Warm-up #49 Apr. 3

1. Warm-up #49 Apr. 3 • May 18, 1980 – Mount St. Helens erupted with tremendous force • Blew off the entire north flank of the mountain • Ejected a cubic km of ash and rock debris • Yakima, Washington (130 km away) was covered with ash that blocked out the sun • Why do volcanoes like this erupt so explosively, while others like Kilauea in Hawaii are relatively quiet?

2. Volcanoes Unit 9

3. What is a volcano? • An opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface

4. Factors that Affect Eruptions • Factors that determine the intensity of a volcano include: • Magma composition • Magma temperature • The amount of dissolved gases in the magma

5. VISCOSITY • Viscosity is a substance’s resistance to flow • Ex. Maple syrup is more viscous than water. • Magma from an explosive eruption may be thousands of times more viscous. • As lava flow cools and begins to harden, its viscosity increases, its mobility decreases, and halts. • Directly related to its silica content

6. Dissolved Gases • The gases trapped in magma provide the force to eject molten rock from the vent, or opening on the surface. • Water vapor and carbon dioxide • Viscous magmas slow the movement of gases going upward creating explosive bubbles and pockets

7. Lava Flows • Basaltic Lava • Very fluid (low silica content) • Two forms: • pahoehoe flows resemble twisted braids of ropes • Aa (Ah ah) flows have rough, jagged blocks and sharp edges

8. Pyroclastic Materials • Name given to particles produced in volcanic eruptions • Fragments ejected during eruptions range in size from very fine dust and volcanic ash (less than 2 mm) to pieces that weigh several tons

9. Types of Volcanoes • Three main types: • Shield • Cinder cone • Composite cone

10. Anatomy

11. Shield Volcano • Produced by the accumulation of fluid basaltic lavas. • Shaped of a broad, domed structure • Ex: Hawaiian Islands, Iceland

12. Cinder Cones • Usually a product of relatively gas-rich basaltic magma • Very simple shape and small • Determined by the steep sided slope that loose pyroclastic material maintains as it comes to rest

13. Composite Cones • Considered the most beautiful and most dangerous • Large, nearly symmetrical, composed of layers of both lava and pyroclastic deposits

14. Assignment Your mission is to find information and report on a volcano, other than the ones listed above, that has erupted in the last 100 years. Your report must include: • Type of volcano • Geographic location • Name, distance, and population of nearest major city • Date of most recent eruption and date of most destructive eruption • Other events associated with the last eruption (earthquakes, floods, mudslides, etc)

15. Warm-up #50 Apr. 4 According to geologists what is the most devastating natural event that can take place on Earth? What type of structure is this?

16. Other Volcanic Landforms • Caldera – large depression in a volcano. • Usually formed 2 ways: • Collapse of the top of a composite volcano after erupting. • Collapse of a shield volcano after magma chamber is drained.

17. Other Volcanic Landforms • Necks and Pipes – conduits that connect a magma chamber to the surface. • Rocks in the pipes remain standing after the cone has been eroded called a volcanic neck

18. Other Volcanic Landforms • Lava Plateaus • The greatest volume of volcanic material is extruded from fissures. • Ex. Columbia Plateau

19. Plutons • Plutons– structures that result from the cooling and hardening of magma at depth. • Intrusive igneous bodies are classified according to shape, size, and relationship to surrounding rock layers

20. Sills and Laccoliths – plutons that form when magma is intruded close to the surface. • Sills form when magma is injected along sedimentary bedding surfaces • Laccoliths are similar to sills but magma is more viscous

21. Dikes – form when magma is injected into fractures, cutting across preexisting rock layers. • Many form when magma from a large chamber invades fractures.

22. Batholiths – largest intrusive body • Must have a surface exposure greater than 100 square km • Ex: Idaho Batholith (40,000 square km)

23. Assignment • http://www.mnh.si.edu/earth/main_frames.html • Explore the site, Review plate tectonics and continue on to volcanoes • Once you have read about volcanoes, go to “GeoGallery” at the bottom of the page. • In your journals, write about 5 different types of volcanic activity (caldera, stratovolcano, pyroclastic flow, etc) • You should have a paragraph for each

24. Warm-up #51 Apr. 5 • Why is a volcano fed by a highly viscous magma likely to be a greater threat to people than a volcano fed by very fluid magma?

25. Origin of Magma • Controversial subject • Geologists conclude that magma originates when solid rock (crust and upper mantle) partially melt

26. Role of Heat • Rate of temperature change averages between 20°C and 30°C per km in the upper crust. • 100 km ranges from 1400°C and 1600°C • Close to the melting point of rock. • Sources of heat: • Friction • Crustal rocks descend • Hotter mantle rocks rise

27. Role of Pressure • An increase in confining pressure causes an increase in the rock’s melting temp. Role of Water • Water causes rock to melt at lower temperatures

28. Convergent Plate Boundaries • The basic connection between plate tectonics is that plate motions provide the mechanism by which mantle rocks melt to generate magma

29. Intraplate Igneous Activity • Intraplate Volcanism occurs within a plate, not the boundary. • Most occur where a mass of hotter than normal mantle material called a mantle plume rises toward the surface • Ex: Kilauea in Hawaii

30. Ticket Out the Door • What are the three types of volcanoes?

31. Build a Volcano • Get in groups of 3-4 • Grab a plate, bottle and some play-doh • Shape a volcano around the bottle • Add: • Half-way full of warm water & food color • 4-6 drops of detergent • About 2 tablespoons of baking soda Lastly, add vinegar