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, ~ 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