3 What 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
4 Critical Thinking #1Considering the prior definitions, explain how a volcano first forms.
5 How Volcanoes Form1. Plate Boundaries – Subduction and Seafloor Spreading2. Hot Spots – Extremely hot intraplate regions where mantle rises
6 Where are Volcanoes Located? Most located along the “Ring of Fire” - zone of volcanic activity along edge of Pacific Ocean
7 Critical Thinking #2Why would oceanic-continental convergent boundaries produce a great number of volcanoes?
8 Types of LavaMafic Lava – forms oceanic crust, dark colored, generally runnyFelsic Lava – forms continental crust, light colored, generally thick
9 Two Forms of Mafic LavaPahoehoe – means “ropy”, formed when mafic lava cools slowly
10 Two Forms of Mafic LavaAA – jagged chunks, formed when mafic lava cools rapidly
11 Critical Thinking #3What factors would determine the speed at which mafic lava hardens?
12 PyroclasticsPyroclastics – Rock fragments made of hardened felsic lava, types based on sizeVolcanic Dust – Less than 0.25mm in diameterVolcanic Ash – mm in diameter
13 Pyroclastics Lapilli – 2-64mm in diameter Volcanic Bombs - >64mm in diameter
14 Critical Thinking #4What factors would determine the size of pyroclasts produced?
15 Viscosity 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
16 Critical 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?
17 Critical Thinking #6What is the relationship between gas content and viscosity?
18 Three Types of Volcanoes 1. Shield Volcano2. Cinder Cone3. Composite Volcano (Stratovolcano)
19 Shield Volcano Broad, gently slopped (900)m High) Eruption of fluid low viscosity mafic lava through multiple ventsEx: Kilauea (Hawaii)
20 Cinder Cone Steep cone-shaped volcano (500m high) Medium-viscosity Lava fragments (Cinders) ejected from single ventEx: Paricutin (Mexico)
21 Composite Volcano Large, often snow-capped peaks (3000m high) Most explosive eruptions, high viscosity lava and pyroclastsEx: Mt. St. Helens (Washington)
22 Critical Thinking #7Which volcano type would likely be the most dangerous? Explain why.
23 Craters 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
24 Critical Thinking #8In what situation would a crater form? In what situation would a caldera form?
25 Mount 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
26 Critical Thinking #9What factors would create the greatest landslide in a volcanic eruption?
27 Critical Thinking #10What aspect of the eruption of Mount St. Helens caused the greatest damage and casualties?