1. Volcanic Eruptions Chapter 9 Section 1

2. Volcanic Eruptions During an eruption magma is forced to the Earth’s surface 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

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

4. Nonexplosive Eruptions

5. Explosive Eruptions Much rarer than non explosive eruptions Much rarer than non explosive eruptions Clouds of hot debris, ash and gas rapidly shoot out from a volcano Clouds of hot debris, ash and gas rapidly shoot out from a volcano No lava flows No lava flows

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

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

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

9. What Makes Up Magma? Water and Magma are an Explosive Combination Water and Magma are an Explosive Combination –If the water content is high an explosive eruption is more likely –When magma quickly rises the water and CO 2 turn into gas which expand quickly –When the gasses expand an explosion takes place

10. Silica-Rich Magma Traps Explosive Gases Magma that has high silica content tends to cause explosive eruptions Magma that has high silica content tends to cause explosive eruptions Silica-rich magma has a stiff consistency or a high viscosity Silica-rich magma has a stiff consistency or a high viscosity Flows slowly and tends to harden in volcano’s vents clogging them. As more magma pushes up the pressure increases and an explosion can happen. Flows slowly and tends to harden in volcano’s vents clogging them. As more magma pushes up the pressure increases and an explosion can happen. Stiff magma prevents water and other gasses from easily escaping Stiff magma prevents water and other gasses from easily escaping

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

12. What Erupts from a Volcano Nonexplosive Eruptions Nonexplosive Eruptions –Lava – liquid magma that flows from a volcanic vent Explosive Eruptions 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 Over years or during the same eruption a volcano’s eruption may alternate between lava and pyroclastic eruptions

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

14. Types of Lava

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

16. Types of Pyroclastic Material

17. One important pyroclastic material to be familiar with is Volcanic Ash is makes up most of the pyroclastic material in an eruption. One important pyroclastic material to be familiar with is Volcanic Ash is makes up most of the pyroclastic material in an eruption.

18. Pyroclastic Flows Pyroclastic Flow 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

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