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Chapter 9 Earth Science VOLCANOES Volcanic Eruptions Volcanoes – are areas of Earth’s surface through which magma and volcanic gases pass. Explosion of a volcanic eruption can turn an entire mountain into a billowing cloud of ash and rock in a matter of seconds. Help form fertile land. Create some of the largest mountains on earth. Volcanic Eruptions Magma – melted rock below the Earth’s surface. Lava – Melted rock above the Earth’s surface. Mt. Cleveland- Aleutian Islands, Alaska Stromboli Volcano - Sicily Mount Redoubt - Alaska Nonexplosive Eruptions Flows of lava are calm. Releases huge amounts of lava. Vast areas of the Earth’s surface, are covered with lava from non-explosive eruptions such as Hawaii. Types of Lava Flows Pahoehoe lava flows – sheet like lava flows that result in continuous surfaces. Flow smoothly Move forwards in tongues or lobes and are characterized by a glassy, plastic skin. Pahoehoe Lava Flow Types of Lava Flows Aa – lava pours out quickly and forms a brittle crust. The crust is torn into jagged pieces as molten lava continues to flow underneath. Got its name because of the painful experience of walking barefoot across the jagged surfaces. Associated with lava fountaining. Aa Lava Flow Lava Fountaining Lava Fountaining – spraying of lava into the air pulsing with the pressure of escaping gases. Types of Lava Flows Pillow lava – forms when lava erupts underwater. Lava forms rounded lumps that are the shape of pillows. Types of Lava Blocky Lava – cool, stiff lava that does not travel far from the erupting vent. Normally forms jumbled heaps of sharp-edged chunks. Get it’s shape from oozing from a volcano. Blocky Lava What is Inside a Volcano? Water and Magma Are an Explosive Combination If the water content of magma is high, an explosive eruption is more likely. Underground the water and magma are under extreme pressures. When water and magma rise to the top the gases expand rapidly, an explosion can result. Sometime so rapid a water floating version of lava called Pumice is formed. Silica-Rich Magma Traps Explosive Gases Magma that has a high silica content will tend to cause explosive eruptions. Silica-rich magma has a stiff consistency. Flows slowly and hardens in a volcano’s vents plugging the vents. Gas Trapped in a Volcano The more that is trapped in a volcano the more explosive it will be due to the rapid rate in which the gas escapes from the volcano. What erupts from a Volcano? Pyroclastic material - forms when magma is blasted into the air and hardens. Volcano’s eruptions may alternate between lava and pyroclastic eruptions. Volcanic Bombs Volcanic Bombs – large blobs of magma that harden in the air. The shape of the bomb was caused by the magma spinning through the air as it cooled. Lapilli Lapilli – pebblelike bits of magma that hardened before they hit the ground. Means “little stones” Volcanic Ash Volcanic Ash – forms when the gases in stiff magma expand rapidly and the walls of the gas bubbles explode into tiny, glasslike silvers. Makes up most of the pyroclastic material in an eruption. Volcanic Ash Pyroclastic Flows Pyroclastic Flow – produced when enormous amounts of hot ash, dust, and gases are ejected from a volcano. Can move downhill at 200km/h. Center of the flow can exceed 700 C.