Kimia's+Notes+Chapter+6+Section+2

__Volcanic Eruptions__ __Key Terms__ -Magma Chamber-beneath a volcano,magma collects in a pocket. -Pipe-a long tube through which magma moves from the magma chamber to Earth's surface. -Vent-molten rock and gas leave the volcano through an opening. -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. -Silica-a material found in magma that formed from the elements oxygen and silicon. -Pyroclastic Flow-a type of expolsive eruption that huurls out a mixture of hot gases,ash,cinders,and bombs. -Dormant-a volcano that is not currently active,but that may become active in the future. -Extinct-a volcano no longer active and is unlikely to erupt again. -Geyser-a fountain of water and steam that builds up pressure underground and erupts at regular intervals.

Introduction In Hawaii, there are many myths about Pepe,the fire goddess of volcanoes.According to legend,Pele lives in the depths of Hawaii's erupting volcanoes.When Pele is angry,she causes volcanic eruptions.One result of an eruption "Pele's hair", a fine,threadlike rock formed by lava.Pele's hair forms when lava sprays out of the gorund like water from a fountain.As it colls,the lava stretches and hardens into thin strands.

Where does lava 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 that the solid material around it.Therefore,magma flows upward into the cracks in the rock above.As magma rises,it sometimes becomes trapped beneath layers of rock.But if an opening in weak rock allows the magma to reach the surface,a volcano forms.

Magma Reaches Earth's Surface A volcano is more than a large,cone-shaped mountain.Inside a volcano is a system of passageways through which magma moves.

What's Inside a Volcano? All volcaoes have a pocket of magma beneath the surface and one or cracks through which the magma forces its way.Beneath a volcano,magma collects in a pocket.The magma moves upward through a **pipe**,a long tube in the gournd connects the magma chamber to Earh's surface.Molten rock and gas leave the volcano thourgh an opening.Often,there is one central vent at the top of a volcano.However,many volcanoes also have other vents that open on the volcano's sides.

What is Volcanic Eruption? What pushes magma to the surface? The explosion of a volcano is similar to the soda water bubbling out of a warm bottle of soda pop.You cannot see the carbon dixoide gas in a bottle of soad pop because it is dissolved in the liquid.But when you open the bottle,pressure is released.The carbon dixoide expands and forms bubbles,which rush to the surface.Like the carbon dixoide in the soda pop,dissolved gases are trapped in magma.These dissoled gases are under tremendous pressure.As magma rises toward the surface,the pressure of the surrounding rock ont he magma decreaes.The dissolved gass begin to expand,forming bubbles.As pressure falls within the magma,the size of the gas bubbles increases greatly.These expanding gases exert an enormous force.When a volcano erupts,the force of the expanding gases pushes magma from the magma chamber through the pipeuntil it flows or exlpodes out of the vent.Once magma escapes from the volcano and becomes lava,the remaining gases bubble out.

What Kinds of Eruptions are There? Some volcanic eruptions occur gradually.Others are dramatic expolsions.Geologists classify volcanic eruptions as quiet or explosive.The properties of magma determine how a volcano erupts.Whether an eruption is depends on the magma's **silica** content and whether the magma is thin and runny or thick and sticky. Quiet Eruptions A volcano erupts quietly if its magma is low in silica.Low-silica magma is thin and runny and flows easlily.The gases in the magma bubble out gently.Low-silica lava however, oozes quietly from the vent and can flow for many kilometers.Quiet eruptions can produce two diffferent types of lava that differ in temperture.Fast moving,hot lava that is thin and runny is called Pahoehoe.The surface of a lava flow formed from Pahoehoe looks like a solid mass of wrinkles and ropelike coils.Aa is lava that is cooler and slower-moving.Aa is thicker than Pahoehoe,when Aa hardens,it forms rough surface consisting of jagged lava chunks.The Hawaiian Islands were formed from quiet eruptions.On the Big Isalnds of Hawaii,lava pours out of the rater on Mount KIlauea.But lava also flows out of long cracks,called fissures,on the volcano's sides.Quiet eruptions built up the Big Isalnd over hundereds of thousands of years. Explosive Eruptions A volcano erupts explosively if its magma is high in silica.High-silica magma is thick and sticky.It builds up in the volcano's pipe ,plugging it like a cork in a bottle.Dissolved gases,including water vapor,cannot escape from thick magma.The trapped gases build up pressure until they explode.The erupting gases and steam push the magma out of the with incredible force.An explosive eruption breaks lava into fragments that quickly cool and harden into pieces of different sizes.The smallest pieces are volcanic ash -fine,rocky particles as small as a speck of dust.Cinders are pebble-sized paritcles.Larger pieces,called bombs,may range from the size of a baseball to the size of a car. Volcano Hazards Although quiet eruptions and explosive eruptions producedifferents hazards,both types of eruptions can cause damage far from the crater's rim.During a quiet eruption,lava flows from vents,setting fire to,and then burying ,everything in its path.A quiet eruption can cover large areas with a thick layer of lava.During an expolsive eruptions,a volcano can belch out hot clouds of deadly gases as well as ash,cinders,and bombs.Volcanic ash can bury entire towns.If it becomes wet,the heavy ash can cause roofs to collapes.If a jet plane sucks ash into its engine,the engine may stall.Eruptions can cause landslides and avalanches of mud,melted snow,and rock.

Stages of Volcanic Activity The activity of a volcano may last from less than a decade yo more than 10 million years.Geoligists tyr to determine a volcano's past and whether the volcano will erupt again. Life cycle of a Volcano An active ,or live,volcano is one that is erupting or has shown signs that it may erupt in the near future.A dormant of sleeping,volcano is like a sleeping bear.Scientists expect a dormant volcnao to erupt in the future and become active.An extinct ,or dead volano is unlikely to erupt again.In California,Lassen Peak and Mount Shasta are considered active volcanoes.Lassen Peak erupted between 1915 and 1921,and Mount Shasta erupted during the late 1700s.Craters near Long Valley on the eastern side of the Sierras are dormant but could become active again. What are Hot Springs and Geysers? Hot springs and geysers are often found in areas of present or past volcanic activity.A hot spring forms when water deep underground is heated bya nearby body of magma or by hot rock.The hot water rises to the surface and collects in a natural pool.Sometimes,rising hot water and steam bcome trapped in a narrow crack.Pressure builds until the mixture suddenly sprays above the surface as a geyser. How Do You Monitor Volcanoes? Geologists use insturments to detect changes in and around a volcano.These changes may gave warning a short time before a volcano erupts.But geologists cannot be certain about the type of eruption or how powerful it will be.Geologists use tiltmeters and other instruments to detect slight surface changes in elevation and tilt caused by magma moving underground.They monitor any gases ecaping from the volcano.A temperature increase in underground water may be a sign