Transcript Volcanoes!
Warm Up Why was the Mt. St. Helens Eruption so destructive? What about the explosion was not predicted by most scientists? Earth Science Volcano – A structure which moves magma onto the surface of the Earth Magma – liquid rock formed from heat inside the Earth Lava – Magma that erupts onto the Earth’s surface Considering the prior definitions, explain how a volcano first forms. 1. Plate Boundaries – Subduction and Seafloor Spreading 2. Hot Spots – Extremely hot intraplate regions where mantle rises Most located along the “Ring of Fire” - zone of volcanic activity along edge of Pacific Ocean Why would oceanic-continental convergent boundaries produce a great number of volcanoes? Mafic Lava – forms oceanic crust, dark colored, generally runny Felsic Lava – forms continental crust, light colored, generally thick Pahoehoe – means “ropy”, formed when mafic lava cools slowly AA – jagged chunks, formed when mafic lava cools rapidly What factors would determine the speed at which mafic lava hardens? Pyroclastics – Rock fragments made of hardened felsic lava, types based on size – Volcanic Dust – Less than 0.25mm in diameter – Volcanic Ash – 0.25-2mm in diameter – Lapilli – 2-64mm in diameter – Volcanic Bombs - >64mm in diameter What factors would determine the size of pyroclasts produced? Viscosity – Ability to resist flow – Determined by amount of Silica – Low Viscosity = Thin lava – Medium Viscosity = Medium Thickness – High Viscosity = Thick lava Gas Content – Determines Explosiveness – High Gas Content = Most Explosive – Low Gas Content = Least Explosive What is the average viscosity of mafic and felsic lava? What does this tell you about the silica content of mafic and felsic lava? What is the relationship between gas content and viscosity? 1. Shield Volcano 2. Cinder Cone 3. Composite Volcano (Stratovolcano) Broad, gently slopped (900)m High) Eruption of fluid low viscosity mafic lava through multiple vents Ex: Kilauea (Hawaii) Steep cone-shaped volcano (500m high) Medium-viscosity Lava fragments (Cinders) ejected from single vent Ex: Paricutin (Mexico) Large, often snowcapped peaks (3000m high) Most explosive eruptions, high viscosity lava and pyroclasts Ex: Mt. St. Helens (Washington) Which volcano type would likely be the most dangerous? Explain why. Crater – Funnel-shaped pit at top of volcanic vent – Formed when material is blown out of a volcano by explosions Caldera – Large basinshaped depression – Formed when magma chamber is emptied and entire volcanic cone is collapsed In what situation would a crater form? In what situation would a caldera form? Mount St. Helens Mount St. Helens – Composite Volcano (Stratovolcano) in Washington State – Erupted in 1980 after being dormant – “inactive” for 120 years – North face of mountain experiences largest ever recorded debris landslide – Pyroclastic flow from eruption destroyed 230 square miles – 57 people killed from the blast What factors would create the greatest landslide in a volcanic eruption? What aspect of the eruption of Mount St. Helens caused the greatest damage and casualties?