Objectives

1. Magma Objectives • Describe factors that affect the formation of magma. • Compare and contrast the different types of magma. Vocabulary • viscosity

2. Magma Magma • The ash that spews from some volcanoes can form billowy clouds that travel around the world before raining back down to Earth. • In the last 10 000 years, more than 1500 different volcanoes have erupted, providing evidence that Earth is indeed geologically active.

3. Magma How Magma Forms • Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes. • Magma forms when temperatures are high enough to melt the rocks involved, usually between 800°C and 1200°C. • Such temperatures exist at the base of the lithosphere and in the asthenosphere.

4. Magma How Magma Forms Pressure • Pressure, which increases with depth, is one factor that determines whether rocks will melt to form magma. • As pressure increases, the temperature at which a substance melts also increases. • Due to the effects of pressure, most of the rocks in Earth’s lower crust and upper mantle do not melt to form magma.

5. Magma How Magma Forms Water • The presence of water also influences whether a rock will melt. • At any given pressure, a wet mineral or rock will melt at a lower temperature than the same mineral or rock under dry conditions.

6. Magma Types of Magma • The three major types of magma are: basaltic magma, andesitic magma, and rhyolitic magma. • Basaltic magma has the same composition as basalt and fuels the volcanoes that make up the Hawaiian Islands and Surtsey, which is south of Iceland. • Andesitic magma has the same composition as andesite and fuels Mount St. Helens in Washington State and Tambora in Indonesia. • Rhyolitic magma has the same composition as granite and fueled the dormant volcanoes in Yellowstone National Park.

7. Magma Types of Magma

8. Magma Types of Magma Magma Composition • A number of factors determine the composition of magma. • Viscosity is the internal resistance to flow; the higher the viscosity, the thicker the magma.

9. Magma Types of Magma Basaltic Magma • Basaltic magma typically forms when rocks in the upper mantle melt. • Most basaltic magma rises relatively rapidly to Earth’s surface and reacts very little with crustal rocks because of its low viscosity. • The volcanoes fueled by basaltic magma erupt relatively quietly.

10. Magma Types of Magma Andesitic Magma • Andesitic magma is found along continental margins, where oceanic crust is subducted into Earth’s mantle, and is formed from oceanic crust or oceanic sediments. • Andesitic magma contains about 60 percent silica, resulting in an intermediate viscosity. • The volcanoes it fuels are said to have intermediate eruptions.

11. Magma Types of Magma Rhyolitic Magma • Rhyolitic magma forms when molten material rises and mixes with the overlying silica- and water-rich continental crust. • Rhyolitic magma has high viscosity and fuels very explosive volcanoes.

12. Magma Viscosity • The viscosity of magma and lava depends on both temperature and composition. • The hotter the magma or lava, the lower the viscosity. • Magmas and lavas high in silica have higher viscosities than magmas and lavas low in silica.

13. Magma Section Assessment 1. Match the magma types with their characteristics. ___ basaltic ___ andesitic ___ rhyolitic B A C A. intermediate viscosity content, forms from oceanic crust and oceanic sediments B.low viscosity and gas content, forms from rocks in the upper mantle C.high viscosity, forms from continental crust materials

14. Magma Section Assessment 2. What would be the likely effect if the volcano at Yellowstone National Park were to erupt? Why? It would most likely be a devastating eruption because it would be fueled by rhyolitic magma, which has a very high viscosity and gas content.

15. Magma Section Assessment 3. Identify whether the following statements are true or false. ______ It is unlikely that Mount Kilauea in Hawaii will explosively erupt. ______ Wet granite will melt at a lower temperature than dry granite. ______ A liquid with a high viscosity will also have a high flow rate. ______ Major eruptions of Mount St. Helens in Washington state and Mount Fuji in Japan would probably be similar in nature. true true false true

16. End of Section 1

17. Intrusive Activity Objectives • Explain how magma affects overlying crustal rocks. • Compare and contrast intrusive igneous rock bodies. Vocabulary • pluton • batholith • stock • laccolith • sill • dike

18. Intrusive Activity Intrusive Activity • Magma, because it is molten, is less dense than surrounding rocks. • This density difference forces magma to move upward and eventually come into contact with, or intrude into, the overlying crust.

19. Intrusive Activity Intrusive Activity • Intruding magma can affect the crust in several ways: A. Magma can force the overlying rock apart and enter the newly formed fissures. B. Magma can also cause blocks of rock to break off and sink into the magma, where the rocks may eventually melt. C. Magma can melt the rock into which it intrudes.

