1. Volcanic Eruptions
Chapter 9
Section 1

2. Volcanic Eruptions
Volcanoes – areas of Earth’s surface through which magma and volcanic gases pass
In seconds a volcanic eruption can turn an entire mountain into a cloud of ash and rock
Helps form fertile farmland
Create some of the largest mountains on earth

3. Volcanic Eruptions
During an eruption magma is forced to the Earth’s surface
Magma – molten rock below the Earth’s surface
Lava – magma that flows on the Earth’s surface

4. Nonexplosive Eruptions
Most common type of eruptions
Produce relatively calm flows of lava
Can release huge amounts of lava
Vast amounts of the Earth are covered with lava from non-explosive eruptions
Ex. The sea floor and the Northwest region of the US

5. Nonexplosive Eruptions

6. Explosive Eruptions Much rarer than non explosive eruptions
Effects can be incredibly destructive
Clouds of hot debris, ash and gas rapidly shoot out from a volcano
No lava flows
Molten rock is blown into tiny particles that harden in the air

7. Explosive Eruptions
Ash – dust sized particles of molten rock that have hardened in the air
Ash can reach the upper atmosphere and circle the Earth for years
Larger pieces fall closer to the volcano
Explosive eruption can blas millions of tons of lava and rock from a volcano
In seconds an explosive eruption can demolish a mountainside

8. Explosive Eruptions
In what resembles a nuclear explosion, volcanic ash rockets skyward during the 1990 eruption of Mount Redoubt in Alaska

9. What is Inside a Volcano?
Magma chamber – body of molten rock deep underground that feeds a volcano
Vents – an opening at the surface of the Earth through which volcanic material passes
Magma is released from vents during an eruption

10. What Makes Up Magma?
The composition of magma affects how explosive a volcanic eruption is.
The key to whether an eruption will be explosive lies in the silica, water, and gas content of the magma

11. What Makes Up Magma? Water and Magma are an Explosive Combination
If the water content is high an explosive eruption is more likely
Underground there is a lot of pressure that keeps water and other compounds dissolved in the magma

12. What Makes Up Magma
When magma quickly rises the water and CO2 turn into gas which expand quickly
When the gasses expand an explosion takes place
Pumice – some lava is so frothy with gas when it reaches the surfaces that its solid form, pumice, can float in water

13. Silica-Rich Magma Traps Explosive Gases
Magma that has high silica content tends to cause explosive eruptions
Silica-rich magma has a stiff consistency
Flows slowly and tends to harden in volcano’s vents
Plugs the vent
As more magma pushes up from below pressure increases

14. Silica-Rich Magma Traps Explosive Gases
If enough pressure builds up an explosive eruption takes place
Stiff magma prevents water vapor and other gasses from easily escaping
Gas bubbles in the magma can expand until they explode
When they explode ash and pumice are blasted from the vent

15. Silica-Rich Magma Traps Explosive Gases
Magma with less silica has a more fluid runnier consistency
Because gas particles can escape more easily explosive eruptions are less likely to occur

16. What Erupts from a Volcano
Nonexplosive Eruptions
Lava – liquid magma that flows from a volcanic vent
Explosive Eruptions
Pyroclastic material – forms when magma is blasted into the air and hardens
Over years or during the same eruption a volcano’s eruption may alternate between lava and pyroclastic eruptions

17. Types of Lava High Viscosity Low Viscosity
Lava with high viscosity is stiff
Flows slowly
Ex. Blocky lava and Pahoehoe
Low Viscosity
Lava with low viscosity is more fluid
Flow more quickly
Ex. Aa and Pillow Lava

18. Types of Lava
Aa – lava pours out quickly and forms a brittle crust. The crust is torn into jagged pieces as molten lava continues to flow underneath
Pahoehoe – lava flows slowly, like wax dripping from a candle. Its glassy surfaces has rounded wrinkles

19. Types of Lava
Pillow Lava – forms when lava erupts underwater. Forms rounded lumps that are the shape of pillows
Blocky Lava – cool, stiff lava that does not travel far from the erupting vent. Blockly lava usually oozes from a volcano and forms jumbled heaps of sharp-edged chunks

20. Types of Lava

21. Types of Pyroclastic Material
Forms when magma explodes from a volcano and solidifies in the air
Also forms when powerful eruptions shatter existing rock
Size ranges from boulders the size of houses to tiny particles

22. Types of Pyroclastic Material
Volcanic Blocks – the largest pieces of pyroclastic matieral, pieces of solid rock erupted from a volcano
Volcanic Bombs – large blobs of magma that harden in the air

23. Types of Pyroclastic Material
Lapilli – “little stones” pebblelike bits of magma that hardened before they hit the ground
Volcanic Ash – forms when the gases in stiff magma expand rapidly and the walls of the gas bubbles explode into tiny, glasslike slivers. Makes up most of the pyroclastic material in an eruption

24. Types of Pyroclastic Material

25. Pyroclastic Flows Pyroclastic Flow Dangerous
Produced when enormous amounts of hot ash, dust and gases are ejected from a volcano
Can race downhill at more than 200 km/h
Temperature at center can exceed 700°C

26. Pyroclastic Flows 1991 eruption of Mount Pinatubo in the Phillippines
Scientists predicted the eruption and saved 250,000 people