1 Natural Disasters Volcanoes: Materials & Effects

2 Nature of Volcanic Activity Volcanic eruptions are caused by magma (liquid rock, crystals and dissolved gas) expelled onto the Earth’s surface

3 Types of Magma Basalt Dark color SiO 2 – wt.% 1,000-1,200°C Andesite Intermediate color SiO 2 – wt.% 800-1,000°C Rhyolite Light color SiO 2 – 65-75wt.% °C

4 Magma Viscosity A fluid’s resistance to flow Depends primarily on: –Composition –Temperature Composition – ↑ SiO 2  ↑ viscosity Temperature – ↑ T  ↓ viscosity

5 Magma Summary Magma Type BasaltAndesiteRhyolite SiO %55-65%65-75% Viscosity LowIntermediateHigh Gas content LowIntermediateHigh Temperature °C °C °C

6 Volcanic Behavior The nature of volcanic eruptions depends on composition; hence, its viscosity Viscosity is an important property in determining the eruptive behavior of magmas – ↑ viscosity  explosive eruption – ↓ viscosity  quiet eruption

7 Primary Effects of Volcanic Eruptions Lava Pyroclastic Material Poisonous Gas Emissions

8 Lava Flows Aa –Rough, jagged –High Viscosity –Low temperature Pahoehoe –Ropy, smooth –Low viscosity –High temperature Hawaii Eruption

9 Pyroclastic Material Ash falls –Clouds of gas and volcanic ash rise in eruption column –Ash picked up by wind and falls back to surface Pyroclastic flows –If eruption column collapses, gas and ash rush down flanks of volcanoes at high speeds –Ash become fused and become welded ash

10 Pyroclastic Material If gas pressure is directed outward instead of upward, produce a lateral blast or surge – a high-speed ash-rich shock wave

11 Poisonous Gas Emissions Carbon Dioxide (CO 2 ) –More dense than oxygen –Lethal if concentration >10% –As magma rises, gas comes out of solution

12 Secondary & Tertiary Effects Mudflows Debris Avalanches & Flows Atmospheric Effects Famine and Disease

13 Mudflows Mudflows, also called lahars, is a mixture of water and sediment, which moves rapidly down slope along existing stream valleys

14 Debris Avalanche and Flows Ground-hugging avalanche or flow composed of hot ash, pumice, rock fragments and dust

15 Atmospheric Effects Volcanic ash may stay suspended in atmosphere for years Ash may reflect solar radiation and cause global cooling Gas, like SO 2, also reflects sunlight

16 Famine and Disease Ash falls destroy crops and animals (including humans) that feed on them Decaying organisms may contaminate water and spread disease

17 Case Histories Versuvius, (79 A.D.)

18 Case Histories Mt. Pelée, Martinique (1902)

19 Case Histories Mount St. Helens (1980)