Ch 12 Volcanoes Earth Science Section 1 Volcanoes

Ch. 12 - Volcanoes Earth Science

Section 1 – Volcanoes and Earth’s Moving Plates � Learning Objectives: › Describe how volcanoes can affect people › List conditions that cause volcanoes to form. › Identify the relationship between volcanoes and Earth‘s moving plates.

Section 1 – Volcanoes and Earth’s Moving Plates � Volcano – opening in Earth that erupts gases, ash, and lava.

� B. Volcanoes can kill people, destroy property, and disrupt the environment. › 1. Lava and pyroclastic flows (very hot ash and debris) can bury cities and towns in their paths.

› 2. Sulfurous gases from volcanoes can mix with water vapor and create acid rain, which can kill organisms and pollute water.

(Not in notes) � How do volcanoes form? › Magma is less dense than the rock around it, so it is forced slowly toward Earth’s surface.

� C. Volcanoes form when magma flows out of a surface opening called a vent. � A steep-walled depression around the vent is called a crater.

Where do Volcanoes form? � D. Volcanoes often form where plates are moving together, or moving apart, or at hot spots.

› 1. The Mid-Atlantic Ridge is a divergent plate boundary that forms rifts through which lava can flow

› 2. At convergent plate boundaries, volcanoes tend to erupt more violently than they do in other areas.

› 3. At the boundary between Earth’s mantle and core, unusually hot areas form hot spots.

› Example of a hot spot Hawaiian Islands �The Pacific plate is moving over a stationary hot spot

Discussion Question � Where do volcanoes tend to form? Hint – 3 main areas

Discussion Question � Where do volcanoes tend to form? › Answer – �Where plates are moving together (convergent boundaries) �or moving apart (divergent boundaries) �or at hot spots

Section 2 – Types of Volcanoes � Learning Objectives: › Explain how the explosiveness of a volcanic eruption is related to the amount of silica and water vapor in its magma › List three forms of volcanoes

(Add to the top or bottom of your notes packet) � Two major factors that determine how violent a volcano will erupt… › 1. Amount of gas or water vapor in magma › 2. Amount of silica in magma Silica – compound made of the elements silicon (Si) and oxygen (O)

� A. The amount of water vapor and other gases present is one factor that determines whether a volcanic eruption will be quiet or explosive.

› 1. Gases can be trapped in magma by pressure of surrounding magma and rock; eventually they cause an explosive eruption. › 2. Magma at convergent boundaries can contain a lot of water vapor that can cause explosive eruptions.

� B. The composition of magma (specifically, the amount of silica) is a second factor affecting the nature of a volcano’s eruption.

› 1. Low-silica magma, called basaltic, is fluid and produces a quiet, nonexplosive eruption.

�a. Pahoehoe (Pa-HOY-hoy) lava runs down the side of a volcano ropelike structure �b. Aa (AH-ah) lava is a stiff, slow moving lava. Same kind of lava, just different speeds

› 2. High-silica magma is called granitic › Intermediate-silica magma is called andesitic They both produce explosive eruptions.

� C. Three types of volcanoes form from the three types of lava. Lava flows, hardens, and builds up over time forming volcanoes.

› 1. As quiet eruptions of basaltic lava spread out in flat layers, they form a broad volcano with gently sloping sides called a shield volcano.

› 2. As tephra (bits of rocks or solidified lava) falls to the ground, it forms a steep-sided, loosely packed cinder cone volcano. › Formed from explosive eruptions

› 3. A composite volcano forms from alternating layers of quiet lava and more explosive tephra. (Found at subuction zones)

� Review: › Shield volcano �Basaltic lava) Low silica �Quiet eruptions › Composite volcano �Low and high silica lava �Alternates between quiet and explosive eruptions › Cinder cone volcano �Granitic + intermediate lava) High silica �Explosive eruptions

Composite Shield

Discussion Question � What two factors account for the varying force of volcanic eruptions?

Discussion Question � What two factors account for the varying force of volcanic eruptions? › 1. Amount of gas or water vapor › 2. Amount of silica in magma

Section 3 – Igneous Rock Features � Learning Objectives: › Describe intrusive igneous rock features and how they form › Explain how a volcanic neck and a caldera form

� Intrusive Features – are formed underground � Extrusive Features – are formed at Earth’s surface

� A. Many intrusive igneous features form underground are later exposed › 1. Batholiths – rock bodies formed when magma bodies that are being forced upward from inside Earth cool slowly and solidify before reaching the surface.

� Example of a batholith Granite domes of Yosemite National Park Remember, Granite is a type of igneous rock which is formed from cooled magma

› 2. Dike – magma that hardens after being forced into a crack cutting across rock layers

› Sill – magma that hardens after being forced into a crack parallel to rock layers.

� B. A volcanic neck forms when the cone of an old volcano is eroded away, leaving the solid igneous core. (Devil’s Tower, Wyoming)

� Example of a volcanic neck Ship Rock, New Mexico

› 1. Caldera – large depression formed when the top of a volcano collapses.

� Example of a caldera Crater Lake, Oregon

› 2. Weathering and erosion wear down surface rock and expose igneous rock features.

Discussion Question � How are a dike and sill different?

Discussion Question � How are a dike and sill different? › Dike – forms from magma that hardens after being forced into a crack cutting across rocks › Sill – forms from magma that hardens after being forced into a crack running parallel with rocks