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

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.

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.

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

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.

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!)

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

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

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.

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.)

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.

Andesitic Lava

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.

Rhyolitic lava.

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.

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.

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 +.

Pyroclastic Flow