20. Intrusive Activity Plutons • When magma cools, minerals form and over a very long period of time; they will combine to form intrusive igneous rock bodies. • Plutons are intrusive igneous rock bodies that can be exposed at Earth’s surface as a result of uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks.

21. Intrusive Activity Plutons

22. Intrusive Activity Plutons Batholiths and Stocks • Batholiths, the largest plutons, are irregularly shaped masses of coarse-grained igneous rocks covering at least 100 km2 and take millions of years to form. • Stocks are irregularly shaped plutons that are similar to batholiths but smaller in size. • Both batholiths and stocks cut across older rocks and generally form 10–30 km beneath Earth’s surface.

23. Intrusive Activity Plutons Laccoliths • A laccolith is a mushroom-shaped pluton with a round top and flat bottom resulting from a Magma intrusion into parallel rock layers close to Earth’s surface. • Compared to batholiths and stocks, laccoliths are relatively small; they are, at most, up to 16 km wide.

24. Intrusive Activity Plutons Sills and Dikes • A sill is a pluton, ranging from only a few centimeters to hundreds of meters in thickness, that forms when magma intrudes parallel to layers of rock. • A dike is a pluton, ranging from a few centimeters to several meters wide and up to tens of kilometers long, that cuts across preexisting rocks. • While the textures of sills and dikes vary, many are coarse grained, which indicates they formed deep in Earth’s crust and cooled slowly.

25. Intrusive Activity Plutons and Tectonics • Many plutons, especially batholiths, are formed as the result of mountain-building processes. • Batholiths are thought to have formed as a result of continental-continental and oceanic-oceanic convergence. • The plutons that form deep beneath Earth’s surface represent the majority of igneous activity on our planet.

26. Intrusive Activity Section Assessment 1. Match the following terms with their definitions. ___ batholith ___ stock ___ laccolith ___ sill ___ dike D B E A C A. a pluton that forms when magma intrudes parallel to layers of rock B.an irregularly shaped pluton that is similar to a batholith but smaller in size C.a pluton that cuts across preexisting rocks D. an irregularly shaped pluton that covers at least 100 km2 E.a mushroom-shaped pluton with a round top and flat bottom

27. Intrusive Activity Section Assessment 2. How do sills and laccoliths differ? Sills and laccoliths both result from magma intrusions that are parallel to existing rock. Laccoliths push the overlying layers upward, creating a distinct mushroom-shape when they cool and solidify. Sills are generally thinner and do not cause a noticeable bump in the surface.

28. Intrusive Activity Section Assessment 3. What surface feature are batholiths most associated with? Batholiths are found at the cores of many of Earth’s mountain ranges.

29. End of Section 2

30. Volcanoes Objectives • Describe the major parts of a volcano. • Compare and contrast shield, cinder-cone, and composite volcanoes. • Contrast the volcanism that occurs at plate boundaries. • Explain the relationship between volcanism and hot spots. Vocabulary • cinder-cone volcano • composite volcano • tephra • pyroclastic flow • hot spot • vent • crater • caldera • shield volcano

31. Volcanoes Anatomy of a Volcano • When magma reaches Earth’s surface it is called lava. • A vent is where lava erupts through an opening in the crust. • As lava flows out onto the surface, it cools and solidifies around the vent, eventually accumulating to form a mountain known as a volcano. • A crater is a bowl-shaped depression at the top of a volcano that is connected to the magma chamber by a vent.

32. Volcanoes Anatomy of a Volcano • Volcanic craters are usually less than 1 km in diameter. • Calderas are large depressions up to 50 km in diameter that can form when the summit or the side of a volcano collapses into the magma chamber that once fueled the volcano.

33. Volcanoes Anatomy of a Volcano

34. Volcanoes Types of Volcanoes • The appearance of a volcano depends on two factors: • The type of material that forms the volcano • The type of eruptions that occur • Based on these two criteria, three major types of volcanoes have been identified: • Shield volcanoes • Cinder-cone volcanoes • Composite volcanoes

35. Volcanoes Types of Volcanoes Shield Volcanoes • A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base that forms when layer upon layer of basaltic lava accumulates during nonexplosive eruptions.

36. Volcanoes Types of Volcanoes Cinder-Cone Volcanoes • A cinder-conevolcano is a generally small, steep-sided volcano that forms when material ejected high into the air falls back to Earth and piles up around the vent. • The magma that fuels these volcanoes contains more water and silica than shield volcanoes, which makes them more explosive in nature.

