The Nature of Volcanic Eruptions

1. The Nature of Volcanic Eruptions Chapter 10, Section 1

2. Factors Affecting Eruptions • The primary factors that determine whether a volcano erupts violently or quietly include magma composition, magma temperature, and the amount of dissolved gases in the magma • Viscosity – a substance’s resistance to flow • Something that is more viscous flows more slowly; as a lava flow cools, it becomes more viscous as the lava slows down • The more silica there is in a lava, the more viscous that lava is • During explosive eruptions, the gases trapped in magma provide the force to eject molten rock from the vent, an opening to the surface

3. Factors Affecting Eruptions

4. Volcanic Material • The less silica content there is in a basaltic lava, the faster it will flow, lava flows are common in Hawaii • Magma contains varied amounts of dissolved gases held in the molten rock by confining pressure, it only is about 1 to 6 percent of the total weight of the magma • It is important to study the composition of volcanic gases, because they are the gases that formed the atmosphere in the past • Pyroclastic material – name given to particles produced in volcanic eruptions • The fragments ejected during eruptions range in size from very fine dust and volcanic ash (less than 2 millimeters) to pieces that weigh several tons (blocks and lava bombs)

5. Volcanic Material

6. Concept Check • What is a volcanic bomb? • A large streamlined chunk of pyroclastic material that is larger than 64 mm in diameter.

7. Anatomy of a Volcano • The three main volcanic types are shield volcanoes, cinder cones, and composite cones • Volcanic activity often starts with a fissure, or crack, develops and magma is forced through it • Volcano – a mountain built by repeated eruptions of lava or pyroclastic material often separated by long inactive periods • Crater – steep-walled depression located at the top of many volcanoes • The form of the volcano is largely determined by the composition of the magma

8. Anatomy of a Volcano

9. Shield Volcanoes • Shield Volcano – produced by the accumulation of fluid basaltic lavas • Shape – broad, slightly domed structure that resembles a warrior’s shield • Most have grown from the ocean-floor to form islands (Hawaii and Iceland)

10. Shield Volcanoes

11. Cinder Cones • Cinder Cone – built by ejected lava fragments the size of cinders, which harden in the air • Product of relatively gas-rich basaltic magma • Shape – determined by the steep-sided slope that loose pyroclastic material maintains as it comes to rest • Usually the product of a single eruption that lasts only a few weeks, rarely a few years • The magma in the pipe solidifies after the eruption, and the volcano doesn’t erupt again • Cinder cones are small, ~30-300 meters and less than 700 meters in height • There are thousands of cinder cones around the world

12. Cinder Cones

13. Composite Cones • Composite Cone (Stratovolcano) – large, nearly symmetrical structure composed of layers of both lava and pyroclastic deposits • The silica-rich magmas typical of composite cones generate viscous lavas that can only travel short distances • May generate the most explosive eruptions that eject huge quantities of pyroclastic materials • About 50 of these have erupted in the U.S. in the last 200 years • The most dangerous results of one of these eruptions is a pyroclastic flow (hot gases and rock fragments), coming at a speed of 200 km/hr • Lahars – destructive mudflow created when volcanic debris becomes saturated with water and rapidly moves down the volcano, often following stream valleys

14. Composite Cones

15. Concept Check • What is a lahar? • A mudflow down the slope of a volcano.

16. Profiles of Volcanic Landforms

17. Other Volcanic Landforms • Caldera – a large depression in a volcano • Forms by: (1) collapse of the top of a composite volcano after an explosive eruption, or (2) collapse of the top of a shield volcano after the magma chamber is drained • Most volcanoes are fed magma through conduits, called pipes, connecting a magma chamber to the surface • When the volcano has been eroded, especially cinder cones, the harder solidified magma will remain and become a volcanic neck • Lava Plateau – the greatest volume of volcanic material doesn’t build volcanoes, but is extruded through fissures and flows over a large area

18. Formation of Crater Lake, Oregon

19. Volcanic Areas in the Northwestern United States

20. Assignment • Read Chapter 10 • Chapter 10 Assessment