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Magma and Erupted Materials



Key Terms:

    viscosity    lava    pahoehoe    aa    pillow lava    pyroclastic flow    pyroclastic material

Notes: (9-2)

Types of Magma

No matter what type of magma you have, silica is the main ingredient.  What determines the type of magma is the percentages of silica and present.  Magmas with high silica content have a high viscosity (resistance to flow) and are generally more explosive when eruptions occur.  All magmas also contain some degree of trapped gasses (CO2 & H2O).  The more trapped gas the more violent the explosion.

Explosive eruptions: (General rule)

High silica percentage

Large amount of trapped gasses

CO2 & H2O

     The more trapped gas the larger the explosion  

ex: Andesitic and granitic magma

Magma composition:

Basaltic magma

contains less silica (50%) easy flowing

less explosive Quiet eruptions  

Low viscosity

High melting temperature

Associated with divergent boundaries and  hot spots

HI Islands, Mid Atlantic Ridge

(geoimages.berkeley.edu/GeoImages/ Wells/geomorph/basalt.JPG)

Andesitic / Granitic Magma

Contains lots of silica (60%) thick

Sometimes explosive Moderate amount of trapped gasses  

Intermediate viscosity

Intermediate melting temperature

Associated with subduction zones

High water content

Mt St Helens

(geology.csupomona.edu/alert/ igneous/andesite.jpg)

Rhyolitic Magma

Contains lots of silica (70%) thick

Very explosive lots of trapped gasses  

High viscosity

Low melting temperature

Associated with Continental hot spots

Yellowstone caldera

Lava Flows

All magma that reaches the surface can be classified as Lava.  

Basaltic lava is classified according to its texture.

aa - rough containing many gas bubbles
pahoehoe - smooth
Underwater lava flows - pillow lava






(www.rit.edu/~rhrsbi/GalapagosPages/ Pictures/Geology/aa.jpeg)

(www.isthe.com/.../photo/usgs/kilauea-2000/ pahoehoe-lava.jpg)

(www.bonin-islands.com/ what/pillowlava.jpg)

Ash and Rock Fragments

Solid fragments that are thrown from volcanoes are called pyroclastic materials.  They are ejected by the trapped gases within the volcanoe and are classified according to their size.

Forms of tephra

Ash - diameter < 2mm
Lapilli - diameter 2mm - 64mm
Bombs - diameter > 64mm 
Pyroclastic flow - mixture of superheated gas and ash that travels down a volcano (~100km/hr)

(volcanoes.usgs.gov/Imgs/Jpg/Tephra/ 30410914-076_large.jpg)

(www.educeth.ch/stromboli/perm/ erta/northpit/icons/ja_128.jp)

(www.disastershq.com/.../volcan/ pyroclastic%20flow.jpg)












Last modified: October 17, 2003