Volcanoes form as molten rock, known as magma, rises from below the Earth's crust and erupts at the surface. The composition and viscosity of the magma determines the type of eruption, from gentle to violent explosive eruptions. During an eruption, molten rock (lava), rock fragments of varying sizes, and volcanic gases are ejected from the volcano. The largest rock fragments fall closest to the volcano, while very fine ash can travel much farther and remain suspended in the air for long periods of time. Most volcanic activity occurs at plate boundaries in the oceans, but some volcanoes form independently at hotspots in the crust.

1. Volcanoes Form as Molten Rock Erupts Pages 146-153 (section B)

2. What is a Volcano? Volcano - an opening in the Earth’s crust through which molten rock, rock fragments, and hot gasses erupt. A mountain built up from erupted material is also called a volcano.

3. What happens when a volcano erupts? Volcanoes may erupt violently or gently Even if no magma reaches the surfaces volcanoes can be dangerous Hot rocks can fly out starting fires Rocks can land on roofs in large numbers causing the roofs to collapse

4. Magma Magma contains silica and gases Magma HIGH in silica is resistant to flowing Causes violent eruptions Magma LOW in silica flows easily Magma rises as long as it is less dense than the surrounding rock. When magma erupts it is called lava

5. Volcanoes

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7. Rock Fragments Fragments form as: Escaping gas bubbles pop, tearing magma apart Larger pieces of lava are thrown into the air, cooling and hardening during flight Rocks of all sizes rip loose from volcanoes’ walls during eruptions

8. Rock Fragments Tiny rock fragments form volcanic ash , sized between dust fragments and grain of rice. Stay in the air a long time, but travel far distances Cinders are larger than ash, smaller than bombs Bombs are the largest size rock fragments Fall the fastest Ash Cinder Bomb

9. Volcanic Gases What looks like smoke coming from a volcano is actually a mixture of ash and gases. The main gases in magma are water vapor and carbon dioxide

10. Pyroclastic Flow Volcanic gases can mix with rock fragments and stay near the ground. Pyroclastic flow- a dense cloud of superhot gases and rock fragments. Can reach temperatures of 800 degrees C (1500 degrees F) Can reach speeds of 160 km/hr (100 mi/hr) Pyroclastic flows are the most dangerous type of volcanic eruption.

11. Most volcanoes form along plate boundaries Most of Earth’s volcanic activity takes place under water- along plate boundaries Volcanoes can also occur along hotspots away from plate boundaries- this is less common

12. Types of Volcanoes Shield Volcano – shaped like a broad, flat dome. Built up by man eruptions of lava that is relatively low in silica.

13. Types of Volcanoes Cinder Cone- a steep, cone-shaped hill formed by the eruption of cinders and other rock fragments that pile up around a single crater. Form as gas-rich magma erupts- often on the side of another volcano Ten to hundreds of meters tall

14. Types of Volcanoes Composite Volcano - a cone shaped volcano that is built up of layers of lava and layers of rocks fragments. Steep at the top flattens out at the bottom Tends to have very violent eruptions

15. Caldera Caldera- a huge crater formed by the collapse of a volcano when magma rapidly erupts from underneath it.

16. Active Volcano A volcano that currently vents lava or gases etc, or has the capability of doing so

17. Dormant Volcano A volcano that has not erupted for a considerable length of time, but is still capable of erupting

18. Warning Signs of Volcanic Activity Temperature of rocks in the volcanic cone increases The cone may change shape- swell or bulge just before eruption Small earthquakes and tremors in the area around the volcano Emission or increase of gas from the volcano Often animals may act strangely before the eruption

19. Scientist Monitor Volcanoes Scientist cannot prevent volcanoes from erupting The signs of an eruption can allow people to leave an area so the loss of human life can be minimized

20. http://www.snagfilms.com/films/title/volcano_natures_inferno/