Volcanoes: The Fire Within Chapter 9: Volcanoes

What is a Volcano? A vent that lets out heat from inside the Earth, spewing out lava and eventually forming a mountain. 3 classifications of volcanic activity: extinct (does not erupt), dormant (sleeping), and active (currently erupting). The most active volcano on the Earth is Kilauea on the big island of Hawaii because it has been erupting almost daily since 1983! Kilauea

Where Are They Found? Volcanoes are found wherever plates are being subducted or spreading apart. At these locations, pressure and heat build up and melt the surrounding rock. Since magma is less dense than solid rock, it will find any little crack in the crust to squeeze into to relieve the pressure. Any place there are lots of earthquakes, you will usually find active volcanoes. You will not find volcanoes at transform boundaries!

Converging and Diverging Converging: where subduction is happening, like around the Ring of Fire. The more dense oceanic plate gets re-melted and creates volcanoes along coastlines (Andes Mountains) Diverging: where plates are spreading apart. Along the mid-ocean ridges, lava pours out and gets immediately cooled by the ocean water. (Mid-Atlantic Ridge exposed in Iceland)

Hawaii: A Tropical Paradise Sometimes, very hot parts of the mantle actually burn holes through the crust. These are called hot spots. Hot spots are located in the middle of tectonic plates. Over time, as the plate moves, the “hot spot” will burn new holes through the crust, creating new places of volcanic activity along the way. Two examples: Hawaii (in the middle of the Pacific plate) and Yellowstone National Park (home of Old Faithful, and located in the middle of the North American plate) Old Faithful

Where Are They Most Violent? Subduction zones produce the most violent volcanic eruptions. Why? The oceanic plate is water-logged. Therefore, when it is subducted, the water turns to steam (which is a gas). This steam gets trapped in the magma, and then explodes out of the volcano! Just think about what you feel like when you have gas. Doesn’t it feel better when you let it go? Well, so does the volcano!

Types of Lava Basaltic: flows very easily and has a low silica content. Usually, this type is associated with non- explosive eruptions. Hot spots and rift zones have this type of lava. Hawaiians call basaltic lava “aa” which has sharp edges when it cools, or “pahoehoe” which they also call “grandma lava” because it is wrinkly when it cools. Granitic: very thick, high silica content, associated with very violent eruptions, like around the Ring of Fire Examples are Mt. St. Helens, Mt. Rainier, Mt. Fuji, Mt. Pinatubo Aa Pahoehoe Pyroclastic flow during the eruption of Mt. Pinatubo in the Philippines

Types of Volcanoes Shield: gently sloping sides, basaltic lava, quiet eruptions (but destructive!), example is Hawaii. Cinder Cone: cone-shaped, granitic lava, violent eruptions, throw pyroclastics into the air (ash, bombs, tephra), example is Paricutin in Mexico. Composite: tall, steep sides, grantitic and basaltic lava, violent and quiet eruptions, examples are ALL volcanoes along the Ring of Fire. Mt. St. Helens, Pinatubo, Fuji and some in Europe like Vesuvius, Etna, Stromboli. Mt. Fuji in Japan

Mauna Loa in Hawaii

Paricutin in Mexico

Mt. St. Helens in Washington

Intrusive Features Sometimes magma doesn’t make it out of a volcano and gets trapped below ground. We can only see these features due to weathering and erosion. 5 main features: sill, neck, dike, laccolith, batholith