4 Volcanism Definition: the origin and movement of magma Three categoriesExtrusive: magma is expelled onto surfaceIntrusive: magma solidifies in the shallow crust near the surface. Can be exposed after weathering.Plutonic: magma solidifies deep inside the crust.Viscosity: The ability for lava to flow. Reflective of the amount of silica present in the magma.LOW silica = LOW viscosity = nonexplosiveHIGH silica = HIGH viscosity = explosive
5 Volcanism Extrusive Examples Mt. Shasta, CAMt. Fuji, Japan
8 5 Types of Extrusive Volcanism 1. Shield Volcano Central vent allows magma from chamber to spill out, slowly building the volcano in layers.Produces wide, gently sloping mountainsBest example: Hawai’ian volcanoes (right: Mauna Loa)
9 5 Types of Extrusive Volcanism 2. Composite Volcano “Stratovolcano”Central vent allows magma to come up from chamber belowProduces symmetrical, steep-sloped mountainsMost “explosive” type of volcanoExamples: Mt Fuji, Mt Shasta, Mt St Helens
10 5 Types of Extrusive Volcanism 2 5 Types of Extrusive Volcanism 2. Composite Volcano example: Mt St Helens in 1980
11 5 Types of Extrusive Volcanism 3. Caldera Volcano Formed when the upper part of a volcano collapsesThe magma chamber below is (partially or completely) emptied after an eruptionThe emptied magma chamber can no longer support the weight of the overlying rockThe overlying rock collapses into itself, forming a circular basinVolcanic activity can continue if there is still magma in the chamber belowExample: Crater Lake, OR
12 5 Types of Extrusive Volcanism 3 5 Types of Extrusive Volcanism 3. Caldera Volcano example: Crater Lake, ORCrater Lake was formed after Mt Mazama (a stratovolcano) eruptedand collapsed back in on itself.
13 5 Types of Extrusive Volcanism 4. Cinder Cones Smallest mountain formation of all volcanic landform typesCone-shaped peaks build up from small, solid particles, ejected into air by small volcanic ventGenerally <1,500’ highExample: Wizard Island
14 5 Types of Extrusive Volcanism 4 5 Types of Extrusive Volcanism 4. Cinder Cone example: Wizard Island in Crater Lake, OR
15 5 Types of Extrusive Volcanism 5. Lava Dome “Plug Dome”High-silica lava exits from a vent. Due to its high viscosity (doesn’t flow very fast or far), it builds up into a dome.Effectively “plugs” the main vent. Can still emit ash, gases, and even lava.Example: Lava Dome in Mt St Helen’s crater.
16 3 Types of Lava Flows ’A’a Pahoehoe Pillow Lava “Clinky” Blocky, choppy lava flows that does not flow well over landPahoehoe“Smooth, unbroken lava”Ropy surfacePillow LavaLava emerges from underwater volcanic ventsLava cools & crusts when it hits colder waterWhen crust fractures, lava oozes out and creates billowing pillows
19 Dangers of Volcanic Eruptions Burning Hot LavaEarthquakes, Landslides and TsunamisThese are often related – an earthquake might jar some land loose, resulting in a landslide. The earthquake may set off the tsunami. Also, if the landslide falls into the ocean, can trigger a tsunami.Mudslides (“Lahars”)Ash FalloutPyroclastic Flow (“nuée ardente”)
20 Dangers of Volcanic Eruptions Mudslide (Lahar) examples Mt. Pinatubo, Philippines1991 eruptionMt. St. Helens, WA1980 eruption
22 Dangers of Volcanic Eruptions Ash fallout example: Mt. St Dangers of Volcanic Eruptions Ash fallout example: Mt. St. Helens 1980 eruption
23 Dangers of Volcanic Eruptions Pyroclastic flow example: Mt St Helens 1980 eruption Hot gases, ash, rock that become a “fluid cloud” that move down the side of the volcano. Can travel great distances.“nuée ardente”: French for “glowing cloud”