Download Notes 13.2 Studying the composition of rocks, scientists determine

Notes 13.2
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Studying the composition of rocks, scientists determine there are 2 types of magma
o Mafic- rich in mg/fe and dark in color (commonly makes up oceanic crust)
o Felsic- rich in silicate minerals and light in color (more common in continental crust)
What determines the force of eruptions?
o Viscosity (resistance to flow)
o Viscosity is determined by the magma’s composition.
o Mafic magma produces runny lava with a low viscosity….produces quiet eruptions.
 Magma that contains small amts of trapped gases ….less explosive
 Due to the low viscosity, gases can easily escape
 Oceanic volcanoes form from mafic magma
o Felsic magma produce sticky lava with a high viscosity…produces explosive eruptions.
 Magma that contains large amts of trapped gases…..explosive
 Due to high viscosity, gases are easily trapped
LAVA FLOWS
o When mafic lava cools rapidly, a crust forms on the surface of the flow. If lava continues
to flow after the crust forms, the crust wrinkles to form a volcanic rock called pahoehoe.
Means ropy in Hawaiian
o When crusted over lava cools too rapidly or grows too thick to form wrinkles, the
surface breaks into sharp volcanic rock. This is called Aa lava. Same composition as
pahoehoe, texture is different because the gas content, rate and slope of the lava flow
are different.
o The most viscous type of lava is blocky lava. The high viscosity causes the cooled lava to
break at the surface in large chunks, while the hot lava underneath continues to flow.
EXPLOSIVE ERUPTIONS
o Continental volcanoes contain felsic lava, like Mt. St. Helens.
o Felsic lavas contained large amts of trapped gases, like water vapor and Co2.
o When a volcano erupts, the dissolved gases within the lava escape and send molten and
solid particle into the air.
o PYROCLASTIC MATERIAL- consists of fragments of rock that form during a volcanic
eruption
 Classified according to the sizes of the particles
 Volcanic Ash- < 2mm in diameter
 Volcanic Dust – volcanic ashe <0.25 mm
 Lapilli- larger pyroclastice particles <64 mm
o Generally fall near the vent
 Volcanic Bombs- large clots of lava that are thrown out of an erupting
volcano, cool in the air and develop a round or spindle shape
 Volcanic Blocks- from solid rock blasted from the vent…largest particles
(some size of a small house)
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Types of Volcanoes
o CRATER- funnel shaped pit at the top of a volcanic vent.
 Forms when material is blown out fo the volcano by explosions
o SHIELD VOLCANOES Volcanic cones that are broad around the base and have gentle sloping sides.
 Quiet eruptions
 Hot mafic lava flows out of the vent, hardens and builds up to form the cone.
 Hawaiian Islands chain of shield volcanoes
o CINDER CONE Steep slopes
 Rarely more than a few 100 m high
 Small explosive eruptions
 Made of pyroclastic material
o COMPOSITE VOLCANO Made of alternating layer of hardened lava flows and pyroclastic material.
 During quiet eruption, lava flows cover the side of the cone
 During explosive eruption, large amts of pyroclastic material builds up around
the vent.
 Quiet-explosive-quiet-explosive
 Also known as STRATOVOLCANOES, form large volcanic mountains
o CALDERAS
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When the magma chamber below a volcano empties, the volcanic cone may
collapse and leave a large basin depression CALDERA
 Calderas may later fill with water to form lakes.
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PREDICTING VOLCANIC ERUPTIONS
o Scientists look for change in earthquake activity around the volcano
 Pressure on the rock from magma that is moving upward causes small
earthquakes
 Temperature changes and fracturing of the rock around the volcano can also
cause small earthquakes
 May be a signal an eruption is about to occur
o Before an eruption, the upward movement of magma may cause volcano to bulge
outward.
o Scientists compare past behaviors with current measurements of earthquakes, surface
bulges and changes in the amt and composition of the gases that a volcano emits.
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