Chapter 9

1. Chapter 9 Volcanoes

2. 9.1 What causes volcanoes?

3. Volcano: an opening in the Earth’s surface which forms a mountain when layers of ash & lava build up • More than 600 active volcanoes • Most volcanoes are inactive

6. Two Examples of Volcanoes • May 18, 1980 • Mount St. Helens in Washington • One of North America’s most damaging eruptions • 59 people were killed—most by landslides & flooding due to snow on the mountain melting

7. Two Examples of Volcanoes • Mount Kilauea in Hawaii • World’s most active volcano • It has been erupting continuously since 1983

8. Volcano Terms • Magma: melted rock INSIDE THE EARTH • Melts due to high levels of heat & pressure • Composed of a mixture of molten rock, minerals, & gases • Melting occurs in the mantle • Which layer of the Earth is this?

9. Volcano Terms • Density: m/v (which means?) • Mass divided by volume • This means how much something weighs (m), in relation to how much space it takes up (v) • Magma is less dense than the rock which surrounds it…so… • Magma rises to the surface of the Earth

10. How do volcanoes form? • After long periods of time magma may reach the surface of the Earth • Then it will flow out of an opening called a vent • Lava will flow out & cool continuously • This can make large mountains made of lava & ash packed together • Crater: steep-walled depression around a volcano’s vent

11. Anatomy of a Volcano • Magma Chamber: area underneath the Earth where magma comes from • Lava: magma which reaches the surface of the Earth • Vent: opening in the crust of the Earth where magma is released • Crater: top of a volcano (bowl-shaped) which is connected to the magma chamber by the vent

15. Where do volcanoes occur? • Most volcanoes occur along plate boundaries—areas where plates interact with one another

16. 1. Divergent Boundary: where plates move apart • Plates move apart and magma is forced upward through faults (?) • Rifts: openings in plates caused by divergent boundaries • Ex. Iceland—Mid-Atlantic Ridge

18. 2. Convergent Boundary: plates come together • Plates move together and the plate which is more dense will sink and melt under the other one. • Then the magma will rise up. • 80% of volcanoes are convergent • Ex. Pacific Ring of Fire, Mt. St. Helens

20. 3. Hot Spots: areas in the mantle which are hotter than others • Plates move over a hot spot and this causes the plate to melt—then magma rises to the surface • Ex: Hawaiian Islands

23. Hot spots also form… • Seamounts: underwater islands • Once an island is past a hot spot, its volcanoes will become dormant (meaning they will no longer erupt)

25. 9.2 Types of Volcanoes

26. Styles of Eruptions • Some volcanic eruptions are explosive & some are quiet. • There are 3 factors of magma that affect the type of eruption.

27. 1.Trapped Gases • Gases such as water vapor & CO2 are trapped in magma by the pressure of the surrounding magma & rock • Gas escapes easily from some magma during quiet eruptions. • Other eruptions are explosive because the gas builds up to a very high pressure

28. 2. Magma Composition: • Magma low in silica (an explosive compound found in magma that traps gas) is fluid (flows easily) & produces quiet eruptions • Basaltic magma • Lava pours from vents (Hawaii)

29. Magma with a lot of silica produces very explosive eruptions (Mt. St. Helens) • Silica-rich magma is thick & gas gets trapped causing pressure to build up • Granitic magma

30. 3. Magma Water Content: High water content will cause explosive eruptions • Magma at some converging zones contains a lot of water vapor

31. Types of Volcanoes • The appearance of a volcano depends on the material that forms it and the type of eruption.

32. 1. Shield Volcano: mountain with gently sloping sides and a circular base • Forms when layer upon layer of lava accumulates after non-explosive eruptions • Ex: Kilauea (Hawaiian Islands)

33. 2. Cinder-Cone Volcano: generally small volcanoes with steep sides • These volcanoes throw rock fragments called tephra high into the air • The tephra lands around the vent and begins to pile up creating a steeply-sided, loosely packed pile.

34. 3. Composite Volcano: very large, steep-sided mountains • Begin with extremely explosive eruptions (magma with high silica content) • Tephra is thrown into the air • Next there is a quiet period with lava flows

35. This repeating cycle of tephra-lava, tephra-lava forms the mountain • Ex: Mt. St. Helens

38. 9.3 Igneous Rock Features

39. Intrusive Rock Activity • Because magma rises but only sometimes reaches the surface, it will intrude on rock layers which already exist • These igneous (“born of fire”) structures are called plutons • Can range in size from 1km to 100 km • These areas can be seen in places that have been eroded or uplifted

40. Batholith: largest intrusive igneous rock bodies • Form when magma cools underground before reaching the surface • One that can be seen is in Yosemite National Park

42. Dikes and Sills: • Dikes: magma that is squeezed into a vertical crack which cuts across other rock layers • Sills: magma squeezed into a horizontal crack between rock layers and hardens

45. Volcanic neck: • Forms when a volcano stops erupting • Magma hardens inside the vent • Soft cone erodes away first leaving behind a solid igneous core—volcanic neck • Ex: Ship Rock, New Mexico

49. Many times the tops of volcanoes will sink into magma chambers. • The hole left behind is called a Caldera. • The caldera usually fills with water • Example: Crater Lake in Oregon