 More than 600 active volcanoes on Earth  Kilauea in Hawaii erupts continuously.  Iceland is a country that is made entirely from volcanoes.

 Volcanic ash and debris can  Kill crops and forests  Kill people  Destroy habitats  Sulfuric acid from volcanic gases mixes with water vapor to create acid rain.  Gases and ash particles can block sunlight from entering Earth’s atmosphere lowering the overall temperature of the planet.

 Soufriere Hills, Montserrat 1995  Volcanologists knew it was about to erupt.  Some folks refused to leave.  Pyroclastic flows wiped out cities and towns in its path, killing 20 people who ignored the evacuation order.

 Plates moving apart—divergent boundaries  Iceland’s volcanoes emerged from seafloor spreading  Eyjafjallajökull eruption 2010   Plates moving together—convergent boundaries  Soufriere Hills, Montserrat 2007   Hot Spot Volcanoes  Hawaii’s Kilauea is the most active volcano in the world. 

 Over a period of about 5 million years, the Pacific plate has moved over a hot spot where mantle material is particularly hot and blasts through Earth’s crust like a torch.  The lava that leaks out at the hot spot built up and formed a volcanic island.  As the plate moves over the hot spot, the magma punches through a new spot in the crust, forming new islands.  Volcanoes that are no longer over a hot spot become dormant or extinct.  The volcanic islands that are quiet begin to erode by the ocean waves.

1. Water vapor and other gases a. Water dragged under with a subducting plate turns to water vapor. b. Water vapor and other trapped gases take up more room, creating more pressure. c. Trapped gases such as water vapor and carbon dioxide build up pressure in magma.

2. Silica a. The more silica present in the magma, the thicker and stickier it is. b. The thicker and stickier the magma, the more likely it is to trap gases. c. Think of blowing a straw into the bottom of a thick milkshake compared to a runny milkshake.

 Silica-poor magma is basaltic magma.  Produces runnier magma and quiet eruptions  Pahoehoe is ropelike or hairlike in structure and comes from hotter lava.  Aa is rough and jagged and comes from cooler lava.

 Silica-rich magma is granitic.  Thick, sticky magma that is good at trapping gases.  Gases can expand rapidly, triggering an explosive eruption  Soufriere Hills, Montserrat

 Amounts of silica can vary in andesitic volcanoes.  They often have more silica than basaltic, less than granitic  These volcanoes often form at convergent plate boundaries.  They an alternate between explosive and quiet eruptions  Krakatau, Indonesia is an example.

 Circular, broad base, gentle slopes, basaltic, non-explosive.  Hawaiian Islands

Alternating layers of lava and tephra Mt. Ranier, WA Soufriere Hills, Montserrat

Explosive eruptions throw rock and bits of tephra into the air. Falls to the ground and collects into a cone shape. Paracutin, Mexico

Types of Volcanoes Shield Volcano KilaueaKilauea, Hawaii Composite Volcano Mt. St. HelensMt. St. Helens, Washington Cinder Cone Volcano ParícutinParícutin, Mexico