1. Volcanoes and Plate Tectonics
“an opening in the Earth’s crust through which an eruption takes place”

2. Nature of a volcanic eruption depends on the Magma
Volcanoes
Nature of a volcanic eruption depends on the Magma
Magma – hot, liquid rock beneath the Earth’s surface.
Lava – hot, liquid rock that reaches (and goes above) the Earth’s surface.

3. Volcanoes (cont.) Possible sources of heat below the surface:
Radioactive decay is a breaking apart of the nucleus of an atom; as a nucleus breaks apart, it releases energy which is changed into heat.
Original Heat inside the earth may have been trapped when the earth first formed.
Friction – results from the movement of lithospheric plates.
Fusion in the core – nuclear fusion, similar to the sun, is occurring in the core

4. Volcanoes (cont.)
Active Volcano – 1300 volcanoes on land around the world are thought to be active - have erupted within the last few hundred years.
Dormant Volcano – ‘sleeping’ volcano, but capable of being activated
Extinct Volcano – has not erupted for the last several thousand years.

5. Volcanoes (cont.) Types of volcanoes
Types of volcanoes are determined by the strength and explosiveness of the eruption.
Eruption Depends on
a) temperature,
b) composition, and
c) amount of dissolved gases
these affect the magma’s viscosity (ability to flow – more viscous –greater resistance to flow)

6. Volcanoes (cont.)
Temperature – the hotter-the runnier (less viscous)…

7. Volcanoes (cont.)
Composition – movement of magma depends on the amount of silica
The more silica the greater the viscosity.
Felsic - High silica – granitic – 70% silica
Mafic - Low silica – basaltic – 30% silica;
(molecular silica links to make long chains, even before crystallization begins)

8. Volcanoes (cont.)
Dissolved gases – this provides the force of an eruption.
As magma moves closer to the surface (like in a volcano) the pressure is reduced and the dissolved gases are released – sometimes suddenly i.e. Soda bottle
Gases are:
water vapor Carbon Dioxide (CO2)
Sulfur Carbon Monoxide (CO)
Sulfur Dioxide, SO2 Hydrogen Sulfide, H2S

9. Volcanoes (lava) Felsic – high silica
- Flows slowly, gas held within; does NOT allow gasses to escape readily –
EXPLOSIVE

10. Volcanoes (cont.)
Mafic – low silica; more fluid, DOES allow gasses to escape readily – NON-Explosive

11. Volcanoes (cont.) Overall… Lava (magma that reaches the surface)
Felsic - flow slowly, gas is held within – explosive
Mafic - more fluid, gas easily escapes – lava pours out slowly

12. Volcanoes (cont.) Lava type Felsic Mafic Silica % High Low Viscosity
High (thick)
Low (thin)
Color
Light
Dark
Rate of movement
Slow
Quick
Dissolved gasses
Cannot easily escape
Escapes easily
Nature of eruption
Explosive
Quiet

13. Eruptive Fragments Pyroclastic
Tephra: lava that is blasted into the air by violent volcanic eruptions and solidifies as it falls to the ground as ash, lapilli, cinders, blocks, and volcanic bombs
Ash – less than 2mm diameter
Lapilli (also cinders) – up to 64 mm
Blocks more than 64 mm (solid)
Bombs more than 64 mm (liquid)

14. Tephra
(Actual size of tephra)

16. Anatomy of a Volcano
Vent – in volcanic regions, an opening in the Earth’s surface through which lava, ash and steam flow
Crater – opening at the top of a volcano
Dike – igneous rock that forms when magma is squeezed into a vertical crack and solidifies (crosses layers)
Sill – small body of igneous rock that forms when magma is squeezed into a horizontal crack and then solidifies (parallel to layers)

17. Anatomy of a Volcano

18. Crater at the top of the vent

19. Evidence of Volcano
Volcanic neck: the core of a volcano’s vent that remains after the outer layers of lava and tephra have been eroded away from an extinct volcano
Caldera: the large opening formed at the top of a volcano when the crater collapses into the vent following an eruption

20. Volcanic Neck

21. Crater Lake

22. Crater Lake