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Plate Tectonics and Volcanoes Earth Science Chapter 18 Volcanoes • Volcanoes – Hills or mountains made from hardened magma Volcanoes • Magma – Molten rock that is below the surface of the earth – Located in the mantle – Forms when rocks melt due to subduction • Lava – Molten rock that has reached the surface of the earth – Forms when volcanoes erupt Volcanoes • Magma/lava is made of: – Silica – Water vapor – Magnesium – Iron Volcanoes • 3 Types of magma/lava – Felsic (Rhyolitic) • Made of 60% or more silica – Intermediate (Andesitic) • Made of 50-60% silica – Mafic (Basaltic) • Made of 50% or less silica Volcanoes • Properties of Felsic (Rhyolitic) magma – Light in color – High silica (SiO2) content – Contains a lot of water vapor – Does not flow easily – Highly viscous (thick) – Often solidifies before reaching the earth’s surface – High amount of gases (bubbles stuck in thick liquid) Volcanoes • Properties of Mafic (Basaltic) magma – Dark in color – High in iron – Low silica content – Flows easily – Does not contain a lot of water vapor – Low amount of gases (bubbles escape easily) Mafic Lava Volcanoes • 2 Manners of Eruptions 1. Pyroclastic • Highly explosive • Felsic lava (trapped gases build up pressure) • Lots of poisonous gases and tephra, little magma erupted • Pyroclastic flow – hot stream of gases and tephra that flows down the volcano Volcanoes • Types of Tephra: – Bombs • Extremely large (> 64 mm) chunks erupted from volcano • Ejected as liquid, harden as they fall – Blocks • Erupted as solid pieces (> 64 mm) – Lapilli • Smaller than bombs ( up to 64 mm) – Ash • Fine grained/very small chunks (< 2 mm) Volcanoes 2. Non-Pyroclastic – Non-explosive – Mafic lava (very little trapped gases) – Lava erupts more continuously, but flows out quietly – Mid-Atlantic Ridge is best example Volcanoes • 3 Types of Volcanoes – Shield cone • Broad or wide • Mafic lava • Mellow eruptions • Hawaii (caused by a hot spot) Shield Cone Volcanoes – Cinder cone • Narrow • Tall (not as tall as composites) • Felsic lava • Pyroclastic eruptions –Lots of tephra and gases, not much lava • Paricutin, Mexico Cinder Cone Volcanoes – Composite Cone • Alternating layer of lava and cinders (ash) • Alternates pyroclastic and non-pyroclastic eruptions • Tall, snow-capped peaks • Mount St. Helen’s Composite Cone Volcanoes and Boundaries – Subduction Boundaries • Pyroclastic eruptions –Felsic lava –Slabs are pushed down into the mantle –Pressure and temp rise – water turns to steam –Hot fluids melt mantle rock & magma migrates upward. –Cinder and composite cones have pyroclastic eruptions Ring of Fire Volcanoes and Boundaries • Subduction Volcanoes – Ocean/Ocean • Chain of volcanoes called an island arc – Ocean/Continent • Volcanic arc • Continental crust has higher silica content • Magma varies in composition (what it’s made of) Volcanoes and Boundaries • Subduction Volcanoes Volcanoes and Boundaries • Divergent Boundary – Non-pyroclastic • Mafic lava • Magma produced during sea floor spreading • Magma rises to fill in rift where plates have separated • Sometimes called basaltic (most of the sea floor is made of basalt) Volcanoes and Boundaries • Big Idea: –Plate motions provide the mechanism by which mantle rocks melt to generate magma. Plutonic Structures • There is much more magma under the surface of the Earth than we see with volcanoes. • That magma forms other Igneous structures (plutons) that we can identify. • It solidifies in the Earth’s crust, hardening in other rocks. Plutonic Structures • Types: • Batholiths – Hardened magma that forms the cores of many mountain ranges – Largest type of intrusion – Sometimes exposed at the surface due to erosion Plutonic Structures • Stock – Hardened magma exposed at the surface due to erosion – Small batholith Plutonic Structures • Dike – Magma cools inside a fracture • A fracture is a break in the crust with no movement – Cuts vertically across the layers of rock Plutonic Structures • Sill – Magma goes into rock layers horizontally and hardens Plutonic Structures • Laccolith – Goes into layers horizontally, but the stiff magma is unable to spread to form a sill – Instead it pushes the land up to form a dome Plutonic Structures • Volcanic Neck – Plug of hardened magma left in the vent – Cone is completely eroded