Introduction to Volcanoes 6th Grade
Define Volcano using your own words. • A weak spot in the Earth’s crust where molten material comes to the surface
3 Types of Volcanoes • Active Volcano –one that is erupting or has shown signs that it may erupt in the near future • Dormant –a “sleeping” volcano that scientists expect to awaken in the future and become active • Extinct –a “dead” volcano that is unlikely to erupt again
What kind of volcanic eruptions exist? • Quiet Eruptions --if there is magma that is low in silica • Ex. Hawaiian Islands • Explosive Eruptions –if there is magma that is high in silica • Ex. Mount St. Helens
Magma –molten material inside the volcanoLava—molten material that has erupted and cools into rock
Parts of a Volcano • Magma chamber –beneath a volcano, magma collects in a pocket called the magma chamber • Pipe –a long tube in the ground that connects the magma chamber to Earth’s surface • Vent –an opening where molten rock and gas leave the volcano • Lava Flow—the area covered by lava as it pours out of a vent • Crater—a bowl-shaped area that may form at the top of a volcano around the central vent
Volcanoes and Plate Boundaries • There are about 600 active volcanoes on land • There are MANY MORE beneath the sea • Volcanoes tend to occur in belts that extend across continents and oceans • Volcanic belts form along the boundaries of Earth’s plates. WHY is this?
Volcanic 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 surface • Most volcanoes form along diverging plate boundaries such as mid-ocean ridges and converging plate boundaries where subduction takes place
What is the Ring of Fire? • A major volcanic belt that is formed by many volcanoes that rim the Pacific Ocean
How can Subduction form volcanoes? • When 2 plates collide, the older, denser plate sinks back into the mantle • The rock from subducting plates melts and forms magma • Because magma is less dense than surrounding rock, it rises toward the surface • Eventually the magma breaks through the crust and creates a volcano
Island Arcs: a string of islands formed by volcanoes along a deep-ocean trench
Major island arcs include Japan, New Zealand, Indonesia, the Philippines, the Aleutians, the Caribbean islands
Note: Volcanoes do NOT occur along transform boundaries, only earthquakes occur there
Hot 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 magma • A volcano forms above a hot spot when magma erupts through the crust and reaches the surface
Do hot spots have to occur along plate boundaries? • No! They CAN occur on or near plate boundaries but they do not have to • Hot spots can also form under oceanic floor or continents
Hawaiian Islands formed one by one over millions of years as the Pacific plate drifted over a hot spot
Yellowstone National Park in Wyoming marks a hot spot under the North American plate –example of a hot spot that formed under the continents
Lets look at the properties of magma/lava. Why is lava different based on the volcano?
Viscosity: a physical property of liquids –the resistance of a liquid to flowing • Because liquids differ in viscosity, some liquids flow more easily than others • The greater the viscosity, the more slowly lava flows • Which lava has a greater viscosity?
Types of Lava • http://www.youtube.com/watch?v=Z9iW_oXMBB8 (pahoehoe) • http://www.youtube.com/watch?v=iyIV5fd1Aww (aa) • Which type of lava has a higher viscosity?
Aa has a higher viscosity • Pahoehoe: fast-moving, hot lava with low viscosity • Looks like a solid mass of wrinkles, billows, ropelike coils • Aa: cooler, slow-moving lava with higher viscosity --when it hardens, it forms a rough surface consisting of jagged lava chunks
Why 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 silicon • Silica content of magma ranges from 50-70 percent
The 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
The 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.
Where 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
What 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
When 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.
When is an eruption over? • When the pressure eases after the gases bubble out of the magma
What 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