| (Ch. 4, cont.) Ways to form magmas 1. Source Rock Composition -decompression melting of upper upper mantle rock U --> U M--> M 2. Partial Melting -not all of the source rock melts, composition is skewed U-->M M-->I I-->F 3. Contamination -magma incorporates other rock components as it moves through the lithosphere -another magma merges with the magma 4. Fractional Crystallization -as magma cools, it crystallizes -assuming our magma doesn't mix, doesn't change composition: crystals of minerals form & sink b/c they're more dense & segregate themselves from the melt Crystals: Felsic: Al, K, Na, Si last Intermediate: Ca, Fe, Mg, O, Na Mafic: Fe, Mg, O, Ca Ultramafic: Fe, Mg, O first Factors that influence the extrusion of magma 1. Magma composition *silica content 2. Temperature 3. Amount of dissolved gasses Bowen's Reaction Series (fig 4.23 p 129) -Bowen demonstrated that minerals tend to crystallize in a systematic fashion based on their melting points * Discontinuous -olivine, pyroxene, amphibole, biotite mica -these minerals form in a specific order; once they form, they stay like that * Continuous -continuous change in type of plagioclase feldspar -some Ultramafic & Mafic are calcium rich -sodium & calcium are exchangeable so they trade out sometimes Chapter 5 - Volcanoes & Igneous Activity Viscosity -the more viscous the material, the greater its resistance to flow (syrup is more viscous than water) Factors that affect Viscosity 1. Silica content: more silica = more viscous 2. Temperature: higher temp = less viscous (heating syrup thins it) 3. Gasses (volatiles): more dissolved gas = less viscous Materials extruded during an eruption 1. Lava a. Pahoehoe- ropy appearance (think about the gravy, the top layer of it bunching up as you try to pour it) -basaltic (mafic) magma; fluid b. Aa- cinder like pieces, rough rubble -formed by escaping gas that breaks up the crystallized top layer -basaltic (mafic) c. Lava tube- underneath the hardened top, the lava flows in a tube, outward from the volcano d. Pillow- pillow shaped structures, one after another -formed under water 2. Gasses 70% H2O, 15% CO2, 5% N, 5% SO2, Cl, Ar, H -helps propel magma from a volcano -help create conduits -new variety of life living off volcanic vent; S=energy, like sun is for us 3. Pyroclastics a. ash and dust particles b. welded tuff- glassy shards/ash/dust fuse together when it hits the ground c. lapilli- 2-64mm cinders d. block- 64+mm, hardened when they are ejected e. bomb- 64+mm, incandescent (not totally solidified), shapes itself on the fall down * scoria- basaltic lava, had gasses in it that came out after it crystallized * pumice- like scoria, but intermediate and felsic lava; extremely light Anatomy of a Volcano * Conduit/Pipe- pipe like opening through which magma moves toward Earth's surface * Vent- the surface opening of a conduit or pipe * Volcano- a mountain formed around the vent from lava and/or pyroclastics * Crater- the depression at the summit of a volcano * Caldera- a large depression typically caused by collapse or ejection of the summit area of the volcano * Parasitic Cone- a volcanic cone that forms on the flank of (near) a larger volcano * Fumarole- a vent in a volcanic area from which just fumes or gasses escape Types of Volcanoes 1. Shield Volcano (fig 5.10 p 148) -produced by the accumulation of fluid basaltic (mafic) lavas -broad slightly domed structure; very big 2. Cinder Cone Volcano (fig 5.14 p 151) -built from ejected lava fragments (pyroclastics) -small, steep sides 3. Composite Cones (fig 5.9 p 148) -aka: stratovolcano -both lava & pyroclastic deposits -most picturesque, yet potentially dangerous **Nuee Ardente -ash flow containing bombs **Lahar -volcanic debris supersaturated with water; snow melts & mud flows @ the beginning of an eruption |