Volcanoes

Chapter 13, Section 1 - Volcanoes and Plate Tectonics                                                                                                                        

Most rock in the mantle is solid due to the pressure on it.  (High pressure raises the melting point.)

Magma - Liquid rock produce below Earth's surface.

Three conditions that affect magma formation...
  • Temperature - if it rises above the melting point of the minerals in the rock.
  • Pressure - if enough is removed, the melting point will drop and the rock will melt.
  • Fluids - the addition of fluids (i.e. - water) can decrease the melting point of some minerals.

Volcanism - Any activity that includes the movement of magma toward or onto the Earth's surface.
  • Magma is less dense than the surrounding rock, so it rises.
  • As magma rises, it melts some of the surrounding rock.
  • Magma is forced into cracks in the surrounding rock, causing blocks of rock to break off and melt.
Lava - Magma that flows onto Earth's surface.
Volcano - A vent or fissure in the Earth's surface through which magma and gases are expelled.

Major Volcanic Zones
  • Most volcanoes form along convergent and divergent plate boundaries.
  • Pacific Ring of Fire - subduction zones around the edges of the Pacific Plate.
  • Subduction Zones
    • As one plate moves under another, water is pulled under and mixes with crust/mantle material.  This lowers the melting point and creates magma.
    • Oceanic-Continental Subduction
      • Forms a chain of volcanic mountains along the plate edge.
    • Oceanic-Oceanic Subduction
      • Forms a chain of volcanic islands along the plate edge.
        • Early stages - small islands (ex. - Aleutian Islands, Alaska)
        • Later stages - islands grow and combine (ex. - Japan)
  • Mid-Ocean Ridges
    • As plates pull apart, lava flows out and creates new lithosphere.
    • Mostly underwater volcanoes - an exception is Iceland.
  • Hot Spots
    • Hot Spot - A volcanically active area of Earth's surface, usually far from a tectonic plate boundary.  (ex. - Hawaii)
    • Mantle Plume - A column of hot, solid material from the deep mantle that rises towards the lithosphere.
      • Mantle plumes can generate magma, which can form volcanoes.
      • Mantle plumes remain stationary; the plates above them move.
        • This can generate a chain of hot-spot volcanoes, with the one directly over the hot spot being active.
          • Older ones, which are no longer over the mantle plume, become inactive.
      • Some mantle plumes are long and straight, creating a chain of active volcanoes.
        • These volcanoes do not have specific age-relationships to each other.

Intrusive Activity
  • As magma rises, it intrudes into the overlying rock.  [See Chapter 6]
  • Intruding magma can...
    • Melt surrounding rock.
    • Change the type of surrounding rock (contact metamorphism).
    • Create fractures/fissures in the rock.
    • Cause pieces of rock to fall into the magma (which are then included in the new igneous rock that forms as magma cools).
  • Pluton - Large formation of igneous rock, formed by intruded magma that has cooled.
    • Dikes - small plutons, only a few cm wide.
    • Batholiths - large plutons that cover at least 100 km2 at the surface


Chapter 13, Section 2 - Volcanic Eruptions                                                                                                                                            

Two general types of magma...
  • Mafic - rich in Mg and Fe, dark in color, more common in oceanic crust.
  • Felsic - rich in feldspar and silica, light in color, more common in continental crust.
Viscosity - The resistance to flow of a fluid.  (How "thick" the fluid is.)
  • Mafic magma has low viscosity (thin, runny, flows easily) less trapped gas, which leads to quiet eruptions.
  • Felsic magma has high viscosity (thick, sticky) and more trapped gas, which leads to explosive eruptions.
Quiet Eruptions
  • Low viscosity allows gases to escape easily.
  • Quickly cooling lava forms a crust, with lava flowing below.
    • Pahoehoe - Lava with a smooth, ropy surface.  Crust wrinkles as lava flows beneath.
    • Aa - Lava crust too thick for wrinkles, so it breaks into jagged chunks.
      • Same composition as pahoehoe, different gas content and/or rate/slope of lava flow.
    • Blocky Lava - Higher silica content, so more viscous.  Cooled surface lava breaks into large chunks.
Explosive Eruptions
  • High viscosity traps gases (H2O, CO2), causing pressure to build up.
  • During an eruption, gas escapes and throws solid/molten particles into the air.
    • Pyroclastic Material - Rock fragments formed during a volcanic eruption.
      1. Volcanic Dust - Particles <0.25mm.  Can travel around the world.
      2. Volcanic Ash - Particles <2mm in diameter.  Mostly settles on land around volcano.
      3. Lapilli - Particles <64mm in diameter.  Usually fall near vent.
      4. Volcanic Bombs - Large blocks, thrown out while molten.  Spin & cool, develop a round/spindle shape.
      5. Volcanic Blocks - Solid rocks blasted from the vent.  Can be as large as a small house.
Volcano Anatomy

There are three main parts to a volcano:

  • Vent - This is the crack in the crust where magma comes up from the mantle.
  • Crater - The funnel-shaped pit, at the top of the vent, where the lava comes out.  Formed as material is blown out by explosions.
  • Cone - This is the "mountain" that is built up around the volcano's crater.

Types of Volcanoes
  1. Shield Volcanoes
    • Broad at base, with gently sloping sides.
    • Form from quiet eruptions.
    • Mafic lava flows out, hardens, and builds up layers around the vent.
    • ex. - Hawaii
  2. Cinder Cones
    • Steep slopes, close to 40o.
    • Form from small explosive eruptions.
    • Made up of pyroclastic material.
    • Usually small - a few hundred meters tall.
    • ex. - Paricutin, Mexico.
  3. Composite Volcanoes (a.k.a. - Stratovolcanoes)
    • Volcano alternates between quiet and explosive eruptions.
    • Made of alternating layers of lava and pyroclastic material.
    • Often are large volcanic mountains.
    • ex. - Vesuvius, Italy
Caldera - A large, circular depression that forms when the magma chamber below a volcano partially empties and causes the ground above to sink.
                ex. - Krakatau.
                ex. - Crater Lake, Oregon.

Predicting Volcanoes
  • Changes in earthquake activity, especially increases in strength and frequency.
  • Changes in the tilt of the volcano's slopes, which may indicate magma moving towards the surface (causing it to bulge).
  • Changes in amount and composition of gases released by the volcano.



Chapter Assignment
  • Pg. 362-363 #5-24, 35-37.


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