Chapter 8 (P1)

Chapter 8:

I. Volcanic Eruptions

   A. Factors that determine the violence of an eruption

      1.Composition of the magma

      2. Temperature of the magma

      3.Dissolved gases in the magma

   B.Viscosity of magma 

      1.Viscosity is a measure of a material's resistance to flow

      2.Factors affecting viscosity 

         a. Temperature (hotter ,ag,as are less viscous) 

         b.Composition (silica content)

            1. High Silica (high viscosity (granitic lava)

            2.Low silica (more fluid) (basaltic lava)

         c.Dissolved gases 

            1.Mainly water vapor and carbon dioxide 

            2.Gases expand near the surface

            3.Provide the force to extrude lava

            4.Violence of an eruption is related to how easily gases escape from magma 

               a.Easy escape from fluid magma

               b.Viscous magma produces a more violent eruption

II. Materials associated with volcanic eruptions

   A. Lava flows

      1.Basaltic lavas are more fluid 

      2.Types of lava

         a.Pahoehoe lava

         b.Aa lava

   B. Gases 

      1.One to five percent of magma by weight

      2. Mainly water vapor and carbon dioxide 

   C. Pyroclastics 

      1. Fire Fragments

      2.Types of pyroclastic material 

         a.Ash- fine glassy fragments

         b. Pumice- from frothy lava

         c.Lapilli- walnut size

         d.Cinders- pea sized with voids

         e.Particles larger than lapilli

            1.Blocks- hardened lava

            2. Bombs- ejected as hot lava

III. Volcanoes 

   A. General features

      1.Opening at summit 

         a. Crater (steep-walled depression at the summit)

         b. Caldera- ( a summit depression greater than 1 km diameter)

      2.Vent (conduit that connects the crater to the magma chamber)

   B.Types of volcanoes 

      1.Shield volcano

         a.Broad slightly domed 

         b.Primarily made of basaltic lava

         c. Generally large

         d. Generally produce a large volume of lava

         e. e.g Mauna Loa in Hawaii

      2.Cinder cone 

         a.Built from ejected lava fragments 

         b.Steep slope angle 

         c. Rather small size

         d. Frequently occur in groups

      3. Composite cone 

         a. Most are adjacent to the Pacific Ocean

         b.Large Size

         c.Interbedded lavas and pyroclastics 

         d.Most violent type of activity 

         e.OFten produce nuee ardente 

            1.Fiery Cloud

            2.Hot gases infused with ash

            3. Flows down sides of a volcano

            4.Speeds up tp 200 km per hour

         f. May produce a lahar a type of mudflow

IV. Volcanic landforms

   A.Crater of caldera

      1.Steep walled depression at summit 

      2.Caldera- a crater that exceeds one km in diameter

   B.Volcanic neck

      1.Resistant vent left standing after erosion 

      2.e.g. Ship rock, new mexico

   C.Fissure eruption and lava plateau 

      1.Volcanic material extruded from fractures

      2. e.g. Columbia Plateau

   D.Pyroclastic flow 

      1.From silica-rich magma 

      2.Consists of ash and pumice fragments

      3.Material is propelled from the vent at a high speed

      4. e.g. Yellowstone plateau