1. The Nature of Volcanic Eruptions
Earth Science
10-2
The Nature of Volcanic Eruptions

2. Factors Affecting Eruptions
The Primary factors that determine whether a volcano erupts explosively or quietly include characteristics of the magma and the amount of dissolved gases in the magma.
Mount Saint Helens before and after eruption

3. Viscosity Viscosity Is a substance’s resistance to flow
Viscosity is strongly affected by temp.
If lava cools and begins to harden, its Viscosity increases

4. Continued… Chemical composition
Chemical composition on magma has a more important effect on the type of eruption
Because of high silica content, rhyolitic lavas are very viscous and erupt explosively
Less silica has a calmer eruption

5. Dissolved Gases Vent An opening to the surface
During explosive eruptions, the gases trapped in magma provide the force to propel molten rock out of the vent.
These gases are mostly water vapor and carbon dioxide

6. Volcanic Material Lava Flows
Silica content and the temperature affect the characteristics of lava flows.
Temperature differences produce 2 types of basaltic lava: pahoehoe and aa
2. Pahoehoe
Is hotter, fast moving basaltic lava
Pahoehoe forms relatively smooth skin that wrinkles as the still-molten lava continues to flow
3. Aa
Is cooler, slower-moving basaltic lava
Aa forms a surface of rough, jagged blocks with sharp, spiny projections

7. 5. Pyroclastic Materials
4. Gases
Magmas contain varied amounts of dissolved gases held under pressure in the molten rock
The gaseous portion of the magma is a small portion about 1 to 6%
5. Pyroclastic Materials
Particles produced in volcanic eruptions are called pyroclastic materials
The fragments ejected range in size from very fine dust and volcanic ash to pieces that weigh several tons

8. Types of volcanoes
Anatomy of a volcano
Volcanic activity usually begins when a fissure, or crack, develops in the crust
Volcano- repeated eruptions of lava or pyroclastic material eventually build up a mountain
Crater- steep-walled depression
The three main volcanic types of volcanoes are shield volcanoes, cinder cones, and composite cones.

10. Shield volcanoes
Are produced by the accumulation of fluid basaltic lavas.
Shield volcanoes have a shape of a broad, slightly domed structure that resembles a warrior’s shield.

11. 2. Cinder cones
Ejected lava fragments that harden in the air build a cinder cone
The shape of a cinder cone is determined by the steep-sided slope that forms as loose pyroclastic material builds up around the vent

12. 3. Composite cones
Is a large, nearly symmetrical volcano mountain composed of layers of both lava and pyroclastic deposits
Most composite cones are located in a relatively narrow zone that rims the Pacific Ocean, the Ring of Fire

13. How many of these Volcanoes are located in the U.S.

14. Other Volcanoc Landforms
Volcanic landforms include calderas, volcanic necks, and lava plateaus
Long after eruptions have ended, these landforms can provide evidence of volcanic activity

15. Calderas A caldera is a depression in a volcanic mountain.
Most calderas form in two ways:
By the collapse of the top of a composite volcano after an explosive eruption
From the collapse of the top of a shield volcano after the magma chamber drained

16. Volcanic necks
Is a landform made of magma that hardened in a volcanoes pipe and later was exposed by erosion
Weathering and erosion work constantly to wear away the volcanoes
When a volcanoes activity ends, magma remaining in the pipe hardens to form igneous rock

17. Lave plateaus
Is a volcanic landform produced by repeated eruptions of very fluid, basaltic lava
Instead of building a cone the lava spreads out over a wide area
The lava that forms the lava plateaus erupts through long cracks called fissures

18. Volcanic Hazards
Volcano hazards include lava flows, volcanic ash, pyroclasitc flows, and mudflows.
Lahar
Occurs when water-soaked volcanic ash and rock slide rapidly downhill
Composite volcanoes are among the most dangerous. They can cause a lahar
An explosive eruption can also create a pyroclastic flow
Volcanoes usually give some warnings that an eruption is near
Usually most people settle near a volcano because of the fertile land from the ash