1. Volcanic Activity Chapter 18

2. Magma Formation Forms deep beneath the Earth’s surface
Temperature must be high enough to melt rocks
What factors, other than temperature, affect the formation of magma?
Magma Formation

3. Factors Affecting Magma Formation
Rocks in Earth’s lower crust and upper mantle do not melt to form magma, even though temperatures are high enough.
Pressure
Increases with depth
As pressure increases, melting point increases
Factors Affecting Magma Formation

4. Factors Affecting Magma Formation
2. Water:
Water lowers the melting point of rocks
Factors Affecting Magma Formation

5. Classifying Magma Magma is classified based on the composition
Composition of magma impacts magma’s viscosity
Types of Magma:
Basaltic
Andesitic
Rhyolitic
Classifying Magma

6. Types of Magma Basaltic Magma Forms in upper mantle Low viscosity
Small amounts of dissolved gas and silica
Quiet eruptions
Types of Magma

7. Types of Magma 2. Andesitic Magma Found in subduction zones
~60% Silica
Intermediate viscosity
Source material
Ocean crust
Ocean Sediments
Types of Magma

8. Types of Magma 3. Rhyolitic Magma
Forms from mixing of molten material with silica and water-rich continental crust
High Viscosity
Large volume of trapped gas
Explosive volcanoes
Types of Magma

9. Viscosity Higher temperatures = lower viscosity
More silica = higher viscosity
Viscosity

10. Plutons Form when magma cools beneath the Earth’s surface
Intrusive igneous rock bodies
Can be exposed at Earth’s surface
Classified by:
Size
Shape
Relationship to surrounding rock
Plutons

11. Types of Plutons Batholiths Stocks Irregular shape
Largest
Common in mountain chains
Mainly composed of granite
Gabbro and diorite also
California
Stocks
Irregular shape
Smaller than batholiths
Types of Plutons

12. Batholiths in Real Life

13. Types of Plutons 3. Laccoliths Rounded side and flat side
Mushroom shaped
Up to 16 km wide
Utah, South Dakota, Montana
How did this laccolith become exposed at Earth’s surface?
Types of Plutons

14. Types of Plutons 4. Sills 5. Dikes Intrudes parallel layers of rocks
Few cm to hundreds of meters
Svalbard
5. Dikes
Cuts across preexisting rocks
Arizona
Types of Plutons

15. Sills and Dikes in Real Life

16. Label

17. Anatomy of a Volcano Vent: hole where lava comes out
Crater: depression around vent
Less than 1 km
Calderas: larger depressions
Crater Lake
Anatomy of a Volcano

18. Types of Volcanoes Shield Volcanoes: Broad, gently sloping sides
Circular base
Largest type
Layered basaltic lava
Hawaiian Islands
Types of Volcanoes

19. Types of Volcanoes 2. Cinder-Cone Volcanoes:
Material ejected piles up around vent
Steep sides
Usually small <500m high
More water and silica than shield
Hawaii
Types of Volcanoes

20. Types of Volcanoes 3. Composite Volcanoes
Layered volcanic fragments with lava
Magma has large amounts of silica, water, and gases
Mount St. Helens
Types of Volcanoes

21. Tephra: rock fragments thrown in the air during a volcanic eruptions
Classified by size
Ranges from ash to the size of a car or house
Volcanic blocks: angular fragments
Volcanic bombs: rounded tephra
Volcanic Material

22. Clouds of gas, ash, and other tephra moving at incredible speeds
Nearly 200 km/hr
Can contain poisonous gases
Temperatures can exceed 700°C
Pyroclastic Flows

23. Where Do Volcanoes Occur?
Convergent Volcanism
Magma is forced upwards through the plate and forms volcanoes
Pacific Ring of Fire – Does the affect our country?
Where Do Volcanoes Occur?

24. Where Do Volcanoes Occur?
Divergent Volcanism
Magma is forced upward into fractures and faults
Rift volcanism occurs along ocean ridges
This is how new seafloor is formed
Iceland
Divergent volcanism above water
Where Do Volcanoes Occur?

25. Where Do Volcanoes Occur?
Hot Spots
Regions of the mantle where high temperature material moves to the surface
Moves vertically, not laterally
Hawaiian Islands
Oldest island = Kauai
Where Do Volcanoes Occur?