37. Volcanoes Types of Volcanoes Composite Volcanoes • Composite volcanoes are large volcanoes that form when layers of volcanic fragments alternate with lava. • The magma that forms composite volcanoes commonly contains large amounts of silica, water, and gases, making these volcanoes violently explosive.

38. Volcanoes Types of Volcanoes Sizes and Shape • Shield volcanoes are by far the largest, and cinder-cone volcanoes are the smallest. • Cinder-cone volcanoes have the steepest slopes, while shield volcanoes have the gentlest slopes. • The slopes of cinder-cone and composite volcanoes are concave, and the slopes of shield volcanoes are straight.

39. Volcanoes Volcanic Material • Tephra are rock fragments thrown into the air during a volcanic eruption. • Tephra are classified by size, the smallest being dust (less than 0.25 mm) and ash (0.25–2 mm). • Somewhat larger fragments are called lapilli, or “little stones” (2–64 mm in diameter). • The largest tephra thrown from a volcano include angular volcanic blocks and rounded or streamlined volcanic bombs, both of which can be the size of a house or larger.

40. Volcanoes Volcanic Material Pyroclastic Flows • Some tephra cause tremendous damage and kill thousands of people. • A pyroclastic flow is a cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly 200 km/h. • The temperature at the center of a pyroclastic flow can exceed 700°C.

41. Volcanoes Where do volcanoes occur? • Most volcanoes form at plate boundaries. • About 80 percent of all volcanoes are found along convergent boundaries. • About 15 percent are found along divergent boundaries. • Only about 5 percent of extrusive igneous activity occurs far from plate boundaries.

42. Volcanoes Where do volcanoes occur? Convergent Volcanism • Convergence involving oceanic plates creates subduction zones, and the magma generated is forced upward through the overlying plate and forms volcanoes when it reaches the surface. • The volcanoes associated with convergent plate boundaries form two major belts: • The larger belt, the Circum-Pacific Belt, is also called the Pacific Ring of Fire. • The smaller belt is called the Mediterranean Belt.

43. Volcanoes Where do volcanoes occur?

44. Volcanoes Where do volcanoes occur? Divergent Volcanism • At divergent plate boundaries, magma is forced upward into the fractures and faults that form as the plates separate. • These areas of major faults and fractures are called rift zones. • Most of the world’s rift volcanism occurs under water along ocean ridges.

45. Volcanoes Where do volcanoes occur? Hot Spots • Some volcanoes are located far from plate boundaries and form as the result of hot spots. • Hot spots are unusually hot regions of Earth’s mantle where high-temperature plumes of mantle material rise toward the surface. • A plume does not move laterally, which results in a trail of progressively older volcanoes that formed as a plate moved over a hot spot.

46. Volcanoes Where do volcanoes occur? Hot Spots • The Hawaiian Islands continue to rise above the ocean floor as the Pacific Plate moves slowly over a hot spot. • The rate and direction of plate motion can be calculated from the positions of volcanoes in a chain that has formed over a hot spot.

47. Volcanoes Where do volcanoes occur? Hot Spots • Hot spots can result in the formation of flood basalts. • Flood basalts erupt from fissures rather than a central vent and form flat plains or plateaus. • Volcanic activity is proof that Earth is a dynamic planet.

48. Volcanoes Section Assessment 1. Match the following terms with their definitions. ___ vent ___ crater ___ tephra ___ pyroclastic flow ___ caldera A C D E B A. an opening in the crust from which lava flows B.depression caused by a collapsed magma chamber C.a bowl-shaped depression around an opening in the crust D. volcanic materials that are thrown into the air during a volcanic eruption E.a cloud of rapidly moving, extremely hot volcanic material

49. Volcanoes Section Assessment 2. How can chains of volcanoes that form over a hot spot track plate movement? The hot spot is in a fixed location. All of the volcanoes in the chain were over the hot spot when they formed. The volcanoes’ movement and the direction of the chain’s alignment indicates the movement of the plate.

50. Volcanoes Section Assessment 3. Identify whether the following statements are true or false. ______ Many cinder-cone volcanoes are less than 500 m high. ______ Volcanism is more common along divergent boundaries as compared to convergent boundaries. ______ The Mediterranean Belt is also known as the “Ring of Fire”. ______ It can be 700ºC in the center of a pyroclastic flow. true false false true