Geology Volcanoes test

the origins of basalt and granite

- basaltic magma forms by melting of the asthenosphere (peridotite creates basaltic magma though partial melting-- basaltic magma leaves silica depleted asthenosphere as it rises to surface) - basaltic magma is hot enough to melt granitic crust into granitic magma near the surface (rising basaltic magma from asthenosphere heats crust) -andesite/intermediate magma forms by similar processes, with less silica than granite

magma behavior

- once magma forms, it rises to surface bc it is less dense than surrounding rock. as it rises, it cools and pressure drops (opposite effects). SO does it remain solid or liquify then? - Basaltic magma and granitic magma behave differently as they rise toward the surface-- - basaltic magma rises to the surface to erupt from a volcano, and granitic magma usually solidifies within the crust - When granitic magma does reach the surface, the eruption is violent/EXPLOSIVE bc of a BUILD UP OF GASES - Granitic magma: higher silica content (70%), long chains of silica, tangle more to give it higher viscosity, ample time to solidify beneath crust before reaching surface, more water (10%) for a lower melting point. As the magma rises, pressure decreases and water escapes. Because the magma loses water it's solidification temp rises and causes it to crystallize (solidify in the crust). - Basaltic magma: lower silica (50 %), reach surface more, less water (1-2%). In basaltic magma, with lower water content, water loss isn't important as it remains liquid.

volcano monitoring

- seismicity: monitoring small earthquake tremors, "volcanic tremor" (low level quake), result of magma pushing toward surface, also increasing volume of rock as it comes to surface - deformation: changes in ground measured by "tiltmeters"- measure changes in slope (shows ground filling up w magma) - gas: commonly gas output increases (water vapor, CO2, sulfur, nitrogen dioxide)

Mount St. Helens

1980, 2 months before great eruption, puffs of steam and volcanic ash rose from the crater and earthquakes occurred beneath the mountain... area evacuated in the spring

Mount Vesuvius

Ad 79, it destroyed roman cities of Pompeii and Herculaneum and other villages had been inactive so farmers had cultivated vineyards on side of mountain during eruption, ash flow buried towns, and preserved molds of inhabitants Mount V is a stratovolcano formed by many eruptions, from gentle or violent still at high risk for another eruption (moving magma)

shield volcano (chart)

BASALT. less silica. FLOW accumulation w small pyro. GENTLE eruptions HOT SPOTS/OCEANS setting ex: Hawaii, Iceland

fissure flows (chart)

CONTINENTAL setting, separating plates BASALT, successive flows QUIET eruptions ex: Columbia Plateau of wash-OR-idaho

composite (stratovolcano) cone (chart)

GRANITIC/ANDESITE. high silica. sticky lava. SUBDUCTION, coastal, alternating PYROCLASTIC and LAVA FLOWS ex: Pacific ring of fire

LAB Introduction notes

Geology in the Philippines: - in Philippine sea, ocean crust is being SUBDUCTED down the Philippine trench - Manila trench: convergent plate boundary w subduction that creates island arc volcanism - result is stratovolcanoes on the Luzon volcanic arc Mount Pinatubo erupts: - April 1991- small explosions create craters, small earthquakes daily (brittle rock fractures) in May, sulfur dioxide emissions grew, ash/gas increased, volcanic plume rose - June- explosive eruption, ash cloud/caldera, pyroclastic flows/lahar... ash blanketed towns. largest eruption since Krakatau in 1883 Global effects: - 3.5 cubic km ash/gas ejected into stratosphere, largest eruption of the century, ash circled the globe, sulfuric acid, global temp decline bc of ash reflecting energy back into space, thinning ozone layer, greater UV radiation possible

processes that form magma

Increasing temperature: increasing temperature melts rock Decreasing pressure: if rock is heated to its melting point on the earth's surface, it melts readily bc there is little pressure to keep it from expanding. if the pressure in the asthenosphere were to decrease, large volumes of the it would melt. Melting caused by decreasing pressure is called pressure-release melting Addition of water: wet rock melts at a lower temp than and identical dry rock

cinder cone (chart)

