1. Volcanoes There are 3 types of volcanoes: 1.Shield – VERY large 2.Composite – medium 3.Cinder - small

2. Differences between volcanoes Origin Where does magma come from? Shield – Basaltic magma forms deep down Composite – andesitic and granitic magma forms from subduction zone Cinder – granitic and andesitic magma formed at subduction zone. Content What magma is made of? Silica content – – Shield – low silica content- basaltic – “runny” – Composite – high silica content – granitic/rhyolitic – viscous (thick) – Cinder – high silica content – granitic/rhyolitic – viscous (thick)

3. Differences continued Gas Content – Gas is held in rock because of high pressure underground (like CO2 in soda). When pressure is reduced the gas tries to escape. Shield – low gas content because it cools so quickly – volcano therefore doesn’t have the explosive “power”. Composite – high gas content kept underground therefore has much explosive power since gas escapes all at once. Cinder – high gas content kept underground therefore has much explosive power since gas escapes all at once.

4. Explosive Power Shield – called “quiet” volcano due to its lack of explosiveness ( low silica and gas content) sputters out not viscous Composite – explosive – very viscous so pressure builds up inside when gas is released the volcano explodes releasing pressure immediately. Cinder - explosive – very viscous so pressure builds up inside when gas is released the volcano explodes releasing pressure immediately.

5. Pyroclastic materials Definition – particles produced in volcanic eruptions – Basaltic lava – viscous rhyolitic magma is charged with gases as the gas expands it blows fragments from the vent.

6. Factors Composition – Temperature of magma – Dissolved gases -

7. Review Questions 1. What factors determine the type of volcanic eruption? 2. List the materials that can be ejected from volcanoes. 3. Describe the three types of volcanoes.