1. Objectives Vocabulary
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 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. 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. 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. 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. 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. 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
false

16. End of Section 1

17. Objectives Vocabulary
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. Plutons Batholiths and Stocks
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. Plutons Sills and Dikes
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. Section Assessment 2. How do sills and laccoliths differ?
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. Objectives Vocabulary Describe the major parts of a volcano.
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. 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. 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. Types of Volcanoes Cinder-Cone Volcanoes
A cinder-cone volcano 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. 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. 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. Volcanic Material Pyroclastic Flows
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. 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. 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. Where do volcanoes occur?

44. 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. 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. 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. 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

51. End of Section 3

52. Chapter Resources Menu
Study Guide
Section 18.1
Section 18.2
Section 18.3
Chapter Assessment
Image Bank
Chapter Resources Menu

53. Section 18.1 Study Guide
Section 18.1 Main Ideas
Temperature, pressure, and the presence of water are factors that affect the formation of magma.
As pressure increases, the temperature at which a substance melts also increases. At any given pressure, the presence of water will cause a substance to melt at a lower temperature than the same substance under dry conditions.
There are three major types of magma: basaltic magma, andesitic magma, and rhyolitic magma. These magmas differ in the source rock from which they form, viscosity, silica content, gas content, and explosiveness. Basaltic magma is the least explosive magma; rhyolitic magma is the most explosive.

54. Section 18.2 Study Guide
Section 18.2 Main Ideas
Magmatic intrusions affect the crust in several ways. Magma can force overlying rock apart and enter the newly formed fissures. Magma can also cause blocks of rock to break off and sink into the magma chamber. Magma can melt the rock into which it intrudes.
Batholiths, stocks, sills, dikes, and laccoliths are plutons that are classified according to their size, shape, and relationship to surrounding rocks. Batholiths are the largest plutons and often form the cores of many of Earth’s major mountain chains.

55. Section 18.3 Study Guide
Section 18.3 Main Ideas
Lava flows onto Earth’s surface through a vent. Over time, multiple lava flows may accumulate to form a volcano. A crater is a depression that forms around the vent at the summit of a volcano. A caldera is a large crater that forms when a volcano collapses during or after an eruption.
There are three types of volcanoes: shield volcanoes, cinder-cone volcanoes, and composite volcanoes.
Rock fragments ejected during eruptions are called tephra.

56. Section 18.3 Study Guide
Section 18.3 Main Ideas
Most volcanoes form along convergent and divergent plate boundaries. Volcanoes also form over hot spots, which are unusually hot areas in the mantle that are stationary for long periods of time.
Flood basalts form when lava flows from fissures to form flat plains or plateaus.

57. Chapter Assessment
Multiple Choice
1. Andesitic magma has a silica content of about ___ percent.
a. 50 c. 70
b. 60 d. 80
Basaltic magma has a silica content of about 50%. Rhyolitic magma has a silica content of about 70%.

58. Chapter Assessment
Multiple Choice
2. Which of the following is the largest type of pluton?
a. stock c. batholith
b. laccolith d. sill
Batholith plutons are common in the interiors of many mountain chains and are the result of the magma generated during subduction at convergence zones.

59. Chapter Assessment
Multiple Choice
3. Shield cone volcanoes are fueled by what kind of magma?
a. basaltic c. tephra
b. andesitic d. rhyolitic
Basaltic magma has a low viscosity and erupts with low explosiveness. This allows the lava to flow down the mountain’s slopes and continually add to the mountain.

60. Multiple Choice 4. Where is divergent volcanism generally found?
Chapter Assessment
Multiple Choice
4. Where is divergent volcanism generally found?
a. ocean rifts c. ocean trench
b. mid-plate d. Abyssal plains
Divergent volcanism is almost entirely found underwater (Iceland is the major exception) at ocean rifts. Through divergent volcanism, new sea floor is created as the plates spread apart.

61. Multiple Choice 5. Where are most active volcanoes located?
Chapter Assessment
Multiple Choice
5. Where are most active volcanoes located?
a. divergent c. hot spots boundaries
b. convergent d. transform boundaries boundaries
Most active volcanoes are in the either the Circum-Pacific or Mediterranean Volcanic Belts, which are located on convergent boundaries.

62. Chapter Assessment
Short Answer
6. How does the presence of water influence 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.

63. Chapter Assessment
Short Answer
7. What is the relationship between viscosity and the temperature of magma or lava?
The hotter the magma or lava, the lower the viscosity.

64. Chapter Assessment
True or False
8. Identify whether the following statements are true or false.
______ Andesitic magma has more explosive potential than rhyolitic magma.
______ A dike forms when magma intrudes parallel to layers of rock.
______ Cinder-cone volcanoes can form on the flanks of a shield volcano.
______ Many of the volcanoes in the Cascade range are composite volcanoes.
______ The hot spot under Hawaii has existed for at least 75 million years.
false
true

65. Image Bank
Chapter 18 Images

66. Image Bank
Chapter 18 Images

67. Image Bank
Chapter 18 Images

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