1. Guided Notes on Volcanoes
Section 18.1
Magma

2. In the last 10,000 years, more than 1500 different volcanoes have erupted, providing evidence that the Earth is indeed geologically active.

3. 2. All volcanoes are fueled by magma deep beneath the Earth’s surface.

4. 3. What is the asthenosphere?
The asthenosphere is the plasticlike portion of the mantle directly beneath the lithosphere. It is hot enough to contain molten magma.

5. Basaltic Magma 50% silica Low viscosity Least violent eruptions
Low amounts of trapped gases
Occur in Hawaii, Iceland, and areas with oceanic crust

6. Andesitic Magma 60% silica Intermediate viscosity
Some violent eruptions, some quiet ones
Are found along subduction zones
An example is Mt. St. Helens, in Washington

7. Rhyolitic Magma 70% silica High viscosity
Usually have violent eruptions
Occur in areas with continental crust
An example is Yellowstone Park

8. 5. The viscosity of magma depends upon its temperature and composition
5. The viscosity of magma depends upon its temperature and composition. The hotter the magma, the lower the viscosity. Magmas high in silica have higher viscosities than magmas low in silica.

9. Section 18.2
Intrusive Activity

10. 1. Why does magma come into contact with overlying rock?
Magma is less dense, so it moves upward where it eventually comes in contact with overlying rock.

11. 3 Ways That Intruding Magma Can Affect the Crust
It can force the rock apart and enter the newly formed fissures
It can cause blocks of rock to break off and sink into the magma, where they melt
It can melt the rock into which it intrudes

12. Define Pluton
A pluton is an intrusive igneous rock body formed through mountain-building processes and oceanic-oceanic collisions. It can be exposed at the Earth’s surface due to uplift and erosion.

13. Types of Plutons Sills When magma intrudes into parallel rock layers
Between a few centimeters and hundreds of meters thick
An example is the Palisades Sill in New York

14. Types of Plutons
Dikes
When magma cuts across pre-existing rock layers, magma invades the cracks in rock
Usually 10 km. long and a few meters wide
An example is the Great Dike in Zimbabwe

15. Types of Plutons Laccoliths
Mushroom-shaped plutons with rounded tops and flat bottoms
Less than 16 km. wide
An example is the Black Hills of South Dakota

16. Types of Plutons Batholiths Large plutons that are at least 100 km2
Irregularly shaped
Coarse-grained
An example is the Coast Range Batholith in British Columbia

17. Types of Plutons Stocks
Similar to batholiths, but smaller than 100 km2

18. 5. What texture do most plutons have, and why?
Most plutons are coarse-grained because they have cooled slowly under the earth’s surface.

19. 6. How are plutons usually formed?
Plutons are usually formed as a result of mountain-building processes.

20. 2 Types of Convergence that Create Batholiths
Continental-Continental: creates chains of mountains
Oceanic-Oceanic: create batholiths that are later uplifted and eroded

21. Guided Notes about Volcanoes
Section 18.3
Guided Notes about Volcanoes

22. How Volcanoes Form
Step 1: magma chambers deep within the earth fuel volcanoes.

23. How Volcanoes Form
Step 2: magma that reaches the surface is called lava. Lava erupts through an opening called a vent.

24. How Volcanoes Form
Step 3: Lava cools and solidifies around the vent

25. How Volcanoes Form
Step 4: over time, lava accumulates to form a mountain called a volcano

26. How Volcanoes Form
Step 5: a crater forms at the top of the volcano near the vent.

27. 2. What is a caldera?
A caldera is a large volcanic crater that forms when the summit or side of a volcano collapses into the magma chamber of the volcano

28. 3. The appearance of a volcano depends upon:
The type of material that forms the volcano
The type of eruptions that occur

29. The 3 Types of Volcanoes Cinder-Cone
Forms when tephra is ejected high in the air, falls back to earth, and piles up around the vent
They are usually small, with steep sides
They have viscous magma, high in silica and trapped gases.

30. The 3 Types of Volcanoes Composite
Forms when layers of volcanic fragments alternate with lava
The magma contains high amounts of water, with moderate levels of silica and trapped gases
Are larger than cinder-cone volcanoes

31. The 3 Types of Volcanoes Shield
They have broad, gently sloping sides and a nearly circular base
They are made when basaltic lava accumulates during quiet eruptions
They are the largest volcanoes

32. 5. What is tephra?
Tephra are rock fragments thrown into the air during a volcanic eruption.
Tephra can be newly cooled and hardened lava, mineral grains that have crystallized, or pieces of the volcanic cone.

33. The 5 Classifications for Tephra
Dust: less than .25 mm in diameter
Ash: between .25 and 2 mm in diameter
Lapilli: between 2 and 64 mm in diameter
Blocks: angular, greater than 64 mm.
Bombs: rounded, greater than 64 mm.

34. 7. Differences between blocks and bombs
Blocks are angular fragments of lava. Bombs are blobs of lava that are squeezed out of a vent and form a rounded, streamlined shape.

35. 8. Describe a pyroclastic flow.
Clouds of gas, ash, and other tephra that move down a volcanic slope at tremendous speeds is a pyroclastic flow.
Speeds can be 200 km. per hour and temperatures can be 700 degrees Celcius.

36. 9. Worldwide Distribution of Volcanoes
80% of volcanoes are found at convergent boundaries
15% are found at divergent boundaries
Only 5% are found away from plate boundaries

37. 10. How does convergence lead to the formation of a volcano?
Convergence causes the crust to descend into the mantle and melt. The magma generated is forced upward and forms volcanoes when it reaches the surface.

38. 11. How does divergence lead to the formation of a volcano?
Magma rises into the fractures and cracks formed when two plates separate. Usually this occurs at ocean ridges.

39. 12. What is a hot spot?
A hot spot is an unusually hot region of the mantle, where plumes of magma rise to the surface, creating volcanoes
An example is Hawaii

40. 13. How do hot spots create volcanoes?
The intense heat of the plumes melt crustal rock, which is forced through a vent to form volcanoes

41. 14.How do the chains of volcanoes form over hot spots?
As the earth’s plates move across hot spots, they create chains of volcanoes
The rate and direction of motion can be calculated from the positions of volcanoes formed.