1. Plate Tectonics and Volcanoes Earth Science Chapter 18

2. Volcanoes Volcanoes –Hills or mountains made from hardened magma

3. 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

4. Volcanoes Magma/lava is made of: –Silica –Water vapor –Magnesium –Iron

5. 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

6. Properties of Felsic (Rhyolitic) magma –Light in color –High silica (SiO 2 ) 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

7. Properties of Mafic (Basaltic) magma –D–Dark in color –H–High in iron –L–Low silica content –F–Flows easily –D–Does not contain a lot of water vapor –L–Low amount of gases (bubbles escape easily) Volcanoes

8. Mafic Lava

9. 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

10. 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)

11. 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

12. Volcanoes 3 Types of Volcanoes –Shield cone Broad or wide Mafic lava Mellow eruptions Hawaii (caused by a hot spot)

13. Shield Cone

14. Volcanoes –Cinder cone Narrow Tall (not as tall as composites) Felsic lava Pyroclastic eruptions –Lots of tephra and gases, not much lava Paricutin, Mexico

15. Cinder Cone

16. Volcanoes –Composite Cone Alternating layer of lava and cinders (ash) Alternates pyroclastic and non-pyroclastic eruptions Tall, snow-capped peaks Mount St. Helen’s

17. Composite Cone

18. 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

19. Ring of Fire

20. 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)

21. Volcanoes and Boundaries Subduction Volcanoes

22. Volcanoes and Boundaries Divergent Boundary –N–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)

23. Big Idea: –Plate motions provide the mechanism by which mantle rocks melt to generate magma.

24. 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.

25. 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

26. Plutonic Structures Stock –Hardened magma exposed at the surface due to erosion –Small batholith

27. 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

28. Plutonic Structures Sill –Magma goes into rock layers horizontally and hardens

29. 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

30. Plutonic Structures Volcanic Neck –Plug of hardened magma left in the vent –Cone is completely eroded