2Define Volcano using your own words. A weak spot in the Earth’s crust where molten material comes to the surface
33 Types of VolcanoesActive Volcano –one that is erupting or has shown signs that it may erupt in the near futureDormant –a “sleeping” volcano that scientists expect to awaken in the future and become activeExtinct –a “dead” volcano that is unlikely to erupt again
4What kind of volcanic eruptions exist? Quiet Eruptions --if there is magma that is low in silicaEx. Hawaiian IslandsExplosive Eruptions –if there is magma that is high in silicaEx. Mount St. Helens
8Parts of a VolcanoMagma chamber –beneath a volcano, magma collects in a pocket called the magma chamberPipe –a long tube in the ground that connects the magma chamber to Earth’s surfaceVent –an opening where molten rock and gas leave the volcanoLava Flow—the area covered by lava as it pours out of a ventCrater—a bowl-shaped area that may form at the top of a volcano around the central vent
9Volcanoes and Plate Boundaries There are about 600 active volcanoes on landThere are MANY MORE beneath the seaVolcanoes tend to occur in belts that extend across continents and oceansVolcanic belts form along the boundaries of Earth’s plates. WHY is this?
10Volcanic belts form along the boundaries of Earth’s plates because… At plate boundaries, huge pieces of the crust diverge (pull apart) or converge (push together)These movements cause fractures in the crust that allow magma to reach the surfaceMost volcanoes form along diverging plate boundaries such as mid-ocean ridges and converging plate boundaries where subduction takes place
11What is the Ring of Fire?A major volcanic belt that is formed by many volcanoes that rim the Pacific Ocean
15How can Subduction form volcanoes? When 2 plates collide, the older, denser plate sinks back into the mantleThe rock from subducting plates melts and forms magmaBecause magma is less dense than surrounding rock, it rises toward the surfaceEventually the magma breaks through the crust and creates a volcano
20Note: Volcanoes do NOT occur along transform boundaries, only earthquakes occur there
21Hot Spot Volcanoes Results from a “hot spot” in the Earth’s mantle Hot spot : an area where material from deep within the mantle rises and then melts, forming magmaA volcano forms above a hot spot when magma erupts through the crust and reaches the surface
28Lets look at the properties of magma/lava Lets look at the properties of magma/lava. Why is lava different based on the volcano?
29Viscosity: a physical property of liquids –the resistance of a liquid to flowing Because liquids differ in viscosity, some liquids flow more easily than othersThe greater the viscosity, the more slowly lava flowsWhich lava has a greater viscosity?
30Types of Lava http://www.youtube.com/watch?v=Z9iW_oXMBB8 (pahoehoe) (aa)Which type of lava has a higher viscosity?
31Aa has a higher viscosity Pahoehoe: fast-moving, hot lava with low viscosityLooks like a solid mass of wrinkles, billows, ropelike coilsAa: cooler, slow-moving lava with higher viscosity--when it hardens, it forms a rough surface consisting of jagged lava chunks
32Why don’t all types of magma have the same viscosity? The viscosity of magma depends upon its silica content and temperature.Silica = a compound that is made up of oxygen and siliconSilica content of magma ranges from percent
33The more silica magma contains, the higher its viscosity. Magma that is high is silica produces light-colored lava that is too sticky to flow very far.When this type of lava cools, it forms rhyolite
34The less silica magma contains, the lower the viscosity. Low-silica magma flows readily and produces dark-colored lava.When this kind of lava cools, it forms basalt.
37Where does lava usually come from? --lava begins as magma, which usually forms in the asthenosphere --the materials of the asthenosphere are under great pressure --liquid magma is less dense than solid material around it, so magma flows upward into any cracks in the rocks above
38What pushes magma to the surface? --Like the carbon dioxide in soda pop, dissolved gases are trapped in magma --The dissolved gases are under tremendous pressure
39When a volcano erupts, the force of The expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.
40When is an eruption over? When the pressure eases after the gases bubble out of the magma
41What happens when the volcano stops erupting? The lava at the surface will have turned to rock, magma will have solidified in the vent and pipe, and some magma might remain in the chamber