Volcanic Eruptions

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. 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 What Erupts from a Volcano • Over years or during the same eruption a volcano’s eruption may alternate between lava and pyroclastic eruptions

17. High 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 Types of 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