 Volcanoes  Areas of Earth’s surface through which magma & volcanic gas passes  Creative Forces  forming fertile farmland & large mountains  Destructive Forces  Turning mountains into clouds of ash & rock, destroying forests & homes

Structure of a typical volcano. Notice the alternating layers of ash and hardened lava. Why does this occur?

 Nonexplosive Eruptions  Most common type of eruption  Release large amounts of lava  Calm lava flow  Pacific NW of USA covered from these eruptions

 Explosive Eruptions  Rare & incredibly destructive  Debris/molten rock blown out of volcano  Large debris falls close to volcano  Smaller, dust-sized particles (ash) can travel miles

 Inside a Volcano  Magma  Body of molten rock (underground) that feeds a volcano  Vents  Opening through which volcanic material passes

 Magma  Molten rock  Composition affects how explosive a volcano is  Silica (SiO 2 ), Water, Gases (CO 2, water vapor)  When pressure decreases as magma rises, the CO 2 and water vapor expand rapidly  Silica Rich (Felsic) Magma Traps Gases  Thicker, viscous  Difficult for gases to escape, so pressure builds up

 What Erupts from a Volcano?  Explosive Volcano  Mainly pyroclastic material  Magma that is blasted into the air and hardens  Nonexplosive Volcano  Mainly lava Eruptions can (and do) alternate between lava & pyroclastic eruptions (even within the same eruption)

 Types of Lava  Aajagged surface; brittle crust is torn by lave flow underneath  Pahoehoeflows slowly; glassy surface has rounded wrinkles  Pillow Lavarounded lumps formed under water  B locky Lavadoes not travel far; heaps of sharp-edged chunks

 Types of Pyroclastic Material  Formed when magma explodes and solidifies in the air  Volcanic Bombs  Large blobs that cool & harden in the air  Lapilli  Smaller, pebble-like stones that harden before hitting the ground  Volcanic Ash  Most of the material in an eruption  Walls of gas bubbles explode into tiny, glass-like slivers

 Pyroclastic Flows  Produced when enormous amounts of hot ash, dust, and gases are ejected from a volcano  Speeds up to 200 km/h  Temperatures over 700 C

 Review 1)Which of the following factors influences whether a volcano erupts explosively? A. The concentration of bombs in the magma B. The concentration of phosphorous in the magma C. The concentration of aa in the magma D. The concentration of water in the magma

 Review 2) How is lava classified? How are pyroclastic materials classified? 3) Explain how the presence of silica (felsic) and water in magma increases the chances of an explosive eruption.

 Answers 1) D 2) Lava is classified by its surface texture; the way it flows may also be used to help classify lava. Pyroclastic material is classified by its size and how it forms. 3) As the magma moves towards the surface, the water changes to water vapor and expands rapidly; silica rich (felsic) magma is viscous and tends to trap volcanic gases and plug vents, the results in high pressures forming