SMALL, form fast, PYROCLASTIC materials, explosive ex: Sunset crater, AZ

batholith

a pluton exposed over more than 100 sq km of the earth's surface (on avg, 10 km thick)

columnar joint

a regularly spaced crack that commonly develops in lava flows forming five or six sided columns

lava plateau

a sequence of horizontal basalt lava flows that were extruded rapidly to cover a large region of the earth's surface - see Columbia River Plateau (in Pacific northwest) and Siberia

dike

a tabular, sheetlike, intrusive rock that forms when magma oozes into a fracture

crater

bowl like depression at the summit of the volcano

fissure

cracks in the land surface where lava oozes - gentle type of volcanic eruption, often basaltic magma (low viscosity) - fissures common on Hawaiian and Icelandic volcanoes

volcanoc eruptions and global climate change

eruptions of mt. Pinatubo in 1991 produced the greatest ash and sulfur clouds in the latter half of the 20th century temperatures slightly decreased as a result correlation btwn temp and eruptions at end of permian period, mass extinction and volcanic eruption in Siberia at same time etc (see textbook pg 151)

lava

fluid magma that flows onto earth's surface

shield volcano (textbook)

formed by fluid basaltic magma gently sloping mountain huge eruptions are gentle and rarely life threatening, slow lava flow

nuèe ardente

french for "glowing cloud" used to describes an ash-flow

cinders

glassy pyroclastic volcanic fragments 4-32 mm in size

volcano

hill or mountain created when lava is too viscous to spread out as a flood

pluton

large mass of granite

sill

magma that oozes btwn layers of country rock and forms a sheetlike rock parallel to the layering

pressure-release melting

melting caused by decreasing pressure

ash-flow tuff

mixture of volcanic ash and rock fragments left after the eruption

stratovolcano (textbook)

name for composite cone

ash flow

nueé ardente; a mixture of volcanic ash, larger pyroclastic particles, and gas that flows rapidly along the earth's surface as a result of an explosive volcanic eruption

flood basalt

occurs with huge fissures that gently pour thousands of cubic km of lava onto earth's surface (covers landscape like a flood) (forms lava plateau)

vent

opening where lava and rock erupt from

Predicting volcanic eruptions

regional-- understand tectonic environment: subduction zones, spreading centers, hot spots. Violence based on geology-- continental crust (violent eruption, granitic magma) vs oceanic crust (gentle eruption, basaltic magma) short term-- time/place. based on instruments that monitor active volcanoes to detect signals (changes in shape, earthquake swarms (moving magma), increasing gas/ash emissions, increasing temp of nearby springs, signs of magma approaching surface))

pyroclastic flow

rock formsed from liquid magma abnd solid rock fragments (explosive eruption)

stock

similar to a batholith but exposed over less than 100 sq km

cinder cone (textbook)

small volcano composed of pyroclastic fragments forms when large amounts of gas accumulate in rising magma when gas pressure builds up enough the entire mass erupts explosively, hurling cinders, ash, and molten magma into the air active for a short amount of time bc once the gas escapes, the driving force is gone symmetrical and can be steep

volcanic ash

smallest particles consisting of class that formed when liquid magma exploded into the air

environments of magma formation

spreading center: as lithospheric plates separated at a spreading center, hot, plastic asthenosphere oozes upward to fill the gap. as it rises, pressure drops and pressure-release melting forms basaltic magma. because magma is lower in density than the surrounding rock, it rises toward the surface. hot spot: mantle plume rises vc it is hotter than the surrounding mantle and is less dense and more buoyant. as the plume rises, pressure-release melting forms magma that erupts onto the earth's surface. hot spots (above mantle plumes) can occur within a tectonic plate. subduction zone: water rises from oceanic crust on top of the subducting plate, circulation in the asthenosphere decreases pressure on the hot rock, friction heats rocks in the subduction zone

composite cone (textbook)

stratovolcano; forms over a long period of time from alternating lava flows and pyroclastic eruptions, can be very high

caldera

the circular depression that forms when nothing remains after the eruption to hold up the roof of the magma chamber. NOT a crater. Yellowstone National Park and the Long Valley Caldera:

partial melting

the process in which a silicate rock only party melts as it is heated, to form magma that is more silica rich than the original rock

aa

type of lava with high viscosity whose surface partially solidifies as it flows and the solid breaks up as deeper molten lava continues to move, which results in a jagged, rubbly, broken surface

pahoehoe

type of lava with low viscosity that continues to flow as it cools, which forms smooth glassy-surfaced wrinkled or ropy ridges