1. Volcanoes
Chapter 13

2. Volcanoes & Plate Tectonics
Volcanic eruptions can be caused by the movement of tectonic plates
Plates are moved by the internal heat of the Earth
A volcanic eruption can be more powerful than an atomic bomb

3. Formation of Magma
Magma – liquid rock that forms under Earth’s surface
Comes from melted crust and mantle

4. Volcanism
Magma naturally rises to the surface because it is less dense
The amount of magma increases because it melts rock along the way

5. Volcanism
Volcanism – any activity that includes the movement of magma onto Earth’s surface
Lava – magma that has reached the surface
Volcano – the vent in Earth’s surface where magma reaches the surface

6. Major Volcanic Zones
Like earthquakes, volcanoes tend to occur near convergent or divergent plate boundaries

7. Subduction Zones Where one plate goes beneath another
Oceanic crust + oceanic crust = island arc chain
Oceanic crust + continental crust = mountain range

8. Mid-Ocean Ridges
The largest amount of magma comes to the surface at mid-ocean ridges
Forms under water volcanoes
Water quickly cools the lava creating pillow lava

9. Hot Spots
Volcanically active areas in the interior of a tectonic plate
The plate moves over the hot spot, causing a volcano to form
That volcano eventually moves away and a new one forms

10. Intrusive Activity
Magma that cools and solidifies under the ground (intrusion)
Forms plutons
Dikes (thin areas)
Batholiths (large areas >100 km2)
Devil’s Tower
Wyoming

11. Exit Ticket 1. What is the difference between lava and magma?
2. Which of the following is the most accurate definition of volcano:
A. Any activity that includes the movement of magma toward or onto Earth’s surface.
B. Magma that flows onto Earth’s surface; the rock that forms when lava cools and solidifies
C. A mountain of rock that spews hot lava onto Earth’s surface
D. A vent or fissure in Earth’s surface through which magma and gases are expelled.

12. (Use I think… because… form) !
Do Now
Create a hypothesis that answers the question:
Which liquid will be able to trap the most bubbles when I blow into the liquid through a straw: water, cooking oil or honey?
(Use I think… because… form) !

13. Viscosity A fluids resistance to flow.
Magma can have different viscosities, which affect the force with which a particular volcano will erupt.

14. Volcanic Eruptions 2 types of magma
Mafic – magma or rock rich in magnesium and iron; dark colored
Felsic - magma or rock rich in silicates; light colored

15. Types of Eruptions Quiet Eruptions
Usually oceanic volcanoes (ex. Hawaii)
Lava has a low viscosity (runny)
Gases easily escape from the lava
Formed from mafic magma

16. Types of Eruptions Lava Flows Crust may form on top of the lava
If the lava keeps flowing, it forms wrinkles called pahoehoe
Aa lava – surface breaks into jagged chunks
Blocky lava – lava is more viscous (thicker)

17. Types of Eruptions Explosive Eruptions
Usually continental volcanoes (ex. Mt. St. Helens)
Lava has a high viscosity (thick & sticky) formed from felsic magma
Gases trapped inside lava
Pyroclastic material – fragments of rock that form during a volcanic eruption
Thrown into the air during an explosive eruption

18. Pyroclastic Material Volcanic ash Lapilli Volcanic bombs
<2mm in diameter
Lapilli
<64mm in diameter
“little stones”
Volcanic bombs
Form when red hot lava is thrown and cools in the air
Volcanic blocks
Largest material
Pieces of rock blasted from the vent

19. Quick Check
Using the term viscosity, explain why a milkshake is harder to drink with a straw then a can of pop.

20. Calderas
Can form when the magma chamber empties and the cone collapses
Lakes can form in the caldera basin
Crater Lake (Mount Mazama)
Oregon

21. Predicting Volcanic Eruptions
Increase in strength and frequency of earthquakes may be a sign of an eruption
Before an eruption, the surface of a volcano may bulge outward from the magma
Scientists compare a volcano’s previous behavior to current measurements

22. Types of Volcanoes
The shape of a volcano is related to the composition of its matter.

23. Types of Volcanoes Shield volcano Broad base; gently sloping sides
Covers wide area
Quiet eruption
Layers of mafic lava build up cone
Ex: Hawaiian Islands

24. Types of Volcanoes Cinder cones Very steep slopes
Rarely less than a few hundred meters high
Explosive eruptions
Cone made of pyroclastic material
Ex: Parícutin, Mexico

25. Types of Volcanoes Composite volcano
Cone made of alternating layers of…
lava flows (quiet eruptions)
pyroclastic material (explosive eruptions)
Form large volcanic mountains
Ex: Mt. Hood, Mt. St. Helens, Mt. Rainier, Mt. Fuji

26. Exit Ticket
The depression that results when a volcanic cone collapses over an emptying magma chamber is a:
A. Crater
B. Caldera
C. Vent
D. Fissure
Compare the three main types of volcanic cones: Shield, Composite, and Cinder-Cone.