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Understanding Intrusive and Extrusive Igneous Rocks: From Mt. St. Helens to Volcanic Types

This chapter delves into the characteristics of intrusive and extrusive igneous rocks, using Mt. St. Helens as a key example two months post-eruption. It explains how magma viscosity, temperature, and silica content influence eruption types, from explosive to non-explosive. Distinctions between types of volcanoes, including shield, cinder cone, and composite volcanoes, highlight their respective hazards. Additionally, the chapter outlines the formation of various igneous features such as plutons, dikes, and batholiths, emphasizing their geological significance.

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Understanding Intrusive and Extrusive Igneous Rocks: From Mt. St. Helens to Volcanic Types

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  1. CHAPTER 5 INTRUSIVE AND EXTRUSIVE IGNEOUS ROCKS

  2. Mt. St. Helens, Washington

  3. Mt. St. Helens – two months after the main event

  4. VOLCANOES – WASHINGTON TO CALIFORNIA

  5. WHY SO EXPLOSIVE? • Type of eruption depends on viscosity of the magma • Temperature of magma • Silica content • Rhyolitic magmas are more siliceous and more viscous • Basaltic magmas are less siliceous and less viscous

  6. IMPORTANCE OF DISSOLVED GASES • Dissolved gases (mostly water) increase fluidity of magma. • High gas (volatile) content, mostly water, escapes easily from a fluid basaltic magma. Quiet Hawaii shield volcanoes. • Escaping gas from viscous intermediate to ryholitic magma produces explosion. Shield volcanoes in western U.S.

  7. PRODUCTS OF VOLCANO • Fluid basaltic lava flows – non-explosive • Pyroclasic airborne material • Ash, dust • Lapilli, blocks, bombs • Molten ash and dust make welded tuff • Flows – pumice, scoria

  8. TYPES OF VOLCANOES • Cinder cones. • Gas-rich basaltic magma ejected as ash, cinders, bombs • Shield volcanoes • Mostly fluid basaltic lavas flow long distances to form large flat volcanoes • Composite or stratovolcanoes • Both lava and pyroclastics form symmetrical cones, as in CA-OR-WA • Mostly gas-rich andesitic flows, with some basaltic and rhyolitic lavas, that don’t flow far. • Large volumes of explosive pyroclasics

  9. CINDER CONE

  10. CINDER CONE

  11. SHIELD VOLCANO

  12. COMPOSITE VOLCANO

  13. VOLCANO TYPES

  14. NOT MUCH DANGER

  15. NOT MUCH DANGER

  16. VERY, VERY BAD

  17. Mt. Vesuvious, city of Pompeii

  18. Mt. Pelee, city of St. Pierre, Martinique

  19. CALDERAMt. Mazama, Oregon

  20. BASALT PLATEAUS

  21. BASALT PLATEAU

  22. BASALT PLATEAUSWashington & Oregon

  23. LAVA DOME

  24. VOLCANIC NECKShiprock, New Mexico

  25. INTRUSIVE IGNEOUS ROCKS • PLUTON – Igneous magma solidifies underground. • Dike – discordant; small; any composition • Sill – concordant; small; any composition • Laccolith – concordant; small; any composition • Stock – discordant; <40 sq. mi. in area; intermediate to granitic compostion • Batholith; area > 40 sq. miles. Can be hundreds of sq. mi. in area; extend to great depth into crust; intermediate to granitic composition; core of mountain range • Idaho batholith - > 40,000 sq. km in area • Canadian Shield - many batholiths; cores of Precambrian mountain ranges

  26. INTRUSIVE & EXTRUSIVE IGNEOUS FEATURES

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