1. Title: 18.2 Eruptions Page #: Date 5/24/2013
Table of Contents
Title: 18.2 Eruptions
Page #:
Date 5/24/2013

2. Objective
Students will be able to: Explain how magma type influences volcanic activity.
Students will be able to describe the role of pressure and dissolved gases in eruptions.
Students will be able to recognize classifications of material ejected by eruptions.

3. Word of the Day
Silica: A white or colorless crystalline compound, SiO2, occurring abundantly as quartz, sand, flint, agate, and many other minerals and used to manufacture a wide variety of materials, especially glass and concrete.

4. 18.2 Eruptions
Main Idea
Pg. 508
The composition of magma determines the characteristics of a volcanic eruption.

5. 18.2 Eruptions
Pg. 508
Making Magma: Not all volcanic eruptions are the same because different types of magma create different eruptions.
Temperature: Increase as depth into Earth increases.
Most rocks melt between 800⁰ C and 1200⁰.
Temperatures found in crust and upper mantle.
Pressure: Pressure increases with depth because of weight of overlying rock.
As pressure increases so does the temperature at which a substance melts.

7. 18.2 Eruptions Composition of Magma Pg. 509
Composition of Magma: Determines a volcano’s explosivity – how it erupts and how its lava flows.
Factors that determine the composition of magma:
Overlying crust
Temperature
Pressure
Amount of dissolved gases
Amount of silica in the magma (VERY IMPORTANT!)

8. Which type of lava (magma on Earth’s surface) is thicker, left or right?

9. 18.2 Eruptions Dissolved Gases Pg. 509
Dissolved Gases: The more dissolved gases, the more explosive an eruption will be.
Gases:
Water vapor: The most common, determines where magma forms, lowers temperature at which minerals melt.
Carbon dioxide
Sulfur dioxide
Hydrogen sulfide

10. 18.2 Eruptions Viscosity Pg. 509
Viscosity: The physical property that describes a material’s resistance to flow.
Hot temp = Low viscosity
Cool temp = High viscosity
Lots of silica = High viscosity – Thick and sticky magma, trapped gases, LARGE EXPLOSIONs.
Low silica = Low viscosity – Runny and thin, easy flow, NON-EPLOSIVE and QUIET ERUPTIONS.

11. 18.2 Eruptions Types of Magma Pg. 510 Types of Magma:
More silica = more viscous, more explosive.
Less silica = less viscous, less explosive.
Basaltic Magma: When rock in upper mantle melts, basaltic magma forms.
Less than 50% silica.
Location: Hot Spots.
Reacts with little overlying continental crust.
Gases escape easily.
Quiet eruptions.
Example: Kilauea (Hawaii.)

13. 18.2 Eruptions Types of Magma Pg. 510
Andesitic Magma: 50 – 60% silica. Medium viscosity.
Location: Oceanic-continental subduction zone (Andes Mountains, Chile.)
Source material: oceanic crust and sediments.
Explosive eruptions.
Example: Colima Volcano in Mexico.

14. Andesitic Lava

15. 18.2 Eruptions Types of Magma Pg. 510 Rhyolitic Magma: 60% + silica.
Location: Above a subduction zone.
Source material: Continental crust (lots of silica and water.)
Very high viscosity.
Very explosive.
Example: Yellowstone National Park.

16. Rhyolitic lava.

17. 18.2 Eruptions Explosive Eruptions Pg. 512
Explosive Eruptions: Viscous lava builds pressure, until it causes a volcano to explode. Explosions throw lava and rock into the air.
Tephra: Erupted materials.
Can be pieces of lava that solidified during eruption, or pieces of crust that didn’t melt into magma.
Classified by size:
Ash: less than 2 mm wide.
Blocks: Largest pieces of tephra thrown. Can be the size of a car.

19. 18.2 Eruptions Explosive Eruptions Pg. 512
Explosive Eruptions send tephra all over the planet.
Ash shoots up to 40 km into the sky.
Ash may change global climates if there is enough of it.

20. 18.2 Eruptions Pyroclastic Flows Pg. 513
Pyroclastic Flows: Rapidly moving clouds of tephra mixed with hot gases.
Very dangerous.
Fast, 200 km / hour.
Hot, 700° C +.

21. Pyroclastic Flow