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Chapter 14 – Volcanoes – Review Questions

Vocabulary terms you should know (some you have seen before) – igneous, felsic, mafic, intrusive/plutonic, extrusive/volcanic, texture, porphyry, magma, lava, subduction boundary eruption, divergent boundary / rift eruption, hot spot, volcano, volcanic neck, tephra, ash, lapilli, blocks, bombs, batholith, dike, laccolith, sill, pluton, cinder cone, shield cone, composite cone


1.      What are the similarities and differences between felsic and mafic molten material?


Similarities – magma contains dissolved silica and other elements such as iron; and gases such as carbon dioxide, water vapor and sulfur; and ranged in temperatures from 600-1200°C

Differences – felsic magma is thick, stiff, slow moving (viscous), high percentage of silica, large amounts of dissolved gases, which causes more explosive eruptions

Mafic magma is thin, fluid (less viscous), fast moving, lower percentage of silica and more iron (darker color when cooled), dissolved gases can easily escape so smooth flowing less violent eruptions


2.      How is the cooling rate of lava/magma related to texture?  Explain fully.


Slower cooling rate = larger crystals = course grained texture

Faster cooling rate = smaller crystals = fine grained texture


3.      What two factors affect the texture of an igneous rock?


Texture effected by cooling rate and amount of dissolved gases


4.      Which types of eruptions are most likely to be eject tephra?  What are the 3 types of tephra?


Ash (under 2mm in diameter); lapilli (2 to 64 mm in diameter); blocks and bombs (more than 64 mm in diameter)


5.      How can mafic and felsic rocks be related to the density of oceanic and continental plates?


Oceanic plates are mostly mafic rocks, which have high percentage of iron and magnesium making them more dense.  Continental plates are mostly felsic rocks, which have a high percentage of silica, making them less dense.


6.      Describe 3 environments which can cause volcanoes, and name an example of each.


1.  Rift eruptions at divergent boundaries (such as Iceland, Mid-Atlantic ridge or Columbia River Plateau)

2.  Subduction eruptions occur at subduction boundaries (such as Cascades, Andes Mts., Phillipines, etc.)

3.  Hot Spot eruptions that occur in the middle of a plate, away from plate boundaries (Hawaii Islands)


7.      What is a hot spot, and what is believed to be the cause?

Area of volcanic activity in the middle of a plate.  Thought to be caused by radioactive minerals in the earth’s asthenosphere.  They seem to remain in one place even though the plate above it moves.


8.      How are the volcanic environments (question #6) related to plate tectonics?


9.      Describe and compare the 3 types of volcanic cones that form in terms of type of eruption, composition, size, and steepness of slope.


1.  Cinder cones – explosive eruptions – felsic lava, cinders and rock particles; creates small volcano with steep sides; for example Paricutin in Mexico

2.  Shield cones – quiet eruptions – mafic lava, flows smoothly from vent and covers large area; broad base and dome shape, gentle slope; for example – Mauna Loa, Hawaii

3.  Composite cone – unpredictable, usually explosive violent eruptions – felsic and mafic lava with bombs, lapilli, cinders and ash.  Alternating layers.  Quiet flow follows explosion.  For example – Mt. St. Helens


10.  Identify the proper shape and positions of plutonic intrusions (see page 264 in text to review).  Draw a picture.