Warm Up 11/29

1. Warm Up 11/29 • The broad, slightly dome-shaped volcanoes of Hawaii are ____. a. cinder cone volcanoes c. pyroclastic volcanoes b. composite cone volcanoes d. shield volcanoes • The most violent volcanic eruptions are associated with what type of volcano? a. composite cones c. fissure eruptions b. shield volcanoes d. cinder cones • A magma’s viscosity is directly related to its ____. a. age c. silica content b. depth d. color Answers: 1) d. 2) a. 3) c.

2. Intrusive Igneous Activity Chapter 10, Section 2

3. Plutons • Plutons – structures that result from the cooling and hardening of magma at depth • They can only be studied after uplift and erosion have exposed them to the surface • Intrusive igneous bodies, or plutons, are generally classified according to their shape, size, and relationship to the surrounding rock layers

4. Sills and Laccoliths • Sills and laccoliths are plutons that form when magma is intruded close to the surface • Sill – forms when magma is injected along sedimentary bedding surfaces, parallel to the bedding planes • For a sill to form, the sedimentary rock must be lifted to a height equal to the thickness of the sill • Laccoliths – forms when magma intrudes into sedimentary bedding surfaces, collects as a lens-shaped mass that pushes the overlying strata upwards

5. Sills

6. Laccoliths

7. Concept Check • How are sills and laccoliths alike? • They are both plutons formed by magma intrusions close to the surface • How are sills and laccoliths different? • They differ in shape and usually composition

8. Dikes and Batholiths • Dikes – plutons that form when magma is injected into fractures, cutting across preexisting rock layers • Many dikes form when magma from a large magma chamber invades fractures in the surrounding rocks • Batholiths – the largest intrusive bodies, must have a surface exposure greater than 100 square kilometers • Batholiths may form the core of mountain ranges

9. Dikes

10. Batholiths

11. Basic Igneous Structures

12. Origin of Magma • Earth’s crust and mantle are composed primarily of solid, not molten, rock, and the liquid iron in the outer core is too dense to travel all the way to the surface • Geologists conclude that magma originates when essentially solid rock, located in the crust and upper mantle, partially melts • The most obvious way to generate magma from solid rock is to raise the temperature above the level at which the rock begins to melt

13. The Role of Heat • The rate of change of temperature as you move down into the upper crust is ~20-30º per kilometer • Geothermal Gradient – the change in temperature with depth • At a depth of ~100 km, temperature ranges from 1400ºC and 1600ºC, almost the melting point of rock • Subduction zones add friction (another way to produce heat) and water (which lowers the melting temperature of rock) to give the added heat needed to melt the crustal rocks • Hotter mantle rocks can rise and intrude into crustal rocks, melting them

14. Geothermal Gradient

15. Concept Check • What is a geothermal gradient? • The change in temperature with depth

16. Magma Formation at Subduction Zones

17. The Role of Pressure • Pressure also increases with depth • Melting occurs at higher temperatures as you go deeper into Earth, because an increase in the confining pressure causes an increase in the rock’s melting temperature • Reducing that confining pressure lowers a rock’s melting temperature • Decompression Melting – triggered when the confining pressure drops enough to melt the rock

18. Decompression Melting

19. Assignment • Read Chapter 10, Section 2 (pg. 289-292) • Do Section 10.2 Assessment #1-6 (pg. 292)