batholiths: large intrusive rock body that forms when magma being forced upward toward Earth's crust cools slowly and solidifies underground. Dikes: magma that is forced into a crack that cuts across rock layers and hardens. Sills: magma that is forced into a crack parallel to rock layers and hardens.

10. Other features Volcanic neck: solid igneous core of a volcano left behind after the softer cone has been eroded. When a volcano stops erupting, the magma hardens inside the vent. Erosion, usually by water and wind, begins to wear away at the volcano. Caldera: large circular- shaped opening formed when the top of a volcano collapses. Crater Lake in OR is a caldera that filled with water and is now a lake.

11. Effects of Eruptions When volcanoes erupt, they often have direct, dramatic effects on the lives of people and their property. Lava flows destroy everything in their path. Falling volcanic ash can collapse buildings, block roads and can cause lung disease in people and animals. Sometimes, volcanic ash and debris rush down the side of a volcano: pyroclastic flow. Temperature inside a pyroclastic flow can be high enough to ignite wood. When sulfurous gases from volcanoes mix with water vapor in the atmosphere, acid rain forms.