Volcanism Behavior of Magma - Granitic magma - 70% silica: thick - 10-15% water: has to degas - explosive if at surface - usually solidifies in crust - Basaltic magma - 50% silica: thin - 1-2% water: not gassy - eruptions are mild and gentle - usually rise to surface - How silica effects magma - silica tends to form large molecules from smaller molecules: polymers - therefore, they will be thicker and will not flow easily - therefore, granitic magma will not rise very easily; basaltic magma will rise easily because it is thinner - How pressure and water will effect magma - Pressure - as magma rises the pressure drops - as pressure drops, the melting point of rock drops, therefore, magma may form due to a decrease in pressure not an increase in temperature: pressure relief melting - this helps if it is dry. Ex. basaltic !1

- Water - water lowers melting point of magma. - remember, granite contains a lot of water - instead of melting at 700o, it may melt at 600o - as it rises, it may lose water - melting point rises and the magma now may cool * this is why basaltic magmas will erupt at* the surface even though it is denser than granitic magma

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Volcanoes - Volcanic Gases - 50 to 80% is water vapor, also carbon dioxide, nitrogen, sulfur dioxide, hydrogen sulfide, carbon monoxide - Gases contained in rising magma expand and can contribute to violent explosions

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- Lava flows - Lava Tubes: When the sides and top of a lava flow solidifies forming a conduit allowing the lava to flow more rapidly

! An active lava tube in Hawaii. Part of the tube’s roof has collapsed, forming a skylight.

! Jot Dean Cave is a lava tube on Medicine Lake Volcano in northeastern California. Notice that part of the lava tube’s roof has collapsed, thus allowing access. This and several other lava tubes in the area are called ice caves, because ice that forms during the winter persists throughout the summer. !4

- Columnar joints: as lava cools cracks form downward to form columns that are 5-6 sided

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! Columnar joints in a basalt lava flow in Washington State.

! Surface view of columns at Devils Postpile National Monument, California. The straight lines and polished surface resulted from abrasion by a sedimentladen glacier that moved over this surface. !6

- Pahoehoe: relatively low viscosity magma. Top partially solidifies and magma continues to flow - glassy, wrinkled surface (ropy)

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- aa: magma is a mixture of solid rock and thick liquid lava - jagged, broken surface

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- pillow lava: fast cooling, happens under water - sphere shaped

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- pyroclastic rock: rock formed when lava or solid rock erupts explosively

Sizes .06 mm

Fine ash; glassy

.06-2mm

Coarse ash

2-64mm

Cinders (lapilli)

+64mm

bombs

! Pyroclastic materials are all particles ejected from volcanoes, especially during explosive eruptions. The volcanic bomb is elongate, because it was molten when it descended through the air. The lapilli was collected at a small volcano in Oregon, whereas the ash came from the 1980 eruption of Mount St. Helens in Washington. !10

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Tuff: The walls of the Grand Canyon of the Yellowstone River are made up of the hydrothermally altered pyroclastic materials that partly fill the Yellowstone caldera.

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Anak Krakatau, which means “daughter of Krakatau,” is a small volcano in the Sundra Strait between Java and Sumatra that is the remnant of a much larger volcano known as Krakatau. In 1883, Krakatau erupted with such explosive force that most of the island disappeared and the volcano collapsed in on itself forming a huge depression known as a caldera. Renewed volcanism built up Anak Krakatau which rose from the ocean in 1927. It is shown here erupting on November 3, 2010. !12

- Types of Volcanoes - Caldera: circular depression caused by an explosive eruption - How a caldera forms 1) Magma rises and pushes up on the rock overhead producing a dome. Gas rise to the top of the magma chamber. Eruption begins as huge quantities of ash are ejected from the volcano.

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2) Pressure pushes upward through cracks or fractures in rock. Magma, rock and gas are vented. The eruption continues as more ash and pumice are ejected into the air and pyroclastic flows move down the flanks of the mountain.

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3) The top of the magma chamber has less “stuff” now. The collapse of the summit into the partially drained magma chamber forms a huge caldera when overlying rock above magma chamber collapses.

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4) Post caldera eruptions partly cover the caldera floor, and a lava dome and/or a small cinder cone/lava dome may form.

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View from the rim of Crater Lake showing Wizard Island. The lake is 594 m deep, making it the second deepest in North America.

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