Volcanoes Cleveland Volcano, Alaska, May 2006 Taken from the ISS Apr 20 2:26 PM The term volcano can refer to two things: 1. the opening in Earth's crust through which molten rock, gases, and ash erupt 2. the landform that develops around this opening Apr 20 2:28 PM 1
Volcanoes can be classified as either: 1. 2. 3. Active: has erupted in recorded history (including those currently erupting) Dormant: has not recently erupted, but is considered to be likely to do so in the future Extinct (Dead): has not recently erupted and is not likely to in the future. Apr 20 2:28 PM Magma Formation (3 ways) A decrease in pressure can lower the melting temperatures of materials in the asthenosphere. Such a decrease takes place along the rift valley at a mid ocean ridge, where the lithosphere is thinner and exerts less pressure. An increase in temperature can cause materials in the asthenosphere to melt. Such an increase occurs at a hot spot. An increase in the amount of water in the asthenosphere can lower the melting temperatures of materials there. Such an increase occurs at subduction boundaries. Apr 20 2:37 PM 2
April 23, 2013 Formation of Volcanoes Since volcanoes are a product of magma, they form under the same circumstances as magma: 1. at subduction boundaries 2. at divergent boundaries 3. over hot spots Apr 20 3:56 PM Subduction Boundaries Magma chambers are produced by the melting lithosphere (water in the subducted rock lowers the melting temperature) Stresses in the less dense crust produces cracks. Since magma tends to be less dense than the material around it, the magma rises through the cracks to the surface. Apr 20 4:01 PM 3
April 23, 2013 Divergent Boundaries As two plates move apart, the pressure at the surface decreases, which in turn lowers the melting temperature. The material which rises below a rift rises from deeper, hotter regions within Earth. This combination causes large amounts of magma to form. So much, that most of the magma that reaches the surface does so at divergent boundaries. Apr 20 4:08 PM Hot Spots This image (not to scale) shows the location of extinct, dormant, and active volcanoes in the Pacific. Can you see a pattern? Within this pattern, the volcanoes are older and extinct the further northwest you go. These volcanoes were formed as the plate moved over "hot spots" in the mantle. Apr 20 4:20 PM 4
April 23, 2013 Apr 20 4:45 PM Types of Magma The type of molten rock contained in the lava has a large effect on the type of volcano generated. Silica, the principal ingredient in all magmas, determines the magma's viscosity (resistance to flow). The more silica the magma contains, the more resistant to flow it is. Basaltic Magma Andesitic Magma Rhoylitic Magma Silica Content Least Intermediate Most Gas Content Least Intermediate Most Viscosity Least viscous (ie. flows easily) Intermediate Most viscous (ie. flows poorly) Type of eruption Rarely explosive Intermediate Usually explosive Melting Temperature Highest Intermediate Lowest Location Rifts Oceanic hot spots Subduction boundaries Continental hot spots Apr 20 4:50 PM 5
Types of Volcanoes Fissure Eruptions (Lava Plateau) > Flow of magma between cracks in the lithosphere (like between plates). > This is the cause of the rock responsible for sea floor spreading. Shield Volcanoes > Built from magma that is low is silicas, it flows out easily in many layers. > This creates volcanoes that are very wide and very tall with a gentle slope. Apr 20 5:06 PM Types of Volcanoes Composite Volcano > These develop when layers of materials from successive eruptions accumulate around a vent. > They tend to have violent eruptions, spaced between periods of relative quiet. Cinder Cones > The simplest type of volcano, forms when lava is thrown from a vent into the air. The lava hardens before hitting the ground and accumulates. > These tend to be smaller than other types of volcanoes, and typically form in groups and on the sides of larger volcanoes Apr 20 5:06 PM 6
Caldera Sometimes magma beneath a volcano is released, and the top of the volcano can collapse, forming a large crater shaped basin. This basin is called a caldera, and in some cases fills with water to form a lake. Many active volcanoes have calderas at their summit. Apr 20 5:06 PM Homework: 1. "Iron Catastrophe" and Scale Diagram no later than Wed. morning 2. Epicentre Lab Thursday (suggest you complete in MSIP) 3. Chapters 9 and 10 from the text are fair game for quizzes and tests, so take some time and read them! Apr 22 8:50 PM 7