3What is a Volcano?Volcano – A structure which moves magma onto the surface of the EarthMagma – liquid rock formed from heat inside the EarthLava – Magma that erupts onto the Earth’s surface
4Critical Thinking #1Considering the prior definitions, explain how a volcano first forms.
5How Volcanoes Form1. Plate Boundaries – Subduction and Seafloor Spreading2. Hot Spots – Extremely hot intraplate regions where mantle rises
6Where are Volcanoes Located? Most located along the “Ring of Fire” - zone of volcanic activity along edge of Pacific Ocean
7Critical Thinking #2Why would oceanic-continental convergent boundaries produce a great number of volcanoes?
8Types of LavaMafic Lava – forms oceanic crust, dark colored, generally runnyFelsic Lava – forms continental crust, light colored, generally thick
9Two Forms of Mafic LavaPahoehoe – means “ropy”, formed when mafic lava cools slowly
10Two Forms of Mafic LavaAA – jagged chunks, formed when mafic lava cools rapidly
11Critical Thinking #3What factors would determine the speed at which mafic lava hardens?
12PyroclasticsPyroclastics – Rock fragments made of hardened felsic lava, types based on sizeVolcanic Dust – Less than 0.25mm in diameterVolcanic Ash – mm in diameter
13Pyroclastics Lapilli – 2-64mm in diameter Volcanic Bombs - >64mm in diameter
14Critical Thinking #4What factors would determine the size of pyroclasts produced?
15Viscosity and Gas Content Viscosity – Ability to resist flowDetermined by amount of SilicaLow Viscosity = Thin lavaMedium Viscosity = Medium ThicknessHigh Viscosity = Thick lavaGas Content – Determines ExplosivenessHigh Gas Content = Most ExplosiveLow Gas Content = Least Explosive
16Critical Thinking #5What is the average viscosity of mafic and felsic lava? What does this tell you about the silica content of mafic and felsic lava?
17Critical Thinking #6What is the relationship between gas content and viscosity?
19Shield Volcano Broad, gently slopped (900)m High) Eruption of fluid low viscosity mafic lava through multiple ventsEx: Kilauea (Hawaii)
20Cinder Cone Steep cone-shaped volcano (500m high) Medium-viscosity Lava fragments (Cinders) ejected from single ventEx: Paricutin (Mexico)
21Composite Volcano Large, often snow-capped peaks (3000m high) Most explosive eruptions, high viscosity lava and pyroclastsEx: Mt. St. Helens (Washington)
22Critical Thinking #7Which volcano type would likely be the most dangerous? Explain why.
23Craters and CalderasCrater – Funnel-shaped pit at top of volcanic ventFormed when material is blown out of a volcano by explosionsCaldera – Large basin-shaped depressionFormed when magma chamber is emptied and entire volcanic cone is collapsed
24Critical Thinking #8In what situation would a crater form? In what situation would a caldera form?
25Mount St. HelensMount St. Helens – Composite Volcano (Stratovolcano) in Washington StateErupted in 1980 after being dormant – “inactive” for 120 yearsNorth face of mountain experiences largest ever recorded debris landslidePyroclastic flow from eruption destroyed 230 square miles57 people killed from the blast
26Critical Thinking #9What factors would create the greatest landslide in a volcanic eruption?
27Critical Thinking #10What aspect of the eruption of Mount St. Helens caused the greatest damage and casualties?